text:
As you know, we make no pretense at being experts in the world of hockey. (Or anything, really.) Because the season is starting, like, this weekend, we've asked Eric McErlain, of Off Wing Opinion, to preview the upcoming season for us. We don't know anyone who knows hockey better than he does, so he's your man. Enjoy.
While plenty of folks in the hockey blogosphere might not always find Tom Benjamin's Canucks Corner to their taste, one thing is for sure — you can never accuse Tom of sloppy thinking. And it's thanks to that commitment to hard analysis that helped uncover one of the absolute truths about the league last season: In the NHL, the West is simply the best.
How much better? Well, let's just say that once the Anaheim Ducks battered their way into the Stanley Cup Finals, they had already endured the toughest stretch in their schedule. Better yet, it's safe to say that at least two other Western Conference clubs — the Detroit Red Wings and the San Jose Sharks — could have made equally quick work of the Eastern Conference Champion Ottawa Senators in the Finals if they had only had the chance.
Over at FanHouse, we've been previewing the entire league since the end of August. Looking at the Western Conference along with me (Pacific) were James Mirtle (Central) and Jes Golbez (Northwest). Teams are listed in my predicted order of finish. To see what the rest of FanHouse has to say, including our picks for the Cup and all the other major hardware, stop by on Friday morning. Along with each of the capsules below, I'll also give you at least one solid suggestion on where you ought to be getting your inside scoop.
As for the East, check back here at Deadspin on Friday for the rest of the skinny.
Central Division: While Chicago, St. Louis and Columbus all come into the season marginally improved, don't expect them to do much more than slow down a Red Wings club that's still the class of the division. With Andy Murray able to enjoy his first full season in St. Louis, the Blues may challenge for a playoff spot, but the real sideshow in the Central is in Nashville. With ownership questions still unresolved and a severely denuded lineup, one wonders whether or not Head Coach Barry Trotz will be able to prevent his team from disintegrating before the home stretch. Come April, there might not be much of a team to move to Vegas, Kansas City, Hamilton or points in between.
Detroit Red Wings: Ho-hum. Another year, another division title. Leave it to the Red Wings to respond to their one major personnel loss (Mathieu Schnieder to Anaheim) by picking up Brian Rafalski, who's a Michigan native to boot. Expect him to enjoy the best season of his career playing on a blue line with Nicklas Lidstrom. Anyone else who left town, in particular Todd Bertuzzi, Kyle Calder and Robert Lang, is simply a matter of addition by subtraction. And while there will always be questions raised about Dominik Hasek, Chris Osgood is more than good enough to plug the gaps as long as "The Dominator" isn't on the shelf for too long. As for the playoffs, that's another story entirely, where nothing but bad luck kept the Wings from advancing to the Finals last season. For the inside scoop, try Abel to Yzerman (part of the Kukla empire), or if that isn't your style, check in with everybody's plucky little sister, Christy Hammond, at Behind the Jersey.
Nashville Predators: While there might be more tragic stories in hockey , there isn't any one sadder than Nashville. From the beginning, General Manager David Poile took the blueprint that he developed in Washington and put it to work building another hard working, defensively-responsible squad. Then, when the time was right, ownership stepped up with the money to import the superstars to take the team over the top (Paul Kariya, Peter Forsberg). The result was one of the best regular season performances in the league over the last two seasons (an eye-catching 216 points), combined with a pair of first round playoff exits to a San Jose team that was simply better. Now the highlight reel goalie — Tomas Vokoun — is gone, along with the superstar imports and a number of character players who kept the squad glued together. If Trotz can make the survivors believe, this team may still be able to make the playoffs. But while they may die hard, I'm afraid they'll still die. Watch the drama unfold at On The Forecheck.
St. Louis Blues: At the start of last season in St. Louis it was easy to laugh. From the drop of the first puck, most of North America couldn't understand what new Team President John Davidson really had in mind going into the season with an aging core of veterans. The results were predictable as former head coach Mike Kitchen helmed a cacophonous mess that was going nowhere. But credit Davidson for stepping in when he did and replacing Kitchen with ex-Kings Head Coach Andy Murray, who responded at one point by leading the team on a 13-2-4 run. Safe to say, there's hope in St. Louis, where the core of a decent blue line will be supplemented by Calder candidate Erik Johnson. Up front, Nashville refugee Paul Kariya ought to add some scoring punch to go along with aging vets Doug Weight and Keith Tkachuk — a pair always wiling to be traded at the deadline as long as they know they'll be back in St. Louis the following season. Share the hope at St. Louis Game Time.
Chicago Blackhawks: Everything that's happened in the offseason pales in comparison with the news today of the death of Blackhawks owner Bill Wirtz of cancer at the age of 77. A stubborn traditionalist in every sense of the word, Wirtz did it his way every time, all the time, something that earned him the ire of plenty of Chicago hockey fans, never more than when he gradually broke up a team in the 1990s that had been built around Chris Chelios, Jeremy Roenick and Ed Belfour. In recent years, as the team missed the playoffs in eight of the last nine seasons and crowds at the United Center continued to dwindle, anger among the true believers — few though they are these days — continued to harden. In light of his death, news earlier this year that he was considering a sale of the team has to be seen in a new light. One thing is for sure, no matter what happens to the team, life is going to change radically after an owner with a tenure of 41 years passes from the scene.
As for this year's club, there's reason to believe that things are going to improve, just not all that much. The high draft picks are beginning to pile up, including the two most recent top picks, Patrick Kane and Jonathan Toews. Free agents Yannic Perrault and Robert Lang will help up front, but not enough to get this club within spitting distance of the playoffs.
Columbus Blue Jackets: With ex-GM Doug McLean gone forever, and his destructive powers now firmly focused on the Tampa Bay Lightning (Hey Doug, the fruit basket is from Francois Beauchemin!), the BJs can begin their long arduous climb to respectability. Yes, things are looking up, and a full season with Head Coach Ken Hitchcock behind the bench will help. But there are some things that even Hitch can't do, and one of those is coaxing a .500 performance out of a roster that's severely overmatched. Otherwise, there are some mistakes from the past that are going to take a few cycles to come out in the wash — like Sergei Federov. The good news: The owner is committed to the rebuilding plan and the fans are an absolute revelation. Let's just say their greeting to the Commissioner on Draft Night 2007 was one for the ages. If/when this team finally makes the playoffs, we're all in for a treat. Until then, grit it out at the End of the Bench.
Northwest Division: While it might not be the toughest division in hockey — that's the Pacific — the arrival of Ryan Smyth in Colorado and Mike Keenan in Calgary may make the Northwest the deepest in the league. If you're looking for offensive fireworks, this is not the place to be, as three of the top goalies in all of hockey — Roberto Luongo, Niklas Backstrom and Miika Kiprusoff — all call it home. Expect the four top teams to qualify for the playoffs, with "lowe"-ly Edmonton settling into a comfortable position in the draft lottery. One last thought: The over/under on Keenan making an example of retro-scapegoat Kristian Huselius is December 20, 2007.
Vancouver Canucks: Life's a lot easier when you know you can count on the best goaltender on the planet to save your bacon. That was a lesson the Vancouver Canucks learned early and often last season as Roberto Luongo settled in just fine after being liberated from his tropical hockey prison in South Florida. Unfortunately, while the Canucks are still the best team that Luongo has ever played behind, they don't have nearly enough punch up front — and that's with Markus Naslund and the Sedin twins on the roster. To get deep into the playoffs, Luongo is going to have to lay off the caffeine and put in regular super-human efforts like he did last spring in the first round against the Dallas Stars. For more, visit Canucks Hockey Blog.
Minnesota Wild: The Minnesota Wild are an object lesson in what life is like in the Western Conference for teams that are merely good and not great. After a fine regular season where they totaled an impressive 104 points, the defensively responsible Wild — a team that boasts one of the candidates for toughest man in the NHL in Derek Boogaard — were simply brutalized in a five game playoff series with the eventual Stanley Cup Champion Ducks. Coming into this season, all the keys to success are still present in Minnesota, including a passel of offensively-talented players who have proven they'll submit to the dictates of Head Coach Jacques Lemaire's defense-first system, as well as the surprising play of mid-career Finnish import goalie Niklas Backstrom. If this were the mid-1990s I'd say that the Wild were perfectly suited for a deep run in the postseason, but now I'm afraid that the one thing they lack — physical play nearly to the point of brutality — will hold them back come April and May. Discuss their chances at Wild Puck Banter.
Calgary Flames: Mike Keenan is nothing if not entertaining, which should make following the Calgary Flames a lot more interesting than last year's edition under the vanilla Jim Playfair. Making it even more interesting is the thought that Keenan has to work for Darryl Sutter, one of the few GMs in the league who can look Keenan in the eye and tell him where to go. As for changes, the blue line has been completely re-shuffled, but when you're playing in front of a goalie like Miika Kiprusoff, worries there tend to fall by the wayside. And while the scoring up front was a little better last season, it's still not good enough to get the job done. When in Calgary always remember to travel the Red Mile with The Battle of Alberta.
