A volcano is an opening in the Earth where melted rock and gas can come out; melted rock under the ground is called magma; a volcano is called active if it has erupted in about the last 10,000 years, even if it is quiet now. Subduction zones make magma: as one plate sinks beneath another, rock melts; magma rises because it is lighter than the rock around it. Many of Earth's volcanoes and earthquakes are found along the Ring of Fire, a path that partly circles the Pacific Ocean. The Hawaiian Islands formed over a hot spot: the Pacific Plate slowly carries volcanoes away from it, so the chain gets older in one direction. Mid-ocean ridges, volcanic chains hidden far beneath the sea, produce an estimated 75% of Earth's volcanism by magma output.

An eruption can be explosive or effusive, and which one happens depends largely on how much gas is in the magma. As magma rises toward the surface, dissolved gas bubbles out of it as the pressure drops, like soda fizzing when a can is opened. Magma with a lot of silica is very thick and sticky, and eruptions of it are likely to be explosive; Hawaiian volcanoes such as Kilauea tend to ooze runny lava rather than burst violently.

Basalt, the dark rock that forms from runny lava, makes up most of the ocean floor. Andesite lava typically forms thick, stubby flows with steep fronts, unlike the long, thin flows of basalt. Rhyolite lava is so viscous that it forms thick, blocky flows or steep-sided domes instead of spreading out.

Cinder cones, composite volcanoes and shield volcanoes are the three best-known types of volcano. Scientists can group volcanoes in more than one way, so there is no single agreed count of volcano types. Composite volcanoes, also called stratovolcanoes, get their name from the layers of lava and ash that build them. Shield volcanoes are wide with gentle slopes because their runny lava flows far before it hardens. Cinder cones are small volcanoes, usually less than 1,000 feet (330 meters) tall.

A lava dome is a mound of thick, sticky lava that piles up over a vent instead of flowing away. Many volcanoes have a crater, a round dip at the top or around a vent where the eruption happens. A caldera forms when a very large eruption empties a magma chamber and the ground above collapses into a huge bowl. Calderas are far bigger than craters; the biggest are tens of miles (tens of kilometers) across.

Pahoehoe is lava with a smooth, ropy surface. Lava called 'a'a has a rough, jagged, chunky surface. Rock columns called columnar joints form when lava cools, shrinks and cracks. Lava that erupts under water often piles into rounded mounds called pillow lava.

Tephra is the word for all the pieces of rock thrown into the air by an eruption, whatever their size. Volcanic ash is made of tiny pieces of volcanic rock; wind can carry volcanic ash hundreds of miles from the volcano. Volcanic ash can melt inside a jet engine and make the engine stall or fail. Explosive eruptions can produce pyroclastic flows, fast-moving mixtures of hot gas, ash and rock that sweep down valleys. Volcanic eruptions can cause tsunamis when a volcano is near a large body of water. A lahar is a mudflow of water and volcanic debris that rushes down the slopes and valleys of a volcano. Heavy rain falling on loose volcanic material can set off a lahar.

The Volcanic Explosivity Index (VEI) rates eruptions on a scale that starts at 0, and the biggest eruptions in history are rated 8. The 1815 eruption of Mount Tambora in Indonesia was the largest volcanic explosion in recorded history. The 1783 Laki eruption in Iceland came from a fissure 27 kilometers long and was one of the largest lava eruptions in historic times. After the 1883 eruption of Krakatau, sunsets and the sky took on unusual colors around the world.

Ash and aerosols from Tambora's 1815 eruption helped cause 1816 to become known as the Year Without a Summer. As of November 2026, Mount Pinatubo's 1991 eruption in the Philippines sent more than 10 million tons of sulfur dioxide into the stratosphere. After Pinatubo's 1991 eruption, global temperatures dropped for a year or two by about 1 degree F (about half a degree C). Aerosol droplets from a big eruption can cool the whole planet for a time by blocking some sunlight.

Volcano observatories combine several kinds of monitoring (watching, listening for earthquakes, measuring gas and measuring ground movement) and compare the results rather than trusting one warning sign. Magma moving underground can make a volcano swell or tilt, and GPS and tiltmeters can measure the change. InSAR uses radar images from a satellite: the change in distance between the ground and the satellite from one pass to the next shows how far the ground moved. Scientists watch changes in volcanic gases such as sulfur dioxide, because rising magma releases gas before an eruption. Scientists measure how much sulfur dioxide gas a volcanic plume holds with ultraviolet spectrometers, by seeing how much of the Sun's light passes through the plume. Before Mount Pinatubo's big eruption in June 1991, scientists' forecasts and recommendations led to more than 60,000 people being evacuated from the area around the volcano in the Philippines.

Words to know

• subduction: plate sinking
• viscous: thick and sticky
• rhyolite: thick lava type
• lahar: volcanic mudflow