Earth From Space: Floating city, originally uploaded by europeanspaceagency.
The islands that make up the Italian city of Venice and the surrounding Venetian Lagoon are pictured in this image. Snaking through the central districts is the Grand Canal, with the Santa Lucia train station at its northern end and the Saint Mark Basin at its southern end. Zooming in, we can see water buses and taxis navigating the canal and gondolas docked along the edge. The square island to the north is San Michele. Once a prison island, it became a cemetery when Napoleon’s occupying forces declared burial on the main islands unsanitary.
This image was acquired on 22 June 2008 by Ikonos-2, a commercial satellite that provides panchromatic and multispectral high-resolution imagery. ESA is supporting Ikonos-2 as a Third Party Mission, which means that the Agency uses its multi-mission European ground infrastructure and expertise to acquire, process and distribute data from the satellite to its wide scientific user community.
High resolution versions of this image can be found at: www.esa.int/esaEO/SEM44R0UDSG_index_1.html
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Caldera formation
A collapse is triggered by the emptying of the magma chamber beneath the volcano, usually as the result of a large volcanic 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 area that collapses may be hundreds or thousands of square kilometers.
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 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.
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 Kīlauea on the island of Hawaii, form calderas in a different fashion. In the case of Kilauea, 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, and the magma chamber is drained by large lava flows rather than by explosive events. The resulting calderas are also known as subsidence calderas, and can form more gradually than explosive calderas. For instance, the caldera atop Fernandina Island underwent a collapse in 1968, when parts of the caldera floor dropped 350 meters. Kilauea Caldera has an inner crater known as Halema‘uma‘u, which has often been filled by a lava lake. The largest volcano on Earth, Mauna Loa is also capped by a subsidence caldera called Moku‘āweoweo Caldera.
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