Colorado Avalanche: Thanks to a furious finish and an offseason that saw them add free agent winger Ryan Smyth up front, the Avs have to be a favorite to return to the postseason — until you turn to the goaltending. Jose Theodore is simply done, it's just too bad former GM Pierre Lacroix isn't around to see the damage the former Vezina winner is doing to Colorado's salary cap. And while Peter Budaj is mildly better, few teams in the NHL quake in fear when they see him starting in net. So while Colorado might have enough to sneak into the postseason, they don't have nearly enough to make an impression. Feel the hockey fellowship at Jerseys and Hockey Love.
Edmonton Oilers: Just as the New York Jets and their fans have paid a dear karmic price for victory in Super Bowl III, so have fans of the Edmonton Oilers paid for their surprise trip to the 2006 Stanley Cup Finals. Since then, they've seen two of the biggest names in the game (Chris Pronger and Ryan Smyth) shipped out of town as they dropped thorugh the standings like a rock. After the deadline trade of Smyth to Long Island, the team simply fell apart down the stretch, leaving all of the offseason pressure firmly on the back of GM Kevin Lowe. He responded by importing free agent defenseman Sheldon Souray from Montreal. While he might have a cannon for a slap shot, Souray's deficiencies on defense are well known. Lowe later devoted another chunk of cash to luring restricted free agent winger Dustin Penner away from Anaheim with an insane five-year $21.25 million offer sheet. What it all adds up to, I'm at a loss to say. It's easy to get intimidated looking at the "Oilgosphere", one of the most intense online hockey communities around, so don't forget, it's safest to wade in at Lowetide.
Pacific Division: Scratch Los Angeles and Phoenix off of your NHL Center Ice schedule, as the Pacific is a three dog race (to those who are interested, LA will be better, but not so much as to disrupt the balance of power — watching Phoenix may be brutal). Anaheim is a proud — and bare knuckled — defending champ, but they're dealing with a number of key personnel losses and early injuries. Don't expect the team that takes to this ice this weekend for a two-game series with the Kings in London that will open the regular season to look like the squad that hoisted the Cup barely three months ago. San Jose hasn't done much to its lineup, but it's young talent is one year older, and the Ducks are wounded. Dallas made few changes, and is counting on increased production from a number of the usual suspects. They've also added some toughness around the margins (please say a prayer for Todd Fedoruk), but it might not be enough.
San Jose Sharks: Say hello to a team and a head coach that's running out of excuses. Wilson has gotten the Sharks to the 2nd round of the playoffs in all his three full seasons in San Jose, but that won't be enough for him to keep his job this time. Either he gets this team to within shouting distance of the Cup or he's done. With the lineup that the Sharks boast — one that includes former MVP Joe Thornton, Team Captain Patrick Marleau and Goalie Evgani Nabokov down the middle — most other head coaches have to be pretty envious. The rest of the lineup is young, big and fast — with the exception of curious offseason pickup Jeremy Roenick, who Wilson insists will play meaningful minutes. And if it isn't enough come the trade deadline, the Sharks have plenty of cap space for GM Doug Wilson to burn. Few hockey bloggers are as equally adept at the keyboard and behind the camera lens as Jon Swensen at Sharkspage.
Anaheim Ducks: I'm sure Anaheim GM Brian Burke, a man who seems to be getting more credit for last June's Stanley Cup win than Head Coach Randy Carlyle, found a way to enjoy his brief offseason. Getting to spend a day with the Cup must certainly help take the edge off of your day. Then again, seeing your top goal scorer (Teemu Selanne) and a Conn Smythe Trophy winning defenseman (Scott Niedermayer playing the role of Bret Favre) unable to make up their minds about retirement couldn't have been fun. Neither is with the team. Niedermayer has been suspended, while Selanne is without a contract. Then there was the Dustin Penner defection to Edmonton. Both goalie J.S. Giguere and Sami Pahlsson, the best defensive forward in the game last season, are recovering from hernia surgery. And the one critical part you imported to shore up the blue line in case Niedermayer didn't come back, Mathieu Schneider, is out with an injury to start the season. With the loss of that much scoring, I'm expecting more physical play from the Ducks. Did I forget to mention that Todd Bertuzzi has found a home in Anaheim? The drink of choice is Jack and Coke at Battle of California. Some find out the hard way, you don't have to.
Dallas Stars: In a way, the Stars are simply a hockey team in the wrong place at the wrong time. They have one of the most underrated coaches in the game in Dave Tippet, and there were few things that the Stars didn't do well last season outside of putting the puck in the net, and that was mostly attributable to injuries that kept Team Captain Brendan Morrow and ex-Captain Mike Modano on the shelf for extended periods. They've got the goaltending in Marty Turco, they've got the total team concept down and they have one of my favorite agitators in Krys Barch. Unfortunately, they simply don't have the firepower to challenge either San Jose or Anaheim. Expect them to make the playoffs, but it won't be a very long stay. Until then, have some laughs with Daryl Reaugh.
Los Angeles Kings: I'd like to tell you that the Kings will be a much improved team this season, as Head Coach Mark Crawford works to meld an impressive young core (Anze Kopitar, Dustin Brown, Alexander Frolov, Mike Cammalleri, Jack Johnson and Patrick O'Sullivan) with mid-tier free agents (Kyle Calder, Brad Stuart, Michael Handzus, Tom Preissing and Jon Klemm) and some solid veterans (Rob Blake, Derek Armstrong and Lubomir Visnovsky). Unfortunately, it all starts in goal, and I don't believe that Jonathan Bernier, J.S. Aubin or Jason LaBarbera (much as I might want to root for him) present any sort of solution either alone or in concert. One of the best kept secrets in hockey blogging has to be Rich Hammond's work at Inside the Kings.
Phoenix Coyotes: So this is what it was like when Bill Russell coached the Sacremento Kings. After the first two years of the second act of his NHL career, Wayne Gretzky has to be wondering where it all went wrong. Outside of Shane Doan and Ed Jovanovski, there's just not a whole lot of NHL caliber talent here, and while new General Manager Don Maloney already has plans in place to right the ship, the changes aren't going to come in time to help this season. Maybe if Phoenix played in another division the situation wouldn't be so dire. Unfortunately, they have 24 games against San Jose, Anaheim and Dallas and those squads tend to be unforgiving when it comes to standings points. Will Gretzky, who will never be fired, make it through the season — especially if he has to do it without his buddy Rick Tocchet? Read the tea leaves at One Fan's Perspective.


text:
DAB 2 KVC AD 85/120T Twin Pump Booster Sets
DAB 2 KVC AD 85/120T Twin Pump Booster Sets
DAB 2 KVC AD Twin Pump Booster Set is equipped with Active Driver are designed and manufactured to meet the constant pressure requirements imposed by modern water distribution system technology. Constant pressure control is used in a many number of applications in a various range of sectors. The basic principles that guided our technical engineers in developing these pump sets simplicity,flexibility and reliability.
- Water pipelines
- Irrigation
- Industry
- Hotels
- Residential buildings
- Spa centres
Short Induction to the Active Driver
The Active Driver module is an all-inclusive device that contains water pipeline connections, pressure sensor, flow sensor and an electronic inverter. When this is added to the discharge line of each electric pump the Active Driver controls the speed of the pump. It is connected in a way that ensures constant pressure irrespective of changes in the water flow rate needs. The water that flows though the Active Drivers connections also helps decrease the heat output created by the internal electronic components.
There are no reviews yet.
Add review
Delivery Information
Free delivery relates to MOST products on the Website over the value of £100.00 (excluding VAT).
*Unless otherwise stated on individual items*
Products purchased under £100.00 will incur a £6.95 delivery charge.
Standard Delivery Orders over £100.00 (3-5 working days)* - Free shipping for in stock items to mainland UK*.
Next day shipping is only available for stock items which are ordered before 14:00 at a charge of £12.50*. Any orders that are placed after this time will be processed the following day.
Specified delivery times are based on orders placed before 2pm Monday to Friday (excludes Weekends and Bank Holidays) - for more details please see "UK Shipping Charges" below.
Weekend or orders placed on a Bank holiday will be dispatched on the next working day.
*Excludes UK Islands, Republic of Ireland and Scottish Highlands.
During Christmas and New Year our warehouse will be closed. Orders over this period will be dispatched on the first working day after New Year.
Delivery times are subject to change if the item is out of stock with our supplier. There are products on our Website that are not stocked at our warehouse, nor with our manufacturer's UK warehouses, therefore lead times can vary. This is why we recommend you contact us prior to purchasing to check current stock levels and lead times.
If you are not available to take a delivery on the specified date, you must let us know immediately. If we attempt to deliver your order and no-one is in, a re-delivery charge may be incurred.
As we use 3rd party delivery agents, Complete Pump Supplies are unable to guarantee that all deliveries will be the next working day. Complete Pump Supplies will not be liable for any loss of income, loss of profits, loss of markets, loss of reputation, loss of customers, loss of use, loss of an opportunity even if we had knowledge that such damages or loss might arise or for any indirect, incidental, special or consequential damages or loss howsoever arising.

