What Is a Volcano?

Start beneath the ground where magma, gas, heat, and pressure interact.

About What Is a Volcano?

A volcano is an opening or system where magma, gases, ash, and rock can reach Earth's surface. Volcanoes form in several tectonic settings and range from small cones to enormous shield volcanoes and calderas.

Visual learning: These pages use real volcanic landscape and eruption photographs from Wikimedia Commons. Final image URLs should be browser-tested before deployment.

Kilauea Lava — Kilauea LavaLava at Kilauea shows molten rock reaching Earth's surface.

Mount St. Helens — Mount St. HelensMount St. Helens is a famous Cascade volcano shaped by explosive eruptions.

Mauna Loa — Mauna LoaMauna Loa is one of the largest active volcanoes on Earth.

Volcanic Crater — Volcanic CraterVolcanic craters form around vents where material reaches the surface.

Magma

Magma is molten or partly molten rock beneath Earth's surface.

Vents

A vent is an opening through which volcanic material escapes.

Craters

A crater is a bowl-shaped depression around a vent.

Cones

Repeated eruptions can build volcanic mountains.

Plate Tectonics

Many volcanoes form where tectonic plates separate or collide.

Volcanic Gases

Magma contains dissolved gases.

Active and Dormant

Volcano labels describe eruption history and potential.

Volcanic Landscapes

Volcanism creates islands, mountains, plateaus, craters, and new crust.

Magma

It can contain crystals and dissolved gases as well as liquid rock.

Vents

Volcanoes can have one main vent plus side vents, cracks, or fissures.

Craters

Large collapsed depressions called calderas form through different processes and can be many kilometers wide.

Cones

Layers of lava, ash, cinders, and other debris gradually create different shapes.

Plate Tectonics

Others form above hot spots within plates.

Volcanic Gases

Water vapor, carbon dioxide, sulfur dioxide, and other gases strongly influence eruption behavior.

Active and Dormant

An apparently quiet volcano can still be considered active if geologic evidence shows it may erupt again.

Volcanic Landscapes

It is one of the major processes reshaping Earth's surface.