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. Felsic calderas are surrounded by thick blankets of pumice derived from the eruption of voluminous pyroclastic sheet flows. Variations in form and genesis allow calderas to be subdivided into three types:. This caldera type is generated after the main phase of a Plinian eruption , during collapse of a stratovolcano into the void of the underlying, depleted magma chamber. Although the waning phase of a Plinian eruption is often associated with the generation of pyroclastic flows, piston-like collapse of the volcanic edifice can generate the additional eruption of voluminous, pumice-dominated sheet flows along ring fractures surrounding the collapsing mass.
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. Calderas vary in size from one to kilometers 0. Some calderas form a lake as the bowl-shaped depression fills with water. A famous example is Crater Lake, in Oregon. This caldera formed about 7, years ago when a stratovolcano , Mt. Mazama, violently erupted.
Crater Lake formed about years ago when a massive volcanic eruption of Mount Mazama emptied a large magma chamber below the mountain. The fractured rock above the magma chamber collapsed to produce a massive crater over six miles across. Centuries of rain and snow filled the caldera, creating Crater Lake.
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.