A volcano is a feature in Earth’s crust where molten rock is squeezed out onto the Earth’s surface. This molten rock is called magma when it is beneath the surface and lava when it erupts, or flows out, from a volcano. Along with lava, volcanoes also release gases, ash, and, solid rock.

Volcanoes come in many different shapes and sizes but are most commonly cone-shaped hills or mountains. They are found throughout the world, forming ridges deep below the sea surface and mountains that are thousands of meters high. About 1,900 volcanoes on Earth are considered active, meaning they show some level of occasional activity and are likely to erupt again. Many others are dormant volcanoes, showing no current signs of exploding but likely to become active at some point in the future. Others are considered extinct.

Volcanoes are incredibly powerful agents of change. Eruptions can create new landforms, but can also destroy everything in their path. About 350 million people (or about one out of every 20 people in the world) live within the “danger range” of an active volcano. Volcanologists closely monitor volcanoes so they can better predict impending eruptions and prepare nearby populations for potential volcanic hazards that could endanger their safety.

Plate Tectonics

Most volcanoes form at the boundaries of Earth’s tectonic plates. These plates are huge slabs of Earth’s crust and upper mantle, which fit together like pieces of a puzzle. These plates are not fixed, but are constantly moving at a very slow rate. They move only a few centimeters per year. Sometimes, the plates collide with one another or move apart. Volcanoes are most common in these geologically active boundaries.

The two types of plate boundaries that are most likely to produce volcanic activity are divergent plate boundaries and convergent plate boundaries.

Divergent Plate Boundaries 

At a divergent boundary, tectonic plates move apart from one another. They never really separate because magma continuously moves up from the mantle into this boundary, building new plate material on both sides of the plate boundary.

The Atlantic Ocean is home to a divergent plate boundary, a place called the Mid-Atlantic Ridge. Here, the North American and Eurasian tectonic plates are moving in opposite directions. Along the Mid-Atlantic Ridge, hot magma swells upward and becomes part of the North American and Eurasian plates. The upward movement and eventual cooling of this buoyant magma creates high ridges on the ocean floor. These ridges are interconnected, forming a continuous volcanic mountain range nearly 60,000 kilometers (37,000 miles)—the longest in the world.

Another divergent plate boundary is the East Pacific Rise, which separates the massive Pacific plate from the Nazca, Cocos, and North American plates.

Vents and fractures (also called fissures) in these mid-ocean ridges allow magma and gases to escape into the ocean. This submarine volcanic activity accounts for roughly 75 percent of the average annual volume of magma that reaches Earth’s crust. Most submarine volcanoes are found on ridges thousands of meters below the ocean surface.

Some ocean ridges reach the ocean surface and create landforms. The island of Iceland is a part of the Mid-Atlantic Ridge. The diverging Eurasian and North American plates caused the eruptions of Eyjafjallajökull (in 2010) and Bardarbunga (in 2014). These eruptions were preceded by significant rifting and cracking on the ground surface, which are also emblematic of diverging plate movement.

Of course, divergent plate boundaries also exist on land. The East African Rift is an example of a single tectonic plate being ripped in two. Along the Horn of Africa, the African plate is tearing itself into what is sometimes called the Nubian plate (to the west, including most of the current African plate) and the Somali plate (to the east, including the Horn of Africa and the western Indian Ocean). Along this divergent plate boundary are volcanoes such as Mount Nyiragongo, in the Democratic Republic of Congo, and Mount Kilimanjaro, in Kenya.

Convergent Plate Boundaries

At a convergent plate boundary, tectonic plates move toward one another and collide. Oftentimes, this collision forces the denser plate edge to subduct, or sink beneath the plate edge that is less dense. These subduction zones can create deep trenches. As the denser plate edge moves downward, the pressure and temperature surrounding it increases, which causes changes to the plate that melt the mantle above, and the melted rock rises through the plate, sometimes reaching its surface as part of a volcano. Over millions of years, the rising magma can create a series of volcanoes known as a volcanic arc.

The majority of volcanic arcs can be found in the Ring of Fire, a horseshoe-shaped string of about 425 volcanoes that edges the Pacific Ocean. If you were to drain the water out of the Pacific Ocean, you would see a series of deep canyons (trenches) running parallel to corresponding volcanic islands and mountain ranges. The Aleutian Islands, stretching from the U.S. state of Alaska to Russia in the Bering Sea, for instance, run parallel to the Aleutian Trench, formed as the Pacific plate subducts under the North American plate. The Aleutian Islands have 27 of the United States’ 65 historically active volcanoes.

