How Do Volcanoes Work?

In 1815, Mount Tambora in Indonesia erupted with a force so large that it is estimated to have killed directly and through subsequent famine more than 71,000 people. The eruption injected so much sulfur dioxide into the atmosphere that 1816 became known as the Year Without a Summer across Europe and North America. Frosts and snowfalls occurred in June and July. Crops failed. Prices for grain soared. The eruption changed the global climate for a year and contributed to the worst famine of the 19th century.

This is the power a single volcano can exert on the world. Volcanoes are not simply mountains that explode. They are the vents through which the Earth releases heat and gases that have been building deep beneath the surface, as described by the USGS Volcano Hazards Program. Understanding how they work helps explain why 800 million people worldwide live within 100 kilometers of an active volcano, and why scientists monitor them so carefully.

The short answer

A volcano forms where magma from deep beneath the Earth’s surface finds a path to the surface and erupts. As magma rises, dissolved gases expand and drive the eruption. The style of eruption depends on the magma’s chemistry and gas content. Runny basaltic magma produces gentle lava flows typical of shield volcanoes like those in Hawaii. Thick, viscous silica-rich magma traps gas under pressure and produces explosive eruptions typical of stratovolcanoes like Mount Fuji. The most dangerous hazards are pyroclastic flows, ash fall, and tsunamis triggered by volcanic activity.

The full picture

Why volcanoes exist: plate tectonics and magma formation

Volcanoes are a direct consequence of plate tectonics. Most volcanoes form at plate boundaries where the physical conditions allow magma to rise to the surface.

At convergent boundaries, where one tectonic plate descends beneath another in a process called subduction, the descending plate carries water and other volatile compounds with it. As it sinks into the hot mantle, these volatiles lower the melting point of the surrounding rock, generating magma. This magma, rich in silica and gas, is buoyant and rises. The result is a chain of volcanoes along the subduction zone. The Andes Mountains contain dozens of active volcanoes built above the subduction of the Nazca Plate beneath South America. Japan, the Philippines, and the Cascades of the Pacific Northwest all sit above subduction zones.

At divergent boundaries, where plates pull apart, magma rises to fill the gap created by the separating plates. The Mid-Atlantic Ridge, where the North American and Eurasian plates separate, is a 16,000-kilometer chain of submarine volcanoes and volcanic vents. Iceland sits atop a particularly active section of the ridge, where divergent plate movement and a mantle hotspot combine to produce frequent eruptions.

Hotspots are a third source of volcanoes, distinct from plate boundaries. A hotspot forms where an unusually hot plume of mantle rises from deep in the Earth, punching through the crust regardless of plate boundaries. As the plate moves over the hotspot, a chain of volcanoes forms. The Hawaiian Islands are the surface expression of the Pacific Plate moving over the Hawaiian hotspot. The oldest islands are northwest, progressively younger islands to the southeast, marking the path the plate has traveled over the hotspot.

Magma: what is underground

Magma is molten rock beneath the Earth’s surface. Its composition determines how it behaves.

Basaltic magma is the most common type, low in silica and very fluid. It flows like warm honey, traveling long distances before solidifying. Basaltic magma produces the gentle, effusive eruptions characteristic of shield volcanoes. Because the lava flows easily, gas escapes without explosive violence.

Andesitic magma is intermediate in composition, found at subduction zone volcanoes. It is more viscous than basaltic magma, trapping gases more effectively. This leads to more explosive eruptions with pyroclastic material.

Rhyolitic magma is the most silica-rich and the most viscous. It moves very slowly and traps gas under high pressure. When it erupts, the gas expands violently, producing the most explosive eruptions on Earth. The ash from rhyolitic eruptions can reach the stratosphere and affect global climate.

The gas content of magma is as important as its chemistry. Most volcanic gas is water vapor, followed by carbon dioxide and sulfur dioxide. As magma rises and pressure decreases, these dissolved gases come out of solution and expand. This expansion is what drives the eruption. A magma with low gas content might produce a gentle lava flow. The same magma with high gas content, under pressure, will produce an explosive eruption when the gas finally escapes.

Types of volcanic eruptions

Volcanologists classify eruptions by their style, which depends on the magma type and gas content.

