Volcanoes are some of the most powerful and fascinating features on Earth. They can create new land, as in Hawaii and Iceland, build towering mountains and enrich soils for farming. They can also destroy cities, change the climate for years and send ash clouds around the world. More than a billion people live within range of an active volcano. Understanding how volcanoes work, where they form and why they erupt in different ways helps scientists protect communities and reveals how our planet is constantly reshaping itself. This explainer describes volcanoes from the deep source of magma to the eruption at the surface.
What is a volcano?
A volcano is an opening in Earth's crust through which molten rock, gases and fragments of rock escape to the surface. Over time, the material erupted from the opening can build up into a mountain or hill. Molten rock below the surface is called magma; once it reaches the surface, it is called lava.
Where magma comes from
Earth's interior is hot, with temperatures increasing with depth. Yet most of the mantle, the thick layer beneath the crust, is solid rock, because the enormous pressure keeps it from melting. Magma forms in specific situations where rock can partially melt:
- Decompression melting: When hot mantle rock rises toward the surface, the pressure on it decreases, allowing it to melt partially without becoming hotter. This happens at mid-ocean ridges, where tectonic plates pull apart, and at hotspots.
- Flux melting: At subduction zones, where one tectonic plate sinks beneath another, water from the sinking plate is released into the hot mantle above. Water lowers the melting temperature of rock, causing it to melt.
- Heat transfer: Hot magma rising into the crust can melt the surrounding rock.
Magma is less dense than the solid rock around it, so it rises, collecting in magma chambers a few miles beneath the surface. When pressure builds enough, magma may push its way to the surface and erupt.
Where volcanoes form
Most volcanoes are found along the boundaries of tectonic plates, the large slabs of Earth's outer shell that move slowly over the mantle.
- Subduction zones: Where an oceanic plate sinks beneath another plate, chains of volcanoes form. The Pacific Ring of Fire, a horseshoe-shaped belt around the Pacific Ocean, contains about 75 percent of the world's active volcanoes, including those in Japan, Indonesia, the Andes and the Cascades of North America.
- Divergent boundaries: Where plates pull apart, magma rises to fill the gap, as along the Mid-Atlantic Ridge. Iceland sits on top of this ridge, which is why it has so many volcanoes.
- Hotspots: Some volcanoes form far from plate boundaries, over plumes of unusually hot mantle rock. As a plate moves over a stationary hotspot, a chain of volcanoes forms, like the Hawaiian Islands. Yellowstone in the United States also sits above a hotspot.
Why some eruptions are gentle and others explosive
The style of an eruption depends mainly on two properties of the magma: its viscosity, or how thick and sticky it is, and how much gas it contains.
- Magma rich in silica is thick and sticky, like peanut butter. Gas bubbles cannot escape easily, so pressure builds until the magma explodes violently, shattering into ash and fragments.
- Magma low in silica, such as basalt, is runny, like warm syrup. Gas escapes easily, so eruptions tend to be gentler, with lava flowing out in streams and fountains.
Gases dissolved in magma, mainly water vapor, carbon dioxide and sulfur dioxide, behave like the gas in a bottle of soda. Under pressure deep underground, they stay dissolved. As magma rises and pressure drops, the gases form bubbles and expand. If they cannot escape, they tear the magma apart in an explosion.
Types of eruptions
- Effusive eruptions produce flowing lava, as in Hawaii and Iceland. They are dangerous to property but usually allow people time to move away.
- Explosive eruptions blast ash, gas and rock high into the atmosphere. Famous examples include Mount Vesuvius in 79 CE, Mount St. Helens in 1980 and Mount Pinatubo in 1991.
- Phreatic eruptions happen when magma heats groundwater, which flashes into steam and causes explosions.
Types of volcanoes
| Type | Shape | Typical magma and eruptions | Examples |
|---|---|---|---|
| Shield volcano | broad, gently sloping | runny basaltic lava, mostly effusive | Mauna Loa and Kīlauea, Hawaii |
| Stratovolcano (composite) | steep, cone-shaped, layered | thicker magma, alternating lava and explosive ash | Mount Fuji, Mount Rainier, Mount Vesuvius |
| Cinder cone | small, steep cone | gas-rich bursts of lava fragments | Parícutin, Mexico |
| Caldera | large depression | forms when a magma chamber empties and the ground collapses | Yellowstone, Crater Lake |
| Lava dome | rounded mound | very thick lava piling up near the vent | domes in the crater of Mount St. Helens |
Shield volcanoes are the largest on Earth by volume. Mauna Loa rises about 13,680 feet (4,170 meters) above sea level but extends far below the ocean, making it taller than Mount Everest when measured from its base on the seafloor.
