Volcanoes Explained

What is a volcano?

A volcano is an opening in Earth’s crust where molten rock (magma), gas and rock fragments escape to the surface. Once magma reaches the surface it is called lava. Over thousands of years, lava flows, ash and debris pile up around the vent to build the mountain most people picture as a volcano. The study of all this is volcanology.

How do volcanoes form?

Volcanoes are not randomly scattered. Most form in three settings: where tectonic plates pull apart (Iceland and the mid-ocean ridges), where one plate sinks beneath another (subduction zones such as Japan, Indonesia and the Andes), and above hotspots — plumes of hot mantle such as the one beneath Hawaiʻi. About three-quarters of active volcanoes ring the Pacific in the Ring of Fire.

Why do volcanoes erupt?

Magma is less dense than the rock around it, so it rises. As it climbs, pressure drops and dissolved gases come out of solution — like opening a shaken soda bottle. Runny, gas-poor magma oozes out as lava flows; sticky, gas-rich magma shatters into ash in explosive eruptions. Learn more about the gases that drive eruptions.

Types of volcanoes

The main types of volcanoes are shield volcanoes (broad and gentle, like Mauna Loa), stratovolcanoes (steep cones such as Mount Fuji and Vesuvius), cinder cones, lava domes and calderas — huge collapsed craters like Yellowstone and Campi Flegrei. Many more lie hidden on the seafloor.

Quick facts

• ~1,350 potentially active volcanoes on land (USGS), plus many under the sea.
• 700–1,200 °C is the temperature range of erupting lava, depending on its chemistry.
• Pyroclastic flows, not lava, cause the most volcanic deaths. See volcanic hazards.
• 40–50 volcanoes are erupting on a typical day — see what’s active in 2026.

New to the subject? Follow our beginner’s study path, check terms in the glossary, or jump to the volcano FAQ.