Big question
Why do volcanoes erupt?
Volcanoes erupt because hot, molten rock called magma, along with dissolved gases, builds up pressure underground until it forces its way out through a weak spot in the Earth's crust. Once released, rising magma and rapidly expanding gases often blast ash, rock and lava into the air.
Magma Under Pressure
Deep beneath the Earth's crust, temperatures and pressures are high enough to melt solid rock into a thick, flowing liquid called magma. Magma is usually a little less dense than the solid rock around it, so it tends to rise slowly upward through cracks and weaknesses in the crust, similar to how a bubble of oil rises up through water. As magma rises, dissolved gases trapped inside it, mostly water vapour and carbon dioxide, start to come out of the liquid and form bubbles, exactly like the bubbles that appear when you open a fizzy drink and the pressure inside suddenly drops. The more gas that builds up and tries to escape, the more forceful an eruption is likely to be.
Volcanoes Form Where Plates Meet
The Earth's outer shell is broken into huge pieces called tectonic plates, which slowly move, collide and pull apart over millions of years. Most volcanoes form along the edges of these plates: where two plates push together, one can be forced down beneath the other, melting into magma that later rises and erupts, and where two plates pull apart, magma can well up to fill the widening gap directly. A smaller number of volcanoes, called hotspot volcanoes, form in the middle of a plate instead, above an unusually hot region deep inside the Earth that melts rock straight through the crust above it, which is how the Hawaiian Islands formed far from any plate boundary.
Gentle Flows or Violent Blasts
Not every volcano erupts the same way, and the difference mostly comes down to how thick and gas-rich the magma is. Thin, runny magma with low gas content, common in places like Hawaii, tends to ooze out fairly gently as flowing streams of lava that people can often watch safely from a distance. Thick, sticky magma packed with dissolved gas, more common around the edges of the Pacific Ocean in what is nicknamed the Ring of Fire, traps that gas until pressure becomes extreme, then releases it in sudden, explosive eruptions that can blast ash and rock high into the sky, sometimes affecting weather and air travel far beyond the volcano itself.
Try it yourself
Ask an adult to help shake a bottle of fizzy drink gently, then slowly loosen the cap and watch the bubbles rush out under pressure. That escaping gas is a safe, small-scale version of what drives a volcanic eruption underground.
Questions children ask
What causes a volcano to erupt?
A volcano erupts when hot magma and the gases dissolved inside it build up enough pressure underground to force their way out through a weak point in the Earth's crust. The trapped gas expanding rapidly is often what makes an eruption explosive.
Why do most volcanoes form along certain lines on a map?
Most volcanoes form along the edges of tectonic plates, the huge pieces that make up Earth's outer shell, because plate edges are where magma can most easily reach the surface. This is why many volcanoes cluster around the Pacific Ring of Fire.
Why are some volcanic eruptions gentler than others?
Some eruptions are gentler because their magma is thin and low in trapped gas, so it flows out steadily instead of building up pressure. Thick, gas-rich magma traps pressure until it releases suddenly, causing explosive eruptions instead.