Volcanoes: How to Measure Volcanic Eruptions

May 10, 2018
Volcano Eruption
Volcano explosion, 2011, Tungurahua Volcano, Ecuador.
Ammit Jack/Shutterstock

How do we determine the explosiveness of a volcanic eruption? Traditionally, the Volcanic Explosivity Index (VEI) helps us measure volcano activity in order to forecast future eruptions and predict how widely the damage could reach. 

What Are Volcanoes?

The word “volcano” comes from the Roman name “Vulcan”—the Roman god of fire. Put simply, a volcano is an opening in the Earth’s surface from which gas, hot magma, and ash can escape. Magma is the name given to hot liquid rock inside a volcano. Once it leaves the volcano, it’s known as lava. 

Today, there are about 1,900 volcanoes on Earth are considered active. Each year, some 50 to 60 of these volcanoes erupt. About 350 million, or one in 20 people in the world live within “danger range” of an active volcano. The soil near the slopes of volcanoes is actually quite rich and fertile.

Volcanic eruptions have incredible variation in size—ranging slow-moving red lava to staggeringly violent explosions that blot out the Sun for years. So how do we determine the “bigness” when there are such extremes?

Image: Lava Flows, Hawaii

As you can see from the chart below, small volcanoes occur more frequently and truly colossal volcanoes don’t happen very often (whew). However, even nonexplosive or gentle eruptions can be incredible dangerous and costly, destroying dozens of homes and displacing hundreds of people. Lava can reach 1,250°C which would burn absolutely everything in its path.

Volcanic Explositivity Index (VEI)

The VEI determined by using one of more of the following critieria: Volume of ejecta, Height of the eruptive column, Qualitative descriptions (“gentle”, “effusive”, “explosive”, “cataclysmic”, etc.), Style of past activity, and Height of spreading of the eruptive plume head (in troposphere or stratosphere).  The VEI numbers below correspond with the following eruption characteristics:


Volcano Facts

  • When Krakatoa, a volcano on the Indonesian island of Rakata erupted, it released 200 megatrons of energy, the equivalent of 15,000 nuclear bombs. the explosions heard in the eruption remain the loudest noise on human record! The sound was heard across the Indian Ocean as far away as Rodriguez Island and Australia.
  • Mauna Loa in Hawaii is the world’s largest active volcano (4,169 meters).
  • The “Ring of Fire”, a 40,000 km Horseshoe shaped area of the Pacific Ocean, is home to 90% of all volcanoes on the Earth.
  •  In A.D. 79, Mount Vesuvius erupted in Pompeii, Italy. The ash deposits preserved the town and the remains of the people within it. You can still see them today! 

Hawaii’s Kilauea Update

In May, 2018, Hawaii’s Kilauea, a shield volcano, began releasing lava from vents about 25 miles (40 kilometers) east of the summit crater, destroying 26 homes (and counting) in its path.

The risk of a big explosion rises as magma drains down the flank of the volcano toward these vents. If the lava lake in the crater drops so low that groundwater is able to flow into the conduit that feeds magma to the crater, then the magma would heat the water, sending steam into the air that would push any accumulated rocks out in an explosion. 

Scientists predict that the magma is likely to drop below the water table around the middle of the month but don’t know how long after that it an explosion could occur. In anticipation of explosions, the areas are being cleared so that no one’s life is in danger.

Read all about “The Year Without a Summer,” caused by the 1816 volcanic eruption of Mt. Tambora.


Reader Comments

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Great update. I really hope the Hawaii explosion won't be too large. The Big Island is a magical place with wonderful people.

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Thank you for helping me with my science project :}