Volcano is a place where ash, gases, and molten rock from deep underground erupt onto the surface. The word volcano also refers to the mountain of erupted rock and ash that often accumulates at such a place.
Volcanic eruptions result from magma (molten rock below the ground). Magma usually forms 30 to 120 miles (50 to 200 kilometers) beneath Earth’s surface. It rises because it is pushed upward by underground gases. Rising magma can collect below or inside a volcano in a region called a magma chamber. As the magma accumulates, the pressure inside the chamber increases. When the pressure becomes too great, the chamber breaks open, and magma rises in the volcano. If magma reaches the surface, an eruption occurs. The hole through which the magma erupts is called a vent.
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If magma accumulates at a high enough rate, the volcano erupts almost continuously. With magma that accumulates more slowly, the eruption may halt for periods while new magma replaces that which has erupted.
The violence of an eruption depends largely on the amount of gas dissolved in the magma and the magma’s viscosity (resistance to flow). Magmas with little gas produce relatively calm eruptions in which lava flows quietly onto the surface. Magmas with much gas can shoot violent jets of gas and ash high into the air. Viscous (thick and sticky) magmas tend to erupt more violently than runnier, more fluid magmas. Water mixing with the erupting magma can make any eruption more explosive.
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Volcanoes can create many dangers. Hot ash, gas, lava, and mud can bury or burn people and buildings near an erupting volcano. The most violent eruptions launch large clouds of ash and gas high into the atmosphere, causing problems far from the volcano itself.
Volcanoes also provide benefits. Erupted materials contain many nutrients and can break down to form fertile soils. Volcanic activity provides an important source of geothermal energy, energy from Earth’s interior heat. Geothermal energy can power electric generators and heat water and buildings. Undersea volcanoes have built up over time to form islands on which millions of people live. Volcanoes also have inspired myths and legends in many cultures. The word volcano comes from Vulcan, the Roman god of fire. Scientists who study volcanoes are called volcanologists.
This article discusses volcanic eruptions, the dangers of volcanoes, where volcanoes form, the types of volcanoes, and how scientists study volcanoes. Most of the article deals with volcanoes on Earth. For information on volcanoes elsewhere, see the section Volcanoes in the solar system at the end of this article.
Volcanic eruptions
Volcanic eruptions differ in their violence and in the materials they produce. Some eruptions involve calm outpourings of lava. Other eruptions produce powerful explosions and large volumes of rock, ash, and gas.
Products of eruption.
During an eruption, a variety of materials can come from a volcanic vent. These include lava, pyroclasts (rock fragments), and gases.
Lava
is magma that flows onto Earth’s surface. As lava spreads from a vent, parts of it begin to cool and harden. The resulting stream of lava and rock is called a lava flow. Lava flows vary greatly in appearance depending on their viscosity, temperature, and rate of advance.
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Fluid lava flows spread easily from the vent. Two common types of fluid lava flows are pahoehoe << `PAH` hoy hoy or pah `HOY` hoy >> and aa << ah AH or ah ah >>. Pahoehoe flows have smooth, glassy surfaces and wavy, ropelike ridges. They form when hot, fluid lava advances relatively slowly. Aa flows have rough, broken surfaces. They form when less fluid lava advances rapidly. Pahoehoe and aa are Hawaiian terms adopted by most volcanologists.
Highly viscous lavas cannot flow easily. They pile up around the vent to form thick mounds called lava domes or short, stubby flows with rugged surfaces. These domes and flows advance extremely slowly.
Pyroclasts,
also called pyroclastics, form when fragments of magma are thrown into the air by expanding gas. More explosive eruptions tend to produce finer pyroclasts. Pyroclasts that settle to the ground can cement together to form a rock called tuff.
Volcanologists often classify pyroclasts by their size. The finest pyroclasts, dust-sized and sand-sized grains, make up volcanic ash. Volcanologists call pebble-sized pyroclasts lapilli. Rock-sized and boulder-sized fragments are known as volcanic bombs.
Volcanologists also classify pyroclasts by texture. Pumice, a lightweight pyroclast, contains many tiny cavities left behind by gas bubbles in the magma. The cavities trap air, enabling some pumice to float on water. Cinder, also called scoria, another pyroclast, also has many tiny cavities, but it does not float on water. Pumice and cinder come from vigorous eruptions that hurl magma fragments high into the air. They solidify before landing, often forming a loose pile around the vent called a cinder cone or pumice cone.