The mighty Andes Mountains of South America run parallel to the Peru-Chile Trench. These mountains are continually built up as the Nazca plate subducts under the South American plate. The Andes Mountains include the world’s highest active volcano, Nevados Ojos del Salado, which rises to 6,879 meters (over 22,500 feet) along the Chile-Argentina border. 

Hot Spots

For many years, scientists have been trying to explain why some volcanoes exist thousands of kilometers away from tectonic plate boundaries.

The dominant theory, framed by Canadian geophysicist J. Tuzo Wilson in 1963, states that these volcanoes are created by exceptionally hot areas fixed deep below Earth’s mantle. These hot spots are able to independently melt the tectonic plate above them, creating magma that erupts onto the top of the plate.

In hot spots beneath the ocean, the tectonic activity creates a volcanic mound. Over millions of years, volcanic mounds can grow until they reach sea level and create a volcanic island. The volcanic island moves as part of its tectonic plate. The hot spot stays put, however. As the volcano moves farther from the hot spot, it goes extinct and eventually erodes back into the ocean. A new and active volcano develops over the hot spot, creating a continuous cycle of volcanism—and a string of volcanic islands tracing the tectonic plate’s movement over time.

For Wilson and many scientists, the best example of hot spot volcanism is the Hawaiian Islands. Experts think this volcanic chain of islands has been forming for at least 70 million years over a hot spot underneath the Pacific plate. Of all the inhabited Hawaiian Islands, Kauai is located farthest from the presumed hot spot and has the most eroded and oldest volcanic rocks, dated at 5.5 million years. Meanwhile, on the “Big Island” of the U.S. state of Hawai‘i—still fueled by the hot spot—the oldest rocks are less than 0.7 million years old and volcanic activity continues to create new land.

Hot spots can also create terrestrial volcanoes. The Yellowstone Supervolcano, for instance, sits over a hot spot in the middle of the North American plate, with a series of ancient calderas stretching across southern part of the U.S. state of Idaho. The Yellowstone hot spot fuels the geysers, hot springs, and other geologic activity at Yellowstone National Park, Wyoming, United States.

While some data seem to prove Wilson’s hot spot theory, more recent scientific studies suggest that these hot spots may be found at more shallow depths in the planet's mantle and may migrate slowly over geologic time rather than stay fixed in the same spot.

Principal Types of Volcanoes 

While volcanoes come in a variety of shapes and sizes, they all share a few key characteristics. All volcanoes are connected to a reservoir of molten rock, called a magma chamber, below the surface of Earth. When pressure inside the chamber builds up, the buoyant magma travels out a surface vent or series of vents, through a central interior pipe or series of pipes. These eruptions, which vary in size, material, and explosiveness, create different types of volcanoes.

Stratovolcanoes 

Stratovolcanoes are some of the most easily recognizable and imposing volcanoes, with steep, conic peaks rising up to several thousand meters above the landscape. Also known as composite volcanoes, they are made up of layers of lava, volcanic ash, and fragmented rocks. These layers are built up over time as the volcano erupts through a vent or group of vents at the summit’s crater.

Mount Rainier is an impressive stratovolcano that rises 4,392 meters (14,410 feet) above sea level just south of the U.S. city of Seattle, Washington. Over the past half million years, Mount Rainier has produced a series of alternating lava eruptions and debris eruptions. These eruptions have given Mount Rainier the classic layered structure and conic shape of a composite volcano. The volcano’s peak has also been carved down by a series of glaciers, giving it a craggy and rugged shape.

Volcan de Fuego and Acatenango are a pair of stratovolcanoes that stand more than 3,700 meters (12,000 feet) above sea level near Antigua, Guatemala. While the volcanoes are considered twins because of their similar shape and size, they are made of different types of lava and have distinct eruption histories. While Acatenango erupts infrequently today, Fuego is considered to be the most active volcano in Central America, erupting more than 60 times since 1524.

Shield Volcanoes 

Shield volcanoes are built almost exclusively of lava, which flows out in all directions during an eruption. These flows, made of highly fluid basalt lava, spread over great distances and cool in thin layers. Over time, the layers build up and create a gently sloping dome that looks like a warrior’s shield. While they are not as eye-catching as their steep stratovolcano cousins, shield volcanoes are often much larger in volume because of their broad, expansive structure.

Shield volcanoes make up the entirety of the Hawaiian Islands. The Kilauea and Mauna Loa shield volcanoes, located on the “Big Island” of Hawai‘i, rise from the ocean floor more than 4,500 meters (15,000 feet) below sea level. The summit of Mauna Loa stands at 4,168 meters (13,677 feet) above sea level and more than 8,500 meters (28,000 feet) above the ocean floor, making it the world’s largest active volcano—and, by some accounts, the world’s tallest mountain. The smaller volcano, Kilauea, has been erupting continuously since 1983, making it one of the world’s most active volcanoes. 

