How Do Volcanoes Work?
Mountains that breathe fire — how molten rock from deep underground finds its way to the surface.
Earth Is Like a Giant Egg
Imagine Earth as a hard-boiled egg. The thin, cracked shell is the crust — the solid rock we live on. Below that is the mantle, a thick layer of extremely hot, slowly moving rock (like the egg white). At the very center is the core, which is scorching hot metal (like the yolk). The deeper you go, the hotter it gets — the inner core reaches about 5,400°C, nearly as hot as the surface of the Sun.
A volcano is an opening in Earth's crust where hot, melted rock from the mantle pushes up to the surface. When that melted rock is still underground, it's called magma. Once it erupts and reaches the surface, we call it lava. Same stuff, different name depending on where it is.
Why Do Volcanoes Form?
Earth's crust isn't one solid piece — it's broken into large sections called tectonic plates that float slowly on top of the mantle. These plates move just a few centimeters per year, but over millions of years, that adds up. Most volcanoes form at the edges where two plates meet.
When one plate slides beneath another (a process called subduction), the sinking plate heats up, melts, and creates magma. That magma rises because it's less dense than the surrounding solid rock — like a bubble rising in a lava lamp. When enough pressure builds up, the magma breaks through the surface and a volcano erupts.
Some volcanoes form far from plate edges, over hot spots — places where an extra-hot plume of magma rises from deep in the mantle. The Hawaiian Islands were created this way: the Pacific Plate slowly drifts over a stationary hot spot, and each island is an old volcano that formed as the plate passed overhead.
Types of Volcanoes
Shield volcanoes are wide and gently sloping, like an upside-down saucer. They produce runny lava that flows long distances before hardening. Mauna Loa in Hawaii is the world's largest shield volcano — measured from its base on the ocean floor, it's actually taller than Mount Everest.
Stratovolcanoes (composite volcanoes) are the tall, steep, cone-shaped mountains most people picture when they think of a volcano. They're built from alternating layers of lava and ash. Mount Fuji in Japan and Mount St. Helens in Washington are classic examples. These volcanoes tend to erupt explosively because their thick, sticky lava traps gas under tremendous pressure.
Cinder cone volcanoes are the smallest type — steep, cone-shaped hills usually under 300 meters tall. They form when chunks of lava blast into the air and pile up around the vent. They often erupt once and then go quiet forever.
What Happens During an Eruption?
When pressure from trapped gas and rising magma overwhelms the rock above it, the volcano erupts. Some eruptions are gentle — lava oozes out slowly and flows downhill like a thick, glowing river. Other eruptions are explosive, blasting clouds of ash, rock, and gas high into the atmosphere. The most dangerous part of an explosive eruption is often the pyroclastic flow — a superheated avalanche of gas and rock that races down the volcano's slopes at hundreds of kilometers per hour.
There are about 1,350 potentially active volcanoes on Earth, but around 80% of the planet's volcanic activity actually happens underwater along mid-ocean ridges. The ocean floor is constantly being reshaped by eruptions that most people never see. In fact, the entire island of Iceland sits on a mid-ocean ridge and is still growing from volcanic activity.
Last reviewed: April 2026