Hawaiian eruptions are the gentlest. They produce spectacular fountains of fluid basaltic lava that can shoot hundreds of meters into the air, but the lava flows calmly downhill. Kilauea in Hawaii has been erupting continuously since 1983, producing lava flows that periodically reach the ocean. No one has been killed by Kilauea’s direct eruption effects in decades, thanks to monitoring and evacuation systems.

Strombolian eruptions are moderate. They produce bursts of lava fragments expelled from the vent, like fireworks. These bursts repeat every few minutes or seconds, creating a characteristic rhythm. Stromboli, one of the Aeolian Islands off Italy, has been erupting in this pattern for at least 2,000 years, earning it the nickname Lighthouse of the Mediterranean.

Vulcanian eruptions are more explosive. They produce dense clouds of ash and rock fragments that shoot upward from the vent. Between eruptions, the vent can become blocked by solidified lava, building pressure until the blockage is blown clear in a sudden explosion. The 1995 eruption of Soufriere Hills volcano on Montserrat destroyed the capital of Plymouth and rendered half the island uninhabitable.

Plinian eruptions are the most violent. Named after Pliny the Younger, who described the 79 AD eruption of Mount Vesuvius that buried Pompeii, these eruptions produce massive columns of ash and gas that rise into the stratosphere. The 1991 eruption of Mount Pinatubo in the Philippines injected 20 million tons of sulfur dioxide into the stratosphere, creating a haze that lowered global temperatures by about 0.5 degrees Celsius for two years.

Types of volcanoes

The shape of a volcano reflects the type of magma that built it.

Shield volcanoes have gentle slopes like a warrior’s shield lying face-up. They form from thousands of thin, fluid lava flows that spread far from the vent before solidifying. Mauna Loa in Hawaii rises 9,170 meters from the ocean floor, making it the largest active volcano on Earth, though its slope is so gentle that it takes 50 kilometers to climb from sea level to the summit. Mauna Loa’s neighbor, Kilauea, has been erupting so continuously that it has added new land to the island where its lava flows into the ocean.

Stratovolcanoes, also called composite volcanoes, are the classic conical peaks with steep sides. They form from alternating layers of lava flows and pyroclastic deposits, built up over thousands of eruptions. Mount Fuji in Japan, Mount Rainier in the United States, and Mount Vesuvius in Italy are stratovolcanoes. Their steep slopes and tendency toward explosive eruptions make them particularly dangerous.

Cinder cones are the smallest and simplest volcano type. They form from a single vent that ejects fragments of lava that fall around the vent and pile up. They rarely exceed 400 meters in height and typically erupt only once. Many cinder cones form on the flanks of larger volcanoes.

Volcanic hazards

Volcanoes kill in several ways, and not always in the ways people expect.

Pyroclastic flows are among the deadliest hazards. These superheated clouds of gas, ash, and rock fragments can travel at speeds exceeding 300 kilometers per hour and reach temperatures of 700 degrees Celsius. They hug the ground, flowing downhill and hugging valleys. The 1902 eruption of Mount Pelee produced a pyroclastic flow that struck the city of Saint-Pierre, killing about 30,000 people in under a minute. Only two people in the city survived, one of them a prisoner in a below-ground cell.

Lahars are volcanic mudflows, a mixture of water, ash, and rock debris that flows downhill with the consistency of wet concrete. They can occur during an eruption when ice and snow on a volcanic peak melt, or when heavy rainfall mobilizes loose volcanic ash. The 1985 eruption of Nevado del Ruiz in Colombia generated lahars that traveled 100 kilometers, reaching the town of Armero at night and killing about 23,000 people. The eruption itself was relatively small, but the lahars caught the sleeping town completely by surprise.

Ash fall from large eruptions can collapse roofs, damage crops, and disrupt aviation. The 2010 eruption of Eyjafjallajokull in Iceland produced an ash cloud that forced the cancellation of more than 100,000 flights across Europe, stranding millions of passengers. Volcanic ash is not like wood ash; it is made of sharp rock fragments that can damage aircraft engines.

