What is a volcano?
A volcano is an opening in the Earth's surface through which lava, volcanic ash, and gases escape. Ancient people believed that volcanoes were under the control of the gods. In fact, the word volcano comes from Vulcan, the Roman god of fire.
Currently there are about 1,500 potentially active volcanoes worldwide. 169 of these are in the United States.
Video courtesy of Spotmatik Ltd / ShutterstockVolcanic eruptions can cause death and widespread devastation. They can also bring benefits and provide:
- Valuable mineral deposits like gold, aluminium, and nickel.
- Nutrient-rich fertile soils from volcanic ash.
- Geothermal energy—much of Iceland’s energy comes from geothermal sources.
- Lava flows that create land—in 2018, a new island appeared off the coast of Hawaii as a result of lava from Kilauea eruptions.
Anatomy of a Volcano
All volcanoes have some things in common:
Did you know?
What is the difference between magma and lava?
- Magma is molten rock stored in the Earth's crust.
- Lava is molten rock that has reached the Earth’s surface through a volcanic vent.
Where volcanoes are found
The lithosphere is the outermost layer that surrounds the Earth. It consists of the crust and part of the mantle. This is broken into extremely large slabs called tectonic plates. For example, the North American Plate (includes most of North America, Greenland, and part of Siberia) is 75,900,000 square kilometers (29,305,000 square miles). These move around on the ductile layer beneath.
Look at the map above to see how the Earth’s surface is divided into seven major plates. Volcanoes are mostly (but not always) found where these tectonic plates meet, called the plate boundaries.
About 75 percent of the Earth’s active volcanoes are in the Ring of Fire. This 40,000-kilometer (25,000-mile) chain of volcanoes and seismically active sites circles the edges of the Pacific Ocean.
Video courtesy of National Geographic PartnersHow volcanoes form
We already know that volcanoes are formed mostly, but not always, at the boundaries of tectonic plates. Let’s look at the mechanisms behind this.
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Cleveland Volcano, Aleutian Islands.
Image credit: NASA
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The islands of Hawaii and Samoa were formed in this way. Plates move about 5 centimeters a year so this is a slow process.
Satellite view of Hawaii archipelago (USA).
Image credit: NASA
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Hills covered by volcanic ashes, Great Rift Valley, Tanzania, East Africa.
Image credit: Aghezzi -
Thingvellir Valley and Lake Thingvallavatn, Thingvellir National Park, Thingvellir, Iceland.
Image credit: Emory Kristof
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Volcanoes of Bromo National Park, Java, Indonesia.
Image credit: Manamana
Types of volcano
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Video courtesy of Wead / Shutterstock
Cinder Cone
Cinder cone volcanoes are relatively small, steep, cone-shaped hills made up mostly of partly burned ash and lava cinders. Cinder cones tend to be explosive volcanoes but they can also produce lava.
Although larger volcanoes form very slowly, a cinder cone can develop rapidly. A good example is the Paricutín volcano in Mexico, which grew from a crack in a corn field to a cone over 300 meters (984 feet) high over the course of one year in the 1940s. Cinder cones also have a shorter lifespan than slower-growing types of volcano.
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Cinder Cone
Photograph by J. Baylor Roberts
Cinder cone volcanoes are relatively small, steep, cone-shaped hills made up mostly of partly burned ash and magma cinders. Cinder cones tend to be explosive volcanoes but they can also produce lava.
Although larger volcanoes form very slowly, a cinder cone can develop rapidly. A good example is the Paricutín volcano in Mexico, which grew from a crack in a corn field to a cone over 300 meters (984 feet) high over the course of one year in the 1940s. Cinder cones also have a shorter lifespan than slower-growing types of volcano.
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Cinder Cone
Photograph by Saro17 / Getty Images
Cinder cone volcanoes are relatively small, steep, cone-shaped hills made up mostly of partly burned ash and magma cinders. Cinder cones tend to be explosive volcanoes but they can also produce lava.
Although larger volcanoes form very slowly, a cinder cone can develop rapidly. A good example is the Paricutín volcano in Mexico, which grew from a crack in a corn field to a cone over 300 meters (984 feet) high over the course of one year in the 1940s. Cinder cones also have a shorter lifespan than slower-growing types of volcano.
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Video courtesy of Rick Ray
Shield
Shield volcanoes get their name from their shape which resembles a warrior's shield lying on the ground. The shape comes from streams of lava that flow from a summit vent or group of vents and then cool down, forming smooth, gentle slopes. Their summits are nearly flat.
Examples are Kilauea in Hawaii and Piton de la Fournaise on Reunion Island in the Indian Ocean. Areas of the world with the most shield volcanoes include Hawaii, the Galapagos Islands, and Iceland.
