Inside a Volcano: The Parts of a Stratovolcano

A volcano is an opening in Earth's surface where molten rock, rock fragments and gases escape. Over many eruptions the erupted material piles up around the opening and builds a hill or mountain. Underneath, cracks and channels connect the opening to a store of molten rock, which is why a volcano can erupt again and again from the same place.

Molten rock is called magma while it is underground and lava once it reaches the surface.

Magma forms many tens of kilometres below the surface. It is a mixture of molten rock, crystals and dissolved gas. Two things push it upwards:

• Buoyancy: magma is less dense than the solid rock around it, so it rises, a bit like a blob in a lava lamp.
• Gas pressure: as magma rises and the pressure on it falls, its dissolved gas forms bubbles that expand, like the fizz when you open a shaken bottle of fizzy drink.

Magma often collects in a magma chamber in the crust. If it forces its way up through weak rock and reaches the surface, an eruption begins.

The volcano on this infographic is a stratovolcano, also called a composite volcano. These are the tall, steep, cone-shaped mountains most people picture, such as Mount Fuji in Japan and Cotopaxi in Ecuador. The tallest on Earth is Ojos del Salado in Chile, about 6,890 m above sea level.

The key feature, according to the US Geological Survey, is a conduit system: a pipe that carries magma from a reservoir deep in the crust up to the surface. At the top is a crater holding the main vent. Lava can also escape through cracks in the crater wall or from side vents on the slopes.

The cone itself is built from alternating layers of lava flows and of ash, cinders, blocks and volcanic bombs thrown out in explosions. Each eruption adds new layers, so the mountain grows taller and wider over time. Its sides are steep, often about 30 to 35 degrees.

• Shield volcanoes are built almost entirely of runny lava that spreads a long way before it cools, so their slopes are gentle, usually under 10 degrees. The Hawaiian Islands are a chain of shield volcanoes, including Mauna Loa, the world's largest active volcano.
• Cinder cones are the simplest kind: small, steep piles of bubbly lava fragments that fall around a single vent. They are rarely more than about 300 m high.

How a volcano erupts depends mostly on its magma. Thin, runny lava lets gas escape easily, so shield volcanoes usually pour out lava flows. Stratovolcanoes erupt stickier, more viscous lava that traps gas until it bursts out explosively.

An explosive eruption blasts out tephra: rock fragments of every size. Ash, the finest tephra, can be carried miles into the sky and around the world by high winds. The most dangerous hazard is a pyroclastic flow, a glowing avalanche of hot gas, ash and rock that can race downhill at up to about 160 km/h (100 mph).

Many volcanoes on land grow above subduction zones, such as those around the Pacific's Ring of Fire, where melting above a sinking plate feeds chains of stratovolcanoes. Long belts of volcanoes also line the mid-ocean ridges, hidden under the sea. A few, such as those of Hawaii, sit over hotspots far from any plate boundary.