Stratovolcanoes are built from layers of lava, ash and rock from many eruptions. They are often the most dangerous because of their explosive eruptions.
Volcanic hazards
Volcanoes produce several different hazards:
- Pyroclastic flows: Fast-moving avalanches of hot gas, ash and rock that can race down a volcano's slopes at more than 60 miles per hour (100 kilometers per hour) and reach temperatures of hundreds of degrees. They are the deadliest volcanic hazard. Pyroclastic flows buried the Roman cities of Pompeii and Herculaneum in 79 CE.
- Lava flows: Usually slow enough to walk away from, but they destroy everything in their path.
- Ash fall: Fine particles of volcanic glass and rock can collapse roofs, damage lungs and machinery, contaminate water and disrupt air travel. In 2010, ash from Iceland's Eyjafjallajökull volcano grounded flights across Europe for days.
- Lahars: Mudflows of volcanic debris mixed with water from rain or melting snow and ice. In 1985, a lahar from the Nevado del Ruiz volcano in Colombia destroyed the town of Armero, killing more than 20,000 people.
- Volcanic gases: Carbon dioxide, sulfur dioxide and other gases can be toxic or suffocating.
- Tsunamis: Large eruptions or collapses of volcanic islands can trigger tsunamis, as when Krakatoa erupted in 1883.
Volcanoes and climate
Large explosive eruptions can affect the global climate. They inject sulfur dioxide into the stratosphere, where it forms tiny droplets of sulfuric acid that reflect sunlight. This can cool the planet for a year or two. The eruption of Mount Pinatubo in 1991 lowered global average temperatures by about 0.5 °C (0.9 °F) for about two years. The eruption of Tambora in Indonesia in 1815 led to 1816 being called the "Year Without a Summer" in Europe and North America, with crop failures and famine.
Over millions of years, volcanoes have also released carbon dioxide that helped shape Earth's long-term climate. Today, however, human activities release far more carbon dioxide each year than all the world's volcanoes combined.
Monitoring volcanoes
Scientists called volcanologists watch active volcanoes for signs that an eruption may be coming. Warning signs include:
- Earthquakes: Swarms of small earthquakes can indicate magma moving underground.
- Ground deformation: The ground may swell or bulge as magma accumulates beneath it. GPS instruments and satellite radar can detect changes of a few millimeters.
- Gas emissions: Increases in sulfur dioxide or carbon dioxide can signal rising magma.
- Temperature changes: Satellites and thermal cameras detect hot spots.
Although scientists cannot predict the exact time of an eruption, monitoring often provides enough warning to evacuate people. The evacuation before the 1991 Pinatubo eruption is estimated to have saved thousands of lives.
Volcanoes as creators
Volcanoes are not only destructive. They have built entire islands, from Hawaii to Iceland. Volcanic ash breaks down into some of the most fertile soils in the world, which is why many people farm on the slopes of volcanoes. Volcanic heat provides geothermal energy, used in countries like Iceland and New Zealand. And over Earth's history, volcanic gases contributed to the formation of the atmosphere and oceans.
Questions readers ask
What is the difference between magma and lava? Magma is molten rock underground; lava is molten rock that has reached the surface.
How many active volcanoes are there? About 1,350 potentially active volcanoes exist on land worldwide, and many more on the ocean floor. Roughly 50 to 70 erupt each year.
Why do some volcanoes explode? Thick, sticky, gas-rich magma traps gas bubbles, building pressure until it fragments explosively.
Can scientists predict eruptions? They cannot predict the exact moment, but monitoring earthquakes, ground swelling and gas emissions often provides advance warning.
Is Yellowstone going to erupt soon? Yellowstone is closely monitored. Scientists see no signs of an imminent large eruption, and the annual chance of a major eruption is very low.
In short
Volcanoes form where magma, created by decompression, added water or heat in Earth's interior, rises to the surface. Most occur along plate boundaries, especially the Pacific Ring of Fire, or over hotspots like Hawaii. Runny, low-silica magma produces gentle lava flows; thick, gas-rich magma produces explosive eruptions. Volcanoes take many shapes, from broad shields to steep stratovolcanoes, and create hazards from pyroclastic flows to ash and lahars. Through careful monitoring, scientists can often warn communities before eruptions. Volcanoes destroy and create, reshaping the land and reminding us that Earth is a living planet.