The Galapagos Islands are also made up of a series of shield volcanoes. Isabela and Fernandina islands have flatter tops than other shield volcanoes because lava erupts from fissures around their tops and along ridges at their bases. As a result, the volcanoes rise at the top and grow outward at the bottom—but not in the middle, making them look like an “inverted soup bowl.”

Pyroclastic Cones

Pyroclastic cones are the most prolific type of volcano on Earth. They can develop as part of stratovolcanoes, shield volcanoes, or independently. Also known as cinder cones, they form after violent eruptions blow lava into the air. In the atmosphere, the lava fragments solidify and fall as “cinders” around a singular vent. Often formed from a single eruption or short series of eruptions, pyroclastic cones only stand at heights of tens of meters to hundreds of meters.

Parícutin, Mexico, is a unique pyroclastic cone. It was the first volcano to be studied for its entire life cycle. Emerging from a cornfield in 1943, Parícutin’s explosive eruptions caused it to reach 80 percent of its height of 424 meters (1,391 feet) during its first year of activity. In that time, lava and ash buried the nearby town of San Juan. Over the next eight years, Parícutin built the remainder of its cone—and then went quiet. Geologists learned a great deal about the evolution of volcanoes in Parícutin’s short, nine-year life.

Lava Domes

Lava domes are like shield volcanoes in that they are built entirely of lava. This lava, however, is too thick and sticky to move great distances. It just piles up around the volcano vent. Lava domes are often found on the summit or flanks of a volcano, but they can also develop independently. Like pyroclastic cones, they only reach a few hundred meters, as they are formed during singular eruptions or slow lava releases.

One of the most iconic lava domes developed after the devastating 1902 eruption of Mount Pelée on the island of Martinique. For almost a year, a lava dome grew out of a summit crater created from the eruption, reaching a height of more than 300 meters (1,000 feet). Known as the Tower of Pelée, the obelisk-shaped structure was twice the height of the Washington Monument. It ultimately collapsed into a pile of rubble after 11 months of growth.

Other Important Volcanic Features 

Calderas

Some volcanoes experience such large, explosive eruptions that they release most of the material in their magma chamber. This causes the land around the erupting vent or vents to collapse inwardly, creating circular depressions called calderas. Depending on their intensity and duration, volcanic eruptions can create calderas as much as 100 kilometers wide.

Crater Lake, Oregon, United States, is in a caldera about 10 kilometers (six miles) wide. Crater Lake’s caldera resulted from an eruption that occurred more than 7,000 years ago. The volcano's magma chamber collapsed, then filled with water from rain and snow, creating the lake. Crater Lake is the deepest lake in the United States. 

Deception Island, located off the coast of Antarctica, experienced a violent eruption roughly 10,000 years ago. The volcano summit collapsed, forming a caldera seven kilometers (4.4 miles) wide and flooded with seawater. The caldera gives Deception Island its horseshoe shape, which opens to the sea through a narrow channel. Deception Island’s unique geologic structure makes it one of the only places in the world where ocean vessels can sail directly into an active volcano.

Craters

Much like calderas, craters are depressions left after a volcano experiences a large eruption. While calderas are formed by the collapse of material inside a volcano, craters are formed as materials explode out from a volcano. Craters are usually much smaller than calderas, only extending to a maximum of about one kilometer (0.62 mile) in diameter.

Many volcanoes have multiple craters caused by different eruptions. The Maly Semiachik volcano, located on the Kamchatka Peninsula in far eastern Russia, has six craters at its summit. The youngest of these craters, Troitsky, filled in with water and snowmelt, creating a lake 140 meters (459 feet) deep. The lake is highly acidic, as volcanic gases continue to be released into the water from the active volcano below.

Lava lakes are also found in volcanic craters. Erta Ale, a volcano in Ethiopia, has a lava lake in its summit crater. Lava lakes are where magma has bubbled up to the surface and pooled in a crater. Volcanologists can fly over Erta Ale’s summit crater to see how the lava lake is behaving and predict future behavior.

Types of Volcanic Eruptions

Volcanic eruptions are as diverse as volcanoes themselves—which is to say, very diverse! Some of the ways volcanologists have classified these eruptions are based on the heights they reach, the types of materials they eject, and the explosiveness of these ejections.

Hawaiian 

Hawaiian eruptions are the calmest eruption type. They are characterized by steady lava eruptions known as lava fountains or fire fountains. Lava fountains are able to reach heights of up to two kilometers (1.2 miles). The highly fluid lava associated with Hawaiian eruptions flows easily away from the volcano summit, often creating fiery rivers and lakes of lava within depressions on the surrounding landscape.

