Volcanoes: Facts

U.S. Geological Survey/photo by J. D. Griggs

Volcanoes can be found all over the earth. They are also located on other bodies in our Solar System. They have played a role in creating land, contributing to fertile soil for growing food, making amazing history and also many disasters. Volcanoes are fascinating to scientists who study them. Are you fascinated by volcanoes, too?

What is a volcano?

U.S. Geological Survey/photo by J. D. Griggs

A volcano is a vent from which the melted rocks under the surface of the earth's crust can escape the heat and pressure that has built up. But to understand volcanoes, first we need to learn a little bit about the structure of the earth itself. Let's look at the earth from the very center outward…

The Inner Core

Deep down inside the center of the earth, lies a solid mass known as the inner core. This inner core is hot – very hot – so hot that it is actually melted rock. But because of its location, this melted rock is pressed by the weight of all of the layers above it, into a solid mass. Scientists have no way of measuring the exact temperature of this layer. This inner core is made up of iron and nickel. It rotates to create our magnetic field.


The Outer Core

The outer core surrounds the inner core. It is very similar to the inner core, but is actually liquid rather than a solid. This layer is also made of iron and nickel. It too, rotates – perhaps faster than the earth rotates in space.

The Mantle

The next layer out from the center is the mantle. This layer is filled with what is called magma. Magma is a thick layer of melted rock and minerals. The mantle layer is the deepest and largest of the layers. The magma layer is always in a state of motion caused by heating and cooling. It heats up along the core layer and then rises towards the surface where it begins to cool. The cooler matter begins to sink back towards the hot core again and the cycle repeats. This is called convection. These hot and cool areas are all still hot by our standards – they are melted rock, after all!!


The Crust

The crust is the outer most layer of the earth. It is where we live. It is the completely cooled magma covered by rocks and soil, plants and animals, and lots of water. The crust is a solid and is broken into giant chunks that float on the mantle – sort of like sponges floating in the sink. These chunks are called tectonic plates. The plates are thicker where the mountains exist and thinner under the ocean. They float and bump together because of the motion created underneath them by the convection. They can crash into one another with such force that early in the history of the earth, they formed many mountains by forcing rocks upward in the collision. The earth's surface looked a whole lot different in its early history. Our continents rest on these plates. Once upon a time, the continents were collected together in a single continent known as Pangaea. For more information about Pangaea you can visit the USGS website. The plates continue to move on the mantle and are the reason for earthquakes and yes, volcanoes!!


Because of the intense heat under the crust and the convection moving the plates all over the surface of the mantle, the earth is in constant change. The place on the crust where the plates meet can often be an area of weakness in the crust's surface. In some of these places, the crust can slide under another plate, or the plates can be pulled apart. It is here that the magma and gases can erupt through the crust. The heating of the magma is like a pot ready to boil over; causing huge amounts of pressure. When the magma finds a weak spot in the crust, it tries to escape and magma breaks through the surface. It might squeeze out in a slow stream or it might be more violent and explode in a huge spray of melted rock. Either way, it's still called a volcano.

U.S. Geological Survey/photo by E. Klett

Magma or Lava?

U.S. Geological Survey/sketch by B. Myers

The melted rock is called magma. It can be varying degrees of runny – from thick like peanut butter to thinner like syrup, depending on many conditions. This is called its viscosity. But one thing is true – once it leaves the mantle layer it is no longer called magma. Once this material comes to the surface of the earth, it is then called lava. The word magma comes from the Greek for “kneaded mass.” Lava is Italian from the word for “avalanche” and means to slide down.

U.S. Geological Survey/photo by J. D. Griggs

Lava either moves as a slow flow of liquid material or it can be thicker and have edges that have begun to cool in a rocky-like texture. When lava has cooled completely, it is easy to see these two forms. They are known by the terms pahoehoe (pronounced – pa-hoy-hoy) and a'a (pronounced ah-ah). These terms come from Hawaii. Both types of lava can sometimes be found at the same location and coming from the same eruption.

Pahoehoe Lava

Pahoehoe lava is formed from obviously runny, almost gooey material. It has a rope-like or bubbly quality to it with folds and wrinkles in it. One can picture it oozing as it moved. The cooled material often contains many gas bubbles that are round in appearance and has a frothy texture inside.

U.S. Geological Survey/photo by T. N. Mattox

A'a Lava

Department of the Interior/USGS

A'a lava is jagged and rough. It has a more massive appearance and probably crashed through the area with all the grace of a wild animal. It may have taken much longer for it to cool. It tends to lack the gas bubbles found in the pahoehoe lava and if it does, they are not round, but misshapen or stretched. A'a often has crystals formed inside.

