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Volcanoes: Formation & Locations

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After completing this lesson, you will be able to describe how volcanoes form, and why they form in the particular locations they do. A short quiz will follow.

What are Volcanoes?

A volcano is a mountain-shaped mass, formed when red-hot liquid magma finds its way up to the Earth's surface. That magma falls along the sides of the volcano, then hardens into rock. This forms the characteristic cone shape that people think of when you mention volcanoes. But where does that magma come from?

A volcano
A Volcano

The Earth is layered like an onion, with one thing beneath another. If you go down below the Earth's crust (the part we live on), you find that the chemical composition gradually changes. But you also find that that matter moves differently. Going deeper into the Earth, the temperature and pressure increases until rock starts to flow almost like a liquid. This flow of red, glowing rock is called magma. And when that magma reaches the surface, bursting out of a volcano, we call it lava.

So that explains what happens when a volcano forms, but not why it happens. So next we're going to talk about how the magma gets to the surface.

How and Where Do Volcanoes Form?

When magma erupts at the surface of the Earth to form a volcano, it does so due to pressure. Deep below the Earth, the pressure is so high, if a pathway to the surface opens up, the magma will spurt out. This is kind of like how if you squeeze a bottle of ketchup too hard, you'll make a huge mess.

There are certain places where these pathways to the surface tend to open up: three places to be exact. Those places are hot spots, divergent plate boundaries, and convergent plate boundaries.

The Earth is split into sections called tectonic plates. These plates float around on the magma below, like floats on a swimming pool. When those plates move towards each other, it's called a convergent plate boundary. When those plates move apart, it's called a divergent plate boundary. In both these cases, volcanoes can form.

Map of the tectonic plates of the Earth
Map of the Tectonic Plates of the earth

In the case of a divergent boundary like an ocean ridge or rift, this happens because the two plates move apart and the magma rushes up to fill the gap. For a convergent boundary, where an oceanic plate goes under another plate, water and other materials are allowed to find their way deep into the Earth. We think that this lowers the boiling point of the mantle, causing magma to be released. Extra magma is also formed as the two plates rub together: this high heat and pressure can cause the two plates to melt.

Cross-section Showing the Formation of a volcano
Cross-Section Showing the Formation of a Volcano

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