subset: commoncrawl


text:
Free Entry
Swim all-year round at Gould Adams Park Aquatic Centre in the City of Logan. Visit this modern indoor facility take your pick of three heated pools. You can swim laps in the 25 metre pool, testing your fitness or just getting some exercise. The youngsters can play in the kiddies adventure pool with its fun water features. There's also a learn-to-swim pool.


text:
Photo by Susie Fitzhugh
SEATTLE – June 6, 2013 – Fred Hutchinson Cancer Research Center scientists have, for the first time, mapped a young gene's short, dramatic evolutionary journey to becoming essential, or indispensable. In a study published online June 6 in Science, the researchers detail one gene's rapid switch to a new and essential function in the fruit fly, challenging the long-held belief that only ancient genes are important.
"We really haven't paid much attention to what is new, because there's so much emphasis on what is old," said Harmit Singh Malik, Ph.D., a member of the Hutchinson Center's Basic Sciences Division and senior author of the study. "This work breaks the paradigm that new genes by definition are not really that important, because if they were important they would be much older."
The less a gene has mutated over the millions or billions of years of its lifespan, the more likely it is to play a key role for its host. But it turns out that the converse is not necessarily true. Young and rapidly evolving genes can be indispensable too. Scientists at the University of Chicago were surprised to find in 2010 that young genes acquire essential functions in the same proportion as old genes, but nobody had explored the biology behind the phenomenon.
Malik and his colleagues examined one such case in detail, starting from the gene's birth and its pathway to a new purpose and evolutionary importance. The fruit fly Drosophila melanogaster, a common model organism in laboratory studies, is host to the relatively new gene Umbrea, which duplicated and began to diverge from its parent gene a mere 15 million years ago.
Courtesy of the Malik Lab, Fred Hutchinson Cancer Research Center
The scientists compared Umbrea with its parent gene, HP1B, by looking at where each gene's protein product tracks within the cell. Both proteins coat chromosomes, but the Umbrea protein specifically hones to centromeres, the specialized centers that help chromosomes duplicate and segregate when cells divide.
In contrast to its parent gene, fruit flies need Umbrea to survive, and the researchers found that cells missing the Umbrea protein cannot shuffle their chromosomes to the correct cell during cell division. They then tracked the gene's evolutionary progress from dispensable to crucial by examining Umbrea's genetic differences between related fruit fly species and pinpointed a short stretch of DNA that led to the gene's essentiality.
Understanding how Umbrea accomplished its quick transition to its new role could help scientists pinpoint other processes in the cell that are subject to the same form of rapid evolution as centromere proteins, including genes that mutate to become essential for the survival of cancer cells. Such evolution of cancer genes happens extremely quickly, in the lifetime of one individual.
Umbrea's rise was not quite as fast, but 15 million years is brief when compared to the majority of essential genes four to five times its age in the fruit fly, and a few important genes that are more than a billion years old.
"The genetic conflicts that lead to quick changes like those found in Umbrea can also shed light on evolutionary arms races between chromosomes, or even between viruses and host immune genes," said Benjamin Ross, a graduate research assistant in Malik's laboratory and lead author of the study.
The team's findings also point to potential limitations for laboratory model organisms as windows into our own health. While many genes do perform similar tasks in such disparate animals as fruit flies and humans, scientists may be overlooking genes unique to humans that are nevertheless vital. Although Drosophila melanogaster needs Umbrea for its survival, several other very closely related fruit fly species lack the gene entirely.
"What might be essential in one species or even in one tissue type, like a cancer cell, might not be essential in another tissue type or related species," said Malik, also a Howard Hughes Medical Institute Early Career Scientist.
Researchers from the University of Connecticut in Storrs, Conn., and Ludwig Maximilians University of Munich in Germany also contributed to the study. The National Institutes of Health, the National Science Foundation, the Howard Hughes Medical Institute, the European Union Network and the Mathers Foundation funded the research.
Editor's note: To obtain a copy of the embargoed Science paper, "Stepwise evolution of essential centromere function in a Drosophilia neogene," please contact the journal at 202-326-6440 or firstname.lastname@example.com.
At Fred Hutchinson Cancer Research Center, home to three Nobel laureates, interdisciplinary teams of world-renowned scientists seek new and innovative ways to prevent, diagnose and treat cancer, HIV/AIDS and other life-threatening diseases. Fred Hutch's pioneering work in bone marrow transplantation led to the development of immunotherapy, which harnesses the power of the immune system to treat cancer. An independent, nonprofit research institute based in Seattle, Fred Hutch houses the nation's first National Cancer Institute-funded cancer prevention research program, as well as the clinical coordinating center of the Women's Health Initiative and the international headquarters of the HIV Vaccine Trials Network.


text:
Precambrian geology
Major subdivisions of the Precambrian System
By international agreement, Precambrian time is divided into the Archean Eon (occurring between roughly 4.0 billion years ago and 2.5 billion years ago) and Proterozoic Eon (occurring between 2.5 billion and 541 million years ago). After the Precambrian, geologic time intervals are commonly subdivided on the basis of the fossil record. The paucity of Precambrian fossils, however, precludes the creation of small-scale subdivisions (epochs and ages) in this time period. Instead, relative chronologies of events have been produced for different regions based on such field relationships as unconformities (interruption in the accumulation of sedimentary rock due to erosion or nondeposition) and crosscutting dikes (intrusions of igneous rock that burrow through cracks in the original structures of surrounding rock). These field relationships, combined with the isotopic age determinations of specific rocks, allow for some correlation between neighbouring regions. The International Commission on Stratigraphy (ISC) and International Union of Geological Sciences (IUGS) divide the Archean Eon into the Eoarchean (approximately 4.0 billion to 3.6 billion years ago), Paleoarchean (3.6 billion to 3.2 billion years ago), Mesoarchean (3.2 billion to 2.8 billion years ago), and Neoarchean (2.8 billion to 2.5 billion years ago) eras. Likewise, they divide the Proterozoic Eon into the Paleoproterozoic (2.5 billion to 1.6 billion years ago), Mesoproterozoic (1.6 billion to 1 billion years ago), and Neoproterozoic (1 billion to 541 million years ago) eras. These definitions are based on isotopic age determinations.
Oldest minerals and rocks
The oldest minerals on Earth, detrital zircons from western Australia, crystallized about 4.4 billion years ago. They occur within sedimentary sandstones and conglomerates dated to about 3.3 billion years ago, but the environment in which they were formed is totally unknown. The rocks from which they came may have been destroyed by some kind of tectonic process or by a meteorite impact that spared individual zircon crystals. On the other hand, rocks containing these minerals may still exist on Earth's surface but simply have not been found. Perhaps their very absence is indicative of something important about early terrestrial processes. Comparisons with the Moon indicate that the Earth must have been subjected to an enormous number of meteorite impacts about 4 billion years ago, but there is no geologic evidence of such events.
The oldest known rocks on Earth are the faux amphibolite volcanic deposits of the Nuvvuagittuq greenstone belt in Quebec, Canada; they are estimated to be 4.28 billion years old. The age of these rocks was estimated using a radiometric dating technique that measures the ratio of the rare-earth elements neodymium and samarium present in a sample.
The Acasta gneisses, found near Canada's Great Slave Lake, are also among the world's oldest rocks. Their age has been established radiometrically at 4.0 to 3.9 billion years. The Acasta gneisses are granitic and contain a single relict zircon crystal, which has been dated to 4.2 billion years ago and formed from granitic magma. They are thought to have evolved from older basaltic material in the crust that was melted and remelted by tectonic processes.
Significant geologic events
Distinctive features
The Archean and Proterozoic eons within Precambrian time are very different and must be considered separately. The Archean-Proterozoic boundary constitutes a major turning point in Earth history. Before that time the crust of the Earth was in the process of growing, and so there were no large, stable continents. Afterward, when such continents had emerged, orogenic belts were able to form on the margins of and between continental blocks.
There are two types of Archean orogenic belts. The first occurs in upper crustal greenstone-granite belts rich in volcanic rocks that are probably primitive types of oceanic crust and island arcs (long, curved island chains associated with intense volcanic and seismic activity) that formed during the early rapid stage of crustal growth. The second occurs in granulite-gneiss belts that were recrystallized in the Archean mid-lower crust under metamorphic conditions associated with high-temperature granulite and amphibolite facies. Thus, granulites, which typically contain the high-temperature mineral hypersthene (a type of pyroxene), are a characteristic feature of many Precambrian orogenic belts that have been deeply eroded. In Phanerozoic orogenic belts, granulites are rare.
There are several other rock types that developed primarily during the Precambrian but rarely later. This restriction is a result of the unique conditions that prevailed during Precambrian time. For example, banded-iron formations are ferruginous sediments that were deposited on the margins of early, iron-rich oceans. Anorthosite, which consists largely of plagioclase, forms large bodies in several Proterozoic belts. Komatiite, a magnesium-rich, high-temperature volcanic rock derived from very hot mantle (part of the Earth between the crust and the core), was extruded in abundance during the early Precambrian when the heat flow of the Earth was higher than it is today. Blueschist, which contains the blue mineral glaucophane, forms in subduction zones under high pressures and low temperatures, and its rare occurrence in Precambrian rocks may indicate that temperatures in early subduction zones were too high for its formation.
