Why Do Volcanoes Erupt?

According to the lore of Australia's Gunditjamara people, the continent's Budj Bim volcano formed when a giant being crouched down over the earth so long that its body became a volcanic mountain and its teeth transformed into the lava the volcano spat out. But as the science of geology explains it, the 60 to 80 volcanic eruptions that occur each year are actually driven by magma's journey from Earth's interior towards its surface. How calm or calamitous an eruption is depends on the features and behavior of the magma that triggers it, says the U.S. Geological Survey (USGS).

What Happens During a Volcanic Eruption?

Because magma is lighter than the solid rock around it, pockets of it occasionally rise through the mantle layer. As it pushes up through Earth's lithosphere, the gases within the magma (including water vapor, carbon dioxide, sulfur dioxide, and others), which remain mixed in at deeper levels, increasingly want to escape as the pressure being exerted on them lessens. How these gases escape determines how violent an eruption results once the magma finally pushes up through the volcano's belly and breaks through weak areas in the Earth's crust, such as vents, fissures, and the summit.

Types of Volcanic Eruptions

While not all volcanic eruptions are alike, they generally fall into one of two categories: effusive or explosive.

Effusive Eruptions

Effusive eruptions are those where lava oozes out of a volcano relatively gently. As the USGS explains, these eruptions are less violent because the magma that produces them tends to be thin and runny. This allows the gases within the magma to more easily escape from the surface, thereby minimizing explosive activity.

Geologists have noticed that effusive eruptions generally behave in one of a few ways. If the molten lava streams out of long fissures (deep linear cracks in Earth's crust), the eruption style is called "Icelandic," after the volcanic activity in Iceland where such behavior commonly occurs.

If a volcano exhibits lava "fountaining" and lava flows streaming from its mouth and surrounding fissures, it's described as "Hawaiian."

Explosive Eruptions

When magma has a thicker, more viscous consistency (think of toothpaste), gases trapped within it aren't as easily released. (Magmas with higher silica contents tend to have thicker consistencies, according to the American Museum of Natural History.) Instead, the gases form bubbles which expand rapidly, causing explosions of lava. The more bubbles the magma develops, the more explosive the eruption will be.

• Strombolian eruptions, or those that spew clumps of lava low into the air in small, continuous bursts, are the mildest explosive eruptions.
• Vulcanian eruptions are characterized by moderate explosions of lava and volcanic ash.
• Pelean eruptions exhibit explosive outbursts that produce pyroclastic flows—mixtures of volcanic fragments and gases that roll down a volcano's slopes at high speeds.
• Plinian (or Vesuvian) eruptions, such as Washington State's Mount St. Helens' eruption in 1980, are the most powerful eruption type. Their gases and volcanic fragments can shoot up over 7 miles into the sky. Eventually, these eruption columns may collapse into pyroclastic flows.

Hydrovolcanic Eruptions

As magma rises through the Earth's crust, it sometimes meets groundwater from aquifers, water tables, and melting icecaps. Because the magma is several times hotter than the boiling point of water (212 degrees F), the water superheats, or converts to steam almost instantaneously. This flash conversion from liquid water to water vapor leads the inside of the volcano to over-pressurize (recall that gases exert a greater force on their containers than do liquids), but because this pressure buildup has nowhere to escape, it pushes outward, fracturing the surrounding rock, and rushes up through the volcano conduit until it reaches the surface, expelling a mixture of lava plus steam, water, ash, and tephra (rock fragments) in what's called a "phreatomagmatic" eruption.

If magma-heated hot rocks, rather than magma itself, interacts with subsurface groundwater or snow and ice, only steam, water, ash, and tephra are expelled without lava. These lava-less, steam-blast eruptions are known as "phreatic" eruptions.

Why Are Some Volcanoes Dormant?

If a volcano hasn't erupted in some time, it's dubbed as "dormant," or inactive. Dormancy can happen whenever a volcano gets cut off from it's magma source, such as when a tectonic plate shifts over a hotspot. For example, the Pacific Plate, which houses the Hawaiian Islands, is moving to the northwest at a rate of 3 to 4 inches per year. As it does so, Hawaii is slowly being dragged away from its oceanic hotspot, which remains stationary. This means that the currently active Hawaiian volcanoes might become dormant in the distant future.

Because it's often difficult to tell if a volcano will remain inactive or just isn't active at the moment, geologists typically won't consider a volcano extinct until it's been dormant for over 10,000 years.