Hotspot volcanoes

Why do volcanoes erupt?

According to the traditions of the Australian Gunditjamara people, the mainland’s Budj Bim volcano formed when a giant crouched on the earth for so long that its body became a volcanic mountain and its teeth turned into lava that the volcano spat out. But as the science of geology explains, the 60 to 80 the volcanic eruptions that occur each year are actually caused by the travel of magma from the Earth’s interior to its surface. The degree of calm or calamity of an eruption depends on the characteristics and behavior of the magma that triggers it, according to the US 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 rises in the earth’s lithosphere, the gases contained in the magma (including water vapor, carbon dioxide, sulfur dioxide and others), which remain mixed at deeper levels, increasingly want to escape as the pressure on them decreases. How? ‘Or’ What these escaping gases determine the violence of an eruption once the magma finally enters the volcano’s belly and passes through weak areas of the earth’s crust, such as vents, cracks, and the summit.

What is magma?

Magma is molten rock from the earth’s mantle, between the superheated core and the outer layer of the crust. The subterranean temperatures of the magma are of the order of 2,700 degrees F. After erupting from the mouth of a volcano on the surface of the Earth, it is known as “lava”.

Types of volcanic eruptions

Although not all volcanic eruptions are the same, they generally fall into one of two categories: effusive or explosive.

Effusive eruptions


Lava flows from a fissure near the Bardarbunga volcano in Iceland.

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Effusive eruptions are those where lava oozes from 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 contained in the magma to escape more easily from the surface, thus minimizing explosive activity.

Geologists have noticed that effusive eruptions typically behave in several ways. If molten lava flows from long fissures (deep linear cracks in the earth’s crust), the eruption style is called “Icelandic”, after volcanic activity in Iceland where such behavior commonly occurs.

If a volcano has a “fountain” of lava and lava flows from its mouth and surrounding cracks, it is described as “Hawaiian.”

Explosive eruptions


Mount St. Helens erupts.

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When magma has a thicker, more viscous consistency (think toothpaste), the gases trapped inside are not 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 that expand rapidly, causing lava explosions. 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 gusts, are the mildest explosive eruptions.
  • Vulcanian eruptions are characterized by moderate explosions of lava and volcanic ash.
  • Pelean eruptions feature explosive explosions that produce pyroclastic flows, mixtures of volcanic fragments and gases that tumble down the slopes of a volcano at high speed.
  • Pliny (or Vesuvian) eruptions, such as the Mount St. Helens eruption in Washington state in 1980, are the most powerful type of eruption. Their gases and volcanic fragments can rise more than 7 miles into the sky. Ultimately, these eruption columns can collapse into pyroclastic flows.

Hydrovolcanic eruptions


A phreatic eruption of the Mount Bromo volcano in Indonesia.

heroris maulidio / Getty Images


As magma rises through the earth’s crust, it sometimes encounters groundwater from aquifers, water tables, and melting ice caps. Because magma is several times hotter than the boiling point of water (212 degrees F), water overheats or turns to vapor almost instantly. This flash conversion of liquid water to water vapor causes the interior of the volcano to overpressurize (remember that gases exert a greater force on their containers than liquids), but because this build-up of pressure does not has nowhere to escape, it pushes outward, fracturing the surrounding rock and rushing through the volcano’s conduit until it reaches the surface, expelling a mixture of lava plus steam, d water, ash and tephra (rock fragments) in what is called a “phreatomagmatic” eruption.

If the hot rocks heated by the magma, rather than the magma itself, interact with underground groundwater or snow and ice, only steam, water, ash, and tephra are expelled without lava. These lava-free, steam-jet eruptions are known as “phreatic” eruptions.

How long do rashes last?

Once an eruption occurs, it lasts until the local magma chamber is emptied, or until enough substance escapes for the pressure inside the volcano to equalize. That said, a single eruption can last from a day to several decades, but according to the Smithsonian Institute’s Global Volcanism Program, seven weeks is about the average.

Why are some volcanoes dormant?

If a volcano has not erupted for some time, it is referred to as “dormant” or inactive. Dormancy can occur any time a volcano is cut off from its source of magma, such as when a tectonic plate moves over a hot spot. For example, the Pacific Plate, home to the Hawaiian Islands, moves northwest at a rate of 3-4 inches per year. In doing so, Hawaii is slowly dragged away from its oceanic hotspot, which remains stationary. This means that currently active Hawaiian volcanoes could go dormant in the distant future.

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


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