The bulk of many of the world's valuable mineral deposits (for example, those of gold, nickel, chromite, copper, and iron) also formed during the Precambrian. These concentrations are a reflection of distinctive Precambrian sedimentary and magmatic rocks and their environments of formation.
Archean crustal growth
During the first third of geologic history (that is, until about 2.5 billion years ago), the Earth developed in a broadly similar manner. Greenstone-granite belts (metamorphosed oceanic crust and island arc complexes) formed in the upper Archean crust, and granulite-gneiss belts formed in the mid-lower crust. This was a time when the overall rate of heat production by the breakdown of radioactive isotopes was several times greater than it is today. This condition was manifested by very rapid tectonic processes, probably by some sort of primitive plate tectonics (more-modern plate-tectonic processes could not occur until the crust became cooler and more rigid). Most of the heat that escapes from Earth's interior today does so at oceanic ridges. This manner of heat loss probably occurred during the Archean in much larger amounts. The oceanic ridges of the Archean were more abundant, longer, and opened faster than those in the modern oceans, and oceanic plateaus derived from hot mantle plumes (slowly rising currents of highly viscous mantle material) were more common. Although the amount of newly generated crust was probably enormous, a large part of this material was inevitably destroyed by equally rapid plate subduction processes. The main results of this early growth that still remain today are the many island arcs and oceanic plateaus in greenstone-granite belts and the voluminous Andean-type tonalites (a granitic-type rock rich in plagioclase feldspar) that were deformed to orthogneiss (gneiss derived from igneous rocks) in granulite-gneiss belts. Although most of the Archean oceanic crust was subducted, a few ophiolitic-type complexes have been preserved in greenstone-granite belts.
The late Archean (Neoarchean Era) was an important interval of time because it marks the beginning of the major changeover from Archean to Proterozoic types of crustal growth. The formation of the first major rifts characterized the significant events of this time. The first major rift valley known in the world, the Pongola Rift, emerged along the border of present-day Swaziland and South Africa; the intrusion of the first major basic dikes (such as the Great Dyke, which transects the entire Zimbabwe craton) and the first large stratiform layered igneous complexes (such as the Stillwater in Montana) formed; and the formation of the first large sedimentary basins (for example, the Witwatersrand in South Africa) also occurred. All of these structures indicate that the continental crust had reached a mature stage with considerable stability and rigidity for the first time during the late Archean. The Neoarchean represents the culmination that followed the rapid tectonic processes of the early Archean (Eoarchean and Paleoarchean) and middle Archean (Mesoarchean) eras. Because crustal growth took place at different times throughout the world, similar structures can be found in the early Proterozoic (Paleoproterozoic) Era.
Proterozoic plate movements
During the early Proterozoic, large amounts of quartzite, carbonate, and shale were deposited on the shelves and margins of many continental blocks. This would be consistent with the breakup of a supercontinent into several smaller continents with long continental margins (combined areas of continental shelf and continental slope). Examples of shelf sequences of this kind are found along the margins of orogenic (mountain) belts, such as the Wopmay, bordering Canada's Slave province, and also the Labrador Trough, bordering the Superior province.
The existence of stable continental blocks by the early Proterozoic allowed orogenic belts to develop at their margins by some form of collision tectonics. This was the first time that long, linear orogenic belts could form by "modern" tectonic processes that involved seafloor spreading, ophiolite obduction, subduction, and landmass collisions. Subduction lead to the creation of island arcs and Andean-type (formed by subduction at the continental margin) granitic batholiths. In addition, the collision of arcs and continents could now give rise to both sutures with ophiolites and to Himalayan-type (formed by continent-to-continent collision) thrust belts with abundant crustal-melt granites. These were key events in the evolution of the continents, and such processes have continued throughout Earth history.
During the late Proterozoic (Neoproterozoic Era), some orogenic belts, like the Pan-African belts of Saudi Arabia and East Africa, continued to develop. The intense crustal growth and the many orogenic belts that formed throughout the Proterozoic began to create large continental blocks, which amalgamated to produce a new supercontinent by the end of the Precambrian. Therefore, in the late Proterozoic many sedimentary basins were infilled with conglomerates and sandstones due to the deposition of material eroded from higher elevations. For example, the Riphean sequence in Russia and also the Sinian sequence in China were able to form on extensive cratons of continental crust.
Occurrence and distribution of Precambrian rocks
Precambrian rocks, as a whole, occur in a wide variety of shapes and sizes. There are extensive Archean regions, up to a few thousands of kilometres across, that may contain either greenstone-granite belts or granulite-gneiss belts or both. These regions are variously designated in different parts of the world as cratons, shields, provinces, or blocks. Some examples include: the North Atlantic craton that incorporates northwestern Scotland, central Greenland, and Labrador; the Kaapvaal and Zimbabwean cratons in southern Africa; the Dharwar craton in India; the Aldan and Anabar shields in Siberia in Russia; the Baltic Shield that includes much of Sweden, Finland, and the Kola Peninsula of far northern Russia; the Superior and Slave provinces in Canada; and the Yilgarn and Pilbara blocks in Western Australia. Linear belts, up to several thousand kilometres long, that are frequently though not exclusively of Proterozoic age include the Limpopo, Mozambique, and Damaran belts in Africa, the Labrador Trough in Canada, and the Eastern Ghats belt in India. Several small relict areas, spanning a few hundred kilometres across, exist within or against Phanerozoic orogenic belts and include the Lofoten islands of Norway, the Lewisian Complex in northwestern Scotland, and the Adirondack Mountains in the northeastern United States. Nevertheless, some extensive areas of Precambrian rocks, such as under the European and Russian platforms and under the central United States, remain overlain by a blanket of Phanerozoic sediments.
Archean rock types
Archean rocks occur in greenstone-granite belts that represent the upper crust, in granulite-gneiss belts that formed in the mid-lower crust, and in sedimentary basins, basic dikes, and layered complexes that were either deposited on or intruded into the first two types of belts.
Greenstone-granite belts
These belts occur on most continents. The largest extend several hundred kilometres in length and measure several hundred metres in width. Today many greenstone-granite belts are regarded as tectonic "slices" of oceanic and island arc crust that have been thrust together to form tectonic collages similar to those in belts found in the present-day Pacific Ocean.
The greenstone sequence in many belts is divisible into a lower volcanic group and an upper sedimentary group. The volcanics are made up of lavas that are ultramafic (silica content less than 45 percent) and basaltic (silica content of 45 to 52 percent). The uppermost sediments are typically terrigenous (land-derived) shales, sandstones, quartzites, wackes, and conglomerates. All the greenstone sequences have undergone recrystallization during the metamorphism of greenschist facies at relatively low temperatures and pressures. In fact, the presence of the three green metamorphic minerals chlorite, hornblende, and epidote has given rise to the term greenstone for the recrystallized basaltic volcanics. Granitic rocks and gneisses occur within, adjacent to, and between many greenstone sequences.
Economic significance of Archean greenstone-granite deposits
Abundant mineralization has occurred in greenstone-granite belts. These belts constitute one of the world's principal depositories of gold, silver, chromium, nickel, copper, and zinc. In the past they were termed gold belts because of the gold rushes of the 19th century that took place in areas such as Kalgoorlie in the Yilgarn belt of Western Australia, the Barberton belt of South Africa, and Val d'Or in the Abitibi belt of southern Canada. The mineral deposits occur in all the major rock groups: chromite, nickel, asbestos, magnesite, and talc in ultramafic lavas; gold, silver, copper, and zinc in basaltic to rhyolitic volcanics; iron ore, manganese, and barite in sediments; and lithium, tantalum, beryllium, tin, molybdenum, and bismuth in granites and associated pegmatites. Important occurrences are chromite at Selukwe in Zimbabwe, nickel at Kambalda in southwestern Australia, tantalum in Manitoba in Canada, and copper-zinc at Timmins and Noranda in the Canadian Abitibi belt.
Greenstone-granite rock types
The volcanics that comprise the lower portion of a greenstone sequence are made up of lavas noted for magnesian komatiites (ultramafic extrusive igneous rocks) that probably formed in the oceanic crust that are overlain by basalts, andesites, and rhyolites whose chemical composition is much like that of modern island arcs. Especially important is the presence in the Isua, Barberton, and Yellowknife belts of sheeted basic dike complexes cutting across gabbros and overlain by pillow-bearing basalts (basalts extruded underwater that form characteristic pillow-shaped hummocks). Volcanic sequences are capped by oceanic cherts and terrigenous sedimentary groups. The overall stratigraphy suggests an evolution from extensive submarine eruptions of komatiite and basalt (ocean floor) to more-localized stratovolcanoes (volcanoes constructed from alternating layers of ash and lava), which become increasingly emergent with intervening and overlying clastic sediments (clay-, silt-, and sand-sized sediments) that were deposited in trenches at the mouths of subduction zones. There are, however, regional differences in the volcanic and sedimentary makeup of some belts. The older belts in southern Africa and Australia have more komatiites, basalts, shallow-water banded-iron formations, cherts, and evaporites and fewer terrigenous (land-derived) sediments. On the other hand, the younger belts in North America have a higher proportion of andesites, rhyolites, and terrigenous and turbidite debris (sediments delivered to the deep ocean by density currents) but fewer shallow-water sediments. These differences reflect a change from the older oceanic-type volcanism (effusion of lava from submarine fissures) to the younger, more arc-type phenomena such as explosive eruption of pyroclastic materials (incandescent material ejected during violent eruptions) and lava flows from steep volcanic cones. Additional changes include an increase in the amount of trench (subduction zone) turbidites and graywackes and an increase in the availability of continental crust as a source for terrigenous debris.