Spatter, a fluid pyroclast, comes from less vigorous eruptions. Blobs of spatter do not fly high and they land while still molten. Spatter collects around vents in steep structures called spatter cones and spatter ramparts.
Gases
from volcanic eruptions include water vapor, carbon dioxide, and sulfur dioxide. Deep underground, the gases are dissolved in the magma. As magma rises, the pressure it is under decreases. The gases come out of solution to form bubbles and may eventually escape.
The violence of an eruption
depends on the amount of gas dissolved in the magma and the magma’s viscosity. Magmas rich in gas develop many bubbles as they rise to the vent. The bubbles increase the pressure in the vent, causing a more explosive eruption. Viscous magmas resist the expansion of bubbles, leading to a buildup of pressure in the magma. When the pressure of the bubbles finally overcomes the magma’s viscosity, an explosive eruption occurs. In more fluid magmas, the bubbles expand without building up excess pressure. The resulting eruptions are relatively mild.
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When external water, such as groundwater or sea water, mixes with magma, the water rapidly turns to steam, expanding in the process. This increases the violence of an eruption. Some volcanologists call eruptions involving external water hydromagmatic eruptions.
As gases and pyroclasts erupt from a volcano, they draw in and heat some of the surrounding air. The heated air, gases, and pyroclasts form an eruption column. If an eruption draws in enough air and heats it sufficiently, the eruption column becomes lighter than the surrounding air. As a result, the column floats upward in a process called convective rise. Convective rise can carry erupted gas and ash high into the atmosphere. Some eruptions do not draw in enough air or heat it sufficiently to produce convective rise. In these eruptions, erupted materials remain closer to the ground.
The dangers of volcanoes
Volcanoes can endanger people, wildlife, and property. Volcanic disasters are much more difficult to avoid once an eruption begins. Instead, volcanologists and disaster planners strive to identify and evacuate dangerous areas before eruptions occur. Most damage results from (1) lava flows, (2) pyroclastic hazards, (3) lahars, (4) dangerous gases, (5) avalanches and landslides, and (6) tsunamis.
Lava flows.
Many people fear being buried by lava, but lava flows rank as the least dangerous volcanic hazard. Lava usually advances at less than 6 miles (10 kilometers) per hour, slow enough for people and animals to escape. Unusually fluid lavas can flow fast enough to be dangerous, but this rarely occurs. However, lava flows can burn and bury buildings, roads, and other structures. Because lava hardens into solid rock, repairing buried areas can prove slow and costly.
Pyroclastic hazards.
Flying bombs pose relatively little danger because they usually fall near the vent. Ash and lapilli, however, can travel on the wind for tens or even hundreds of miles or kilometers. Falling ash and lapilli can contaminate water supplies, damage crops, and collect in great masses on roofs, causing them to collapse. Falling ash also blocks sunlight and reduces visibility, complicating evacuations. Ash can clog the engines of jet aircraft and parts of other machines, causing them to fail. Dispersed ash can remain in the stratosphere for years, where it can cool the atmosphere by blocking some sunlight.
The most dangerous volcanic hazards are pyroclastic flows and surges, clouds of hot ash and gas that travel mostly along the ground. They can choke or poison people with gases, bury people with debris, and burn them with temperatures of up to 1100 °F (600 °C). They advance tens or hundreds of feet or meters per second and can even cross such natural barriers as rivers and ridges. People and animals cannot outrun pyroclastic flows and surges. In 1902, pyroclastic flows and surges from Mount Pelée on the island of Martinique swept through the city of St. Pierre, killing about 28,000 people.
Some pyroclastic flows occur when the column of ash and gas rising from a vent becomes too heavy to be supported by rising air. The column collapses and flows away from the vent. Other pyroclastic flows occur when the edge of a steep-sided lava dome or flow collapses, releasing pressurized gases mixed with lava fragments.
