How Do Volcanoes Erupt?

Deep within the Earth it is so hot that some rocks slowly melt and become a thick flowing substance called magma. Since it is lighter than the solid rock around it, magma rises and collects in magma chambers. Eventually, some of the magma pushes through vents and fissures to the Earth's surface. Magma that has erupted is called lava.

Some volcanic eruptions are explosive and others are not. The explosivity of an eruption depends on the composition of the magma. If magma is thin and runny, gases can escape easily from it. When this type of magma erupts, it flows out of the volcano. A good example is the eruptions at Hawaii’s volcanoes. Lava flows rarely kill people because they move slowly enough for people to get out of their way. If magma is thick and sticky, gases cannot escape easily. Pressure builds up until the gases escape violently and explode. A good example is the eruption of Washington’s Mount St. Helens. In this type of eruption, the magma blasts into the air and breaks apart into pieces called tephra. Tephra can range in size from tiny particles of ash to house-size boulders.

Explosive volcanic eruptions can be dangerous and deadly. They can blast out clouds of hot tephra from the side or top of a volcano. These fiery clouds race down mountainsides destroying almost everything in their path. Ash erupted into the sky falls back to Earth like powdery snow. If thick enough, blankets of ash can suffocate plants, animals, and humans. When hot volcanic materials mix with water from streams or melted snow and ice, mudflows form. Mudflows have buried entire communities located near erupting volcanoes.

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How hot is a Hawaiian volcano?

Very hot!! Here are some temperatures recorded at different times and locations: The eruption temperature of Kīlauea lava is about 1,170 degrees Celsius (2,140 degrees Fahrenheit). The temperature of the lava in the tubes is about 1,250 degrees Celsius (2,200 degrees Fahrenheit). The tube system of episode 53 (Pu'u O'o eruption) carried lava for...

How dangerous are pyroclastic flows?

A pyroclastic flow is a hot (typically >800 °C), chaotic mixture of rock fragments, gas, and ash that travels rapidly (tens of meters per second) away from a volcanic vent or collapsing flow front. Pyroclastic flows can be extremely destructive and deadly because of their high temperature and mobility. For example, during the 1902 eruption of...

Is it dangerous to work on volcanoes? What precautions do scientists take?

Volcanoes are inherently beautiful places where forces of nature combine to produce awesome events and spectacular landscapes. For volcanologists, they're FUN to work on! Safety is, however, always the primary concern because volcanoes can be dangerous places. USGS scientists try hard to understand the risk inherent in any situation, then train...

Will extinct volcanoes on the East Coast of the U.S. erupt again?

No. Through plate tectonics, the eastern U.S. has been isolated from the global tectonic features (tectonic plate boundaries and hot spots in the mantle), that cause volcanic activity. So new volcanic activity is not possible now or in the near future. If you wait around several hundred million years, maybe... Remnants of past volcanism are found...

Where is the largest active volcano in the world?

Rising gradually to more than 4 km above sea level, Hawaii’s Mauna Loa is the largest volcano on our planet. Its submarine flanks descend to the sea floor an additional 5 km (3 mi), and the sea floor in turn is depressed by Mauna Loa's great mass another 8 km (5 mi). This makes the volcano's summit about 17 km (10.5 mi) above its base!

What is the difference between "magma" and "lava"?

Scientists use the term magma for molten rock that is underground and lava for molten rock that breaks through the Earth's surface.

What kind of school training do you need to become a volcanologist?

There are many paths to becoming a volcanologist. Most include a college or graduate school education in a scientific or technical field, but the range of specialties is very large. Training in geology, geophysics, geochemistry, biology, biochemistry, mathematics, statistics, engineering, atmospheric science, remote sensing, and related fields can...

What are some benefits of volcanic eruptions?

Over geologic time, volcanic eruptions and related processes have directly and indirectly benefited mankind. Volcanic materials ultimately break down and weather to form some of the most fertile soils on Earth, cultivation of which has produced abundant food and fostered civilizations. The internal heat associated with young volcanic systems has...

Do volcanoes affect weather?

Yes, volcanoes can affect weather and the Earth's climate . Following the 1991 eruption of Mount Pinatubo in the Philippines, cooler than normal temperatures were recorded worldwide and brilliant sunsets and sunrises were attributed to this eruption that sent fine ash and gases high into the stratosphere, forming a large volcanic cloud that...

How many active volcanoes are there on Earth?

There are about 1,500 potentially active volcanoes worldwide, aside from the continuous belts of volcanoes on the ocean floor at spreading centers like the Mid-Atlantic Ridge . About 500 of those 1,500 volcanoes have erupted in historical time. Many of those are located along the Pacific Rim in what is known as the " Ring of Fire ." In the United...

Can an eruption at one volcano trigger an eruption at another nearby volcano?

