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.
When Yellowstone Caldera (last) erupted 640,000 years ago it released 1,000 cubic kilometers of material, covering all 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 2,800 cubic kilometers of ejecta, the largest known eruption within the Quaternary Period (last 1.8 million years). In the late 1990s, archeologist Stanley Ambrose [1] 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). Even larger caldera-forming eruptions are known, especially La Garita Caldera in the San Juan Mountains of Colorado, where the 5,000 cubic kilometer Fish Canyon Tuff was blasted out in a truly major single eruption 27.8 million years ago.
