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. Because 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 in the Earth's surface. Magma that has erupted is called lava.

Some volcanic eruptions are explosive and others are not. How explosive an eruption is depends on how runny or sticky the magma is. 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.

Explore additional information about volcanic eruptions.

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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 belt of volcanoes on the ocean floor. About 500 of these have erupted in historical time. Many of these are located along the Pacific Rim in what is known as the 'Ring of Fire.' In the United States, volcanoes in the Cascade Range and Alaska (Aleutian volcanic...

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 17, 2017

EarthWord–Phreatic Eruption

This EarthWord is straight up steampunk...

Date published: May 16, 2017

EarthWord–Lahar

Which sounds more dangerous, lava or mud? The answer may surprise you...

Date published: August 22, 2016

EarthWord–Tephra

Look! In the sky! It’s a bird, it’s a plane! Wait, run, it’s this week’s EarthWord!

Date published: February 1, 2016

EarthWord – Nuée Ardente

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

Filter Total Items: 21
Pyroclastic flows descend the south eastern flank of Mayon Volcano, Philippines, during its 1984 eruption
April 19, 2016

Pyroclastic flows descend the south eastern flank of Mayon Volcano, Philippines, during its 1984 eruption

Pyroclastic flows descend the south eastern flank of Mayon Volcano, Philippines, during its 1984 eruption. Credit: USGS

Lassen Peak Eruption
April 18, 2016

Lassen Peak Eruption

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.

Video Sections:

  • Volcanoes: Monitoring Volcanoes
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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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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.

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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