How Do Volcanoes Erupt?

Deep within the Earth it is so hot that some rocks slowly melt and become a thick flowing substance called magma. Since it is lighter than the solid rock around it, magma rises and collects in magma chambers. Eventually, some of the magma pushes through vents and fissures to the Earth's surface. Magma that has erupted is called lava.

Some volcanic eruptions are explosive and others are not. The explosivity of an eruption depends on the composition of the magma. If magma is thin and runny, gases can escape easily from it. When this type of magma erupts, it flows out of the volcano. A good example is the eruptions at Hawaii’s volcanoes. Lava flows rarely kill people because they move slowly enough for people to get out of their way. If magma is thick and sticky, gases cannot escape easily. Pressure builds up until the gases escape violently and explode. A good example is the eruption of Washington’s Mount St. Helens. In this type of eruption, the magma blasts into the air and breaks apart into pieces called tephra. Tephra can range in size from tiny particles of ash to house-size boulders.

Explosive volcanic eruptions can be dangerous and deadly. They can blast out clouds of hot tephra from the side or top of a volcano. These fiery clouds race down mountainsides destroying almost everything in their path. Ash erupted into the sky falls back to Earth like powdery snow. If thick enough, blankets of ash can suffocate plants, animals, and humans. When hot volcanic materials mix with water from streams or melted snow and ice, mudflows form. Mudflows have buried entire communities located near erupting volcanoes.

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Image shows billowing clouds of steam and smoke emanating from the ground
November 30, 2000

Mount St Helens Phreatic Eruption

Phreatic eruption at the summit of Mount St. Helens, Washington. Credit: D.A. Swanson, USGS

Image: Lava flows on Mauna Loa
March 25, 1984

Lava flows on Mauna Loa

Erupting vents on Mauna Loa’s northeast rift zone near Pu‘u‘ula‘ula (Red Hill) on Mar. 25, 1984, sent massive ‘a‘ā lava flows down the rift toward Kūlani.

Volcano erupting and spewing a huge cloud of rock and ash into the sky.
May 18, 1980

Mount Saint Helens eruption

On Sunday, May 18, 1980 at 8:32 a.m., the bulging north flank of Mount St. Helens slid away in a massive landslide -- the largest in recorded history. Seconds later, the uncorked volcano exploded and blasted rocks northward across forest ridges and valleys, destroying everything in its path within minutes.

Tephra-jet explosion hurls spatter onto active lava delta on the so...

Tephra-jet explosion hurls spatter onto active lava delta on the SE...

Tephra-jet explosion hurls spatter onto active lava delta on the southeast coast of Kīlauea Volcano, Hawai‘i