Ultramafic rocks (rocks with a very low silica content—less than 45 percent) are commonly altered to talc schists and tremolite-actinolite schists. There are some indications that several phases of metamorphism exist—namely, seafloor metamorphism associated with the action of hydrothermal brines that could occur at oceanic ridges, syntectonic metamorphism related to thrust-nappe tectonics, and local thermal contact metamorphism caused by intrusive granitic plutons pushing into cooler surrounding rock.
Granitic rocks and gneisses are associated with many greenstone sequences. Some paragneisses (gneisses metamorphosed from sedimentary rocks), as in the Quetico belt in Canada, are derived from wackes. They were probably deposited in an ocean trench or accretionary prism (a mass of accumulating sediments on the inner trench wall in a subduction zone) at the mouth of a subduction zone between the island arcs of the adjacent greenstone sequences. Many early granitic plutons were deformed and converted into orthogneiss (gneisses metamorphosed from igneous rocks). Late plutons commonly intruded the greenstones that were downfolded in synclines (an upward concave fold of rock) between them, or they intruded along the borders of the belts, deflecting them into irregular shapes.
Structure and formation of greenstone-granite belts
The structure of many belts is complex. Their stratigraphic successions are upside-down and deformed by thrusts and major horizontal folds (nappes). They have been subsequently refolded by upright anticlines (convex folds of rock) and synclines. The result of this thrusting is the repetition of the same stratigraphic successions on top of one another, creating a massive deposit of material up to 10 to 20 km (6 to 12 miles) thick. Also, there may be thrusts along the base of the belts, as in the case of Barberton, showing that they have been transported from elsewhere. In other instances, the thrusts may occur along the borders of the belts, indicating that they have been forced against and over adjacent gneissic belts. The conclusion from structural studies is that many belts have undergone intense subhorizontal deformation during thrust transport.
Clearly, there are different types of greenstone-granite belts. To understand their origin and mode of evolution, it is necessary to correlate them with comparable modern analogues. Some, like the Barberton and Yellowknife belts, consist of oceanic-type crust and have sheeted dike swarms that occur in many ophiolites of Mesozoic-Cenozoic origin, such as in the Troodos Mountains in Cyprus. They are the hallmark of a modern oceanic crust that formed at an oceanic ridge. Also, like modern ophiolites, a few seem to have been covered by thrusting onto continental crust. Many belts, such as the Isua belt of Greenland and those in the Superior province of Canada, are very similar to modern island arcs. Geochemical data are revealing that some lavas were derived from depths of 1,000 to 2,700 km (620 to 1,680 miles) in the Earth's mantle and not from shallower subduction zones, which are commonly 600 km (about 373 miles) deep. These rocks are comparable to oceanic plateaus in modern oceanic crust that were formed from plumes of hot magma from the very deep mantle. The Wawa belt, for example, has been shown to consist of an immature island arc built on oceanic plateau crust and overlain by a more mature arc. The Abitibi belt began as oceanic crust with island arcs and oceanic plateaus. Between the Wawa and Wabigoon island arcs lies the Quetico belt, consisting of metamorphosed turbidites and slices of volcanics that probably developed in a regularly overlapping accretionary prism in an arc-trench system, as seen today in the Japanese arcs. The Pilbara belts are similar to modern active continental margins, and they have been interthrust with older continental orthogneisses to form very thick crustal piles intruded by diapiric crustal-melt granites. This scenario is quite comparable to that of a Himalayan type of orogenic belt formed by collisional tectonics. In conclusion, most greenstone-granite belts are today regarded by geologists as different parts of interthrust oceanic crust–accretionary prism structures within island arcs of oceanic plateau systems that collided with continental gneissic blocks.
Age and occurrence of greenstone-granite belts
Greenstone-granite belts developed at many different times throughout the long Archean Eon. The Isua greenstone belt in West Greenland is about 3.85 billion years old. In the Zimbabwean craton, they formed over three successive periods: the Selukwe belt about 3.8 to 3.75 billion years ago, the Belingwean belts about 2.9 billion years ago, and the Bulawayan-Shamvaian belts about 2.7 to 2.6 billion years ago. The Barberton belt in the Kaapvaal craton and the Warrawoona belt in the Pilbara block are 3.5 billion years old. Globally, the most important period of formation was from 2.7 to 2.6 billion years ago, especially in the Slave and Superior provinces of North America, the Yilgarn block in Australia, and the Dharwar craton in India. Some of the better-documented belts seem to have formed within about 50 million years. It is important to note that while the Bulawayan-Shamvaian belts were forming in the Zimbabwean craton, flat-lying sediments and volcanics were laid down in the Pongola Rift and the Witwatersrand Basin not far to the north.
Greenstone-granite belts range from aggregates of several belts (as in the southern Superior province of Canada) to irregular, even triangular-shaped belts (as in the Barberton in South Africa) to synclinal basins (as in the Indian Dharwar craton). The irregular and synclinal shapes are commonly caused by the diapiric intrusion of younger granites.
Important occurrences are the Barberton belt in South Africa; the Sebakwian, Belingwean, and Bulawayan-Shamvaian belts of Zimbabwe; the Yellowknife belts in the Slave province of Canada; the Abitibi, Wawa, Wabigoon, and Quetico belts of the Superior province of Canada; the Dharwar belts in India; and the Warrawoona and Yilgarn belts in Australia.
Granulite-gneiss belts
The granulites, gneisses, and associated rocks in these belts were metamorphosed to a high grade in deep levels of the Archean crust; metamorphism occurred at a temperature of 750 to 980 °C (1,380 to 1,800 °F) and at a depth of about 15 to 30 km (9 to 19 miles). These belts, therefore, represent sections of the continents that have been highly uplifted, with the result that the upper crust made up of volcanics, sediments, and granites has been eroded. Accordingly, the granulite-gneiss belts are very different from the greenstone-granite belts. Granulite-gneiss belts may be regarded as variably preserved sections of continental cratons.
Economic significance of Archean granulite-gneiss deposits
The mid-lower crust is relatively barren of ore deposits as compared to the upper crust with its sizable concentrations of greenstones and granites, and therefore little mineralization is found in the granulite-gneiss belts. The few exceptions include a nickel–copper sulfide deposit at Selebi-Pikwe in the Limpopo belt in Botswana that is economic to mine, and banded-iron formations in gneisses in the eastern Hubei and Liaoning provinces of northwestern China that form the foundation of a major steel industry. There are subeconomic quantities of chromitite in the anorthosites of western Greenland, southern India, and the Limpopo belt; iron from a banded-iron formation at Isua in western Greenland; and tungsten in amphibolites of western Greenland.
Granulite-gneiss rock types
Orthogneisses of deformed and recrystallized tonalite (a granitic-type rock rich in plagioclase feldspar) and granite constitute the most common rock type. The geochemical signature of these rocks closely resembles that of modern equivalents that occur in granitic batholiths in the Andes. Where such rocks have been metamorphosed under conditions associated with amphibolite facies, they contain hornblende, biotite, or a combination of the two. However, where they have been subjected to conditions of higher temperature associated with the granulite facies, the rocks contain pyroxene and hypersthene and so can be called granulites.
The granulites and gneisses enclose a wide variety of other minor rock types in layers and lenses. These types include schists and paragneisses that were originally deposited on the Earth's surface as shales and which now contain high-temperature metamorphic minerals such as biotite, garnet, cordierite, staurolite, sillimanite, or kyanite. There also are quartzites, which were once sandstones or cherts; marbles (either limestones or dolomites); and banded-iron formations. Commonly intercalated with these metasediments are amphibolites, which locally contain relict pillow structures, demonstrating that they are derived from basaltic lavas extruded underwater. These amphibolites have a trace element chemistry quite similar to that of modern seafloor basalts. The amphibolites are often accompanied by chromite-layered anorthosite, gabbro, and ultramafic rocks such as peridotite and dunite. All these rocks occur in layered igneous complexes, which in their well-preserved state may be up to 2 km (1.2 miles) thick and 100 km (60 miles) long. Such complexes occur at Fiskenaesset in western Greenland, in the Limpopo belt of southern Africa, and in southern India. These complexes may have formed at an oceanic ridge in a magma chamber that also fed the basaltic lavas, or they may be parts of oceanic plateaus. In many cases, the complexes, basaltic amphibolites, and sediments were extensively intruded by the tonalites and granites that were later deformed and recrystallized. The result of this is that all of these rocks may now occur as metre-sized lenses in the orthogneisses and granulites.
