Facts for Kids

Many eruptions happen for three main reasons. One is when gas in molten rock expands and bursts out — these are called magmatic eruptions. A second is when hot ground or rock heats nearby water so fast that the water turns to steam and blasts out rocks and ash; this is a steam-driven eruption. A third happens when hot magma meets water and the contact makes both steam and fragments fly out.

Scientists also use a simple scale called the VEI (Volcanic Explosivity Index) from 0 to 8 to describe how strong an eruption is. The VEI helps compare eruptions and often matches the type of eruption and how much material comes out.

A volcanic eruption is when hot rock, ash, gas, or melted rock comes out of a volcano through a hole called a vent or a long crack called a fissure. Eruptions can look very different. Scientists give names to eruption styles after places where they were first seen, so you might hear names like Hawaiian or Strombolian.

Some volcanoes mostly behave the same way each time they erupt, while others can change style during one long event. Because of this variety, volcanoes are interesting to watch and study: their behavior tells scientists about the hidden parts under the ground.

The Hawaiian eruptions are the calmest magmatic style. Very runny lava called basalt flows out smoothly and quietly, so it spreads in wide, thin sheets that slowly build a broad, low hill called a shield volcano. Because the lava has little gas, it usually oozes instead of exploding.

Sometimes Hawaiian eruptions start as a long line of small openings that make a “curtain of fire,” and later focus on a few big vents. Lava can fountain hundreds of meters, and tiny glassy drops called Pele’s tears or thin threads called Pele’s hair can form. There are two common flow textures: smooth pahoehoe and rougher a’a; pahoehoe can change into a’a when it cools and stalls.

Strombolian eruptions get their name from the volcano Stromboli. They happen when gas bubbles inside the magma grow into large slugs that rise and then burst at the surface. Each burst makes a short, loud explosion and throws out glowing bits of magma.

These eruptions are more noisy and jumpy than Hawaiian eruptions. They fling bombs and small stones called lapilli into the air, which pile up around the vent to make a steep little hill called a cinder cone. Strombolian blasts usually send material a few hundred meters high and make clear rings or layers of rockfall around the crater.

A phreatic eruption happens when water suddenly touches very hot rock or magma and turns to steam. The steam expands so fast it blasts out pieces of the volcano that were already there, rather than new lava. These blasts can be quick and surprising and sometimes happen before bigger eruptions, so scientists watch them closely.

Phreatic blasts can send clouds of ash, wet mud flows called lahars, and fast ground-hugging clouds called base surges. They can also release dangerous gas, so they are risky for people nearby. Volcanoes that have had phreatic activity include Mount Ontake, Taal, and Mount St. Helens (as a warning before larger events). Because phreatic eruptions may come without long warning, people are careful near steaming or cracked ground.

A Vulcanian eruption happens when thick, sticky magma traps gas until it explodes. Because the magma is more viscous and full of crystals, the blasts are sharp and powerful. Short explosions that can last minutes to hours throw out big pieces of rock called bombs and blocks instead of long, flowing lava.

These eruptions often send ash and gas high into the sky—sometimes 5 to 10 kilometers. Lava domes may grow slowly after an eruption; if a dome collapses, it can trigger another violent Vulcanian blast with hot rock and ash rushing down the slope.

A Surtseyan eruption happens when lava meets shallow water, such as at the edge of the sea. The water turns to steam very quickly and explodes, breaking the lava into fine ash and building tall, wet plumes. Because water is involved, these eruptions are usually more explosive than dry ones.

They can make new bits of island as erupting material piles up. The eruptions often come in steady bursts or rhythms, and the ash can change into a crumbly rock called palagonite when it cools in water.

When a volcano erupts beneath a glacier, scientists call it a subglacial eruption. Hot rock and lava meet thick ice, so the heat melts the ice and makes lots of hot water. At first the lava behaves a bit like it does under the sea, making rounded pillow shapes and breaking into glassy fragments called hyaloclastite when it cools quickly.

If the eruption keeps going, the meltwater can collect into a lake under the ice. That lake lets explosions build up steep sides of broken rock, and later thick lava flows can pile on top to make a flat top. These flat-topped, steep-sided volcanoes are called tuyas — places in Iceland and the Tuya Butte area of British Columbia show clear examples. Melting ice can rush away as big floods called jökulhlaups, and the lava-and-ice piles help scientists learn where ice used to be and how the climate changed.

A Plinian–Peléan eruption makes very tall columns of ash and gas that can travel far in the sky. In some volcanoes, a thick lava dome or spine grows and then collapses. When that happens, the hot rock and ash can rush down the mountain as a fast, glowing flow called a pyroclastic flow.

These flows move quickly and carry hot gas and fragments that can smash or bury things in their path, so they are among the most dangerous types of eruptions. Scientists watch for dome growth because it often means a stronger, riskier event could follow.

A submarine phreatomagmatic eruption takes place beneath the ocean or under a lake when hot magma meets water. An eruptive column can form and then collapse, creating a low, fast-moving cloud called a base surge that hugs the seafloor and spreads outward like a wedge.

These events leave special marks in the rocks: dune-shaped layers from the moving surge, places where big fragments fell and sagged into soft ash (“bomb sags”), and round, wet ash pellets called accretionary lapilli. These clues help scientists learn how the eruption happened.