The Earth isn't solid all the way through. Beneath the thin rocky crust we live on, there's a layer called the mantle — thousands of kilometres of rock so unimaginably hot that it behaves more like an incredibly thick, slow-moving liquid than solid rock. And deeper still, the core reaches temperatures of up to 5,000°C — hotter than the surface of the Sun.
All that heat needs to go somewhere. One of the ways it escapes is through volcanoes.
What causes magma to rise?
Deep in the mantle, rock is kept solid by the crushing pressure of all the rock above it. If that pressure decreases — because tectonic plates are being pulled apart, or a plate is sinking and displacing material upwards — the rock can melt into magma. Magma is less dense than solid rock, so it rises, like a bubble of oil in water, working its way up through cracks and weaknesses in the crust.
Shake a can of fizzy drink. The pressure inside keeps the gas dissolved in the liquid. When you open the can, the pressure drops suddenly and the gas rushes out explosively. Magma contains dissolved gases too. When it rises and the pressure drops as it approaches the surface, those gases — mostly water vapour and carbon dioxide — expand violently. That's what makes volcanic eruptions explosive. A volcano with low gas content flows gently; one with high gas content can detonate like a bomb.
Where do volcanoes occur?
Most volcanoes sit along the boundaries between tectonic plates — the enormous chunks of crust that float on the mantle and slowly move around. At divergent boundaries (where plates pull apart), magma wells up to fill the gap — this is how Iceland formed. At subduction zones (where one plate dives under another), the descending plate releases water into the mantle, lowering its melting point and creating magma — this is what rings the Pacific Ocean in the "Ring of Fire."
What's the difference between lava and magma?
Same stuff, different location. While it's underground, it's called magma. The moment it exits the volcano, it becomes lava. Lava flows can reach temperatures over 1,200°C and can travel at speeds from a slow walk to — in extremely fluid types — as fast as a car on a motorway. Over time, cooled lava builds up into new rock, which is why volcanic islands exist at all.
The Earth isn't solid all the way through. Under the thin rocky crust we live on is the mantle. The mantle has thousands of kilometres of rock that is incredibly hot. This hot rock moves like very thick honey instead of staying solid. Even deeper down, the core reaches temperatures of 5,000°C. That's hotter than the surface of the Sun!
All that heat needs to go somewhere. One way it escapes is through volcanoes.
What causes magma to rise?
Deep in the mantle, rock stays solid because of crushing pressure. All the rock above it pushes down really hard. Sometimes the pressure gets less when giant pieces of crust move apart. When this happens, the rock can melt into magma. Magma is lighter than solid rock, so it rises up. It's like a balloon filled with air floating up through water. The magma moves up through cracks in the rocky crust.
Think about shaking a bottle of fizzy pop. The pressure inside keeps the gas mixed in with the drink. When you open the bottle, the pressure drops quickly. The gas rushes out and makes the drink explode everywhere! Magma has gases mixed in too. These are mostly water vapour and carbon dioxide. When magma rises up, the pressure gets less. The gases expand very quickly and violently. That's what makes volcanic eruptions so explosive. A volcano with not much gas flows gently like syrup. One with lots of gas can explode like a massive firework.
Where do volcanoes occur?
Most volcanoes sit along the edges between tectonic plates. These are enormous chunks of crust that float on the mantle. They move around very slowly like pieces of a jigsaw puzzle. At some edges, plates pull apart from each other. Magma comes up to fill the gap. This is how Iceland formed. At other edges, one plate dives under another plate. The plate going down releases water into the mantle. This makes the rock melt more easily and creates magma. This happens around the Pacific Ocean in the "Ring of Fire."
What's the difference between lava and magma?
It's the same stuff but in different places. While it's underground, it's called magma. The moment it comes out of the volcano, it becomes lava. Lava flows can be over 1,200°C hot. They can move as slowly as you walk. Some very runny types can move as fast as a car. Over time, cooled lava builds up into new rock. This is why volcanic islands exist at all.