Volcanoes Explained: The 4 Types and How Eruptions Work

A volcano is an opening in the Earth’s crust through which molten rock, gases and solid fragments from deep inside the planet escape. While that molten rock is still underground it is called magma; once it reaches the surface it is called lava.

How a volcano is built

The structure of a volcano is easiest to read from the bottom up, following the path of the magma:

• the magma chamber is the deep reservoir where magma collects;
• the conduit (or pipe) is the vertical channel the magma rises through;
• the crater is the funnel-shaped opening at the top where material escapes;
• the cone is the mountain built up from layers of lava and ash left by each eruption;
• side vents are secondary openings on the flanks, where magma can escape before reaching the summit.

Why volcanoes erupt

Magma is less dense than the surrounding rock, so it tends to rise. But the real engine of an eruption is the gas dissolved inside it, mostly water vapour and carbon dioxide. As magma rises the pressure drops and the gases come out of solution as bubbles, exactly like opening a bottle of fizzy water. If the magma is runny the bubbles escape easily; if it is thick they stay trapped and pressure builds until something gives.

Effusive and explosive eruptions

Eruptions fall into two broad families, and the difference comes down to the viscosity of the magma, or how sticky it is.

Effusive eruptions involve fluid, low-silica magma: lava pours out steadily and travels in flows that can run for kilometres. These are the classic Hawaiian eruptions.

Explosive eruptions involve viscous, silica-rich magma: trapped gases blow out the plug in the conduit and hurl ash, lapilli and volcanic bombs into the air, forming an eruption column kilometres high. The most famous historical example is the eruption of Vesuvius in AD 79, which buried Pompeii and Herculaneum.

What a volcano produces

An eruption releases three categories of material. Lava, which cools as it flows and forms new rock. Pyroclasts, solid fragments thrown into the air and sorted by size: ash (under 2 mm), lapilli (2 to 64 mm) and volcanic bombs (over 64 mm). And gases, largely water vapour, along with carbon dioxide and sulphur compounds.

The 4 types of volcano

The shape of a volcano tells you what kind of lava built it, and geologists usually sort volcanoes into four types:

Type	Shape	Lava	Example
Shield volcano	very broad, gentle slopes	fluid, low in silica	Mauna Loa, Kilauea (Hawaii)
Cinder cone	small, steep, single vent	pyroclasts and lapilli	Parícutin (Mexico)
Composite volcano (stratovolcano)	tall, steep, classic cone	alternating lava and ash	Mount Fuji, Vesuvius, Etna
Lava dome	small, bulging, rounded	very thick, silica-rich	Mount St. Helens dome

Shield volcanoes are broad and gently sloping, because fluid lava travels far before it solidifies. Cinder cones are small, steep hills formed by lapilli piling up around a vent; they are the simplest type and often grow in a single eruption. Composite volcanoes, or stratovolcanoes, have the classic tall, steep cone made of alternating layers of lava and pyroclastic material, and produce the most dangerous eruptions. Lava domes form when magma is so viscous that it cannot flow at all: it squeezes out and piles up over the vent like toothpaste, sometimes inside the crater of a larger volcano.

Volcano or mountain: what is the difference?

Both are raised ground, but they are built by opposite processes. A volcano is first of all an opening in the crust connected to a magma chamber, and it constructs its own relief from the outside in, one eruption at a time: material comes up from below and settles in layers. An ordinary mountain has no vent and no magma supply; it is pushed up by tectonic plates folding and thrusting rock over millions of years, or left standing when erosion removes everything around it.

The practical consequence is that a volcano can grow in a human lifetime — Parícutin rose from a Mexican cornfield to 424 metres in nine years — while a fold mountain grows by millimetres a year. Every volcano with a built-up cone counts as a mountain, but the reverse is almost never true: the Alps and the Himalayas contain no volcanoes at all.

Where volcanoes are found

Volcanoes are not scattered at random: they follow tectonic plate boundaries. They cluster where one plate sinks beneath another (subduction zones) and along mid-ocean ridges, where plates pull apart and magma rises to form new crust. The best-known belt is the Pacific Ring of Fire, which encircles the Pacific Ocean and is home to roughly three quarters of the world’s active volcanoes. There are also hotspots, plumes of magma rising in the middle of a plate: the Hawaiian Islands were built this way.

Volcanoes around the world

Some volcanoes are worth knowing by name. Kilauea in Hawaii is a shield volcano and one of the most active on Earth. Mount Fuji in Japan is the textbook stratovolcano, a near-perfect cone. Mount St. Helens in the United States produced a famously destructive explosive eruption in 1980. Etna in Sicily is the tallest active volcano in Europe, at roughly 3,350 metres, a height that changes with every eruption.

Active, dormant and extinct

Volcanoes are also classified by their state. A volcano is active if it is erupting or showing clear signs of activity; dormant if it is resting but has erupted in historical times and still has a magma chamber; and extinct if it has not erupted for a very long time and is no longer supplied with magma.

The exercises below ask you to order the parts of a volcano, work through a true-or-false on the key ideas, match the types of volcano to the lava that builds them, calculate the speed of a lava flow, and reason about why some eruptions are gentle and others devastating.