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The geologic history of Yellowstone centers around volcanism. The caldera that rings much of the park was formed by a huge explosion about , years ago.
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The magnitude of the eruption is beyond the realm of imagination: cubic miles of ash were blasted into the atmosphere, making this eruption perhaps the largest ever to occur on earth. After this eruption, the magma chamber collapsed into a gigantic pit reaching 28 by 47 miles 45 by 76 kilometers across, and perhaps several thousand feet deep.
Over millennia, the lower parts of this basin filled with water, forming Lake Yellowstone.
- Caldera Eruptions.
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Typical caldera forming eruptions occur in the late stages of stratovolcano evolution. Instead, they subside in increments to produce a nested structure of pits and terraces, as shown in the photos above for the Kilauea caldera and the Erta Al caldera. Basaltic calderas like these are gradually enlarged by episodic collapse, due to the extraction of lava from shallow-level magma chambers underlying the summit areas. In some cases, the extraction of magma may occur through fractures that feed eruptions along the flanks of the shield volcano.
Iceland’s Caldera of Hot Springs
Local collapse along such fractures can generate a linear system of pit craters that emanate away from many basaltic calderas and summit craters. Photo by Vic Camp. Resurgent calderas are the largest volcanic structures on earth. They are associated with massive eruptions of voluminous pyroclastic sheet flows, on a scale not yet observed in historic times. The youngest of these resurgent calderas is the 74,year-old Toba Caldera on the Indonesian Island of Sumatra. The Toba eruption generated times more pyroclastic material than the moderate Plinian eruption of Mt.
What is a Caldera?
Helens in ! There are three resurgent calderas in the United States less than 1. With diameters ranging from 15 to km, resurgent calderas dwarf those of Crater-Lake type. They are similar to Crater-Lake type calderas in that they are also generated by crustal collapse above shallow magma chambers.
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Resurgent calderas, however, are too large to have been associated with a Crater-Lake type central volcano, like Mt. Apart from their large size, the definitive feature of resurgent calderas is a broad topographic depression with a central elevated mass resulting from post-collapse upheaval of the caldera floor. Uplift is commonly more than one kilometer. The caldera floor is typically filled with rhyolitic lavas , obsidian flows , and domes , and the uplifted centers often contain elongate rifts graben along their crests.
Yellowstone volcano: How NASA spotted LARGE anomaly below caldera OVERNIGHT
The evolution of a resurgent caldera is depicted in the illustration shown here modified from Calvin J. Caldera formation begins with crustal uplift associated with the arrival of a large plume of gas-rich rhyolitic magma A. Ring fractures propagate outward from the chamber toward the surface and these are used as conduits for escaping magma.
Decompression of the magma results in massive vesiculation and the explosive eruption of tephra into the high atmosphere B. As the eruption wanes, pyroclastic flows begin to erupt from the ring fractures. The magma chamber is depleted and its crustal roof begins to collapse, generating additional pyroclastic flows C.
Caldera: Crater Formed by Volcanic Collapse or Explosion
As the eruption ends, hundreds of meters of pyroclastic flows have now accumulated within the caldera inflow facies and beyond the caldera walls outflow facies D. The floor of the caldera may quickly be occupied by a lake E. Resurgence of the caldera floor takes about to , years to accomplish F. This period of uplift may be controlled by compression of the remaining magma beneath the collapsed roof of the chamber, or by the arrival of new magma into the chamber.
Crater Lake caldera. Aniakchak caldera.