The subducted oceanic crust triggers volcanism. Lava deltas are generally associated with large-scale, effusive type basaltic volcanism. This process may have caused resurfacing by water volcanism and tectonics, erasing older geological features. This sediment uplift was accompanied by igneous intrusions and volcanism. Mountains are formed through tectonic forces or volcanism. Mid-ocean ridges exhibit active volcanism and seismicity. Subduction zones are sites that usually have a high rate of volcanism and earthquakes. Pyroclastic rocks are the product of explosive volcanism. Many of the volcanic belts bear ore deposits that are related to the volcanism. The subducted oceanic crust triggers volcanism, although the basic mechanisms are varied. Deformational events are often also associated with volcanism and igneous activity. Geologists refer to this local volcanism as the Moraga Volcanics. Gabbro is also found as plutons associated with continental volcanism. Lava flows are widespread and forms of volcanism not present on Earth occur as well. Features common in shield volcanism include lava tubes.