How Are Volcanoes Formed?

Some of the most dramatic mountains in the world are volcanoes. Volcanoes can grow to enormous heights quickly, sometimes just a few years. But have you ever wondered, how are volcanoes formed?

As you probably know, you’re standing on the Earth’s crust; a relatively thin layer of solid Earth. Beneath this is the mantle, an extremely hot region just a few dozen kilometers beneath your feet. Although the mantle is almost entirely solid, it can form small pockets of liquid rock and hot gasses. It’s hard to think of this way, but the molten rock – called magma – is more buoyant than the surrounding solid rock. It slowly forces its way upward through weaknesses in the Earth’s crust. These become volcanoes.

When the magma reaches the surface, it wells out onto the surface forming a volcano. What kind of volcano forms depends on the size of the crack in the crust, and the kind of lava that comes out. Some lava is very fluid, and can flow for long distances. This kind of lava creates shield volcanoes, like in the Hawaiian islands. Other lava is very thick, and doesn’t travel very far, creating the more familiar cinder cone shaped volcanoes.

Some very energetic volcanoes will spew rock, ash and magma high into the air. This material falls back down around the vent, and this is how the volcano builds up. The volcano is like a big debris pile surrounding the volcanic vent.

The largest, most dramatic volcanoes in the world are composite volcanoes, or stratovolcanoes. These can have vast networks of volcanic vents and chambers, with many openings to the surface. They can be made up of lava flows and deposits of ash that build up into huge mountains over millions of years. Some of the most dramatic mountains in the world are composite volcanoes: Mt. Fuji, Mt Kilimanjaro, and Mt. Rainier, for example.

We have written many articles about the volcanoes for Universe Today. Here’s an article about the biggest volcano on Earth, and here’s the tallest volcano.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.