Describe how volcanoes erupt, the materials they produce, and why eruption styles differ

What this dot point is asking

SEAB wants you to describe how volcanoes erupt, the materials they produce, and why eruption styles differ so widely, from gentle lava flows to violent explosions. The central insight is that the type of magma controls everything: runny, low-gas magma erupts gently, while thick, gas-rich magma erupts explosively, and which type a volcano has depends on its plate-boundary setting.

The answer

How a volcano erupts

A volcano erupts through a clear process:

1. Hot molten rock (magma) collects beneath the volcano in a magma chamber.
2. Pressure builds up as more magma and dissolved gas accumulate.
3. When the pressure is great enough, the magma is forced up through a vent or crack toward the surface.
4. At the surface it erupts: the magma is now called lava, and gases, ash and rock fragments may be thrown out.

The materials produced

An eruption can produce several materials:

• Lava: molten rock flowing at the surface, which cools to solid rock.
• Ash: fine fragments of shattered rock and volcanic glass.
• Volcanic gases: such as steam and carbon dioxide.
• Rock fragments and bombs: larger pieces thrown out.
• Pyroclastic flows: in explosive eruptions, fast-moving clouds of hot gas, ash and rock that race downhill.

Why eruption styles differ

The type of magma decides whether an eruption is gentle or explosive:

• Runny (low viscosity), low-gas magma, found mainly at divergent boundaries and hotspots, lets gas escape easily, so it erupts gently as flowing lava. The lava can travel far and build broad, gently sloping volcanoes.
• Thick (high viscosity), gas-rich magma, found at convergent boundaries (from melting subducted crust), traps the gas, so pressure builds until it is released suddenly in a violent, explosive eruption throwing out ash, gas, rock and pyroclastic flows.

This is why eruptions at convergent boundaries (like much of the Ring of Fire) are far more dangerous than the gentle eruptions at divergent boundaries.

Examples in context

Example 1. Mount Pinatubo, the Philippines, 1991. Sitting above a subduction zone, Mount Pinatubo erupted explosively in 1991 in one of the largest eruptions of the century, blasting out a huge ash column and pyroclastic flows and sending ash across the region. Its thick, gas-rich magma, typical of a convergent boundary, made the eruption violent. Timely warnings and evacuation saved many lives, showing both the danger of explosive volcanoes and the value of preparation.

Example 2. Kilauea, Hawaii. Kilauea, a hotspot volcano, erupts the runny, low-gas basaltic lava typical of gentle eruptions, with lava flows that advance slowly enough for people to move out of the way, even as they bury roads and homes. The contrast with Pinatubo highlights how magma type, not just the presence of a volcano, determines whether an eruption is a slow flow or a violent explosion.

Try this

Q1. Explain the difference between magma and lava. [2 marks]

• Cue. Magma is molten rock beneath the Earth's surface (in the chamber and vent); lava is the same molten rock once it has erupted and is flowing at the surface.

Q2. Explain why magma rich in gas tends to erupt explosively. [2 marks]

• Cue. Thick, gas-rich magma traps the gas rather than letting it escape, so pressure builds up until it is suddenly released, blasting out ash, gas and rock in a violent, explosive eruption.

Q3. Name two materials produced by a volcanic eruption and state one danger of an explosive eruption. [3 marks]

• Cue. Materials include lava, ash, volcanic gases and rock fragments; a danger of an explosive eruption is a pyroclastic flow, a fast-moving cloud of hot gas, ash and rock that destroys and burns everything in its path, or heavy ash fall that collapses roofs and harms breathing.