There are a few historic examples of simultaneous eruptions from volcanoes or vents located within about 10 km of each other, but it's very difficult to determine whether one eruption caused the other. To the extent that these erupting volcanoes or vents have common or overlapping magma reservoirs and hydrothermal systems, magma rising to erupt...
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Date published: May 6, 2019

The National Volcano Early Warning System (NVEWS) will help USGS better monitor nation’s most dangerous volcanoes

In September 2004, USGS scientists detected sudden, but unmistakable, signs that Mount St. Helens was waking up. Volcano monitors had picked up the occurrence of hundreds of small earthquakes and other signals that the volcano’s crater floor had begun to rise. Within a week, several eruptions blasted clouds of ash into the atmosphere, and soon after, a new lava dome emerged in the crater.

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EarthWord – Nuée Ardente

A nuée ardente is a turbulent, fast moving cloud of hot gas and ash erupted from a volcano. 

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May 20, 2018

Kīlauea Volcano — Slow Moving Lava Flow

Video of a slow moving lava flow in Kīlauea Volcano's lower East Rift Zone, taken May 20, 2018, at around 2:31 AM HST. The flow is ~3 m (9 ft) high. The HVO

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Tephra blasted from the summit vent on Saturday night included lith...
August 8, 2016

Tephra blasted from the summit vent on Saturday night included lith...

Tephra blasted from the summit vent on Saturday night included lithic (solid rock) fragments from the vent wall as well as spatter (molten lava fragments) ejected from the lava lake. The light-colored lithic in the center of this photo is about 20 cm (8 in) long—the GPS unit is shown for scale. Tephra, the general term for volcanic rock fragments exploded or carried into

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video thumbnail: Volcano Hazards
July 30, 2012

Volcano Hazards

The United States has 169 active volcanoes. More than half of them could erupt explosively, sending ash up to 20,000 or 30,000 feet where commercial air traffic flies. USGS scientists are working to improve our understanding of volcano hazards to help protect communities and reduce the risks.

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  • Volcanoes: Monitoring Volcanoes
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Attribution: Volcano Hazards
video thumbnail: The 20th Century's Greatest Volcanic Eruption- Mt Katmai 100 Years Later 
June 5, 2012

The 20th Century's Greatest Volcanic Eruption- Mt Katmai 100 Years Later 

Bill Burton discusses the June 6-8, 1912 eruption of Mount Katmai in Alaska which was 30 times larger than the eruption of Mt. St. Helens in 1980. This eruption caused widespread devastation, and inspired heroic efforts at survival by the local people. Burton returns to this topic a century later and explains what lessons the Mount Katmai eruption provides for modern-day

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Attribution: Natural Hazards
Image: Northeastern Vent of the Kamoamoa Eruption
March 8, 2011

Northeastern Vent of the Kamoamoa Eruption

View looking down onto the northeastern vent.

video thumbnail: Lava Fountaining from a Dominant Vent
March 6, 2011

Lava Fountaining from a Dominant Vent

Video showing low fountaining from the dominant vent, near the southwest end of the fissure system adjacent to Napau Crater, active during the day on March 7.

Attribution: Natural Hazards
Image: Fissure Eruption
March 6, 2011

Fissure Eruption

This fissure began in the early hours of March 6, erupting spatter and producing lava flows.

USGS
February 24, 2009

How do volcanoes erupt?

Listen to hear the answer.

video thumbnail: Halema'uma'u Explosive Eruption (October 12, 2008)
October 11, 2008

Halema'uma'u Explosive Eruption (October 12, 2008)

On October 12, 2008, an explosive eruption, shown in this video, blasted lithic and juvenile tephra onto the Halema'uma'u crater rim 85 meters (280 feet) above the informally-named Overlook vent (see http://hvo.wr.usgs.gov/kilauea/timeline/ for links describing eruptive activity at the summit of Kilauea Volcano). It

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Attribution:
video thumbnail: Lava Fountaining from MLK Vent (February 9-10, 2005)
February 8, 2006

Lava Fountaining from MLK Vent (February 9-10, 2005)

On February 9, 2005, an eruptive surge at Pu'u 'O'o resulted in episodic spattering and fountaining from the MLK vent, on the southwestern flank of the Pu'u 'O'o cone. The main cone active during this event was 6-7 meters (20-23 feet) high. This suggests that fountain heights reached about 10 meters (33 feet). The time-lapse camera was positioned on the south flank of the

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Attribution:
A gas plume arising from Augustine Volcano during it's eruptive phase 2005-06.
January 24, 2006

A gas plume arising from Augustine Volcano during it's eruptive phase 2005-06.

A gas plume arising from Augustine Volcano during it's eruptive phase 2005-06. This photo was taken during  a FLIR/maintenance flight on January 24, 2006.

Attribution: Natural Hazards, Alaska