Jeff Schmaltz / MODIS / NASA GSFC
A plume of ash and steam rising from the Eyjafjallajökull volcano reached 17,000 to 20,000 feet (5 to 6 kilometers) into the atmosphere on May 10, 2010, when an instrument on NASA's Aqua satellite captured this image.
updated 7/22/2012 6:08:47 PM ET 2012-07-22T22:08:47

When a volcano erupts explosively, molten rock bubbles upward at the speed of sound, forming glasslike rocks that shoot out of the earth as fast as a speeding bullet (literally). 

But not all volcanic eruptions are created equal. The size of the erupting particles helps determine how dangerous the outburst is. A jet of fine-grained ash is more likely to make it into the upper atmosphere, where it can disrupt air traffic and cause short-term cooling by reflecting incoming sunlight back into space. The 1991 eruption of Mount Pinatubo in the Philippines, for example, lofted particles and gases into the stratosphere, where they reflected sunlight and helped reduce global temperatures by about 0.9 degrees Fahrenheit (0.5 degrees Celsius) during the following year.

On the other hand, a payload of large pumice rocks, also known as bombs, is more likely to cause local damage, and form at the same time as a lethal pyroclastic flow of superheated gas and rock.

Scientists had thought that these particles were shaped at the beginning of an eruption, when magma bubbles and forms glasslike tephra. But a new study published in the July 22 edition of the journal Nature Geoscience found that particles are primarily shaped during their swift ride to the surface, during which the rocks ram into each other and break apart. 

After deeply rooted eruptions, particles have more time to crash into one another and break apart, and so are more likely to produce a fine-grained ash, said study author and Georgia Tech researcher Josef Dufek.

Shallow eruptions, on the other hand, are more likely to chuck out pumice bombs, he told OurAmazingPlanet. During field work in Greece, Dufek found basketball-sized chunks of pumice that were thrown 12 miles (20 kilometers) by ancient eruptions.

To find out how much volcanic rocks shatter on their way to the surface, Dufek fired beads of volcanic glass at one another using a "pumice gun" and filmed their collisions with high-speed cameras. After enough collisions, the particles can break each other into a million little pieces, forming fine grains that can rise in a choking plume into the atmosphere.

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This kind of eruption can clog jet engines with clouds of tiny, glasslike shards. The 2010 eruptions of Iceland's Eyjafjallajökull volcano, for example, led to the largest closing of air traffic since World War II, and temporarily stranded millions of passengers.

The results of the new study should help volcanologists better understand what's going on beneath volcanoes, and may one day even help predict what sort of eruption to expect from a certain volcano.

"If you live near a volcano, the pyroclastic flow is more dangerous," Dufek said. "If you're far away, your concern is more about the fine-grained ash getting up into the atmosphere."  

Reach Douglas Main at Follow him on Twitter @Douglas_Main. Follow OurAmazingPlanet on Twitter @OAPlanet. We're also on Facebook and Google+.

© 2012 OurAmazingPlanet. All rights reserved. More from OurAmazingPlanet.

Interactive: Anatomy of a volcano

Explainer: Eight dangerous volcanoes around the world

  • Carlos Gutierre  /  Reuters file

    After 9,000 years of dormancy, the Chaiten volcano in southern Chile awoke in 2008 and began a series of eruptions that spewed ash miles into the sky, as shown in this image. The volcano's namesake town of 4,500, just 6 miles from the spewing crater, was devastated by falling ash and floods. The eruption claimed at least one life and serves as a stark reminder that slumbering volcanoes pose grave dangers. Click on the "Next" label to learn about seven more dangerous volcanoes around the world.

  • Is Mount Vesuvius the most dangerous?

    Courtesy of Digital Globe

    Italy's Mount Vesuvius is most famous for the A.D. 79 eruption that buried the cities of Pompeii and Herculaneum. Despite the dark history, millions of people today live near the volcano. The thriving mass of humanity in such close proximity to the volcano makes Vesuvius a serious contender for the world's most dangerous volcano. Scientists fear that a catastrophic eruption could hurl scalding gas-rich magma, water vapor and debris at the masses with insufficient warning time for an evacuation.

  • In Mexico, they wonder when 'Popo' will blow

    Joel Merino  /  AP file

    Mexico City, a metropolis of 18 million people, sits 40 miles to the east of Popocatepetl, the second tallest volcano in North America. Puebla, a town of 2 million, lies 30 miles to the west. A major eruption, scientists say, could choke the skies with ash and send massive mudslides into the crowded valleys below. The result could prove catastrophic. The volcano has been relatively quiet since a bout of activity between 1920 and 1922, though it rumbled back to life in 2000, as shown in this image, prompting evacuation orders and worries that "Popo" is ready to blow.

  • Will spirits warn when Merapi is ready to go?

    Afp  /  AFP/Getty Images

    Merapi in Indonesia is one of the world's most active volcanoes, regularly spewing hot gas and ash miles into the sky, and sending mud and fragmented rocks down the sides. In 1994, 60 people were killed by a searing gas cloud, and about 1,300 people died when it erupted in 1930. During a bout of eruptions in 2006, many villagers, including the woman in this picture, refused orders to evacuate. They believe the spirits will warn when a catastrophic eruption is imminent.

  • Nyirangongo threatens with fast moving lava

    Image: Aerial view of Nyirangongo
    NASA file

    Lava flows, while hot, are rarely deadly: They usually ooze slow enough that people can easily outrun them. That's not the case with the lava that flows from Nyirangongo in Africa's Democratic Republic of Congo. It has very low levels of silica, the mineral that thickens and slows lavas. In 2002, Nyirangongo's lava suddenly gushed at speeds up to 60 mph into the town of Goma, which is home to half a million people. Scientists fear that lava pooling in the crater could suddenly drain again and cause even more devastation.

  • Will disaster of Nevada del Ruiz be repeated?

    R. J. Janda  /  USGS

    After nearly a year of minor earthquakes and eruptions, Colombia's Nevada del Ruiz volcano exploded on Nov. 13, 1985. Pyroclastic flows melted the summit's snowcap. Mudflows, called lahars, raced down the mountainside. One mudflow wiped out the village of Chinchina and killed 1,927 people, according to reports. A second followed the same path as earlier lahars and swept away the town of Armero, shown in this image. An estimated 23,000 people died, making it Colombia's worst natural disaster. Scientists said an early warning system could have averted the loss of life. Now that one is in place, will it work when the volcano wakes again?

  • Is majestic Mount Fuji overdue for an eruption?

    Shizuo Kambayashi  /  AP file

    The islands of Japan harbor more than 100 volcanoes, and a handful or so erupt every year. The majestic Mount Fuji, shown here, has not erupted since 1707, but a swarm of low-frequency earthquakes in 2000 and 2001 raised the specter that the mountain was awakening from its 300-year slumber. Though Fuji has since quieted down, the risk to Tokyo, a city of 30 million people just 70 miles to the east, is very real, scientists say. A 2004 government study put the price tag of a worst-case eruption at more than $20 billion.

  • Mount Rainier, an attractive danger

    Lyn Topinka / USGS

    Washington's 14,410-foot-tall Mount Rainier, shown in this image, is a big attraction for many people in the Pacific Northwest. It is also a big threat, according to scientists. An estimated 3 million people live in its shadow — at least 100,000 on top of old mudflows from previous eruptions. The flows, known as lahars, are the greatest risk. Though commonly associated with major eruptions that strike with ample warning, an earthquake or small burp of rock, ash and gas could also trigger a lahar, giving residents in the path only 10 to 15 minutes to escape.


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