Volcanoes are mountains built up by the eruption of liquid rock, called magma, onto the Earth's surface. Magma is formed by a variety of processes deep in the Earth, and when it erupts onto the Earth's surface it is known as lava. Volcanoes and volcanism do not occur randomly over the Earth's surface but rather in striking patterns—along the edges of some continents and island chains, along mid-ocean ridges, and in lines along the ocean floors. These patterns reflect the mechanisms inside the Earth that are causing the mantle to melt.

The rock of the Earth's mantle is solid almost everywhere (only the outer core is always liquid, and it made mainly of iron and not silicates). Yet this is the rock that melts to make magmas that erupt through volcanoes, and scientists have spent centuries putting together evidence for how and why these melting events occur. The answers are not all known. First, mantle can melt if it is depressurized. As the mantle flows upward, it largely retains its temperature but progresses to lower and lower pressures. As pressure is released from the rock, eventually it can melt. Depressurization melting occurs at mid-ocean ridges. Second, mantle can melt if other materials are added to it which lower its melting temperature. Water and carbon dioxide are prime examples: Mantle material with a little of either of these volatiles added to it melts far more easily than the dry material alone. Melting by addition of water occurs at subduction zones. Third, mantle can melt if it is heated to higher temperatures.This seems to be the least common way of melting the mantle.

Mid-ocean ridges are the sources of the crust that lines ocean basins. The underlying mantle rises up beneath mid-ocean ridges and melts through decompression.The magma rises as sheets along vertical cracks near the line where the two oceanic plates are moving apart, creating new crust. Some of the magma bubbles to the surface and rolls out into the cold ocean water forming what are called pillow basalts. Pillow basalts form in humps as their surfaces freeze in the cold water and the hot rising magma pushes out of the bottom of the hump, forming a new hump, called a toe. These shapes are definitive of magma that erupted underwater. Magma is rising and forming new crust along the length of all the world's mid-ocean ridges all the time, and each year the new crust totals about one cubic mile (~4 km3).

The new oceanic crust moves away from the mid-ocean ridge. Because the area of the surface of the Earth is a fixed quantity new crust creates a space problem, so the other edges of each oceanic plate necessarily push against neighboring plates where oceanic crust pushes against continental crust, the oceanic crust is forced down into the mantle by the much thicker, more buoyant, and rigid continental crust. The process of a plate sinking down into the mantle when pressing against another plate is called subduction. When oceanic crust presses against another plate made of oceanic crust, one or the other plate will begin to subduct. The top of oceanic crust is filled with minerals holding water, from circulating ocean water heated in the new crust near mid-ocean ridges. The crust is also covered with water-holding sediments that settled onto it as it moved away from the ridge.When the oceanic crust subducts, the water-holding minerals give up their water at a variety of depths and temperatures. The sodium- and potassium-filled water released from these minerals rises buoyantly into the mantle above the subducting crust. Water reduces the melting temperature of mantle rocks, and so when the water percolates upward through the mantle, the mantle at some point begins to melt.

The resulting magma is also more buoyant than the mantle rock it melted from, and it, too, percolates upward. The magma eventually erupts, forming volcanoes that are explosive because they have water under pressure dissolved in the magma, bursting out as the pressure drops during eruption, and also because magmas formed in subduction zones tend to have more silica in them, making them highly vis-cous.The high viscosity also means that a large amount of pressure is required to force the eruption, also leading to explosive eruptions. These eruptions may also consist more of dry ash (flakes of silicate minerals that look like wood ash) or blocks of pumice (solidified magma laced with bubbles). Subduction zones form island arcs like Japan, the Philippines, and parts of the Caribbean. Subduction zones also form continental arcs like the Cascades on the west coast of North America, and the volcanoes that line the western edge of Central and South America.

Large-scale climate changes on Earth have occurred because of terrestrial effects. Volcanic eruptions inject ash and gas into the atmosphere, blocking the Sun's energy from reaching the Earth to the point that climate is changed. Two famous recent examples of catastrophic volcanic eruptions are Tambora, in Indonesia, which erupted in 1815, and Krakatau, also in Indonesia, which erupted in 1883. When Tambora erupted, the top of the volcano collapsed and the volcano ejected about 10 cubic miles (~40 km3) of ash and about double that amount in magma. Tambora's eruption produced pyroclastic flows, in which superheated clouds of ash and volcanic gas flow down the sides of the volcano like waterfalls, faster than a person can outrun. While about 10,000 people were killed by the eruption itself, the ash that blanketed plants or stayed in the atmosphere, blocking the Sun, killed another 80,000 people from crop loss and famine. The reduction of solar energy from ash in the atmosphere

The volcanic eruption cloud from Rabaul Caldera, Papua New Guinea, was estimated to be 60,000 feet (18,300 m) tall, but it was not the largest eruption plume of the decade when it erupted in 1994: Mount Pinatubos 1991 eruption plume was larger. (Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center, eol.jsc.nasa.gov, STS064-116-64)

caused temperatures across the Northern Hemisphere to drop by three degrees, virtually eliminating summer. Krakatau killed the majority of its victims in another way:The extreme seismic shock of its immense explosion created tsunamis that killed 36,000 people. Mount Saint Helens, on the other hand, killed only 57 people when it erupted in 1980.

The many smaller volcanoes usually threaten fewer people, but the sizes and intensities of their eruptions are not always easily predictable. Shown here is an image of the 1994 eruption plume of the Rabaul volcano, in New Britain, Papua New Guinea. This eruption plume is estimated at 60,000 feet (18,300 m) tall. Many volcanoes are erupting on Earth on any given day, most of which are located in the Pacific ring, where multiple subduction zones create explosive volcanoes.

Italy has a long history of active volcanoes and danger to citizens. Mount Etna has been erupting for years, as shown in the lower color insert on page C-5.The view looks southeast over the island of Sicily.

The high ash cloud curves south toward Africa, and ashfall was reported in Libya.

Volcanoes can be studied closely on Earth, and their effects can be seen on other planets. The large, dark portions of the Moon that are easily visible from Earth are giant pools of magma that erupted early in lunar history. Venus, Mercury, and Mars all have volcanoes and lava flows visible on their surfaces. These planets apparently all have internal processes that allow their silicate mantles to melt, as on Earth. Jupiter's moon Io is the most volcanically active body in the solar system, and erupts lavas consisting of both silicates and sulfur.While the gas and ice planets cannot have volcanism in the same way, some of their icy moons are volcanically active (volcanism consisting of ices is called cryovolcanism). Tiny moons cannot hold enough internal heat to create volcanism, but larger moons sometimes do. None of Jupiter's other moons show any signs of volcanism, even Ganymede, the largest moon in the solar system. Icy eruptions on Neptune's moon Triton were actually witnessed by spacecraft, and judging from their smooth, new surfaces, Jupiter's Europa, Uranus's Ariel, and Saturn's Titan and Enceladus probably experience cryovolcanism. While other moons may have been volcanically active in the past, the evidence for this has not yet been discovered.

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