Volcanoes: How to Measure Volcanic Eruptions | The Old Farmer's Almanac

Volcanoes: How Volcanic Eruptions Are Measured

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Volcano explosion, 2011, Tungurahua Volcano, Ecuador.
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Ammit Jack/Shutterstock
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Hawaii’s Mauna Loa volcano erupted on Sunday after nearly 40 years of dormancy. Thankfully, it’s been a fairly docile event! How do we measure 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. Learn more about the VEI and how volcanoes work!

On Sunday, November 27, 2022, the world’s largest active volcano erupted for the first time since 1984. Located on Hawaii’s Big Island, Mauna Loa (Hawaiian for “Long Mountain”) began erupting at around 11:30 PM local time, with lava erupting from the volcano’s summit. Thus far, the lava flows are not a threat to any communities, although residents are encouraged to be prepared and to be on alert for threats to air quality from volcanic gases and fine ash.

Mauna Loa lava flow on Monday, November 28, 2022. Photo by Natalie Deligne, USGS

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?

volcano-1784658_1920_full_width.jpgImage: 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 2282°F (1,250°C), which burns and melts absolutely everything in its path.

The Volcanic Explosivity Index (VEI)

The VEI determined by using one of more of the following criteria: 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:


Biggest Volcanic Eruptions in History

Eruptions of over 6 VEI occur rarely. Since 1700, only five have been recorded. The latest was Mount Pinatubo in the Philippines on June 15, 1991. Fortunately, science had taught us much about volcanic eruptions; 66,000 were able to be evacuated and some 850 people were killed in the eruption. (For comparison, an estimated 92,000 people died after the biggest eruption in the recorded history, Mt. Tambora).

Here are the two most famous eruptions which you may have read about in your history books:

1883, Krakatoa

On August 7, 1883, Krakatoa, a volcano on the Indonesian island of Rakata erupted. It was one of the most deadly volcanic eruptions of modern history with a VEI of 6. When it erupted, Krakatoa released 200 megatons 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 3,000 miles away on Rodrigues Island and Australia.

The VEI 6 eruption created a tsunami 150 feet tall, which killed 36,000 people. As we learned from Krakatoa and other eruptions with a V.E.I. of 6 or higher, the gas and ash particles in the atmosphere can have a cooling effect on the climate for several years, by reflecting more sunlight away from the Earth’s surface. With Krakatoa, particles from the eruption were released into the stratosphere, causing the average temperature of Earth to lower by one degree for the next two years.

The caldera of Mt. Tambora. Photo by NASA.

1815, Mt. Tambora

On April 15, 1815, the biggest volcanic eruption in modern history occurred. Tambora is the only eruption in at least 1,000 years to rate a VEI 7.

Located in what is now Indonesia, Tambora’s eruption caused crops to fail worldwide and the particles in the atmosphere caused global temperatures to decrease by more than 5 degrees Fahrenheit. The result was the “year without a summer” in 1816. Read our fascinating article from the Almanac archives about “The Year Without a Summer.”

More Volcano Facts

  • Mauna Loa in Hawaii (on the Big Island) 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! 
About The Author

Catherine Boeckmann

Catherine Boeckmann loves nature, stargazing, and gardening so it’s not surprising that she and The Old Farmer’s Almanac found each other. She leads digital content for the Almanac website, and is also a certified master gardener in the state of Indiana. Read More from Catherine Boeckmann

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