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What is a Volcano?

Michael Anissimov
Updated Jan 28, 2024
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A volcano is an opening in the Earth's crust where magma from the mantle reaches the surface, sometimes in a slow, dribbling fashion, called an effusive eruption, and sometimes in a violent event called an explosive eruption. Volcanoes usually occur in divergent boundaries between tectonic plates, places where the crust is weak and magma can rise to the surface due to the immense pressure of the mantle below. When magma reaches the surface, it is called lava.

Thousands of volcanoes have been recognized on Earth, and they are found on every continent and scattered across the ocean floor. Among the most famous are Mount Etna in Sicily, Mount Vesuvius in Italy, Mount Merapi in Indonesia, Sakurajima in Japan, Mauna Loa in Hawaii, Mount Rainier in Washington, USA, and Mount Erebus in Antarctica. Volcanic activity is how the world's islands get formed. Measured from the ocean floor, Mauna Loa in Hawaii is actually a taller mountain that Mt. Everest in Nepal.

The main types of volcano include the following:

  1. shield volcanoes, which are broad and shield-shaped, created by the slow eruption and long flow of viscous lavas;
  2. lava domes, formed by viscous lava that does not flow very far;
  3. cinder cones, which are small (98 to 1,312 feet (30 to 400 meters)), cone-shaped hills that occur on the flanks of larger volcanoes;
  4. stratovolcanoes, tall conical ones, like Mt. Fuji in Japan and Vesuvius in Italy;
  5. supervolcanoes, massive structures that explode very rarely;
  6. submarine volcanoes, located on the ocean floor; and
  7. subglacial volcanoes, located beneath continental glaciers.

Due to the


of Greenland and Antarctica, subglacial volcanoes are among the rarest, with only five known in modern times.

Volcanoes and their eruptions have occasionally changed the course of history. In prehistory, it is thought that the eruptions of the supervolcanic Yellowstone Caldera 650,000 years ago and Lake Toba eruption 75,000 years ago almost wiped out the human species by producing especially cold winters for entire centuries. These winters would be caused by volcanic aerosols in the upper atmosphere blocking out sunlight, kickstarting a feedback process of glaciation and ushering in a minor Ice Age.

AllTheScience is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Michael Anissimov
By Michael Anissimov

Michael is a longtime AllTheScience contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

Discussion Comments

By anon305251 — On Nov 25, 2012

There is no volcano near me, but I am worried about what he said about the world ending in the future or even having a future. I hope that is no time soon.

By shell4life — On Sep 08, 2012

@cloudel – I've heard that the volcano at Yellowstone just wouldn't produce enough sulfur to block the sun. Magma has to have a lot of sulfur in it in order to make clouds capable of starving plants and humans of sunlight.

I've been hearing talk about a possible eruption for decades, and I've just lost interest in it. I know that it could happen at any time, but why worry about something that might not happen for thousands of years?

By cloudel — On Sep 07, 2012

I've read a lot lately about the volcano at Yellowstone National Park possibly having a supereruption in the near future and destroying life as we know it. Some experts are afraid that it would be so powerful that all the junk it ejects could block out the sun all over the U.S. What do you guys think about this?

By wavy58 — On Sep 07, 2012

I think it's crazy that in Hawaii, you can actually go on a volcano tour! There is a place called Volcano National Park Hawaii, and you can even camp there to watch the lava flowing when it's dark outside.

What is so amazing is that this park has both the biggest volcano on Earth and the most active one. For me, those are two reasons to stay far, far away!

I understand that volcanoes are fascinating, and the thrill of the danger lures some people. However, it just doesn't seem safe or reasonable to me.

By Perdido — On Sep 06, 2012

I first became aware of volcano hazards as a young child. The eruption of Mt. St. Helens in 1980 was all over the news, and I heard my parents discussing it, as well.

This was the most terrible eruption I ever recall hearing about. It shot gas and ash so far up into the atmosphere that when the wind carried a giant cloud of this stuff, it got totally dark over two-hundred miles away!

That was when my fear of volcanoes began. I used to stand on my bed, imagining that lava was flowing on the floor all around me, and I scared myself with these scenarios. Once I learned that we lived nowhere near any active volcano, my fears were relieved.

By Georgesplane — On Jun 05, 2010

Volcanoes can also form on hot spots. The Hawaiian Islands are a great example. They are actually part of a larger archipelago that extends all the way to the emperor seamounts. These islands, atolls, and seamounts were formed by a stationary magma chamber below the pacific plate that has been erupting continuously for some 65 million years. The pacific plate is currently moving to the northwest at a rate of about 4 inches a year. Every island, atoll, and seamount in this 5000 kilometer chain was at one point in time an active volcano.

By Fiorite — On Jun 05, 2010

@ PelesTears - I would like to add that the viscosity of magma is controlled by the silica content. Basaltic lava has low silica content. This type of less viscous lava flows from the volcanoes in Hawaii. Rhyolite on the other hand is an igneous rock with a high silica content formed from more viscous lava flows. Basaltic flows are characterized by the rivers of lava flowing down a mountain. More viscous lava flows more like molasses or is ejected as chunks of molten rock that tend to roll down hills. When the lava has lower silica content the trapped water vapor and carbon dioxide escape easily; preventing explosive eruptions.

By PelesTears — On Jun 05, 2010

@ Anon21131 - The explosive properties of a volcano are dependent on the viscosity of the magma beneath the surface. More viscous magma produces more explosive volcanic eruptions. The viscosity also dictates how fast, and how much, lava will flow from the volcano. Shield volcanoes (like the ones found in Hawaii) eject low viscosity lava. The eruptions rarely produce pyroclastic flows, and the lava flows easily down the mountain side. The path of the lava flow from shield volcanoes is easier to predict and rarely poses a threat to human life. The more explosive flows occur because the thicker, more viscous, magma does not allow trapped gases to escape easily. This causes a build-up of pressure, heat, and explosive gas. When this type of eruption occurs the results can be catastrophic; literally removing mountain tops, and sending superheated gases and ash down the mountain side. These vulcanian and plinian eruptions produce stratovolcanoes that are identifiable by the steep sloped sides (Think Mt St Helens, Mt Hood, etc.). Although stratovolcanoes may produce less magma, they are often deadlier.

By anon21131 — On Nov 10, 2008

What kind of volcano does not shoot out a lot of lava?

Michael Anissimov

Michael Anissimov

Michael is a longtime AllTheScience contributor who specializes in topics relating to paleontology, physics, biology...

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