Volcanic gases are released during eruptions and also during periods of quiet. Sulfur dioxide, hydrogen chloride, and hydrogen fluoride can be toxic at high concentrations and can damage crops and infrastructure. In 1986, Lake Nyos in Cameroon suddenly released a cloud of carbon dioxide from a volcanic lake, killing about 1,700 people and 3,500 livestock in the surrounding valley.

Tsunamis can be triggered by submarine volcanic eruptions or by the collapse of volcanic flanks into the ocean. The 1883 eruption of Krakatoa generated tsunamis that killed about 36,000 people on the surrounding coastlines of Java and Sumatra.

Why it matters

Volcanoes have shaped human civilization in ways both obvious and subtle. The soil on the slopes of volcanoes is among the most fertile on Earth, enriched by volcanic minerals weathered from fresh rock. This is why so many people live near volcanoes: the land is productive. The ancient Romans built their civilization partly on volcanic soils. Indonesia, the Philippines, and Japan all owe much of their agricultural productivity to volcanic geology.

At the same time, volcanoes are a reminder that the Earth is dynamic and indifferent to human presence. The 2010 Eyjafjallajokull eruption disrupted more flights than any single event since World War II. A larger eruption from an Icelandic volcano could potentially ground flights across the North Atlantic for weeks.

The relationship between volcanoes and climate goes in both directions. Large eruptions cause temporary cooling. But over geological time, volcanic activity also releases carbon dioxide, contributing to the greenhouse effect that keeps the Earth warm enough for liquid water. Without volcanic activity, Earth might look more like Mars.

Common misconceptions

“Volcanoes only explode at the top.” The explosive part of a volcanic eruption can occur at the summit vent, along the flanks of the volcano, or from new vents that open on the surrounding terrain. In 2001, Mount Nyiragongo in the Democratic Republic of Congo erupted from fissures that opened in the flanks, sending lava flows directly into the city of Goma. Residents were caught off-guard because they expected the eruption to come from the summit.

“An eruption that looks small is not dangerous.” The deadliest volcanic disasters often come from relatively small eruptions that catch people by surprise. The 1985 Nevado del Ruiz eruption was a moderate event that produced a relatively small amount of ash. But the ash mobilized into lahars that traveled far and fast enough to reach a town 50 kilometers away. The size of the eruption bears little relation to the size of the lahars it produces.

“Volcanoes are all bad.” The net effect of volcanic activity on Earth is beneficial. Volcanic soils support agriculture that feeds billions. Volcanic gases have helped maintain the Earth’s atmosphere over geological time. Geothermal energy, harnessed in Iceland, New Zealand, and elsewhere, provides clean electricity generated by volcanic heat. Without volcanic activity, the Earth would be a colder, quieter, and significantly less fertile planet.

Key terms

Magma: Molten rock beneath the Earth’s surface, underground. It contains dissolved gases that expand as pressure decreases during eruption.

Lava: Magma that has erupted and reached the Earth’s surface. Once it solidifies, it becomes volcanic rock.

Shield volcano: A broad, gently sloped volcano built from thin, fluid lava flows that spread far from the vent. Hawaii’s Mauna Loa and Kilauea are shield volcanoes.

Stratovolcano: A tall, conical volcano built from alternating layers of lava flows and pyroclastic deposits. Characteristic of explosive eruptions. Also called a composite volcano.

Cinder cone: A small, steep volcano built from fragments of lava that fall from a single vent and pile up around it.

Subduction zone: A plate boundary where one tectonic plate descends beneath another. Subduction produces magma that feeds explosive volcanoes at the surface.

Hotspot: A region where an unusually hot plume of mantle rises through the crust, producing volcanoes regardless of plate boundaries. Hawaii is the classic example.

Pyroclastic flow: A superheated cloud of gas, ash, and rock fragments that flows downhill at hundreds of kilometers per hour, hugging the ground and burning everything in its path. One of the most dangerous volcanic hazards.

Lahar: A volcanic mudflow, a mixture of water, ash, and rock debris that flows downhill with the consistency of wet concrete. Can travel tens of kilometers from a volcano.

Plinian eruption: The most explosive type of volcanic eruption, named after Pliny the Younger, who described the 79 AD eruption of Mount Vesuvius. Produces massive columns of ash that reach the stratosphere.