Lahars
are mudflows that occur when loose ash on a steep volcano mixes with rain or melting snow. Eruptions or earthquakes, which often accompany volcanic activity, can dislodge the resulting mud. As a lahar travels downhill, it can pick up trees, boulders, and other debris. Lahars usually follow river and stream valleys, where there may be many towns. A lahar leaves a thick layer of mud that can harden like cement. Lahars from Nevado del Ruiz volcano in Colombia destroyed the town of Armero in 1985, killing more than 23,000 people.
Dangerous gases
may be nearly invisible. Carbon dioxide causes suffocation in high concentrations. Where air circulation is poor, carbon dioxide can collect in low areas. People in these areas can be overcome before they realize the danger. Sulfur dioxide irritates the eyes, nose, throat, lungs, and skin. Along with other volcanic gases, sulfur dioxide can create thick smog and acid rain.
Avalanches and landslides
often occur on volcanoes because many are steep-sided and covered with loose ash and fractured lava flows. Many volcanoes also produce tremors or explosions that can dislodge snow and rubble. A landslide or avalanche may stem from an eruption or may happen when no eruption is occurring.
Tsunamis.
A tsunami is a series of large ocean waves that can devastate coastal areas. Volcanic tsunamis can begin when a pyroclastic flow or avalanche enters the ocean, when a volcano in or near the ocean collapses, or when an underwater volcano erupts. During the 1883 eruption of Krakatau in Indonesia, the summit of the volcano collapsed, causing a tsunami that killed more than 30,000 people on nearby coasts.
Where volcanoes form
Volcanoes form above regions where magma is produced in the mantle, the rocky layer beneath Earth’s crust. The theory of plate tectonics helps explain why volcanoes form where they do. According to the theory, Earth’s outer shell consists of rigid pieces called plates that slowly move against one another. Nearly all volcanoes form along the edges of plates at subduction zones and divergent boundaries. Some volcanoes appear above locations called hot spots that can be far from plate boundaries.
Subduction zones
are boundaries where two plates push up against each other, forcing an oceanic plate to subduct (sink) beneath another plate. The subducting plate moves downward, carrying water trapped in sediments and rock into the mantle. When the plate reaches a depth of about 60 to 90 miles (100 to 150 kilometers), Earth’s heat causes the water to boil. The boiling water rises into the overlying mantle. There, the water lowers the melting point of the rock, which turns to magma in a process called hydration melting. The magma can then rise through the overlying plate to erupt at the surface.
Subduction zones produce viscous magmas that contain much water vapor and other gases. For this reason, subduction zone volcanoes tend to erupt explosively.
Bands of subduction zone volcanoes often form along the edges of continents where an oceanic plate subducts beneath a continental plate. These include the volcanoes of the Andes Mountains in South America and the Cascade Range in North America. Other bands of volcanoes occur where two oceanic plates meet and one subducts beneath the other. These include the volcanoes of Japan and Indonesia. Subduction zones border most of the Pacific Ocean, creating a region of volcano and earthquake activity called the Ring of Fire.
Divergent boundaries
are areas where two plates are pulling apart. As the plates separate, hot rock from the mantle rises to fill the space between them. The pressure on the rock decreases as the rock rises, causing it to melt in a process called decompression melting. The resulting magma erupts along the fracture between the plates and cools to form new plate material.
Divergent boundaries usually produce fluid magmas that contain few gases. Accordingly, eruptions at divergent boundaries tend to be less violent than eruptions at subduction zones. Most divergent boundaries involve two oceanic plates. For this reason, most eruptions at divergent boundaries occur underwater.
Hot spots
are areas where columns of hot, solid rock rise slowly through the mantle. At the top of the column, decompression melting turns the rock into relatively fluid magma with little gas. This magma rises to erupt through the overlying plate. Many hot spot volcanoes, such as those of the Hawaiian Islands, occur far from plate boundaries. However, some hot spot volcanoes, such as those of Iceland, lie on or near plate boundaries.
Types of volcanoes
Many schemes exist for classifying volcanoes. Most volcanoes fit into one of these types: (1) shield volcanoes, (2) stratovolcanoes, (3) silicic caldera complexes, (4) monogenetic fields, (5) mid-ocean ridges, and (6) flood basalts.
Shield volcanoes
build up over time from hundreds of thousands of lava flows. They erupt large volumes of fluid lava and few pyroclasts. The lava spreads far from the vent, creating a broad volcano with gently sloping sides. The word shield refers to the volcano’s profile, which resembles the shallow curve of a warrior’s shield.
