Collapsing Vesuvius Summit and Powerful Pyroclastic Flows

Collapsing Vesuvius Summit and Powerful Pyroclastic Flows STEM Ancient Places and/or Civilizations Geography Disasters

The cutaway, in this graphic depiction, shows the inside of Vesuvius as a stratovolcano, a layer cake of lava flows, mudflows and ash layers.

After several hours of erupting, the gas pressure propelling Vesuvius’ eruption plume, in 79 AD, begins to falter. The eruption cloud itself begins to collapse and starts to pour down the slope of the mountain as pyroclastic flows.

In this graphic, by Dr. Steven Dutch, we see the pyroclastic flows becoming even more powerful as they race toward Pompeii:

During the collapse, hidden in the ash clouds, the faltering pressure in the magma chamber allows the summit [of Vesuvius] to collapse. The collapse allows still more magma to escape. As it does, escaping gas turns the magma into pumice and the pyroclastic flows become even more powerful.

How could it be that the summit of the volcano just ... collapsed?!

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 ... 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. (See “Mount Vesuvius and Pompeii: Caldera Collapse,” by Steven Dutch, at University of Wisconsin, Green Bay.)

Click on the image for a better view.

Media Credits

Image, by Professor Steven Dutch, as it appears via his online article “Vesuvius, 79 AD.” University of Wisconsin, at Green Bay.

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"Collapsing Vesuvius Summit and Powerful Pyroclastic Flows" AwesomeStories.com. May 08, 2015. Oct 20, 2019.
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