What is a Volcano? - Definition & Eruptions

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  • 0:06 Volcanoes
  • 0:32 Magma and Lava
  • 1:50 Origin of the Name Volcano
  • 2:25 Classification of Volcanoes
  • 5:19 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Volcanoes are vents that allow lava, rock fragments and gases to escape from layers beneath the earth's surface. Learn how volcanoes form both on land and underwater and why they erupt.


Volcanoes are spectacular events and because of this, they have found their way into the plot of many Hollywood movies. While the movies have given most of us the vivid image of red-hot lava spewing out of the top of a towering volcano, they do not share the entire story of volcanoes. In this lesson, we will fill in some of the blanks left by Hollywood, as we learn how volcanoes form and why eruptions occur.

Magma and Lava

So what exactly is a volcano? Well, it can be defined as a vent in the earth's crust through which lava, rock fragments, hot vapor and gases are ejected. In other words, a volcano is the earth's way of letting off a little steam.

The super-heated particles that eject out of a volcano come from deep below the earth's surface where temperatures can become so hot that rock actually melts. Magma is the term used to describe this hot molten rock from deep within the earth.

A volcano begins to form when magma, which is less dense than the rock it originated from, rises toward the earth's surface. This liquid rock collects in chambers called 'magma chambers,' where pressure builds due to expanding steam and gases associated with the magma. As pressure reaches a peak within these chambers, magma finds its way through a vent or fissure in the earth's surface, resulting in a volcanic eruption and the expulsion of the hot molten rock.

We now have hot molten rock outside of a volcano, and its name changes from magma to lava. So you can think of 'magma' as liquid rock in the 'middle' of the earth and 'lava' as liquid rock that's 'leaving' the earth.

Origin of the Name Volcano

When a volcano erupts, it expels lava, gases and rocks with tremendous force. It's no wonder that the Romans thought that volcanoes were the work of the gods. The name 'volcano' comes from a little island in the Mediterranean Sea called 'Vulcano.' Centuries ago, people in the area thought that the island was a chimney that led out of Vulcan's workshop. Vulcan was the blacksmith for the Roman gods and when lava and ash would spew from the mountain, it was a sign that Vulcan was hard at work pounding out weapons for the gods.

Classification of Volcanoes

Volcanoes are classified by their level of activity. If a volcano is deemed to be an active volcano, then it is a volcano that has erupted at least once in the past 10,000 years. As you can see, there is a pretty big window for active volcanoes. With this definition, an active volcano could be erupting right now or might have erupted only once since the last Ice Age. There are about 1,500 active volcanoes on planet Earth, and this includes those under the world's oceans.

You might think it's odd that a fiery volcano could erupt under tons of ocean water, but this is because many active volcanoes, both underwater and on land, occur due to plate tectonics. Plate tectonics is a theory that states the earth's crust is broken up into pieces. With plate tectonics, we see that the outer crust of the earth is actually cracked, much like the fractures that happen when you crack the shell of a hardboiled egg. The resulting plate-like sections of crust are called tectonic plates, and they float on top of the much hotter deep layers of the earth, sort of like a hockey puck glides over an air hockey table.

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