Shield volcanoes include some of the largest volcanoes on Earth. Mauna Loa, a shield volcano on the island of Hawaii, rises about 30,000 feet (9,000 meters) from the ocean floor to its summit.
Most shield volcanoes form at hot spots. These include the volcanoes of Comoros, the Galapagos Islands, Hawaii, and many volcanoes in Iceland. Some of them, such as Westdahl Peak in Alaska, occur at subduction zones. Other shield volcanoes, including Erta Ale in Ethiopia and Nyamuragira in the Democratic Republic of the Congo, appear at divergent boundaries between continental plates.
Stratovolcanoes,
also called composite volcanoes, form from explosive eruptions that produce viscous lava and a large volume of pyroclasts. These materials pile up around the vent to form a steep-sided volcano. Most stratovolcanoes are smaller than shield volcanoes and erupt less often. The name stratovolcano refers to the strata (layers) of pyroclasts and hardened lava that make up the volcano’s cone.
Stratovolcanoes are among the most common volcanoes on Earth. They include many famous historical volcanoes, such as Krakatau in Indonesia, Mount Pinatubo in the Philippines, and Vesuvius in Italy.
Most stratovolcanoes occur along subduction zones. Nyiragongo in Congo (Kinshasa), however, is a stratovolcano that lies on a divergent continental boundary.
Silicic caldera complexes
produce the most violent volcanic eruptions. A silicic caldera complex can be difficult to recognize as a volcano because it consists of a broad, low-lying depression rather than a mountain.
Silicic caldera complexes form when a huge volume of magma collects below the surface in a giant magma chamber. The magma eventually erupts explosively, throwing ash high into the atmosphere and producing pyroclastic flows that damage vast areas. During the eruption, the ground above the chamber usually collapses, producing a large depression called a caldera.
A caldera can also form when part of a shield volcano or stratovolcano collapses into an opening left behind by erupting magma. These calderas are much smaller than silicic caldera complexes and are considered part of the larger volcano.
No silicic caldera complex has produced a major eruption in recent history. However, geologists have found evidence of such eruptions in Earth’s past. These eruptions occur rarely because the large volume of magma they require takes a long time to accumulate. For example, a huge silicic caldera complex at what is now Yellowstone National Park in Wyoming produced three massive eruptions roughly 600,000 years apart.
Silicic caldera complexes are sometimes called resurgent calderas. This is because scientists think the caldera floor can resurge (rise again) as magma accumulates in the magma chamber before an eruption.
Silicic caldera complexes usually form on land near a hot spot or subduction zone. Famous examples include La Primavera in Mexico, Taupo in New Zealand, Toba in Indonesia, and the Yellowstone caldera.
Monogenetic fields
form when magma from a single source flows to the surface through many different vents. Each vent forms during a single eruption. The word monogenetic means of one origin. Casual observers may not realize that what appears to be a single volcano is actually one vent in a large monogenetic field.
Monogenetic fields usually occur near subduction zones and hot spots. The best known is the Michoacan-Guanajuato field in Mexico. The field’s newest vent, a large cone called Paricutín, formed in a thinly populated area during an eruption that lasted from 1943 to 1952. Monogenetic fields also occur in and around the cities of Auckland, New Zealand, and Flagstaff, Arizona.
Mid-ocean ridges
are places at divergent boundaries where erupting magma creates new oceanic plate material. The mid-ocean ridge system forms the longest mountain chain on Earth. Estimates of its total length range from 30,000 to 50,000 miles (50,000 to 80,000 kilometers). Many geologists consider smaller segments of the ridge system to be individual volcanoes. At mid-ocean ridges, the material built up by eruptions spreads out as the plates pull apart. Ridges that spread rapidly, such as the East Pacific Rise, are broad and low. Ridges that spread slowly, such as the Mid-Atlantic Ridge, are narrow and steep.
