Pressure finding a way out
Why Do Volcanoes Erupt?
Molten rock is lighter than the rock above it, and dissolved gas gives it the shove.
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Step 1 of 5
Deep rock is hot enough to melt.
Earth's interior is thousands of degrees, left over from its formation and topped up by radioactive decay. In places, rock becomes liquid magma.
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Step 2 of 5
Melted rock is lighter than solid rock.
Magma is less dense than the rock around it, so it floats upward — the same reason a cork rises through water. Nothing has to push it.
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Step 3 of 5
Dissolved gas is the real explosive.
Magma holds water and carbon dioxide dissolved under pressure. As it rises, pressure drops and the gas fizzes out — exactly like opening a shaken bottle.
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Step 4 of 5
Runny magma oozes; sticky magma explodes.
Thin magma lets gas escape gently, giving lava flows. Thick, sticky magma traps the gas until it bursts, giving ash clouds and blasts.
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Step 5 of 5
They cluster where plates meet.
Most volcanoes sit along the edges of Earth's plates, where one slab dives beneath another and drags water down, which makes rock melt more easily.
The 1815 Tambora eruption threw up so much ash that 1816 became 'the year without a summer' across Europe.
The short version
Magma rises because it is buoyant, and dissolved gas expanding on the way up drives the eruption.
Try it yourself
Open a fizzy drink slowly, then shake one and open it. That is the difference between a lava flow and an explosive eruption.
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