What Is a Volcano?

A volcano is a place where molten rock from deep inside the Earth breaks through to the surface. The molten rock is called magma while it is underground and lava once it reaches the air. Over time, the lava cools and hardens around the eruption point, often building a cone-shaped mountain. There are approximately 1,500 active volcanoes on land today (and many more under the ocean), and roughly 20 of them are erupting at any given moment. Volcanoes are some of the most powerful natural forces on Earth, capable of building entire islands or destroying ancient cities.

  • Active volcanoesApproximately 1,500On land alone
  • Currently eruptingApproximately 20At any given moment
  • Tallest volcanoMauna Loa4,170 m above sea, 9 km from seabed
  • Lava temperature700 to 1,250 degrees CGlowing red to white-hot
  • Most active countryIndonesia120+ active volcanoes
  • Ring of Fire90% of world's volcanoesAround the Pacific

Parts of a volcano

  • Magma chamber: a pool of molten rock deep underground.
  • Main vent: a pipe through the rock that magma rises through.
  • Crater: the bowl-shaped opening at the top.
  • Cone: the mountain itself, built from layers of cooled lava and ash.
  • Side vents and fissures: secondary openings where lava can also break out.

What causes a volcano

Almost all volcanoes form at the edges of tectonic plates. The main settings are:

  • Subduction zones: where one tectonic plate slides under another. Water and rock from the descending plate cause the rock above to melt, forming magma. Most of the Ring of Fire volcanoes work this way.
  • Mid-ocean ridges: where two plates pull apart and magma rises into the gap. Most ocean-floor volcanism is here.
  • Hot spots: places where a plume of unusually hot mantle rock rises from deep below. Hawaii and Yellowstone sit on hot spots.
  • Continental rifts: where a continent is splitting apart, like the East African Rift.

Why volcanoes erupt

Magma rises because it is less dense than the surrounding rock. As it rises, dissolved gases (mostly water vapour and carbon dioxide) come out of solution. If the magma is runny, the gases can escape gently and the volcano produces flowing lava. If the magma is thick and sticky, the gases build up pressure until the whole thing explodes violently.

This is why volcanoes are so different in character. Hawaiian volcanoes have runny basalt magma and produce gentle fountains of lava that even tourists can safely watch. Mount St Helens-style volcanoes have thick sticky magma and can blow themselves apart with the force of nuclear weapons.

Fact The tallest mountain on Earth when measured from base to top is the Hawaiian volcano Mauna Kea. Its summit is 4,207 m above sea level, but its base sits on the Pacific Ocean floor approximately 6,000 m below. From base to top, Mauna Kea is over 10,000 metres tall: more than 1 km taller than Mount Everest, measured from sea level.

Active, dormant, extinct

  • Active: has erupted recently or is showing signs that it might soon. Approximately 1,500 worldwide.
  • Dormant: has not erupted in a long time but could potentially erupt again.
  • Extinct: very unlikely to erupt ever again. The magma source has cooled or moved on.

The line between dormant and extinct is sometimes hard to draw. Mount Vesuvius was thought to be dormant before it destroyed Pompeii in 79 AD. The Yellowstone supervolcano has been quiet for tens of thousands of years but is definitely not extinct.

Did you know? The UK has no active volcanoes today, but it used to have plenty. Almost all the rocky parts of Scotland, including Edinburgh's Arthur's Seat, are the remains of ancient volcanoes from hundreds of millions of years ago. The basalt columns of the Giant's Causeway in Northern Ireland are the remains of a huge eruption around 60 million years ago.
Deeper dive: how volcanic eruptions are predicted

Unlike earthquakes, volcanoes often give signs that they are about to erupt. Modern volcanology uses several techniques to monitor active volcanoes and warn nearby communities.

  • Seismometers: detect tiny earthquakes caused by magma moving underground. A sudden rise in small quakes often comes before an eruption.
  • GPS and tilt-meters: detect the ground bulging as magma rises and inflates the volcano. Even a few millimetres of swelling can be measured.
  • Gas sensors: monitor the kind and amount of gas escaping from vents. A rise in sulphur dioxide often comes before an eruption.
  • Thermal cameras: detect heat patterns on the volcano's surface.
  • Satellites: monitor changes in shape, temperature and gas emissions from space.

Modern monitoring saves many lives. The 1991 eruption of Mount Pinatubo in the Philippines (the second-largest eruption of the 20th century) was successfully predicted, allowing 60,000 people to evacuate in time. Sadly, monitoring is not always available: many volcanoes in poor countries are too remote or under-funded to be properly watched.

For more, see types of volcano, magma vs lava and the Ring of Fire.