A volcano is an opening in the Earth's surface where melted rock, ash and gas escape from deep underground. There are around 1,350 potentially active volcanoes on land, and somewhere between 40 and 50 are erupting at any given moment. Most of them are in a horseshoe around the Pacific Ocean called the Ring of Fire, and the biggest volcanoes of all are on the seafloor where nobody can see them.
What Is Under Your Feet
The Earth has layers, a bit like an onion. The thin outer layer you stand on is the crust, between about 5 and 70 kilometres thick. Under that is the mantle, which is rock so hot it flows very slowly, like extremely thick treacle. In the middle is the core, made mostly of iron and nickel.
The crust is broken into about fifteen large pieces called tectonic plates, plus lots of smaller ones. They float on the mantle and move a few centimetres a year — roughly the speed fingernails grow.
Volcanoes happen where those plates meet, or where a plume of hot rock rises through the middle of one. Melted rock underground is called magma. Once it reaches the surface it gets a new name: lava.
Why It Comes Out
Magma rises for the same reason a cork rises in water: it is less dense than the solid rock around it, so it floats upwards.
As it rises, the pressure on it drops, and gases dissolved in it — mostly water vapour and carbon dioxide — start to bubble out. This is exactly what happens when you open a fizzy drink.
If the magma is runny, the gas escapes easily and the lava flows out gently. If the magma is thick and sticky, the gas cannot get out, pressure builds, and eventually the whole thing lets go at once. That is why some eruptions are rivers of orange lava and others are enormous explosions.
Three Kinds of Volcano
Shield volcanoes are wide and gently sloping, built by runny lava spreading out over a large area. Mauna Loa in Hawaii is one, and it is the largest volcano on Earth by volume.
Stratovolcanoes, also called composite volcanoes, are the classic cone shape. They are built from alternating layers of lava and ash, and they are the explosive ones. Mount Fuji, Vesuvius, Mount St Helens and Krakatoa are all stratovolcanoes.
Cinder cones are the small ones, built from chunks of lava thrown into the air that fall back around the vent. They can appear surprisingly fast. In 1943, a Mexican farmer watched a crack open in his cornfield; within a year the cinder cone Parícutin was over 300 metres tall.
Make a Volcano Erupt
Build a cone from clay, papier-mâché or just a mound of soil, with a plastic bottle or cup buried in the middle. Put two tablespoons of bicarbonate of soda in the cup, add a squirt of washing-up liquid and a few drops of red food colouring.
Now pour in about half a cup of vinegar and stand back. Foam pours out of the crater and runs down the sides.
What you have made is a chemical reaction producing carbon dioxide gas, and the washing-up liquid traps it as bubbles so it foams. A real volcano works on the same principle — gas expanding suddenly and pushing liquid out of a hole — but with rock at 1,000 degrees instead of vinegar.
The Ring of Fire
About 75% of the world's active volcanoes and 90% of its earthquakes happen along a 40,000-kilometre horseshoe around the edge of the Pacific Ocean.
It runs up the west coast of South America and North America, across to Alaska and the Aleutian Islands, down through Kamchatka, Japan, the Philippines and Indonesia, and on to New Zealand.
The reason is subduction. Around the Pacific, dense oceanic plates are sliding underneath lighter continental ones. As the sinking plate goes down it heats up and releases water, which lowers the melting point of the rock above it. That rock melts, rises, and comes out as a volcano.
Hotspots
Some volcanoes are nowhere near a plate boundary. Hawaii is in the middle of the Pacific Plate, thousands of kilometres from the nearest edge.
These sit over hotspots — plumes of unusually hot rock rising from deep in the mantle. The plume stays roughly in place while the plate slides over it, so it burns a line of volcanoes through the plate like a blowtorch under a moving sheet of metal.
That is why the Hawaiian Islands form a chain, with the oldest and most worn-down at the north-west end and the youngest at the south-east. A new one, Kamaʻehuakanaloa, is already growing under the sea and will surface in tens of thousands of years. Yellowstone in the United States and Iceland both sit over hotspots too.
