Volcano Facts

Volcanoes ['väl-'kā-nō-z]

An opening in the surface of the earth through which lava, gases and ash are forced out to form a mountain when cooled.

Bromo Volcano, Java Island Indonesia

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 causing disasters. Volcanoes are fascinating to scientists who study them. Are you fascinated by volcanoes, too?

What is a volcano?

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…

Volcano diagram

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 outermost 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!!

Earths Layers


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.

Volcanic Eruption

Magma or Lava?

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.

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.

Lava flow

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 have a frothy texture inside.


A'a Lava

A'a lava is jagged and rough, made up of broken lava blocks called clinkers. It is formed from thick, fast-moving lava flows that cool into massive piles of spiky fragments. A'a lava tends to lack the gas bubbles found in pahoehoe lava and often has crystals formed inside. A'a lava is sharp, unstable, and very difficult to walk on.

A'a Lava

Pillow Lava

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.

Pillow lava

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

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.”

Composite Volcano

Cinder Volcano

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.

Cinder Volcano

Shield Volcano

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.

Shield Volcano

Lava Dome Volcano

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.

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 earth several times from some eruptions.

Lava Dome Volcanos

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 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).

Lava Sampling

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-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, and then 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.

Devils Tower, Wyoming

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

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.

Ring of fire

Volcanoes in the Northwest

Mt. Saint Helens
Mt. Saint Helens

In the last 200 years, more than 50 volcanoes in the United States have erupted one or more times. 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 Helens erupted in May of 1980, after 123 years of inactivity. It erupted with enormous power, sending clouds of ash over several states and causing 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 Helens' activity over a 19-year period.

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

Craters of the moon
Craters of the moon

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 leftovers have created amazing soil that assists farmers who grow crops of potatoes, barley, sugar beets, fruit, and more. This soil also is beneficial for raising sheep and cattle.

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

Volcanoes in Hawaii

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, which is also the world's tallest mountain when measured from its base on the ocean floor to its summit. Volcanoes National Park is on the "big island" of Hawaii, and there is an observatory for watching and studying volcanic activity which sits near the rim of Kilauea. Learn more about the Hawaiian Volcano Observatory. Hawaii also has an underwater observatory called HUGO for studying Lo'ihi, which is an underwater volcano. Scientists have recorded the sounds made by the eruptions of Lo'ihi which you can listen to online.

Volcanoes national park

In May 2018, Kilauea became very active, with eruptions, flowing lava, multiple earthquakes, and explosions from its summit crater. Some neighborhoods were destroyed by lava, and lava even evaporated a freshwater lake on the island! You can follow scientists' daily monitoring of Kilauea's volcanic activity.


Geysers are also a product of volcanic activity. The 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

Volcanologists study volcanoes and make measurements of earthquake activity, temperature, and gas composition to try to determine 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 University of Oregon's website for more information.

Volcanologist sampling gases at Mount Baker

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. Another famous eruption occurred in 1883 on the island of Krakatoa in Indonesia, with explosions so powerful they could be heard up to 2,000 miles away. The eruption affected the global climate and caused temperatures to drop all over the world.

Volcanoes Around the Solar System

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.

Top 10 Questions

November 2015

Thanks to Jeffrey Johnson, associate research professor of geophysics, Boise State University; and Brittany Brand, assistant professor in the department of geophysics, Boise State University for the answers.

  1. Why do volcanoes erupt?

    Magma is a liquid. It is stored within the crust and because it is a liquid, it is less dense than the surrounding rock. Therefore, it is going to rise buoyantly through the crust toward the surface. When it gets to the surface, the pressure inside that intrusion of magma gets so high that it finally exceeds the ability of the rock to hold it in. It's like filling up a water balloon too full. Eventually it's going to pop and open. So essentially, volcanoes erupt when the lava, or the magma, gets so close to the surface that it punches out. (From Natalie at Cole Valley Christian School in Boise)

  2. How many active volcanoes are there in the world?

    Active volcanism is defined as a volcano that has erupted in the last 10,000 years. This means there are 1,500 active volcanoes, and about 500 of those have erupted during historic times. Today, there are about 20 volcanoes world wide that are in eruption. (From Ashland at Longfellow Elementary School in Boise)

  3. What makes lava rise?

    We call magma when it's in the ground, "molten rock." "Lava," is what we call it when it erupts onto the ground. Magma rises like a hot air balloon rises. This is because it's less dense than the surrounding rock. It rises through the crust until it makes its way to the surface to erupt. (From Alex at Edahow Elementary School in Pocatello)

  4. What are the signs of a volcano when it's about to erupt?

    Volcanologists are very interested in understanding and forecasting volcanic eruptions. They look at four main things to understand if a volcano is getting ready to erupt. They watch the earthquake activity underneath the ground. They study the deformation of a volcano. They also check to see if the volcano is growing in size. Finally, they look at the gasses and temperature coming out of a volcano. These four things together help us determine if a volcano may soon erupt. (From Emma at Lewis and Clark Elementary School in Pocatello)

  5. How far do volcanoes shoot lava?

    If lava is coming out of a volcano, it doesn't shoot up into the sky. It flows across the ground. When lava, or magma, becomes explosive, it's because it has gas in it. The gases expand and rip the lava apart, shooting up into the air. It can go anywhere from a couple hundred feet all the way up to 30 miles into the atmosphere. (From Nathan at River Valley Elementary School in Meridian)

  6. How hot is liquid magma?

    Liquid magma is really hot. In order to melt rock and create magma or lava, you need to have temperatures of at least 1,500 degrees Fahrenheit. Some lava is as hot as 2,200 degrees Fahrenheit. (From Ryan at Cynthia Mann Elementary School in Boise)

  7. How many times can a volcano erupt?

    A volcano can erupt as long as it has a supply of magma from depth. So, really it can erupt many times. They can erupt continuously for years, or they can erupt over the course of a few days and then stop for many hundreds of years. It really depends on the characteristics of the specific volcano. (From Jeff at Cynthia Mann Elementary School in Boise)

  8. How many different kinds of lava are there?

    There are more types of lava on earth than there are flavors of ice cream. You can have lava that is very thick and sticky. This lava doesn't flow very easily and might just pile up inside a volcanic crater. Lava can also be really fluid and flow quite far from the volcano. Two styles of lava that you might be familiar with are pahoehoe and a'a. These lavas have the same composition, but one of them, pahoehoe, moves very slowly across the ground and forms smooth surfaces. A'a, on the other hand, usually moves faster and tumbles around forming really rough surfaces. (From Nick at Anser Charter School in Boise)

  9. How do people know when a volcano is extinct?

    We can't be absolutely certain that a volcano won't erupt again. Generally, when a volcano hasn't erupted for 10,000 years, we declare that volcano to be dead. However, in many cases, we don't know that the volcano hasn't erupted for that many years. In other cases, a volcano that has been quiet for 10,000 years may awaken. There have been plenty of examples of volcanoes that were thought to be extinct that have erupted. (From Adrianna at Cynthia Mann Elementary School in Boise)

  10. Could volcanoes be a renewable thermal energy source?

    Volcanic areas are one of the premier examples of renewable energy resources. A common misconception is that the geothermal energy that comes from volcanic areas taps into the actual magma, and this is not true. We tap into the hot ground surrounding the volcanic area. One country that does an exceptionally good job with harvesting geothermal energy from volcanoes is Iceland. (From Kai at North Junior High School in Boise)