Mount Vesuvius and Pompeii: Caldera Collapse

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

Location: 40o 49' 16" N, 014o 25' 33" W.

Between the weakening of gas pressure, plus the large amount of material blown out by the eruption, the magma chamber was no longer capable of supporting the weight of Mount Vesuvius and it began to collapse. The summit of the volcano sank into the magma chamber, creating a circular depression called a caldera. Although the picture shows part of the landscape, in reality nobody could have seen this phase of the eruption because ash clouds and pyroclastic flows completely covered the area and hid everything from view. In fact, ash clouds spread so far that Pliny the Younger, his mother, and all the household had to flee from their home in Misenum on the far side of the bay.

In the diagram, blocks of rock are shown sinking into the magma. Solid rock is denser than magma and it is quite possible that some blocks sank into the magma during the collapse. In intrusive rocks we frequently find blocks of other rock trapped in the solidified magma. Such blocks are called xenoliths and they can range anywhere from a few centimeters to kilometers in size.

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Created 09 July 2009, Last Update 15 January 2020