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Shield
Photography by USGS / Getty Images
Shield volcanoes get their name from their shape which resembles a warrior's shield lying on the ground. The shape comes from streams of lava that flow from a summit vent or group of vents and then cool down, forming smooth, gentle slopes. Their summits are nearly flat.
Examples are Kilauea in Hawaii and Piton de la Fournaise on Reunion Island in the Indian Ocean. Areas of the world with the most shield volcanoes include Hawaii, the Galapagos Islands, and Iceland.
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Shield
Photography by CORBIS / Getty Images
Shield volcanoes get their name from their shape which resembles a warrior's shield lying on the ground. The shape comes from streams of lava that flow from a summit vent or group of vents and then cool down, forming smooth, gentle slopes. Their summits are nearly flat.
Examples are Kilauea in Hawaii and Piton de la Fournaise on Reunion Island in the Indian Ocean. Areas of the world with the most shield volcanoes include Hawaii, the Galapagos Islands, and Iceland.
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Video courtesy of fitopardo / Getty Images
Composite
Composite volcanoes, also known as stratovolcanoes, are steep-sided, symmetrical cones formed when pyroclastic eruptions (explosions of ash, cinders, and rock fragments) alternate with lava flows and layers build up over time. They usually have a central vent or a cluster of vents in a crater at their summit.
Mount Fuji in Japan, Mount St. Helens in Washington State, and Mount Etna in Italy are all composite volcanoes.
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Composite
Photograph by pxhidalgo
Composite volcanoes, also known as stratovolcanoes, are steep-sided, symmetrical cones formed when pyroclastic eruptions (explosions of ash, cinders, and rock fragments) alternate with lava flows and layers build up over time. They usually have a central vent or a cluster of vents in a crater at their summit.
Mount Fuji in Japan, Mount St. Helens in Washington State, and Mount Etna in Italy are all composite volcanoes.
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Composite
Photograph by Fomin Sergey / EyeEm
Composite volcanoes, also known as stratovolcanoes, are steep-sided, symmetrical cones formed when pyroclastic eruptions (explosions of ash, cinders, and rock fragments) alternate with lava flows and layers build up over time. They usually have a central vent or a cluster of vents in a crater at their summit.
Mount Fuji in Japan, Mount St. Helens in Washington State, and Mount Etna in Italy are all composite volcanoes.
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Video courtesy of BBC Universal / Getty Images
Caldera
A caldera is a large oval or circular hollow that can form soon after an eruption. If the magma chamber has emptied over a short period of time, it won't be able to support the weight of the mountain above it. This leads to a collapse above the magma chamber and the caldera forms.
The caldera may be encircled by steep cliffs and filled with lakes. Calderas can also form on composite volcanoes and on the flat tops of shield volcanoes.
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Caldera
Photograph by Cavan Images
A caldera is a large oval or circular hollow that can form soon after an eruption. If the magma chamber has emptied over a short period of time, it won't be able to support the weight of the mountain above it. This leads to a collapse above the magma chamber and the caldera forms.
The caldera may be encircled by steep cliffs and filled with lakes. Calderas can also form on composite volcanoes and on the flat tops of shield volcanoes.
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Caldera
Photograph by National Parks Service
A caldera is a large oval or circular hollow that can form soon after an eruption. If the magma chamber has emptied over a short period of time, it won't be able to support the weight of the mountain above it. This leads to a collapse above the magma chamber and the caldera forms.
The caldera may be encircled by steep cliffs and filled with lakes. Calderas can also form on composite volcanoes and on the flat tops of shield volcanoes.
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Video courtesy of Stefan Schuetz / Getty Images
Lava Dome
Lava domes are formed when very thick lava erupts through the volcano’s vent and then piles up around it. Like lava flows, there isn’t enough pressure for the volcano to erupt explosively. But unlike lava flows, where the faster-moving lava can flow for miles, the lava is so thick that it doesn't flow very far.
These steep mounds can grow to become several hundred meters high and be more than 1,000 meters (3,280 feet) in diameter.
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Lava Dome
Photograph by Ulet Ifansasti / Getty Images
Lava domes are formed when very thick lava erupts through the volcano’s vent and then piles up around it. Like lava flows, there isn’t enough pressure for the volcano to erupt explosively. But unlike lava flows, where the faster-moving lava can flow for miles, the lava is so thick that it doesn't flow very far.
These steep mounds can grow to become several hundred meters high and be more than 1,000 meters (3,280 feet) in diameter.
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Lava Dome
Photograph by Mike Poland / USGS
Lava domes are formed when very thick lava erupts through the volcano’s vent and then piles up around it. Like lava flows, there isn’t enough pressure for the volcano to erupt explosively. But unlike lava flows, where the faster-moving lava can flow for miles, the lava is so thick that it doesn't flow very far.
These steep mounds can grow to become several hundred meters high and be more than 1,000 meters (3,280 feet) in diameter.
