Calderas - CATACLYSMIC ERUPTIONS The really big ones This...

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1 CATACLYSMIC ERUPTIONS The really big ones! This figure compares the size of some recent, well-known eruptions. Note how small the eruptions of Mount St. Helens and even Vesuvius are compared to Katmai, Krakatau and Tambora. Within the very recent geological past, there have been extremely violent explosive eruptions several orders of magnitude greater in size than any historical eruptions:- Date Volume Km 3 Mount St Helens 1980 2 Pinatubo 1991 8 Krakatoa 1883 18 Santorini 1628 BC 39 Crater Lake 6845 BP 75 Long Valley 700,000 BP 600 Yellowstone 2 million BP 2500
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2 These volcanoes have erupted vast volumes of rhyolite - dacite ash and pyroclastic flows. Some are still potentially active and could very well erupt again (Yellowstone, Long Valley, Rabaul, Pozzoli). Calderas • Calderas are large elliptical volcanic depressions (2 – 80 km in longest dimension). • They occur on all types of volcanoes • Almost all of the large silicic eruptions are associated with calderas. • Early debate on whether they were formed by explosions or collapse.
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There are two basic types:- I. Volcanoes that have completely blown themselves apart, and then collapsed, leaving a huge crater in their place. Crater Lake, Oregon 75 km 3 Santorini, Mediterranean 39 km 3 Krakatau, Indonesia 20 km 3 Classic interpretation of how “Crater Lake” type of calderas form (Williams, 1941). Initial plinian eruption (a,b) is followed by pyroclastic flows (c). Withdrawal of magma from magma chamber leaves volcano unsupported, resulting in collapse to form caldera (d). Subsequent volcanic activity occurs
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This note was uploaded on 02/22/2012 for the course GEA 3000 taught by Professor Staff during the Fall '09 term at UMass (Amherst).

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Calderas - CATACLYSMIC ERUPTIONS The really big ones This...

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