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The cone volcanoes are created by the upwelling of liquid hot rock from the earth's interior. This is called Plutonic rock, along with Granites which however are cooled before they reach the surface. The hollow left at the top of a volcano is formed when the pool of magma withdraws and the volcano becomes quiet.

Some volcanoes do not form cones. Lake Taupo in New Zealand, and Yellowstone National Park in the US are notable examples, where there was a violent explosive eruption which however did not produce a local cone.

The volcanic eruption, whose caldera is now filled in by Lake Taupo occurred about 26000 years ago and ejected over 1 000 cubic kilometres of material! It has had 28 eruptions in the last 27 000 years, and in one it ejected about 30 cubic kilometres in a few minutes. The last eruption was about AD 186.

Naturally, once erupted, erosion takes its toll on the exposed and fairly unstable material.

There is a third type of volcanic material, the Flood Basalts, which are of continental scale! These ejections are of a very liquid material, covering giant areas, but without conspicuous surface relief.

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13y ago

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