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Volcanoes are mountains. But unlike most mountains in the world, formed from folding continental plates, uplift and erosion, volcanoes are created when material from inside the Earth escapes to the surface. Let’s answer the question, “what are volcanoes”.
As you probably know, the ground you’re standing on is the Earth’s crust. It varies in depth between 10 km under the oceans and 30 km beneath the continental plates. Beneath the crust is a vast region called the Earth’s mantle. The mantle is made up of molten rock called magma. At the boundaries between continental plates, volcanic vents can open up, where magma and gasses from inside the Earth can escape.
When it’s still underground, the molten rock is called magma. Although most of the Earth’s mantle is solid, it can create pockets of liquid which escape from underneath through weaknesses in the Earth’s crust. After it escapes to the surface, it’s called lava. Lava has different levels of viscosity – how easily it flows downhill. The least viscous (easy flowing) lava creates shield volcanoes. The lava can flow for great distances in huge rivers, and creates wide volcanoes with gently sloping sides. The most viscous (thickest) lava piles up around the volcanic vent created the familiar cone-shaped volcanoes.
The simplest volcanoes where a single vent breaks through a weakness in the crust, releasing lava, ash and rocks. After being blasted out of the vent, it piles up around on the surface, allowing the volcano to grow up quickly. Some of the largest, most familiar volcanoes are composite, or stratovolcanoes. These are built up of multiple layers of ash and lava, and can have vast networks of vents.
The word volcano comes from the island of Vulcano in the Mediterranean Sea off Sicily. Ancient people thought that this island was the chimney of the forge of Vulcan, the Roman god of blacksmithing.
We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.