These eruptions are named after the Hawaiian Islands, where they most often occur. Kilauea, which has been erupting continuously since 1983, has produced lava flows covering more than 100 square kilometers (37 square miles) on the island of Hawai‘i. These flows continuously destroy houses and communities in their path, while also adding new coastline to the island. 

Strombolian 

Strombolian eruptions are characterized by short-lived outbursts of lava rather than steady fountaining. The lava is thicker and has a higher gas content than that of Hawaiian eruptions. Large gas bubbles rise from the magma chamber, pushing the pasty lava upward until the bubbles explode at the summit vent. These explosions can reach heights up to 10 kilometers (6.2 miles) although most don’t go higher than a few hundred meters into the air.

Strombolian eruptions are named after the Mediterranean island of Stromboli, Italy. Considered by many to be the most active volcano on Earth, Stromboli has been erupting almost continuously for 2,000 years. The island’s eruptions are almost always Strombolian in nature: Small gas explosions eject blobs of lava into the air a couple of times per hour. 

Vulcanian

Vulcanian eruptions are short-lived but much more explosive than Strombolian eruptions. Very thick lava causes gas pressure to build up in the magma chamber. When this pressure is finally released it creates canon-like explosions that can travel faster than 350 meters per second (800 miles per hour). Lava, rock, and ash are propelled up to 20 kilometers (12.4 miles) in the air, although most eruption columns are between five and 10 kilometers high. These plumes of material have the ability to drift moderate distances away from the eruption site.

The 2013 vulcanian eruption of Sakurajima, on the island of Kyushu, Japan, covered the nearby city of Kagoshima in a thick coat of ash.

Plinian

Reaching as high as 50 kilometers (35 miles) in the atmosphere, Plinian eruptions are the largest of all eruption types. Much like vulcanian eruptions, they eject materials at speeds of hundreds of meters per second. Plinian eruptions, however, are more sustained than the coughing fits of vulcanian eruptions. These consistent eruptions result from the volcano’s magma and growing gas bubbles rising at a similar velocity.

Plinian eruptions are the most destructive type of eruption. They release a deadly mixture of lava, ash, and volcanic rocks such as scoria and pumice, which can fall kilometers away from the eruption site. They are also characterized by pyroclastic flow, a fluid mixture of fragmented materials and extremely hot, toxic gases.

In 79 C.E., a series of Plinian eruptions from Mount Vesuvius buried the nearby Roman cities of Pompeii and Herculaneum (in what is today Italy). The cities and their 13,000 inhabitants were buried in volcanic ash and rock. Rainfall mixed with the ash and created a concrete-like substance that preserved the city for thousands of years.

Surtseyan

Surtseyan eruptions occur where magma or lava interacts with water, most often when an undersea volcano reaches the ocean surface. Another term for this sort of interaction is a phreatomagmatic eruption. When heated rapidly by lava, water flashes to steam and expands violently, creating the most explosive of all eruption types. This aggressive interaction between water and heat is able to fragment lava into very fine grains of ash that can reach heights of 20 kilometers (12.4 miles).

Tonga’s islands of Hunga Tonga and Hunga Ha'apai are actually the tops of a single, large underwater volcano. In 2009, the volcano erupted for several days, causing steam and ash to explode from the water to altitudes of five kilometers (3.1 miles). While the eruptions killed all signs of wildlife on and around the islands, it also added hundreds of square meters of land to Hunga Ha’apai.

Volcanic Hazards

Volcanoes are some of Earth’s most potent natural hazards and agents of change. They release enormous amounts of energy and material, engaging natural processes that can modify landscapes at a local, regional, and even global scale.

Many volcanic materials and processes pose a threat to human, animal, and other ecological communities. 

Volcanic Gas

Volcanoes regularly release volcanic gases that can be dangerous at concentrated levels. Carbon dioxide and fluorine can collect in soil or volcanic ash, causing crop failure, animal death and deformity, and human illness.

Volcanic eruptions can also release massive amounts of sulfur dioxide, which rises into the stratosphere. There, it reflects incoming solar radiation while absorbing outgoing land radiation, leading to a cooling of Earth’s temperature.

In extreme cases, these “volcanic winters” can cause crop failures and drastically affect weather. The 1815 eruption of Mount Tambora, Indonesia, cooled the average global temperature by as much as 3° Celsius (5.4° Fahrenheit), causing the “year without a summer.”