Pillow Lava

NOAA PMEL Vents Program

When lava is released underwater it creates what is known as pillow lava. Pillow lava is the most common of all of the formations. Because pillow lava cools quickly in the water, a crust forms over the eruption. The lava continues to fill the crusty pillow and the pillow grows in size until it breaks open like an overfilled balloon. Lava will ooze out and form another pillow.

Kinds of Volcanoes


When we think of a volcano, most of us envision the pointed cone-shaped mountain from dinosaur cartoons. But the truth is, volcanoes come in a number of different forms and shapes.

Composite Volcano

Department of the Interior/USGS

The composite volcano is created from years and years of eruptions and cooling which can add layers that form the conic shape. Composite volcanoes have a series of internal vents where magma rises to the surface. The magma can reach the surface from the central vent or from one of its many secondary vents where it will escape through a crack in the side of the volcano. This is the one most of us picture in our minds when we think “volcano.”

Cinder Volcano

Cinder Cone
U.S. Geological Survey/photo by J. P. Lockwood

Some volcanoes spew cinders; blobs of lava that explode into the air and cool as they fall to the ground. The cinder volcano is the result of cinders piling up over many eruptions to create a volcanic mountain. Cinder volcanoes have a circular or oval vent opening and tend to be shorter than composite volcanoes.

Shield Volcano

Sheild Volcano
U.S. Geological Survey/photo by D. Little

Shield volcanoes never build the mountainous structure, but pour lava out over a large flat area. This flattened layer can stretch for miles. Idaho is famous for its shield volcanic remains located at Black Butte Crater. Visit their website to learn more.

Lava Dome Volcano

U.S. Geological Survey/photo by C. Newhall

Lava domes are unique in that the magma never actually flows, but instead, pushes up from beneath the volcano and rises in a bulbous form. The material is too thick to flow so it simply sits there and cools in a dome shape.

Learn more about the various kinds of volcanoes at PBS's NOVA.

U.S. Geological Survey/photo by C. Newhall

Volcanoes do not just release magma. Gases, ash, cinders, dust and steam can be a part of an eruption. These can be just as deadly as a lava flow. Gases can contain poisonous chemicals and be heated to intolerable temperatures. Cinders can rain down on inhabited areas setting fire to buildings and trees. Ash, dust and steam can roll through the air in a cloud known as pyroclastic flow. Pyroclastic material fills the air with a thick, hot blanket that cuts off oxygen to all life in the nearby area. Pyroclastic material has been known to get caught in the jet stream, rise up into the atmosphere and cycle around the searth several times from some eruptions.

Inside a Volcano


Scientists who study volcanoes are known as volcanologists. Many are so intrigued by the way volcanoes behave, that they are willing to subject themselves to the intense heat of volcanoes. They will dress in special heat resistant suits to go as close to an eruption as they can. Volcanologists have learned a lot about what goes on inside of a volcano. They have a good understanding of the interior structure found in volcanoes and can use this knowledge to predict some future eruptions.

While no one can see below the surface of a volcano, much has been learned using complex equipment and measuring devices. Read more about some of the work involved in understanding volcanoes from the U.S. Geological Survey (USGS).

No two volcanoes are the same, but many have classic structure similarities. A central vent is found in many composite and cinder cone volcanoes. This vent is a direct route from the magma found in the mantle to the surface of the crust. Often there is a cavity – known as a magma chamber or magma reservoir – that fills with magma in the lower layers of the crust. The chamber is trapped and needs to expand due to the heat and pressure it is under. As it expands, magma is pushed up the central vent. This expansion can cause bulging of the surface. That might be the land above a - not as yet formed - volcano or it might mean that the sides of a previously active volcano swell. Often the magma does not stay in the central vent, but finds weak places along its path to try and break through. These are secondary vents and can erupt and make additional volcanic cones. The opening at the top of a vent is known as a crater and can be round, oval or even odd shaped.

When the central vent erupts, the lava can flow down the sides of the volcano, cool, erupt again and again to create layers that add to the shape and size of the volcano cone. Steam, ash, cinders and clouds of smoke can also erupt from the vents. Many volcanoes start out as a rupture in the earth's surface, erupt multiple times, cool, erupt again, cool again, and at some point in time, die altogether. This final stage is known as a dormant volcano. The likelihood of a dormant volcano coming back to life – although it has happened – is slim. Dormant volcanoes can become extinct – or one that scientists don't ever expect will erupt again. Extinct volcanoes often have cooled volcanic rock plugging their vents. Given enough time (like millions of years) volcanoes have been found in which the outer cone has worn away with erosion and weathering only to leave a pillar of volcanic rock. This pillar is the plug that had existed inside the volcanic cone, but is now exposed. Devil's Tower in Wyoming is a great example of such a landform.

