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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

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.

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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.Openings at the plate boundaries where these plates meet each other allow hot molten magma to escape in a volcanic eruption, even if these plate boundaries are under water.

Not all volcanoes occur along plate boundaries, but many of the world’s most active volcanoes are in these locations. For example, the Ring of Fire is an area of the Pacific Ocean Basin that is tectonically active due to the presence of plate boundaries. This area is home to many active volcanoes, including Mount St.

Helen’s, which erupted in 1980 in Washington State. It was the most destructive volcano eruption in the United States history.If a volcano is not active, it could be a dormant volcano, which means that it is a volcano that has not erupted in the past 10,000 years, but has the potential to erupt in the future. Mt. Rainier, which is also located in Washington State, is an example of a dormant volcano. At a height of over 14,000 feet, Mt. Rainier can be considered a sleeping giant that keeps volcanologists on their toes.

A volcano can also be classified as an extinct volcano, which means that it is a volcano that is not expected to erupt again. These past volcanoes no longer have a lava supply, and therefore are deemed unlikely hot spots for volcanic activity. In 2013, researchers discovered the world’s largest volcano lying quietly at the bottom of the Pacific Ocean.

It is estimated that this 400-mile wide volcano last erupted about 144 million years ago and has been extinct ever since.

Lesson Summary

Let’s review. A volcano is a vent in the earth’s crust through which lava, rock fragments, hot vapor and gases are ejected. A volcano begins to form when magma, which is hot molten rock from deep within the earth, rises toward the earth’s surface and collects in magma chambers.As pressure builds within these chambers, magma is expelled through a vent or fissure in the earth’s surface as a volcanic eruption. Hot molten rock outside of a volcano takes on the name 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.There are different classifications of volcanoes. An active volcano is a volcano that has erupted at least once in the past 10,000 years.

Many active volcanoes occur due to plate tectonics, which is a theory that states the earth’s crust is broken up into plates. These plate-like sections of crust are called tectonic plates, and at the boundaries between plates, magma can escape in a volcanic eruption. The Ring of Fire is an area of the Pacific Ocean Basin that is tectonically active due to the presence of plate boundaries.A volcano can also be classified as a dormant volcano, which means that it is a volcano that has not erupted in the past 10,000 years, but has the potential to erupt in the future, or as an extinct volcano, which is a volcano that is not expected to erupt again.

Learning Outcomes

When you you finish this lesson, you should be able to:

  • Define and describe what a volcano is
  • Explain the role of tectonic plates in volcanoes
  • Identify the types of volcanoes

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