How do volcanoes work?

A doorway into the hot Earth

Deep below your feet, the Earth is incredibly hot. In some places the rock gets hot enough to melt. Melted rock underground is called magma. Once it bursts or oozes out onto the surface, it is called lava.

A volcano is an opening in the Earth's crust where magma from deep down reaches the surface in an eruption. Some volcanoes pour out rivers of glowing lava. Others blast out clouds of ash and chunks of rock, which scientists call tephra.

The Earth's outer shell is not one solid piece. It is broken into giant slabs called tectonic plates, which move very slowly. Most active volcanoes are found near the edges of these plates.

Where volcanoes form

Where two plates pull apart, magma rises to fill the gap. These eruptions are usually gentle, and most of them happen underwater along the sea floor.

Where two plates push together, one can be forced down underneath the other. This is called subduction. The sinking plate drags wet mud and seawater down with it. The water makes the surrounding rock melt more easily, and that creates magma. Many of our nearest active volcanoes, in Indonesia, Papua New Guinea, Vanuatu and New Zealand, are in places where plates meet.

A few volcanoes grow far from any plate edge, above a hotspot. Scientists think hotspots are plumes of extra-hot rock rising from deep inside the Earth. As a plate slides slowly over a hotspot, new volcanoes form one after another, making a chain. The Hawaiian Islands formed this way. Iceland, where the photo above was taken, is special: it sits on a hotspot and also where two plates are pulling apart.

Runny or sticky?

Why do some volcanoes explode while others just ooze? It mostly comes down to how thick the magma is. Magma has gas mixed through it. Thick, sticky magma, which scientists call viscous, traps a lot of that gas until the pressure builds and it bursts out in an explosive eruption. Runny magma lets gas escape easily, so the lava flows out more gently.

It is a bit like blowing bubbles through a straw into water and then into thick yoghurt. In water, the bubbles escape easily. In yoghurt, they get stuck, grow bigger, and then pop.

The type of magma also shapes the volcano. Runny lava spreads a long way before it cools, building wide, gently sloping shield volcanoes. Sticky lava and ash pile up in layers, building steep cones called composite volcanoes, or stratovolcanoes, such as Ruapehu and Taranaki in New Zealand. When a huge eruption empties the magma chamber underneath, the ground can collapse into a giant bowl called a caldera, often several kilometres wide.

Scientists call a volcano active if it has erupted in the last 10,000 years, even if it is resting right now. A volcano that has been quiet for longer than that is considered extinct.

Australia's sleeping volcanoes

Mainland Australia has no active volcanoes, because there are no plate boundaries anywhere on our continent. Australia's only active volcanoes are on Heard Island and the McDonald Islands, remote islands in the Southern Ocean about 4,000 kilometres south-west of the mainland.

But Australia has plenty of old volcanoes. Signs of past eruptions can be found in every state and territory, and there were many eruptions down the east coast over the past 60 million years. Scientists think that as the continent drifted slowly north over a hotspot, it left behind a trail of volcanoes that get younger as you travel south.

One of the most famous is Wollumbin, formerly known as Mount Warning, in northern New South Wales. The 1,157-metre peak is what remains of the central vent of an ancient volcano, and it stands in the middle of a huge caldera.

In south-eastern Australia, the volcanoes of the Newer Volcanics Province have been quiet for about 5,000 years. Scientists describe them as dormant: sleeping, but not necessarily finished.

A volcano in a living story

In south-western Victoria stands Budj Bim. More than 30,000 years ago it erupted, and its lava flowed more than 50 kilometres towards the sea. The ancestors of the Gunditjmara people witnessed the eruption. In Gunditjmara tradition, the Ancestral Being Budj Bim transformed himself into part of the landscape.

The lava flow reshaped the land and its waterways. For at least 6,600 years, the Gunditjmara have used the volcanic stone to build channels, weirs and ponds to trap and farm kooyang, or short-finned eels. It is one of the oldest and most extensive aquaculture (fish-farming) systems in the world, and it is now World Heritage-listed.

So volcanoes do not only destroy. Over long periods they build new land, reshape rivers and lakes, and become part of people's stories. Today, scientists keep watch on active volcanoes, looking for signs such as small earthquakes and bursts of gas, to understand when and how one might erupt.

📚 Words to know

Magma

Melted rock underground.

Lava

Melted rock that has come out onto the Earth's surface.

Tectonic plate

One of the giant, slowly moving slabs that make up the Earth's outer shell.

Viscous

Thick and sticky, so it flows slowly, like honey.

Caldera

A huge bowl-shaped hollow left when a volcano collapses after its magma chamber empties.

Dormant

Not erupting now, but could erupt again in the future.

🧪 Try this: sticky magma, runny magma

1. You will need a plate, two cups, a straw, some water, some honey and some thick yoghurt. Ask an adult first, and work over a tray to catch drips.
2. Put a spoonful of water and a spoonful of honey side by side near the top of a plate, then tilt the plate. Which one runs further? Runny lava spreads out the same way, building wide shield volcanoes.
3. Half fill one cup with water and the other with thick yoghurt. Blow gently through the straw into the water. The bubbles escape quickly and easily, like gas from runny magma.
4. Now blow gently into the yoghurt. The bubbles get trapped, grow bigger, then burst with a pop. That is a tiny model of how gas trapped in sticky magma causes explosive eruptions. Always blow out through the straw, never suck in.