Department of the Interior/USGS

Volcanoes are often grouped together in hot spots on the earth's surface. These can be found along the plate boundaries. Sometimes the history of the volcanoes can be identified by these collections of hot spots. The crust can move over the hot spots causing a volcano to form and later die. As the hot spot shifts to a new location, another volcano erupts and dies. If enough of these happen, a path can be seen as to the historic progress of the eruptions. At the end of the path, there is usually an active volcano or two. This progress can be witnessed on the islands of Hawaii and along the west coast of the United States.

Ring of Fire

Ring of Fire
Department of the Interior/USGS

Along the rim of the Pacific Ocean lies an area called the Ring of Fire. The tectonic plates of the Pacific Ocean join here, where 90% of the earth's volcanoes exist and about 81% of the earth's largest earthquakes. The plates slip under one another along this general area causing a lot of geologic activity. There are a number of volcanoes under the ocean along this rim, too.

Volcanoes in the Northwest

U.S. Geological Survey/photo by Tom Casadevall

Because the Ring of Fire runs along the west coast of the United States, many volcanoes can be found in the states of Alaska, Washington, Oregon and California. Most of these volcanoes are inactive, however Mt. Saint Helen's is the most recently active. In May of 1980 it erupted with enormous power, sending clouds of ash over several states leaving a huge chunk of the volcano to slide down its side. The volcano has continued to be active off and on with small explosions and seismic activity. Check out the details of Mt. Saint Helen's activity over a 19 year period.

Alaska is also has an active volcanic region too. At least 6 volcanoes in Alaska have shown activity in the last 100 years.

Southern Idaho was once the site of ancient volcanic activity. Craters of the Moon National Monument is all that remains of this prehistoric activity. The volcanic left overs have created amazing soil that assists farmers who grow crops of potatoes, barley, sugar beets, fruit and more. This soil also is beneficial to raising sheep and cattle.

You might want to learn more about the volcanoes of the Northwest by visiting the USGS site on this subject.

Department of the Interior/USGS

The Hawaiian Islands are famous for their five active Volcanoes; Kilauea, Mauna Loa, Haleakala, Hualalai, and Lo'ihi. Hawaii has one dormant volcano called Mauna Kea. Hawaii has an observatory for watching and studying volcanic activity which sits near the rim of Kilauea. Learn more about the observatory. Hawaii also had an underwater observatory for studying Lo'ihi which is an underwater volcano. This observatory is called HUGO. Scientists have recorded the sound made by the eruptions of Lo'ihi. You can listen to those sounds online – there are many to choose from.


U.S. Geological Survey/photo by S. R. Brantly

Geysers are also a product of volcanic activity. Water heats below the surface of the crust when it comes near magma. The heating can bring water temperatures to boiling. If there is a place for the boiling water to escape, it will. The water and steam will shoot up in a column of hot water spray. This is a geyser.

Geysers are always located in hot spots on the earth. Additional hydrothermal activity is sure to be close by such as steaming hot springs, bubbling pots of mud, growing pools of colorful heat-loving algae and other unique features. To learn more about geysers visit Yellowstone National Park where more than half of all of the earth's geysers exist.

Predicting Volcano Eruptions

Department of the Interior/USGS

Volcanologists work to try and predict volcanic eruptions. They study volcanoes, make measurements of earthquake activity, temperature and gas composition to try to know if and when an eruption will take place. They visit erupting volcanoes to study the lava material; taking samples to examine in laboratories. Their work can be very exciting, but also very dangerous. Volcanologists also make maps, measure gravity, take photographs, and study the changes in the geology and plant life of the land around a volcano. If you thought being a volcanologist might be in your future, visit Oregon State University's website for more information.


Volcanologists study ancient eruptions too in order to learn everything they can about the causes and activities of volcanoes. One historic eruption that is fascinating to study is the eruption of Mt. Vesuvius in Italy. In the year 79 AD Mt. Vesuvius erupted killing nearly everyone who lived in the city of Pompeii. It happened with such violence and with little warning that no one had time to escape. Scientists have dug through the volcanic material to find remains of the artwork, furniture, food, and people who lived so long ago.

Volcanoes Around the Solar System

Io's volcano - image courtesy of NASA

Earth is not the only place where volcanoes exist. Extinct volcanoes have been found all over the Solar System. But only on the moons of several planets have scientists found active volcanoes that erupt currently. NASA has an interactive Space Volcano Explorer where you can click on the places that volcanoes have been found to learn more.

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