Landslides and Lahars

The enormous energy of volcanic eruptions can cause large landslides that move at speeds of more than 100 kilometers per hour (60 miles per hour). Mount St. Helens, Washington, is a stratovolcano that had an explosive Plinian eruption in 1980. The eruption produced the largest landslide in recorded history, covering a 36-kilometer (14-mile) area of land with ash and rocks. Reaching speeds of 50 to 80 meters (165 to 260 feet) per second, the landslide had enough power to surge over a ridge 400 meters (1,312 feet) high.

Landslides can mix with surrounding rivers, ice, snow, or rain to produce watery mixtures called lahars. This mixture of water, rock, and debris creates a sludge that can obliterate almost anything in its path. The 1985 eruption of Nevado del Ruiz, Colombia, caused small lahars of rock, ash, and melted snow to flow down into the surrounding river valleys. The lahars gained momentum and size as they traveled the riverbeds, ultimately destroying more than 5,000 homes and killing more than 23,000 people.

Pyroclastic Flows

Explosive eruptions sometimes produce pyroclastic flows, a mixture of hot rock fragments and toxic gases that move almost like a liquid out and away from the volcano. Reaching speeds greater than 80 kilometers per hour (50 miles per hour) and temperatures between 200-700° Celsius (392-1292° Fahrenheit), pyroclastic flows knock down, shatter, bury, or burn anything in their path.

Pyroclastic flows are responsible for the haunting figures from Pompeii and Herculaneum, Italy. While many scientists thought residents of Pompeii suffocated to death from volcanic gases released during Mount Vesuvius’ eruption in 79 C.E., new studies suggest that they actually died from extreme heat produced by the volcano’s pyroclastic flow. 

Volcanologist Giuseppe Mastrolorenzo and the Italian National Institute for Geophysics and Volcanology recently discovered that the pyroclastic flow that reached Pompeii produced temperatures of up to 300° Celsius (570° Fahrenheit). These extreme temperatures are able to kill people in a fraction of a second, causing them to spasm in contorted postures like those found among the plaster casts of Vesuvius’ victims.

Volcanic Ash 

Huge plumes of volcanic ash can spread over large areas of the sky, turning daylight into complete darkness and inhibiting air traffic. (During the eruption of Iceland’s Eyjafjallajökull in 2011, flights to and from Northern Europe were suspended for more than a week.) Volcanic ash conducts electricity when wet and can contain concentrated levels of toxic materials, posing threats to humans that come in close contact with it on land.

The 1994 double eruption of Vulcan and Tavurvur in Papua New Guinea covered the nearby city of Rabaul in a layer of ash up to 75 centimeters (about two feet) deep. Rains turned the ash into a cement-like substance that was heavy enough to collapse 80 percent of the buildings in the city.

Volcanic Monitoring and Research

Volcanic hazards can be incredibly dangerous to human life. In the United States alone, 54 volcanoes are a very high or high threat to public safety. By closely monitoring volcanic activity, volcanologists can warn people of impending eruptions. While these warnings are not exact predictions, they do provide communities with the valuable time they need to protect themselves against volcanic hazards and ensure their safety.

Volcanologists predict volcanic activity by taking real-time measurements and comparing them against what happened in the past. They use a variety of instruments and technologies to monitor temperatures, gas emissions, water levels, ground movements, and changes in the landscape. These measurements paint a clear portrait of a volcano’s current state, which volcanologists then interpret against historical data. Volcanologists issue eruption warnings when these measurements stray far from the norm or mirror those that preceded a historic eruption.

Different countries use different systems to issue eruption warnings to the public. All of these systems categorize their alerts based on the probability and severity of an impending eruption. Typical volcanic behavior is often represented by the number 1 or the color green, while an imminent and potentially destructive eruption is typically represented by the number 4 or the color red.

A number of international organizations lead the way in volcanic monitoring and research, providing invaluable information to scientists, volcanologists, and the public alike. The Smithsonian Institution’s Global Volcanism Program documents current activity for all the volcanoes on the planet through publicly available data, reports, and images. The program also keeps the world’s only archive of volcanic activity from the last 10,000 years.

As part of the UN International Decade for Natural Disaster Reduction, the International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) created a list of 16 "Decade Volcanoes" to study because of the high risk they pose to public safety. Mount Nyiragongo in Democratic Republic of the Congo, for example, is dangerously close to the city of Goma. Its 2002 eruption killed 50 people and forced roughly 450,000 people to evacuate their communities. The Santa María Volcano, which sits right above the city of Quetzaltenango, Guatemala, has been continuously erupting since 2003. The Decade Volcanoes program has brought together geologists, volcanologists, and government officials to closely study these volcanoes and create risk-mitigation plans for potential eruptions.