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Video courtesy of Ocean Today / Smithsonian Institution / NOAA
Submarine
There are considerably more volcanoes underwater than on land. Most eruptions on the seafloor occur from fissures, for example along mid-ocean ridges and around Iceland. Other eruptions form cone-shaped seamounts or sea mountains.
A seamount will occur where magma erupts through the seafloor and forms new seafloor. If the seamount grows out of the ocean it is known as a volcanic island. Hawaii is an example of a volcanic island.
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Submarine
Photograph by NOAA / NSF / WHOI
There are considerably more volcanoes underwater than on land. Most eruptions on the seafloor occur from fissures, for example along mid-ocean ridges and around Iceland. Other eruptions form cone-shaped seamounts or sea mountains.
A seamount will occur where magma erupts through the seafloor and forms new seafloor. If the seamount grows out of the ocean it is known as a volcanic island. Hawaii is an example of a volcanic island.
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Submarine
Photograph by NOAA Okeanos Explorer Program, INDEX-SATAL 2010
There are considerably more volcanoes underwater than on land. Most eruptions on the seafloor occur from fissures, for example along mid-ocean ridges and around Iceland. Other eruptions form cone-shaped seamounts or sea mountains.
A seamount will occur where magma erupts through the seafloor and forms new seafloor. If the seamount grows out of the ocean it is known as a volcanic island. Hawaii is an example of a volcanic island.
Did you know?
Composite volcano eruptions are the most catastrophic. Lava pours out from fissures or cracks in the volcano's walls, strengthening the cone and forming a seal as it hardens. Pressure gradually builds up within the cone and the result is a violent explosion of pyroclastic material from the volcano's vents.
How volcanoes erupt
We know that volcanic eruptions can be
- fairly calm lava flows, calm enough for scientists to stand near and observe, or
- violently explosive, powerful enough to blow mountains apart.
What decides how violent the eruption will be?
This is decided by the physical properties of the magma:
- Viscosity: how thick or runny it is
- Gas contents: the amount of dissolved gas in it
Viscous magma
Viscosity describes how runny the magma or lava is. Viscosity is affected by the amount of silica (SiO2 also known as silicon dioxide) in the magma.
Silica molecules form long chains in the magma; these get tangled together, making it difficult for the magma to slide past itself. The more silica, the more viscous the magma, the slower it will move.
Dissolved gases
In the Earth's crust the magma is at a higher pressure than at the Earth's surface, so it is possible for gases (water vapour and carbon dioxide) to be dissolved in it.
As lava moves up towards the Earth's surface, the gas tries to escape because the pressure on it is decreasing. Think about what happens when you shake a bottle of soda!
Did you know?
The 1883 eruption of Krakatoa measured 6 out of 8 on the Volcanic Explosion Index with an estimated force comparable to 200 megatons of TNT (trinitrotoluene, an explosive). The atomic bomb that devastated Hiroshima had a force of 20 kilotons. Krakatau was nearly ten thousand times more explosive than the bomb.
Types of Eruption
The following summarizes how the properties of viscosity and the amount of gases dissolved in the magma control what kind of eruption happens and what kind of volcano results.
Select each eruption type to find out more about it.
Fire fountains result in cinder cone volcanoes.
Low viscosity of magma
High dissolved gas
Fire-fountain spewing lava into the air 
Explosive with fluid lava
Mount Etna lava fountains in October 2013.
Video courtesy of Wead / Shutterstock
Explosive eruptions result in composite or stratovolcanoes.
- High viscosity of magma
- High dissolved gas
- Lava explodes out of the volcano
- Explosive eruption with hard pyroclastic material (fragments of rock)
Eruption at Anak Krakatau volcano in afternoon light, Krakatoa, Indonesia, November 2010.
Video courtesy of James Reynolds / Getty Images
Lava flows result in shield volcanoes.
- Low viscosity of magma
- Low dissolved gas
- Fast-moving streams of lava
- Effusive with highly fluid lava flows
Video compilation from helicopter overflight of Kīlauea volcano's lower East Rift Zone on June 6, 2018, around 6:30 a.m. The fissure 8 lava fountain feeds a channelized lava flow that travels northeast around the Kapoho cone, then flows toward the south to enter the ocean at Kapoho Bay and Vacationland. The ocean entry has completely filled Kapoho Bay with lava, building a delta that extends 0.8 miles from shore.
Video courtesy of United States Geological Survey
Slow-moving lava results in lava dome volcanoes.
- High viscosity of magma
- Low dissolved gas
- Slow-moving lava
- Effusive, with slow-moving lava that piles up
The images that make up this movie were acquired by a time-lapse camera positioned on the southeastern rim of the Pu‘u ‘Ō‘ō crater, about 125 meters (137 yards) from the erupting vent, July 13, 2007. Lava can be seen occasionally overtopping levees that formed along the edges of the lava lake.
Video courtesy of United States Geological Survey
