What Is a Volcano?

From Chile to Hawai‛i to Yellowstone to Oregon, volcanoes have been "erupting" in the headlines over the past week. Each one of these powerful eruptions has its own unique story. How do volcanoes form? Are eruptions always dangerous?

Sabrina Stierwalt, PhD
5-minute read
Episode #145

Hi! I’m Sabrina Stierwalt, Ask Science, bringing you Quick and Dirty Tips to help you make sense of science.

Volcanoes remind us that our planet is active, and that even the ground beneath our feet is ever changing. Four volcanoes from Chile, Hawai‛i, Yellowstone, and Oregon have made headlines in the last week alone, but each with its own unique story. Let’s explore the causes of these powerful eruptions.


What Is a Volcano?

A volcano is formed when the pressure below the Earth’s surface builds enough to cause a rupture that then forces the release of the hot magma and gas below. These eruptions can be dramatic, with ejecta traveling at hundreds of miles per hour, or they can be calmer like a slow ooze. While volcanologists disagree over which volcanoes can be labeled active versus extinct, the number of active volcanoes on Earth is somewhere around 1,500.

The Earth’s crust is not solid, but instead is made up of a series of large plates, somewhat like large puzzle pieces, that can move and shift over time. Volcanoes are usually formed where these so-called tectonic plates are either converging (like in the band circling the Pacific Ocean, known as the Pacific Ring of Fire) or diverging (like in the Mid-Atlantic Ridge).

Friction from these shifting plates heats the magma, or molten rock, below the Earth’s surface much like how rubbing your hands together will warm them up. The increased temperature can lead to a build-up in pressure that eventually explodes through the Earth’s crust, forcing the magma (which is called lava once it reaches the surface) to pour out.

Volcanic eruptions can cause mass devastation. Mount Vesuvius, near Naples, Italy, buried the cities of Pompeii and Herculaneum and their residents in an eruption in the year 79 AD. Vesuvius continues to erupt on roughly a 20-year cycle, although the last major eruption was in 1944. The volcano is still classified as one of the most dangerous volcanoes in the world due to the hundreds of thousands of people that live nearby.

But volcanoes aren’t all destruction; they serve a positive purpose as well. Over time they have created a large fraction of the Earth’s surface, including the Hawai‛ian island chain, through the ejection and then the eventual cooling and settling of material. They also provide rich soil for farming, and in some cases, their geothermal energy can be harnessed for our energy needs.

So which volcanoes have been “erupting” in the news lately?

Chile’s Calbuco Volcano

In southern Chile, the Calbuco Volcano has erupted 3 times in a span of 8 days, causing darkened skies in the nearby cities of Puerto Montt and Puerto Varas. Thousands of residents (including animals and salmon, a local staple) have been evacuated.

The ashen debris stacks as high as 2 feet in some areas, and has spewed as far as Argentina. Since the last major eruption for Calbuco was in 1962, this latest event caught many off-guard.

Also included in the 200 million cubic meters of ash and rock bursting from below the surface is a large amount of sulfur dioxide gas, the climate effects of which are not yet understood.


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About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.