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Unformatted text preview: Lecture #11 note; Geology 3950 2010; CR Stern Volcanic landforms Part II super volcanoes and giant calderas Explosive eruptions of andesite and rhyolite with volumes >1 to 100 km 3 typically generate caldera, which then are reconstructed into stratovolcanoes by subsequent dome building eruptions of relatively dry, volatile free magmas. Many such calderas have been formed by this size eruption during the last few thousand years (see lecture #11) However, much bigger explosive eruptions, in the range 500 to 5,000 km 3 in volume of new rock erupted, generate much larger calderas, often so large (100 x 50 km in size) that they can not simple be seen or assessed as calderas without geologic mapping and/or satellite photos. These giant calderas are typically related to eruption of rhyolitic magma and they occur in areas of continental crust, above subduction zones (such as the Toba caldera in Sumartra see below), in areas of continental rifts (such as the Long valley and Valles calderas in the western US), or above hot spots (such as the Yellowstone caldera). Giant calderas do not form in oceans because rhyolites do not form in oceans. No such giant caldera is less than 74,000 years old (Toba) and civilization would certainly be devastated by such an eruption anywhere on the earth. Giant calderas form in the same way as smaller caldera, by collapse of the roof into the magma chamber after an explosive eruption, but typically go through more than one cycle of eruption. As they prepare for a second or third eruption, new magma coming into the very large magma chamber below the caldera causes them to uplift, or resurge, and they are called resurgent calderas (see figure 1 below). Smaller calderas like Crater Lake rebuild themselves into stratovolcanoes by new eruptions, but they are not resurgent. Three giant calderas in the western US, all formed in the last 1 million years include the Yellowstone hot-spot and two rift related supervolcanoes, Long valley caldera in eastern California and Valles caldera along the edge of the Rio Grande river in New Mexico (see figure 2 below) Each covered large areas of the US with volcanic ash when they last erupted. Yellowstone is the currently active volcanic center above a hot spot that breeched the...
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This note was uploaded on 03/02/2010 for the course GEOL 3950 taught by Professor Charles during the Spring '08 term at Colorado.
- Spring '08