# How Volcanoes Form How magma forms deep in the mantle, rises through the crust and builds a volcano layer by layer, from subduction zones to hotspots\. By happy\-comet\-20f0f700 Topic: Earth & Geography [Open the interactive Jig](https://ask.jig.com/j/WHFpDNelYVB3) ## Transcript ### 1. A mountain built from inside A volcano is a place where melted rock from deep inside the Earth reaches the surface\. This model is cut open so you can see where that rock comes from\. Drag to turn it and look around\. Heights are stretched so the volcano is easy to see\. Real volcanoes are tiny next to the layers below them\. ### 2. Earth's cracked shell The Earth's hard outer shell is broken into giant pieces called plates\. They drift a few centimetres a year, about as fast as your fingernails grow\. Here a heavy ocean plate slides down under a continent at a deep trench\. Most volcanoes sit near the edges of plates\. ### 3. The mantle is solid rock Under the plates lies the mantle\. It is very hot, yet it is almost entirely solid rock that creeps very slowly\. To make a volcano, a small part of it has to melt\. Melted rock underground is called magma\. Once it comes out, we call it lava\. ### 4. Water helps rock melt As the ocean plate sinks, it carries seawater locked inside its rocks\. About 100 kilometres down, the heat squeezes that water out\. The water lets the hot rock above melt at a lower temperature, a bit like salt helps ice melt\. Water lowers the melting point of mantle rock by roughly 60 to 100 °C\. ### 5. Magma rises Magma is lighter than the solid rock around it, so it rises slowly in blobs, like oil floating up through water\. It collects in a magma chamber in the crust beneath the volcano\. A magma chamber is not an open lake\. It is more like a sponge of crystals with melted rock in the gaps\. ### 6. Eruption\! Gas dissolved in the magma forms bubbles and builds up pressure\. When the pressure is high enough, magma pushes up the pipe and bursts out of the vent at the top\. Press Erupt and watch it travel\. Lava can be 700 to 1,200 °C\. Ash is not smoke: it is tiny bits of rock and volcanic glass\. ### 7. Layer by layer Each eruption leaves a new layer of lava and ash on the slopes\. Over thousands of years, the layers stack up into a tall cone with steep sides\. Slide to add eruptions and watch the mountain grow\. This kind of volcano is called a stratovolcano\. Its slopes are often 30 to 35 degrees\. ### 8. Hotspots Some volcanoes grow far from any plate edge\. A column of extra\-hot rock, called a plume, rises from deep in the mantle and melts the plate above it\. Runny lava spreads out into a wide, gentle shield volcano\. Hawaii's Mauna Kea rises more than 9 kilometres above the sea floor, more than Mount Everest's height above sea level\. ### 9. Volcanoes everywhere Rock melts when water is added, when a hot plume rises, or when plates pull apart and the pressure on the mantle drops\. Every volcano begins with one of these\. Each eruption then builds the mountain a little more\. Most of Earth's magma, about three quarters, erupts unseen along ridges on the ocean floor\.