Caldera
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Caldera formation
A caldera collapse is usually triggered by the emptying of the magma chamber beneath the volcano, often as the result of a large eruption. If enough magma is erupted, the emptied chamber will not be able to support the weight of the volcanic edifice (the mountain) above. Fractures will form around the edge of the chamber, usually in a roughly circular shape. These ring fractures may in fact serve as volcanic vents. As the magma chamber empties, the center of the volcano within the ring fractures begins to collapse. The collapse may occur as the result of a single massive eruption, or it may occur in stages as the result of a series of eruptions. The total amount of collapse may be hundreds or thousands of meters.
Explosive calderas
If the magma is rich in silica, the caldera is often filled in with ignimbrite, tuff, rhyolite, and other igneous rocks. Silica-rich magma is very viscous. As a result, gases tend to become trapped at high pressure within the magma. When the magma gets near the surface of the Earth, the gas expands quickly, causing explosions and spreading volcanic ash over wide areas. Further lava flows may be erupted, and the center of the caldera is often uplifted in the form of a resurgent dome by subsequent intrusion of magma. A silicic or rhyolitic caldera may erupt hundreds or even thousands of cubic kilometers of material in a single event. Even small caldera-forming eruptions, such as Krakatoa in 1883 or Mount Pinatubo in 1991, may result in significant local destruction and a noticeable drop in temperature around the world. Large calderas may have even greater effects.
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When Yellowstone Caldera erupted 600,000 years ago it released 1000 cubic kilometers of material, covering half of North America in up to two meters of debris. By comparison, when Mount St. Helens erupted in 1980, it released 1.2 cubic kilometers of ejecta. The ecological effects of the eruption of a large caldera can be seen in the record of the Lake Toba eruption in Indonesia. About 75,000 years ago, this volcano released 2800 cubic kilometers of ejecta. In the late 1990s, archeologist Stanley Ambrose [1] (http://www.anthro.uiuc.edu/faculty/ambrose/) proposed that a volcanic winter induced by this eruption reduced the human population to a few thousand individuals, resulting in a population bottleneck (see Toba catastrophe theory).
At some points in geologic time, rhyolitic calderas have appeared in distinct clusters. The remnants of such clusters may be found in places such as the San Juan Mountains of Colorado (erupted during the Tertiary Period) or the Saint Francois Mountain Range of Missouri (erupted during the Proterozoic).
Non-explosive calderas
Some volcanoes, such as Kilauea on the island of Hawaii, form calderas in a different fashion. In the case of Kiluaea, the magma feeding the volcano is relatively silica poor. As a result, the magma is much less viscous than the magma of a rhyolitic volcano. The magma chamber is drained by large lava flows rather than by explosive events. The caldera, known as Halema‘uma‘u, is often filled by a lava lake.
Non-volcanic calderas
It is possible, although rare, for a caldera-like formation to be created by erosion rather than volcanism. It is believed that the Caldera de Taburiente on La Palma in the Canary Islands is an example of this.
Notable calderas
- Askja (Iceland)
- Aso (Kumamoto Prefecture, Japan)
- Caldera de Taburiente (La Palma, Canary Islands, Spain)
- Las Ca񡤡s on Teide (Tenerife, Canary Islands, Spain)
- Crater Lake on Mount Mazama (Crater Lake National Park, Oregon, United States)
- Lake Taupo (New Zealand)
- Lake Toba (Sumatra, Indonesia)
- Long Valley (California, US)
- Mount Tambora (Sumbawa, Indonesia)
- Valle Grande (New Mexico, US)
- Yellowstone Caldera (Wyoming, US)
- Newberry Caldera (Oregon, US)
- Ngorongoro Crater (Tanzania, Africa)
- Olympus Mons Caldera (Mars)
- Santorini (Greece)
- Lake Bracciano (Italy)