Structure and occurrence of granulite-gneiss belts
The structure of the granulite-gneiss belts is extremely complex because the constituent rocks have been highly deformed several times. In all likelihood the basalts and layered complexes from the oceanic crust were interthrust with shallow-water limestones, sandstones, and shales; with tonalites and granites from Andean-type batholiths; and with older basement rocks from a continental margin. All these rocks, which are now mutually conformable (parallel to one another with uninterrupted deposition), were folded in horizontal nappes and then refolded. The picture that emerges is one of a very mobile Earth, where newly formed rocks were routinely compressed and thrust against other rocks.
Granulite-gneiss belts occur in a variety of environments. These may be extensive regions, such as the North Atlantic craton, which measures 1,000 by 2,000 km (about 620 by 1,240 miles) across and, before the opening of the Atlantic Ocean, was contiguous with the Scourian Complex of northwestern Scotland, the central part of Greenland, and the coast of Labrador; the Aldan and Ukrainian shields of continental Europe; the North China craton; large parts of the Superior province of Canada; the Yilgarn block in Australia; and the Limpopo belt in southern Africa. They may be confined to small areas such as the Ancient Gneiss Complex of Swaziland, the Minnesota River valley and the Beartooth Mountains of the United States, the Peninsular gneisses and Sargur supracrustals of southern India, the English River gneisses of Ontario in Canada that form a narrow strip between greenstone-granite belts, the Sand River gneisses that occupy a small area between greenstone-granite belts in Zimbabwe, and the Napier Complex in Enderby Land in Antarctica. Granulite-gneiss belts are commonly surrounded by younger, mostly Proterozoic belts that contain remobilized relicts of the Archean rocks, and the granulites and gneisses must underlie many Archean greenstone-granite belts and blankets of Phanerozoic sediment.
Age and correlation of granulite-gneiss belts
Isotopic age determinations from the granulite-gneiss belts record an evolution from about 4.0 to 2.5 billion years ago—more than a third of geologic time. Most important are the few but well-constrained age determinations of detrital zircons at Mount Narryer and Jack Hills in Western Australia that are more than 4 billion years old. Several regions have a history that began in the period dating from 3.9 to 3.6 billion years ago—western Greenland, Labrador, the Limpopo belt, Enderby Land, the North China craton, and the Aldan Shield. Most regions of the world experienced a major tectonic event that may have involved intrusion, metamorphism, and deformation during the period between 3.1 and 2.8 billion years ago; some of these regions, like the Scourian in northwestern Scotland, show no evidence of any older crustal growth. The best-documented region is in western Greenland, which has a long and complicated history from 3.85 to 2.5 billion years ago.
It is impossible to correlate the rocks in different granulite-gneiss belts. One granitic gneiss is essentially the same as another but may be of vastly different age. There is a marked similarity in the anorthosites in various belts throughout the world, and their similar relationship with the gneisses suggests that the belts have undergone comparable stages of evolution, although each has its own distinctive features. Little correlation can be made with rocks of Mesozoic-Cenozoic age because few modern orogenic belts have been eroded sufficiently to expose their mid-lower crust. The lack of modern analogues for comparison makes it particularly difficult to interpret the mode of origin and evolution of the Archean granulite-gneiss belts.
Sedimentary basins, basic dikes, and layered complexes
During middle and late Archean time (3 to 2.5 billion years ago), relatively stable, post-orogenic conditions developed locally in the upper crust, especially in southern Africa, where the development of greenstone-granite and granulite-gneiss belts was completed much earlier than in other parts of the world. The final chapters of Archean crustal evolution can be followed by considering specific key sedimentary basins, basic (basaltic) dikes, and layered complexes.
Along the border of Swaziland and South Africa is the Pongola Rift, which is the oldest such continental trough in the world; it is 2.95 billion years old, having formed only 50 million years after the thrusting of adjacent greenstone-granite belts. If there were earlier rifts, they have not survived, or, more likely, this was the first time in Earth history that the upper crust was sufficiently stable and rigid for a rift to form. It is 30 km (19 miles) wide, 130 km (81 miles) long, and within it is a sequence of lavas and sediments that is 11 km (7 miles) thick. It seems most likely that the rift developed as the result of the collapse of an overthickened crust following the long period of Archean crustal growth and thrusting in the Kaapvaal craton.
The 200-by-350-km (124-by-217-mile) Witwatersrand Basin contains an 11-km- (7-mile-) thick sequence of lavas and sediments that are 3 billion years old. The basin is famous for its very large deposits of gold and uranium that occur as detrital minerals in conglomerates. These minerals were derived by erosion of the surrounding greenstone-granite belts and transported by rivers into the shoreline of the basin. In all probability, the gold originally came from the komatiitic and basaltic lavas in the early Archean oceanic crust.
The Great Dyke, thought to be about 2.5 billion years old, transects the entire Zimbabwe craton. It is 480 km (about 300 miles) long, 8 km (5 miles) wide, and made up of layered ultrabasic rocks—gabbros and norites. The ultrabasic rocks have several layers of chromite and an extensive platinum-bearing layer that form economic deposits. The Great Dyke represents a rift that has been filled in with magma that was probably derived from a deep mantle plume.
The Stillwater Complex is a famous, 2.7-billion-year-old, layered ultrabasic-basic intrusion in the Beartooth Mountains of Montana in the United States. It is 48 km (30 miles) long and has a stratigraphic thickness of 6 km (3.7 miles). It was intruded as a subhorizontal body of magma that underwent crystal settling to form the layered structure. It is notable for a 3-metre- (9-foot-) thick layer enriched in platinum minerals that forms a major economic deposit.
The basins, dikes, and complexes described above cannot be mutually correlated. They most resemble equivalent structures that formed at the end of plate-tectonic cycles in the Phanerozoic. They represent the culmination of Archean crustal growth.
Proterozoic rock types
What happened geologically at the time of the Archean-Proterozoic boundary 2.5 billion years ago is uncertain. It seems to have been a period of little tectonic activity, and so it is possible that the earlier intensive Archean crustal growth had caused the amalgamation of continental fragments into a supercontinent, perhaps similar to Pangea of Permian-Triassic times. The fragmentation of this supercontinent and the formation of new oceans gave rise to many continental margins upon which a variety of distinctive sediments were deposited. Much evidence suggests that in the period from 2.5 billion to 570 million years ago Proterozoic oceans were formed and destroyed by plate-tectonic processes and that most Proterozoic orogenic belts arose by collisional tectonics. Sedimentary, igneous, and metamorphic rocks that formed during this period are widespread throughout the world. There are many swarms of basic dikes, important sedimentary rifts, basins, and layered igneous complexes, as well as many orogenic belts. The rocks commonly occur in orogenic belts that wrap around the borders of Archean cratons. The characteristic types of Proterozoic rocks are considered below, as are classic examples of their occurrence in orogenic belts. The following types of rocks were formed during the early, middle, and late Proterozoic, indicating that similar conditions and environments existed throughout this long period of time.
Basic dikes
The continents were sufficiently stable and rigid during the Proterozoic Eon for an extremely large number of basic dikes to be intruded into parallel, extensional fractures in major swarms. Individual dikes measure up to several hundred metres in width and length, and there may be hundreds or even thousands of dikes in a swarm, some having transcontinental dimensions. For example, the 1.2-billion-year-old Mackenzie swarm is more than 500 km (311 miles) wide and 3,000 km (1,864 miles) long and extends in a northwesterly direction across the whole of Canada from the Arctic to the Great Lakes. The 1.95-billion-year-old Kangamiut swarm in western Greenland is only about 250 km (155 miles) long but is one of the world's densest continental dike swarms. Many of the major dike swarms were intruded on the continental margins of Proterozoic oceans in a manner similar to the dikes that border the present-day Atlantic Ocean and were similarly the result of the rise of mantle plumes into the crust.
Layered igneous intrusions
There are several very important layered, mafic to ultramafic intrusions of Proterozoic age that were formed by the accumulation of crystals in large magma chambers. The well-known ones are several tens or even hundreds of kilometres across, have a dikelike or sheetlike (stratiform) shape, and contain major economic mineral deposits. The largest and most famous is the Bushveld Complex in South Africa, which is 9 km (5.6 miles) thick and covers an area of 66,000 square km (about 25,500 square miles). It was intruded nearly 2.1 billion years ago and is the largest repository of magmatic ore deposits in the world. The Bushveld Complex consists of stratiform layers of dunite, norite (a type of gabbro rich in orthopyroxene), anorthosite, and ferrodiorite (an iron-rich intrusive igneous rock that is basic to intermediate in composition) and contains deposits of chromite, iron, titanium, vanadium, nickel, and—most important of all—platinum. The Sudbury Complex in southern Canada, which is about 1.9 billion years old, is a basin-shaped body that extends up to 60 km (37 miles) across. It consists mostly of layered norite and has deposits of copper, nickel, cobalt, gold, and platinum. It is noted for its high-pressure structures and other manifestations of shock metamorphism, which suggest that the intrusion was produced by an enormous meteorite impact.
