How Does a Volcano Work?

Volcanic Eruption Basics

Most people are familiar with volcanic explosions like the one that blew apart Mt. St. Helens in Washington State in 1980. That was a dramatic eruption that blew part of the mountain away and showered billions of tons of ash on surrounding states. However, it's not the only one in that region. Mt. Hood and Mt. Rainier are also considered active, although not as much as their sister caldera. Those mountains are known as "back-arc" volcanoes and their activity is caused by plate motions deep underground.

The Hawaiian island chain stems from a hot spot, a weak point in Earth's crust under the Pacific Ocean. The islands were built up over millions of years as the crust moved over the hotspot and lava vented out to the seafloor. Eventually, each island's surface broke the water's surface and kept growing.

The most active Hawaiian volcanoes are on the Big Island. One of them — Kilauea — continues to pump out thick lava flows that have resurfaced much of the south area of the island. Recent eruptions from a vent on the side of that mountain have destroyed villages and homes on the Big Island.

Volcanoes also erupt all along the Pacific Ocean basin, from Japan south to New Zealand. The most volcanic areas in the basin are along plate boundaries, and that whole region is called the "Ring of Fire".

In Europe, Mt. Etna in Sicily is quite active, as is Vesuvius (the volcano that buried Pompeii and Herculaneum in 79 AD). These mountains continue to affect surrounding regions with earthquakes and occasional flows.

Not every volcano builds up a mountain. Some vent volcanoes send pillows of lava out, particularly from undersea eruptions. Vent volcanoes are active on the planet Venus, where they pave the surface over with thick, viscous lava. On Earth, volcanoes erupt in various ways. 

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How Do Volcanoes Work?

Volcanic eruptions provide routes for material deep beneath Earth's surface to escape to the surface. They also allow a world to vent its heat. Active volcanoes on Earth, Io, and Venus are fed by subsurface molten rock. On Earth, the lava comes up from the mantle (which is the layer under the surface). Once there's enough molten rock — called magma — and enough pressure on it, a volcanic eruption occurs. In many volcanoes, the magma rises up through a central tube or "throat," and emerges out the top of the mountain.

In other places, lava, gases and ash flow out through vents. They can eventually create become cone-shaped hills and mountains. This is the style of eruption that most recently occurred on the Big Island of Hawai'i.

Volcanic activity can be fairly quiet, or it can be quite explosive. In a very active flow, clouds of gas may come rolling out of the volcanic caldera. These are quite deadly because they're hot and moving fast, and the heat and gas and kill someone very quickly.

Volcanoes as Part of Planetary Geology

Volcanoes are often (but not always) closely related to continental plate movements. Deep under the surface of our planet, huge tectonic plates are slowly moving and jostling against each other. At the boundaries between plates, where two or more come together, magma creeps up to the surface. The volcanoes of the Pacific Rim have been built up this way, where plates slide together creating friction and heat, allowing lava to flow freely. Deep-sea volcanoes also erupt with magma and gases. We don't always see the eruptions, but clouds of pumice (rock from the eruption) eventually make their way to the surface and create long rock "rivers" on the surface. 

As mentioned earlier, the Hawaiian islands are actually the result of what's called a volcanic "plume" underneath the Pacific Plate. Here are some more scientific details about how that works: the Pacific Plate is moving slowly to the southeast, and as it does, the plume is heating the crust and sending material to the surface. As the plate moves southward, new spots are heated, and a new island gets built from molten lava forcing its way to the surface. The Big Island is the youngest of the islands to rise above the surface of the Pacific Ocean, although there's a newer one being built as the plate slides. It's called Loihi and it's still underwater. 

In addition to active volcanoes, several places on Earth contain what are called "supervolcanoes." These are geologically active regions that lie atop massive hotspots. The best known is the Yellowstone Caldera in northwestern Wyoming in the U.S. It has a deep lava lake and has erupted several times throughout geologic time. 

A Scientific Look at Volcanic Eruptions

Volcanic eruptions are usually heralded by earthquake swarms. They indicate the motion of molten rock beneath the surface. Once an eruption is about to happen, the volcano can spew out lava in two forms, plus ash, and heated gases.

Most people are familiar with the sinuous-looking ropy "pahoehoe" lava (pronounced "pah-HOY-hoy"). It has the consistency of molten peanut butter. It cools very quickly to make thick black rock layers. The other type of lava that flows from volcanoes is called "A'a" (pronounced "AH-ah"). It looks like a moving pile of coal clinkers.

Both types of lava carry gases, which they release as they flow. Their temperatures can be more than 1,200° C. The hot gases released in volcanic eruptions include carbon dioxide, sulfur dioxide, nitrogen, argon, methane, and carbon monoxide, as well as water vapor. Ash, which can be as small as dust particles and large as rocks and pebbles, is made of cooled rock and is flung out from the volcano. These gases can be quite deadly, even in small amounts, even on a relatively quiet mountain.

In very explosive volcanic eruptions, ash and gases are mixed together in what's called a "pyroclastic flow". Such a mixture moves very fast and can be quite deadly. During the eruption of Mt. St. Helens in Washington, the blast from Mount Pinatubo in the Philippines, and the eruptions near Pompeii in ancient Rome, most people died when they were overcome by such killer gas and ash flows. Others were buried in the ash or mud floods that followed the eruption.

Volcanoes Are Necessary to Planetary Evolution

Volcanoes and volcanic flows have affected our planet (and others) since the earliest history of the solar system. They have enriched the atmosphere and soils, at the same time they have posed drastic changes and threatened life. They're part of living on an active planet and have valuable lessons to teach on other worlds where the volcanic activity takes place.

Geologists study volcanic eruptions and related activities and work to classify each type of volcanic land feature. What they learn gives them more insight into the interior workings of our planet and other worlds where volcanic activity takes place.