Calderas usually, if not always, form by the collapse of the top of a volcanic cone or group of cones because of removal of the support formerly furnished by an underlying body of magma molten rock. Often this collapse is of a composite cone that rapidly emptied the underlying magma reservoir by voluminous eruptions of pumice and pumiceous ash. At the end of the eruptions, the top of the mountain has disappeared, leaving an immense hole in its place. It once was believed that the top of the mountain had been blown away by the explosions, but studies showed that only a little of the old rock was thrown out and the rest had dropped down into the void. Subsequent minor eruptions may build small cones on the floor of the caldera, which may still later fill up with water, as did Crater Lake in Oregon. Other depressions, of markedly angular, irregular outline, also occur in volcanic districts and commonly are even larger than calderas. They therefore are referred to as volcano-tectonic depressions. Their collapse also appears to be at least partly related to the rapid extrusion of large amounts of lava.
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What is a Caldera?
A caldera is a large depression formed when a volcano erupts and collapses. During a volcanic eruption , magma present in the magma chamber underneath the volcano is expelled, often forcefully. When the magma chamber empties, the support that the magma had provided inside the chamber disappears. As a result, the sides and top of the volcano collapse inward.
When an erupting volcano empties a shallow-level magma chamber, the edifice of the volcano may collapse into the voided reservoir, thus forming a steep, bowl-shaped depression called a caldera Spanish for kettle or cauldron. These features are highly variable in size, ranging from km in diameter. In most cases, they can be readily differentiated from summit craters , which are generally much smaller and form by explosive erosion of the central vent.
Tuesday, December 16, Yellowstone is Still Hot Yellowstone National Park sits over a hotspot, a place in the Earth's crust where magma sits close to the surface. The Yellowstone Supervolcano, which sits over the hotspot, has had three caldera-forming eruptions: 2. Magma continues to build beneath Yellowstone. It's a question of when, not if, it erupts again.