Shelf-type sediments
Quartzites, dolomites, shales, and banded-iron formations make up sequences that reach up to 10 km (6.2 miles) in thickness and that amount to more than 60 percent of Proterozoic sediments. Minor sediments include sandstones, conglomerates, red beds, evaporites, and cherts. The quartzites typically have cross-bedding and ripple marks, which are indicative of tidal action, and the dolomites often contain stromatolites similar to those that grow today in intertidal waters. Also present in the dolomites are phosphorites that are similar to those deposited on shallow continental margins against areas of oceanic upwelling during the Phanerozoic. Several early-middle Proterozoic examples of such dolomites have been found in Finland and northern Australia, as well as in the Marquette Range of Michigan in the United States, in the Aravalli Range of Rajasthan in northwestern India, and at Hamersley and Broken Hill in Australia. Other constituents of these dolomites include evaporites that contain casts and relicts of halite, gypsum, and anhydrite. Examples occur at Mount Isa in Australia (1.6 billion years old) and in the Belcher Group in Canada (1.8 billion years old). These evaporites were deposited by brines in very shallow pools such as those encountered today in the Persian Gulf.
Phanerozoic ophiolites are considered to be fragments of ocean floor that have been trapped between island arcs and continental plates that collided or that have been thrust onto the shelf sediments of continental margins. They consist of a downward sequence of oceanic sediments such as cherts, pillow-bearing basalts, sheeted basic dikes, gabbros, and certain ultramafic rocks (such as serpentinized harzburgite, which is primarily made of olivine and orthopyroxene; and lherzolite, which is mainly composed of olivine, clinopyroxene, and orthopyroxene). Comparable ophiolites occur in several Proterozoic orogenic belts and provide strong evidence of the existence of oceanic plates similar to those of today. The oldest is an ophiolite in the Cape Smith belt on the south side of Hudson Bay in Canada whose age has been firmly established at 1.999 billion years. There is a 1.96-billion-year-old ophiolite in the Svecofennian belt of southern Finland, but most Proterozoic ophiolites are 1 billion to 570 million years old and occur in the Pan-African belts of Saudi Arabia, Egypt, Yemen, and The Sudan, where they occur in sutures between a variety of island arcs.
Greenstones and granites
Greenstone-granite belts such as those of the Archean continued to form in the Proterozoic, albeit in greatly reduced amounts. They are characterized by abundant volcanic rocks that include pillowed subaqueous basalt flows and subaerial and subaqueous volcaniclastic rocks. Magnesian komatiites are for the most part absent, however. Intrusive plutons are typically made of granodiorite. Examples occur at Flin Flon in central Canada, in the Birrimian Group in West Africa, and in the Pan-African belts of the Arabian-Nubian Shield. Generally, such rocks resemble those in modern island arcs and back-arc basins, and the presence of remnants of oceanic plateau is suspected.
Granulites and gneisses
These highly deformed and metamorphosed rocks are similar to those of the Archean Eon and occur in many Proterozoic orogenic belts such as the Grenville in Canada, the Pan-African Mozambique belt in eastern Africa and Madagascar, the Musgrave and Arunta ranges in Australia, and in Lapland in the northern Baltic Shield. They were brought up from the mid-lower crust on major thrusts as a result of continental collisions.
Orogenic belts
One of the world's classic Proterozoic orogenic belts is the Wopmay Orogen, which is situated in the Arctic in the northwestern part of the Canadian Shield. This beautifully exposed belt formed within a relatively short time (between 1.97 and 1.84 billion years ago) and provides convincing evidence of tectonic activity of a modern form in the early Proterozoic. On the eastern continental margin here are red beds (sandstones) that pass oceanward and westward into stromatolite-rich dolomites deposited on the continental shelf to a thickness of 4 km (2.5 miles); these dolomites pass into submarine turbidite fans that were deposited on the continental rise. An island arc and a continental margin are located to the west. The history of the Wopmay Orogen can be best interpreted in terms of subduction of oceanic crust and collision tectonics.
The Svecofennian Orogen of the Baltic Shield extends in a southeasterly direction from northern Sweden through southern Finland to the adjoining part of western Russia. It formed in the period from 1.9 to 1.7 billion years ago. A major lineament across southern Finland consists of the suture zone on which occur ophiolite complexes representing the remains of oceanic crust. At Outokumpu there is copper mineralization in these oceanic crust rocks similar to that in the Cretaceous ophiolite at Troodos in Cyprus. On the northern side of the suture is a shelf-type sequence of sediments; on the southern side is a volcanic-plutonic arc. To the south of this arc lies a broad zone with thrusted gneisses intruded by tin-bearing crustal-melt granites, called rapakivi granites after their coarse, zoned feldspar megacrysts (that is, crystals that are significantly larger than the surrounding fine-grained matrix). The rocks in this zone probably formed as a result of mantle plume activity.
The Grenville Orogen is a deeply eroded and highly uplifted orogenic belt that extends from Labrador in northeastern Canada to the Adirondack Mountains and southwestward under the coastal plain of the eastern United States. It developed from about 1.5 to 1 billion years ago. Apart from an island arc situated today in Ontario, most of the Grenville Orogen consists of highly metamorphosed and deformed gneisses and granulites that have been brought to the present surface on major thrusts from the mid-lower crust. A result of the terminal continental collision that occurred at about 1.1 billion years ago was the formation of the Midcontinent (or Keweenawan) rift system that extends southward for more than 2,000 km (about 1,240 miles) from Lake Superior.
A type of crustal growth—one very different from that described above—took place in what are now Saudi Arabia, Egypt, Yemen, and The Sudan in the period from 1.1 billion to 500 million years ago. This entire shield, called the Arabian-Nubian Shield, is dominated by volcanic lavas, tuffs (consolidated rocks consisting of pyroclastic fragments and ash), and granitic plutons that formed in a variety of island arcs separated by several sutures along which many ophiolite complexes occur. Some of the ophiolites contain a complete stratigraphy that is widely accepted as a section through the oceanic upper mantle and crust. The final collision of the arcs was associated with widespread thrusting and followed by the intrusion of granitic plutons containing tungsten, tin, uranium, and niobium ore deposits. The island arcs grew from the subduction of oceanic crust in a manner quite comparable to that taking place today throughout Indonesia.
The Mozambique belt is one of the many Pan-African orogenic belts that formed in the period between 1 billion and 500 million years ago. It extends along the eastern border of Africa from Ethiopia to Kenya and Tanzania. It consists largely of highly metamorphosed, mid-crustal gneisses deformed by eastward-dipping thrusts very similar to the thrusts on the southern side of the Himalayas (formed as a result of the collision of India with Tibet during the early Cenozoic Era). To the east on the island of Madagascar, mid-crustal gneisses of similar age were brought to the surface by major late extensional collapse of the orogenic belt.
During the middle and late Proterozoic, thick sequences of sediment were deposited in many basins throughout Asia. The Riphean sequence spans the period from 1.6 billion to 800 million years ago and occurs primarily in Russia. The Sinian sequence in China extends from 800 to 570 million years ago, toward the end of the Precambrian time. The sediments are terrigenous debris characterized by conglomerates, sandstone, siltstone, and shale, some of which are oxidized red beds, along with stromatolite-rich dolomite. Total thicknesses reach over 10 km (6.2 miles). The terrigenous sediments were derived from the erosion of Proterozoic orogenic belts.
Glacial sediments
Evidence of the oldest known glaciation, which occurred 2.9 billion years ago, is preserved in the Pongola Rift in South Africa, though most Precambrian glaciations occurred during the Proterozoic. Evidence that ancient deposits are of glacial origin is obtained by comparing them with those left behind by the Quaternary ice sheets and with deposits associated with modern glaciers. The main sediments left behind by early Proterozoic glaciers are tillites containing rock fragments ranging in size from pebbles to boulders and distributed randomly in a fine-grained silty matrix. The surfaces of some pebbles have parallel scratches caused by having been rubbed against harder pebbles during ice transport. Locally, the basement rocks below the tillite also have been scratched, or striated, by the movement of the overlying boulder-strewn ice. Another type of glacial deposit is a varved (laminated) sediment composed of alternating millimetre-to-centimetre-thick layers of silt and clay, which closely resemble the layered varves that are laid down in modern glacial lakes at the front of retreating glaciers or ice sheets. Each of these layers defines an annual accumulation of sediment. Varved sediments may contain dropstones, which are fragments of rock that have dropped from an overlying floating ice sheet and that have sunk into and depressed the layers beneath them. When all these features are found together, they provide good evidence of ancient glaciations.
The most extensive early Proterozoic Huronian glaciation occurred 2.3 billion years ago in what is now northern North America. Glacial deposits, similar in age to those of the Huronian, are located in the Transvaal and Cape regions of South Africa, where they reach only 30 metres (100 feet) in thickness but extend over an area of 20,000 square km (7,700 square miles). Such deposits are also encountered in the Hamersley Basin of Western Australia, in east-central Finland and the adjoining part of northwestern Russia, near Lake Baikal in Siberia, and in central India, suggesting the occurrence of a wide-spread glaciation.
