Under the Volcano: A Vesuvius Backgrounder
What exactly happened at Mount Vesuvius that summer day in 79 AD? What caused the mountain to blow its top, and why did Pompeii and Herculaneum experience their different fates?
Geologists tell us it all started with the restless motion of the Earth’s crust.
Where Plates Collide
Most volcanoes, like earthquakes, are the result of plate tectonics, the motion of large, rigid sections of the Earth’s crust and uppermost mantle. Riding on a more fluid layer of the mantle, these sections collide, spread apart, or slide past each other. The majority of eruptions on Earth occur when tectonic plates pull apart and magma rises to fill the gap; nearly all of these, however, are deep under the ocean and don’t affect life on land. Vesuvius, like most of the volcanoes we’re familiar with, formed at a “subduction zone,” where plates collide.
“Vesuvius is sitting where the African plate is diving under the Eurasian plate on the order of an inch per year,” explains geologist Phil Janney, Manager of the Field Museum’s Isotope Geochemistry Laboratory. “This causes part of the overlying plate to melt, forming a magma rich in dissolved gases such as water vapor, carbon dioxide, and sulfur dioxide.”
Because the magma is lighter than the surrounding rock, it starts to rise. At shallow depths (1 to 2 kilometers) the gas is holds forms bubbles. This increases the pressure and drives the magma upward at ever-increasing speed.
If the gas content is relatively low and the magma rich in iron and magnesium, it will have low viscosity and may simply flow out from the summit and fissures of the volcano, with little explosive force. But if the gas content is high and the magma rich in silicon dioxide, it will be highly viscous…and the stage is set for an explosive eruption.
“It’s like shaking a bottle of pop,” Janney says. “The gas pressure builds up inside, and when the top comes off, or the blockage is released, it just explodes, taking the soda—or the magma—with it.”
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