Volcano FAQ: Common Questions Answered

A volcano is an opening in a planet's crust where magma (molten rock), gas, and rock fragments reach the surface. Once magma erupts it is called lava. About 1,350 volcanoes on land are active today. Learn more in Volcanology 101.

It is the same molten rock — only the location differs. Underground it is called magma; once it erupts onto the surface it is called lava.

Eruptions are driven by gas and pressure. Dissolved gases in magma form bubbles as it rises and pressure drops. In runny magma the gas escapes gently; in sticky magma it builds pressure until the magma shatters explosively. See how volcanoes erupt.

Lava typically ranges from about 700°C to 1,200°C (1,300–2,200°F). Runny basaltic lava is the hottest; thicker, silica-rich lava is cooler. For comparison, a household oven maxes out around 260°C.

About 1,350 volcanoes on land are considered potentially active, meaning they have erupted in the last roughly 10,000 years. On any given day, 40 to 50 are erupting somewhere on Earth.

An active volcano has erupted recently or shows signs it could. A dormant volcano is quiet but could erupt again because it still has a magma supply. An extinct volcano has no magma supply and is not expected to erupt again.

Not necessarily. A dormant volcano is not currently erupting but could reactivate. Vesuvius was quiet for centuries before AD 79. Only an extinct volcano is considered unable to erupt again.

The Volcanic Explosivity Index ranks explosive eruptions from 0 to 8 based on how much material they erupt. It is logarithmic, so each step is about ten times larger than the one below. See the full VEI breakdown.

The 1815 eruption of Tambora in Indonesia, a VEI 7, was the largest in recorded history. It ejected about 150 km³ of material and caused the 1816 "Year Without a Summer."

A supervolcano is capable of a VEI 8 eruption, ejecting more than 1,000 km³ of material. Yellowstone and Toba are examples. No supereruption has occurred in recorded human history.

There is no sign Yellowstone is heading toward an eruption. The USGS monitors it continuously and estimates the annual chance of a large eruption at roughly 1 in 730,000. Small hydrothermal events are far more likely than a supereruption.

Scientists can often forecast that an eruption is likely within days to weeks by monitoring earthquakes, ground swelling, and gas emissions. They cannot yet give an exact date, but forecasting reliably narrows the window — the 1991 Pinatubo evacuation saved thousands of lives.

Yes. Large eruptions inject sulfur dioxide into the stratosphere, forming aerosols that reflect sunlight and cool the planet. Pinatubo lowered global temperatures by about 0.5°C for over a year in 1991–92.

The Ring of Fire is a horseshoe-shaped belt of subduction zones around the Pacific Ocean that hosts about 75% of the world's active volcanoes and 90% of its earthquakes. Learn why in plate tectonics.

Mostly water vapor, plus carbon dioxide and sulfur dioxide. CO₂ is dangerous because it is invisible and pools in low areas; in 1986 a release from Lake Nyos in Cameroon suffocated over 1,700 people. More in volcanic hazards.

Pyroclastic flows — fast avalanches of superheated gas and rock — cause about a third of all volcanic deaths. They travel over 100 km/h and reach 700°C. Lahars and tsunamis are next. See all hazards ranked.

A ground-hugging avalanche of superheated gas, ash, and rock that races downhill at over 100 km/h and reaches 700°C. It is the deadliest volcanic hazard and destroyed Pompeii in AD 79.

A lahar is a volcanic mudflow — a fast slurry of ash, rock, and water that flows down valleys like wet concrete. Lahars can travel over 100 km; one killed about 23,000 people at Armero, Colombia in 1985.

Yes. Volcanic ash is pulverized rock and glass, not soft like fireplace ash. It can collapse roofs, stall jet engines, contaminate water, and irritate lungs and eyes. An N95 mask offers meaningful protection. See our safety guide.

Usually yes. Most lava advances at walking pace or slower, so people can escape on foot. Lava destroys property but rarely kills. The exception is unusually fluid lava, such as at Nyiragongo.

Follow official evacuation orders immediately. If sheltering from ash, go indoors, seal windows and vents, and wear an N95 mask and goggles outside. Move to high ground away from river valleys if lahars threaten. Full checklist in our volcano safety guide.

Many volcanoes are popular, well-managed tourist destinations, especially effusive ones like Kīlauea and Stromboli. Safety depends on the current alert level, going with reputable guides, and following all official restrictions.