Q How do volcanoes form, and how did the volcano of Pompeii show up?
— Andrew Vigliotta, Mount Horeb, Wis.
A Clifford Thurber, professor of geophysics at the University of Wisconsin-Madison:
Some volcanoes, such as the Cascade volcanoes up in Washington and Oregon, are of the type called a stratovolcano. These steep volcanoes sometimes erupt explosively and other times have calmer lava flows that just spill out on the surface. The material from eruptions, such as lava and ash, build up and cause these volcanoes to have a character like a layer cake.
The volcano of Hawaii, as a shield volcano, is very different. It’s very broad and low, built almost completely by highly fluid liquid lava, flow after flow after flow. Many lava flows over thousands and millions of years build up the island very slowly without generally catastrophic eruptions.
Vesuvius, the volcano of Pompeii, is of the former type. It’s a stratovolcano. Sometimes it just has lava flows that build up the size of the volcano. But other times, such as in 79 A.D., it erupts explosively and produces large volumes of ash. That’s what buried the town of Pompeii and killed many people in the process.
The basic origin of Mount Vesuvius is due to the fact that Africa is drifting northward and actually colliding with Europe. This is producing a very small subduction zone, or area where one tectonic plate moves under another, in southern Italy, leading to a number of active volcanoes.
Volcanologists can use earthquakes as one way to try to predict volcanic eruptions. As volcanoes prepare for eruption, fracturing happens inside the Earth while magma tries to move toward the surface.
As a result, earthquakes are produced. We can use that to actually track the movement of magma inside before it erupts at the surface.