The Story of Tahan River- Part II

This movie presents the geologic research results on the stream capture of Tahan River, and its effects on the economy as well as the lives of people. Geologists were interviewed to discuss the years stream capture occurred, and to reconstruct the process of the stream capture of Tahan River.


Goal of Learning:
To attain an understanding on stream capture, to learn the evolution of Tahan River, and to understand the scientific research results.

Geologic Terms
A wide stretch of flatland raised above its lower-elevation surroundings.  It normally has a gentle slope, but with steep borders. In general; flatlands with lower elevations are known as plains; flatlands of higher elevations are plateaus.  Terrace refers to flatlands with elevations ranging from 100m to 700m.

Geoscientists differentiate the earth structure, according to material strength and behavior, into the crust, the upper mantle, the inner core and the core. The crust and the upper mantle together form the lithosphere. The solid outer lithosphere comprises many solid plates of various sizes. These plates move in different directions and speeds. There are 6 major plates, which are: the African Plate, the American Plate, the Eurasian Plate, the Indo-Australian Plate, the Pacific Plate, and the Antarctic Plate.

A water body located beneath the ground surface within the soil and rock crevasses.

Dating Methods:
Carbon-14 Dating: In 1940, American scientist W. F. Libby at the University of Chicago developed the carbon-14 dating method. The product of carbon-14 fractionation is nitrogen-14. Throughout their lives, plants constantly absorb carbon dioxide. In the process, they also incorporated some carbon-14; when animals eat the plants, they also ingest some carbon-14. As organisms live, their biochemical activity continues exchanging carbon with its natural environment. When organisms die, this process of carbon exchange stops, and they no longer absorb carbon-14. The dead organism became a closed system. Since carbon-14 is a radioisotope, radioactive decay begins in this closed system. Through fractionation decay on a half-life of 5730 years carbon-14 would fractionate into nitrogen-14. And so, the ratio of carbon-14 to nitrogen-14 contents in the dead organism would indicate the date of death of the organism. The carbon-14 dating method is well suited to determining the age of various carbonaceous materials, such as dead wood, bones and shells. This dating method is most helpful in archaeology and studies on the Quaternary Period. Due to instrumental limitations, the upper limit for carbon-14 dating stands at 50,000 years BP. (Note: BP.: before present)
Optical dating: This method makes use of various defects in imperfect crystal cells. Radioisotopes with long half lives (mainly 238U, 235U, 232Th, and 40K) in the stratigraphic column on earth are bombarded by cosmic ray, their electrons may be excited and ionized, and they are eventually captured by ion cloud resulting from crystal defects. In the lab, under luminescence photons of the mineral are released (Aitken, 1985, 1998). The sample is tested in the lab through exposure to strong natural fluorescence, it is then put through simulated regression under artificial radioactive dose, and the equivalent dose is obtained.  Then the trace content of radioactive elements in the test sample was calibrated. The annual dose the environment contributed to the sample is then:

Principles of Stratigraphy:
Sedimentary rock formations follow the “Law of Superposition”.  This law dictates that in a rock sequence, older rock formations occur at the bottom, while new formations occur on the top.

Geomorphology and Geologic History of the Taipei Basin:
The Taipei Basin is the northernmost basin in Taiwan.  Its center is located approximately at 25 degrees North and 121 degrees East. The basin is shaped like an inverted triangle, with a base of approximately 18km long and a height of 10km, the total surface area is approximately 90km2. The Taipei Basin is approximately less than 20m in elevation, and is a rolling flatland. The basin is surrounded on all sides by mountains. To the north, there is the Tatun Volcano Group, and to the south there is the Xueshan Range, the Songshan Hills lie in the southeast, and to the east there is the Linkou Terrace. Meteoric water in the mountain areas collect to form the Keelung River, the Jingmei River, the Xindian River and the Tahan River. These streams confluent as they enter the basin and form the Tamsui River that flows out the northwest corner of the basin at Guandu.

