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Craters & Calderas

Lesson 6

raters and calderas are holes in the ground created by a volcano. Craters are smaller than calderas and are less than two kilometers across. Some craters are so tiny they are only a few meters across. Craters form at the top when eruptions occur repeatedly from the same vent on a volcano. The crater is often located on the summit of these volcanoes.

Cinder cones usually erupt from a central vent on the volcano. After the eruptions cease a crater that is a circular shaped hole with steep sides is left. Over the years rubble falling into the floor of the crater from the steep sides creates a shallow crater with sloping sides.

Shield volcanoes sometimes develop a series of pit craters that form along a fracture on the volcano. Other times a crater will form within a crater. Lava that flows into a pit crater will form a lake of molten rock. The lava lakes sometimes take many years to cool.

The magma inside stratovolcanoes is usually thick and pasty. Lava domes will often plug a vent on a volcano. Pressure builds inside the volcano until there is enough pressure to blow the plug from the opening. This type of an eruption often produces a large crater. Lakes will sometimes form in the depression after the eruption has ceased.

Geologists believe that Mt. Mazama was a large stratovolcano with several vents prior to its last climatic eruption. The eruption began when the mountain began to blow out large volumes of rhyolite. The eruption continued until the lighter colored rhyolite emptied and darker lava rose up in the magma chamber and was expelled. Mt. Mazama erupted so much material that it collapsed inward forming a gaping hole in the ground. Thick deposits of pumice and pyroclastic debris covered the area around the mountain for many kilometers. The gaping hole eventually cooled and filled with rainwater and melting snow forming Crater Lake. Native Americans living in the area still tell stories about the eruption witnessed by their ancestors.

Plinian eruptions sometimes produce large calderas that are easily visible from space but are often difficult to detect standing on Earth. Yellowstone Park is located in an area where three overlapping calderas are visible from space. The magma beneath the caldera, that is producing the thermal hot springs, geysers, and fumaroles, is slowly cooling.

Geologists estimate that during massive caldera eruptions only about ten percent of the magma in the magma chamber is erupted and the rest cools slowly underground. Geologists estimate that one of the great caldera eruptions at Yellowstone Park was over 1000 times larger than the 1980 Mt. St. Helens eruption.

Lesson summary

•  Craters range in size from a few meters up to two kilometers across.

•  Cinder cones usually produce a round steep sided crater on their summit.

•  Large calderas and craters form when domes over volcanic vents create a plug that allows pressure to build inside the volcano until a large explosion blows the dome away.

•  Calderas often form after a large Plinian eruption.

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