How Big Can an Eruption Get?
Scientists use the Volcanic Explosivity Index, or VEI, which runs from 0 to 8. Each step up is roughly ten times more powerful.
Mount St Helens in 1980 was a VEI 5 and blew 400 metres off the top of the mountain. Krakatoa in 1883 was a VEI 6 and was heard 4,800 kilometres away — the loudest sound in recorded history. Tambora in 1815 was a VEI 7 and threw so much ash into the atmosphere that 1816 became known worldwide as the Year Without a Summer, with snow falling in New England in June.
No VEI 8 has happened in human history. The Toba eruption in Indonesia around 74,000 years ago was one, and some scientists think it may have come close to wiping out early humans.
Volcanoes Are Useful
Volcanic soil is among the most fertile on Earth, because eruptions bring up minerals from deep underground. That is why so many people live near active volcanoes despite the risk — the farming is excellent.
Iceland heats around 90% of its buildings with geothermal energy, using hot water from volcanic rock, and generates a large share of its electricity the same way. It costs almost nothing and produces almost no carbon.
Volcanoes also built the atmosphere. Most of the water in the oceans and much of the early air came out of volcanic vents over billions of years. Without them, Earth would probably be a dry rock.
What Comes Out
Lava is the obvious part, but it is often the least dangerous. Lava flows are usually slow enough to walk away from, and the Hawaiian kind can be gentle enough to stand near with the right protection.
The killers are the things that move fast. A pyroclastic flow is a cloud of superheated gas and rock fragments that races down the slope at up to 700 kilometres an hour at temperatures around 1,000 degrees. That is what destroyed Pompeii, and nothing survives one.
A lahar is a mudflow of volcanic ash mixed with water, from melting ice or heavy rain, which sets like concrete when it stops. In 1985 a lahar from Nevado del Ruiz in Colombia buried the town of Armero and killed over 20,000 people. Ash itself is not soft grey powder either — it is tiny shards of volcanic glass, which is why it wrecks aircraft engines and is dangerous to breathe.
Watching for Trouble
Scientists can often tell when a volcano is waking up. Magma moving underground causes small earthquakes, makes the ground swell by a few centimetres, and changes the mix of gases coming out of vents.
Instruments called seismometers, tiltmeters and GPS stations track all three. Satellites measure ground deformation from space and spot heat and ash from orbit.
The warnings work. In 1991, Mount Pinatubo in the Philippines had the second-largest eruption of the twentieth century, and scientists' warnings led to the evacuation of tens of thousands of people. Many thousands of lives were saved. The eruption also cooled the whole planet by about half a degree for a couple of years.
Supervolcanoes and Calderas
Some volcanoes are so large that they do not look like volcanoes at all. When an enormous magma chamber empties in one eruption, the ground above it collapses into a huge basin called a caldera.
Yellowstone in the United States is one. Its caldera is around 70 by 45 kilometres, and the whole of Yellowstone National Park sits inside it. The geysers, hot springs and bubbling mud pools there are heated by magma a few kilometres below.
Lake Toba in Sumatra is another, about 100 kilometres long and now filled with water. Campi Flegrei near Naples is a third, with a town of half a million people living on top of it. These systems erupt on timescales of hundreds of thousands of years, and none has done so in recorded history.
The Ones You Should Know
Vesuvius in Italy buried the Roman towns of Pompeii and Herculaneum in 79 CE under metres of ash, preserving them so completely that archaeologists can read graffiti on the walls.
Mount Fuji in Japan last erupted in 1707. Kilauea in Hawaii is one of the most active volcanoes on Earth and has erupted almost continuously in recent decades. Eyjafjallajökull in Iceland erupted in 2010 and its ash cloud grounded European air traffic for six days, cancelling around 100,000 flights.
The largest volcano in the solar system is not on Earth at all. Olympus Mons on Mars is about 22 kilometres tall, nearly three times the height of Everest, and roughly the size of France. To keep exploring, find out about why the ground shakes and how volcanic islands are born.