Flood basalts,
also known as plateau basalts, consist of layers of a dark volcanic rock called basalt that can cover hundreds of thousands of square miles. No flood basalt has erupted in recorded history, and geologists are still debating how they form. They once thought that the expanses of basalt resulted from rapid “floods” of lava because slow-moving lava would solidify before flowing so far. However, more recent research has shown that slow-moving lava can flow long distances if it develops an insulating skin or crust of rock. Flood basalts include the Columbia River basalts in Washington, Oregon, and Idaho, the Deccan Traps in India, and the Parana basalts in Brazil.
Studying volcanoes
The study of volcanoes offers many benefits. Erupting magma can carry material from deep underground to the surface, providing scientists with samples from Earth’s interior. Hardened lava and ash deposits preserve evidence of major changes in Earth’s history. Studying ancient volcanoes also helps scientists find new deposits of ores. The fluids moving through volcanoes can concentrate valuable metals in deposits called veins.
Perhaps most importantly, scientists study volcanoes to learn how to predict eruptions. This information can help reduce the damage and loss of life eruptions cause.
Predicting eruptions.
To determine whether or when a particular volcano will erupt, volcanologists monitor seismic activity (earthquakes) near the volcano. They also watch for changes in the volcano’s shape caused by pressure from magma below. They analyze the types and amounts of gases coming from vents. Changes may signal that an eruption is coming.
Modern volcanologists can state fairly confidently the probability that a particular volcano will erupt sometime in the next tens, hundreds, or thousands of years. With enough information, they can occasionally provide warnings a few days to a few hours before an eruption. Volcanologists are working to develop the ability to forecast an eruption a few years to a few months before it occurs. Warnings on this time scale would prove most useful to disaster planners.
Most volcanologists consider any volcano that has erupted in the last 10,000 years or so to be active. Some of them use the term dormant to describe an active volcano that is not currently erupting or showing signs of a coming eruption. Volcanologists label a volcano extinct if there is strong evidence it will never erupt again.
Describing eruptions.
Volcanologists sometimes rate the explosive power of an eruption using a scale called the Volcanic Explosivity Index (VEI). The index ranks eruptions according to the volume of magma erupted, the amount of energy released, and the height of the eruption cloud. Most eruptions have VEI ratings from 0, for nonexplosive, to 8, for extremely explosive. Each number on the index represents a tenfold increase in explosive power or the volume of material erupted. For example, a VEI rating of 5 represents 10 times more power or eruption volume than a rating of 4.
Another common method for describing eruptions is based on how they produce pyroclasts. In this system, both Plinian and Hawaiian eruptions produce pyroclasts almost continuously. Plinian eruptions produce convective rise that can carry huge plumes of fine pyroclasts into the atmosphere. Hawaiian eruptions do not cause convective rise. Their pyroclasts are larger and stay closer to the ground, often landing while still molten. Vulcanian and Strombolian eruptions produce pyroclasts in bursts separated by periods of relative calm. Vulcanian eruptions produce convective rise, but Strombolian eruptions do not. Some volcanologists use classification schemes that have additional categories of eruptions.
Volcanoes in the solar system
In addition to Earth, planetary scientists have identified evidence of volcanic activity on the moon and on Mars, Venus, and Io, a large satellite of Jupiter. Much of the activity on these bodies resembles volcanic activity on Earth. Scientists have identified basalt, a rock commonly erupted on Earth, in samples retrieved from the moon and in space probe observations of Mars. Many volcanoes on Mars and Venus resemble giant versions of shield volcanoes found on Earth. The shield volcano Olympus Mons on Mars ranks as the largest volcano in the solar system. It measures more than 370 miles (600 kilometers) in diameter and rises about 16 miles (25 kilometers) above the surrounding plain.
Io ranks as by far the most volcanically active body in the solar system. Space probes and telescopes have identified hundreds of volcanoes on Io, many of them active. Some eruptions on Io measure at least 350 Fahrenheit degrees (175 Celsius degrees) higher than the hottest eruptions on Earth. Other eruptions on Io involve sulfur rather than molten rock. Probes have recorded eruptions shooting sulfur as high as 310 miles (500 kilometers) above Io’s surface.
Scientists studying Neptune’s moon Triton have found evidence of a process called cryovolcanism. Cryovolcanism resembles volcanic activity but is driven by melted ice rather than magma. Volcanolike vents on Triton erupt liquid nitrogen far above the moon’s frigid surface. Scientists think cryovolcanism may occur on other cold bodies in the outer solar system.
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