Volcanoes
There are about 1,350 potentially active volcanoes on land, and far more under the sea. They built the continents, made the air breathable and can bury cities in hours.
In August of the year 79, the people of Pompeii watched a column of ash rise from the mountain they had farmed for generations. Nobody alive knew that Vesuvius was a volcano. Within a day the city was buried under metres of ash and pumice, and it stayed buried for seventeen centuries. Volcanoes are the places where the hot interior of the Earth breaks through to the surface, and they are far more varied, more common and more important than the occasional catastrophe suggests.
Where molten rock comes from
The mantle beneath the crust is solid, but in a few settings parts of it melt. Where tectonic plates pull apart, hot rock rises and the drop in pressure lets it melt, like the fizz escaping an opened bottle. Where one plate sinks beneath another, water carried down with it lowers the melting point of the rock above. And in a few places, plumes of unusually hot rock rise from deep in the mantle and melt their way through, creating volcanoes far from any plate boundary; Hawaii sits on one.
The molten rock, called magma while underground and lava once it erupts, is lighter than the solid rock around it, so it rises, collecting in chambers a few kilometres down. Dissolved gases, mostly water vapour and carbon dioxide, build pressure. When the pressure exceeds the strength of the rock above, the volcano erupts.
Gentle and violent
How a volcano erupts depends on its lava. Basalt lava, formed from melted mantle, is runny; gas escapes from it easily, and it pours out in rivers and fountains, building broad, gently sloping shield volcanoes like those of Hawaii and Iceland. These eruptions are spectacular but usually slow enough to walk away from.
Lava rich in silica, common above subduction zones, is thick and sticky. Gas cannot escape, so pressure builds until the volcano blows apart, hurling ash, pumice and gas tens of kilometres into the sky. These eruptions build steep cone-shaped volcanoes like Fuji, Vesuvius and Mount St Helens, and they are the dangerous ones. The deadliest hazard is the pyroclastic flow: an avalanche of hot gas and ash, several hundred degrees, racing down the slopes at hundreds of kilometres an hour. It was pyroclastic flows, not lava, that killed Pompeii.
The very largest eruptions empty a magma chamber so completely that the ground above collapses into a crater called a caldera, tens of kilometres across. Yellowstone, in the United States, is a caldera; its last colossal eruption was 640,000 years ago.
The Ring of Fire
About three quarters of the world's volcanoes stand along the edges of the Pacific Ocean, where plates are sinking into the mantle all around the rim: Japan, the Philippines, Indonesia, New Zealand, the Andes, Central America, the western United States, Alaska and Kamchatka. This horseshoe is the Ring of Fire. Most of the world's earthquakes happen there too, for the same reason.
Yet the greatest volume of volcanic activity on Earth is almost never seen. The mid-ocean ridges, where new seafloor is made, are a chain of volcanoes 65,000 kilometres long, erupting continuously two or three kilometres beneath the waves.
Volcanoes and climate
A big eruption changes the weather for the whole planet. Ash falls out of the air within days, but sulphur dioxide reaches the stratosphere and forms a haze of tiny droplets that reflects sunlight for a year or two. After Mount Pinatubo erupted in 1991, global temperatures dropped by about half a degree. After Tambora in 1815, the largest eruption in recorded history, crops failed across Europe and North America in the "year without a summer", and the gloom in Switzerland that year helped inspire Mary Shelley to write Frankenstein.
Over far longer timescales, volcanoes do the opposite. The carbon dioxide they release has kept Earth warm enough for liquid water through its entire history, while the atmosphere's water and nitrogen themselves were largely outgassed by ancient volcanoes. Without them Earth would be a frozen, airless rock.
Living beside them
About 800 million people live within 100 kilometres of an active volcano, because volcanic soils are among the most fertile on Earth. Scientists watch dangerous volcanoes with instruments that detect the swelling of the ground, the swarm of small earthquakes and the change in gases that often precede an eruption, and evacuations based on those signs have saved tens of thousands of lives. Pinatubo's eruption in 1991 killed a few hundred people; without the warning it would have killed tens of thousands.
Why it matters
Volcanoes destroy, but they also create. They built the Hawaiian islands, Iceland and Japan from nothing. They made the atmosphere and oceans. They scatter minerals that feed forests and farms. The same fire that buried Pompeii is the reason there is a living planet to bury anything on.
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Test yourself
4 questions · answers explained at the end
1.What is magma called once it reaches the surface?
2.Where are about three quarters of Earth's volcanoes?
3.What is the deadliest volcanic hazard?
4.How can a large eruption cool the planet?