Metamorphic Rock:
Rocks that have been buried deep down in the earth’s crust are subject to tremendous pressures and very high temperatures. To adapt to this new environment of high pressure and high temperature these rocks may become plastic and flow; or they may change their chemical and mineralogical characteristics to become more stable under these new chemical and physical conditions. These new rocks with new sets of minerals, textures and fabrics and physical properties are called metamorphic rocks.
Metamorphic rocks constitute one of three major rock groups that form the lithosphere in the earth’s crust.

Sedimentary Rocks:
Sedimentary rocks are formed when loose clastic sediments go through compaction and cementation processes, and finally become consolidated. Also, dissolved mineral matters in seawater or freshwater bodies may precipitate and form chemical sedimentary rocks such as limestone or gypsum.  Some chemical sedimentary rocks are formed through organic activities, such as radiolarite.

Rocks on the ground surface are in constant contact with the atmosphere, water, and organisms. Prolonged exposure to these elements will break or disintegrate the rocks. Breaking down of rocks may be through physical actions or the rock may be changed chemically and disintegrate.

Loose materials from weathering may be moved by wind, water or ice from their resting place.  Removal of the loose surface cover exposes the rock to further weathering and the rocks are gradually wasted away. This process is call erosion.

Orogeny or Mountain Building Processes:
Orogeny refers to the tectonic structural phenomena resulting from crustal instability that transform continents into oceans and changes the morphology of the land. Orogeny is solely caused by plate activities. Plates that move in opposite direction will converge and collide. The tremendous force of plate collision can uplift, incline or fold rock formations, building high-rising mountain chains in the proximity of the subduction zone.

1. Professor Wen-Shan Chen, Department of Geosciences, National Taiwan University.
Subject: Research Results on the Stream Capture of the Tahan River.
2. Professor Yue-Gao Chen, Department of Geosciences, National Taiwan University
Subject: Research Motivation, Process and Results on the Date of the Stream Capture of the Tahan River.
Learning List
Easy Version
1.    The location of the stream capture of the Tahan River occurred at (1) Yingge; (2) Shihmen; (3) Shulin.
2.    In the movie, it was mentioned that geologists found out that the sediment on top of Taoyuan Terrace was deposited at which date (1) 20,000 years BP; (2) 20,000 ~ 60,000 years BP; (3) 50,000~60,000 years BP.
3.    Professor Wen-Shan Chen’s dating study on bottom sediments in the Taipei Basin indicated that the stream capture of Tahan River occurred at which date (1) 20,000 years BP; (2) 20,000~50,000 years BP; (3) 60,000~70,000 years BP.
4.    The paired terraces along the banks of Tahan River were mainly formed because (1) Tahan River captured the Ancient Shihmen River, (2) down cutting by the Tahan River.
Advanced Version
1.    The Ancient Xindian River alluvial fan is (1) the present-day Taipei Basin; (2) the Linkou Terrace. (3) Taoyuan Terrace.
2.    The Ancient Shihmen River alluvial fan is (1) the present-day Taipei Basin; (2) the Linkou Terrace, (3) Taoyuan Terrace.
3.    Where was the location of river capture?
4.    In the movie it was mentioned that geologists use two methods in dating the ages of the Taoyuan Terrace, what are these methods?
5.    In the movie it was mentioned that geologists had dated the surface layer of Taoyuan Terrace to be deposited at how long ago?
6.    Professor Yue-Gao Chen conducted age dating on Tahan River terraces and Taoyuan terrace; his result indicated that stream piracy occurred at what?
7.    Professor Wen-Shan Chen’s dating study on bottom sediments in the Taipei Basin indicated that Tahan River stream occurred at what time?
8.    The multi-tiered symmetrical terraces along the banks of Tahan River were formed by (1) Tahan River capturing the Ancient Shihmen River, (2) Down cutting by the Tahan River.
9.    What was the main factor that caused the Tahan River change its direction of flow towards the north?
10.    What were the main factors that created the unique water-pond culture on Taoyuan Terrace?

NTU Digital Archives of Geosciences Department
National Taiwan University