Evidence for the largest glaciation in Earth's history, known as the Snowball Earth event, dates from the late Proterozoic between 1 billion and 600 million years ago. The principal occurrences of these global glacial deposits are in Europe (Scotland, Ireland, Sweden, Norway, France, the Czech Republic, and Slovakia), the Western Cordillera (Yukon, Can., to California, U.S.) of western North America and the Appalachians of the United States, eastern Greenland, Brazil, much of Africa (Congo [Brazzaville], Angola, Namibia, Zambia, Congo [Kinshasa], and South Africa), and much of Russia, China, and Australia. In addition to the Flinders Range deposits described above (see Worldwide glaciations), other notable deposits include the Port Askaig tillite on the island of Islay off northwestern Scotland, which is only 750 metres (2,460 feet) thick but records 17 ice advances and retreats and 27 periglacial periods (which are indicated by infilled polygons that formed under ice-free permafrost conditions). There are two major tillites in central Africa and Namibia (910 to 870 and 720 to 700 million years old, respectively) and two other such consolidated tills in eastern Greenland.
Correlation of Precambrian strata
The fact that Phanerozoic sediments have been so successfully subdivided and correlated is attributable to the presence of abundant fossil remains of life-forms that evolved and underwent changes over time. Precambrian sediments lack such fossils, thus preventing any comparable correlations. There are, however, stromatolites in Precambrian sediments ranging in age from about 3.5 billion to 540 million years that reached their peak of development in the Proterozoic. Stromatolites underwent evolutionary changes sufficient for Russian biostratigraphers to use to subdivide the Riphean sequence into four main zones throughout widely separated areas of former Soviet territory. Similar stromatolite-based stratigraphic divisions have been recognized in the Norwegian islands of Spitsbergen, China, and Australia. This stromatolite biostratigraphy still has relatively limited application, however. As a consequence, it is the chronometric time scale that is used to subdivide Precambrian time and to correlate rocks from region to region and from continent to continent.
The rocks within Proterozoic orogenic belts are invariably too deformed to allow correlation of units between different belts. Nonetheless, the techniques of geochronology—in particular, zircon dating—have improved considerably in recent years, with the result that rocks of approximately similar age on different continents can be mutually compared and regarded as equivalent. The isotopic dating of Archean rocks, especially with the use of zircons, has enabled similarities and differences in age to be determined, thereby aiding correlation.
Establishing Precambrian boundaries
There is no record of tectonic activity of any sort at the time corresponding to the Archean-Proterozoic boundary—about 2.5 billion years ago. This probably means that a supercontinent had been created by the amalgamation of innumerable smaller continental blocks and island arcs. Accordingly, this was a period of tectonic stability that may have been comparable to the Permian-Triassic when the supercontinent of Pangea existed. The main geologic events would have been the intrusion of basic dikes and the formation of sedimentary basins such as the Huronian on the U.S.-Canadian border, into which large volumes of clastic sediment (that is, sediment of predominantly clay, silt, and sand sizes) were deposited. Such sediments would have been derived by erosion of high plateaus and mountains that are characteristic of a large continental mass.
Learn More in these related Britannica articles:
North America: Precambrian timeThe oldest rocks in the world occur in the Canadian Shield. Their ages have been calculated from precisely measured ratios of the radioactive decay of trace amounts of certain isotopes in the rock sample. The ratio of…
Asia: Tectonic framework…platforms that stabilized mostly in Precambrian time (between roughly 4 billion and 541 million years ago) and have been covered largely by little-disturbed sedimentary rocks; included in that designation are the Angaran (or East Siberian), Indian, and Arabian platforms. There are also several smaller platforms that were deformed to a…
river: The river system through timeThe Precambrian portion of the record, prior to 542,000,000 years ago, is complicated by the widely metamorphosed character of the surviving rocks, although even here the typical cross-bedding of shallow-water sands can be recognized in many places. The Cambrian and post-Cambrian succession of the last 542,000,000…
plant: Specific variations in photosynthesis…photosynthesis in the cyanobacteria of Precambrian times (about 4.6 billion years ago to 541 million years ago) oxygenated the atmosphere, the ratio of carbon dioxide to oxygen fell drastically, and Rubisco began to function more and more as an oxygenase. This greatly reduced the net fixation of CO2 into sugars…
climate change: Climates of early Earth…pre-Phanerozoic interval, also known as Precambrian time, comprises some 88 percent of the time elapsed since the origin of Earth. The pre-Phanerozoic is a poorly understood phase of Earth system history. Much of the sedimentary record of the atmosphere, oceans, biota, and crust of the early Earth has been obliterated…
More About Precambrian12 references found in Britannica articles
Assorted References
- geological time scale
geological development of
- Appalachian Mountains
- Asia
- North America
- Rocky Mountains
- atmospheric conditions
- climate
- fluvial processes
- fossil record


text:
Customize your site, manage shipping and payments, and list your products on Amazon, Ebay, and Facebook with the #1 ecommerce platform.
Try it free, no credit card required.
Your store is always open for business thanks to one of the best uptimes in the industry, including 100% availability during Cyber Week.
BigCommerce is a quarter of the cost of on-premise solutions like Magento, with no hosting, maintenance or update expenses
According to research by Ipsos, BigCommerce merchants grow 28% year over year, nearly 2x the industry average.
Your store is always open for business thanks to one of the best uptimes in the industry, including 100% availability during Cyber Week.
BigCommerce is a quarter of the cost of on-premise solutions like Magento, with no hosting, maintenance or update expenses.
Our industry-first native integration lets you effortlessly increase sales by connecting your BigCommerce store to eBay.
We'll give you a high-level look at the BigCommerce platform and its key features, plus answer any questions you have.
Solutions for fast-growing brands
All the tools you need to increase sales
and improve your bottom line.
Grow your business
Solutions for high-volume businesses
Advanced features, premium support,
enterprise-grade hosting.
BigCommerce Enterprise
We power ecommerce websites for successful retailers in all over the world
Since moving to BigCommerce our site is now faster, the user experience is better, and we have achieved a higher search rank — showing up as the number one result in organic listings on Google.Paul Yoo, US Patriot Tactical
When we launched our collaboration with Beyoncé, there was a flood of visits to our website. We couldn't be happier with how well the BigCommerce team and our website performed under such high stakes.Kirsten Stoddard, Flash Tattoos
We used to see less than 1% mobile conversion, and on BigCommerce we're above 10%. Our sales overall have increased 55% after switching because of the look and feel of the site and its functionality.Rita Shelley, SISU Guard
Our revolutionary design platform lets you create stores that engage shoppers and make more sales. Choose from the best responsive ecommerce templates on the market or build a fully custom site.
Best-in-class marketing and conversion tools enable our merchants to grow at 2x the industry average. Drive more traffic, convert more visitors and seamlessly sell on multiple channels from a single location.
BigCommerce merchants benefit from industry-leading performance and stability, including an average uptime of more than 99.99% and sites that load fast for maximum conversion.
Our cloud-based platform is 1/4 the cost of on-premise solutions like Magento. You'll spend less time managing technology and get regular updates with new, cutting-edge features.
No matter what size your business,
BigCommerce can help increase your sales
Solutions for fast-growing brands
Get beautiful themes to build an engaging online store, built-in SEO and conversion tools to scale your business, and much more.
Grow your business
Solutions for high-volume businesses
BigCommerce Enterprise is an advanced, reliable platform designed to grow revenue and optimize operations.
BigCommerce Enterprise


text:
You have no items in your shopping cart.
Sodom - Agent Orange printed patch, longer side 10 cm.
The picture is printed professionally on a hard black fabric.
Black thread border sewn all around the patch.
PLEASE NOTE: If you wish to buy in bulk visit our wholesale page or contact us to get a custom quote!
Items are shipped within 2-5 working days of receipt of payment.
The faster/cheaper shipping method is "Posta Prioritaria" (PLEASE NOTE: NO tracking code provided)
You can choose a Registered shipping when checking out.
Estimated delivery time:
Europe: 7-10 working days
Any other: 10-15 working days
Europe: 10-12 working days
Any other: 15-20 working days
PLEASE NOTE: We can't know the exact delivery time as they depend on your country's postal service.
Overseas shippings may take up to 15 working days from date of shipment. Please wait the expiry of this period before contacting us to claim your item/s.
Any resulting inconvenience from your postal service/customs office is to be dealt with them directly.
All orders will be shipped to the address you provide through our checkout system.
Please be sure this address is correct!
Mad Printing is not responsible in respect of any delivery failure due to incorrect/unavailable address.


text:
Door glass, back glass, quarter panels, windshields, and more. What does it all mean? Do I have tempered glass or safety glass? How can I tell if my windshield has extra features? It has been many years since auto glass was created and by far one of the best enhancements made to the automobile. In general, the auto glass on the sides and back of your car are tempered, meaning they were specially treated to break into hundreds of tiny pieces, keeping you from being exposed to dangerously sharp edges. It's not to say you cannot still get hurt from broken auto glass, but it is much safer today than it was in the past. Read more about auto glass on our auto glass and windshield pages.
Your windshield is made differently than any other piece of auto glass. This piece of auto glass is comprised of two layers of safety glass with a layer of composite material in between. Modern windshields can include a myriad of options, including rain sensors for your windshield wipers, solar coated windshields for those in warmer climates, antennas to support your GPS, and more. The most important part of your windshield remains the laminate in between the layers of glass. Your car is much safer today because of this lamination, which keeps the glass together, and allows repairs to be made when the damage is just a small chip or crack.