The processes by which molten rock material, or magma, rises from the interior of the earth on to or towards its surface, and by which associated gases are released into the atmosphere are called Volcanism. The study of these processes, and of the structures, deposits, and landforms they create is called volcano logy.
Magma and gases exploit weak zones in the earth's outermost layer, the lithosphere, in order to reach the surface. Such weaknesses are found primarily along the boundaries between the earth's tectonic plates, and this is where most volcanism occurs. Where magma and gases do reach the surface, through vents or fissures in the earth's crust, they form geological structures known as volcanoes, of which there are several types. The classic picture of a volcano, exemplified by Mount Fuji in Japan or Mount Mayon in the Philippines, is of a conical structure with a hole (crater) at the top, from which (in the case of active volcanoes) ash, steam, gases, molten rock, and solid fragments erupt, often explosively. In fact, volcanoes of this type, though not uncommon, account for less than 1 percent of the earth's volcanic activity.

At least 80 per cent of volcanism takes place through lengthy vertical fissures in the earth's crust. Such fissure volcanism occurs predominantly along the constructive boundaries between the plates into which the lithosphere is divided. Oceanic ridges where new lithosphere is continuously being created and the plates pushed apart mark constructive boundaries. Indeed, it is the rising, cooling magma produced by fissure volcanism that makes the new ocean floor. Most of the world's volcanism therefore takes place unseen, beneath the oceans.

Surface Volcanism.  Surface, or continental, volcanism is much less important than sub-oceanic volcanism in terms of the volume of magma ejected, but much more is known about it because it is visible and directly affects human beings. It has been known since ancient times that volcanic activity ranges from violent explosions to the gentle extrusion of magma, which becomes known as lava when it is on the earth's surface.

Fissure Volcanoes.  Fissure volcanism is mostly associated with oceanic ridges, but it also occurs on land, and in some cases has led to spectacular results. Fissure volcanoes emit large volumes of very fluid material, which spreads out to cover large areas; successive eruptions can build up great plains or plateaux. Today, fissure volcanoes are probably best seen in Iceland, which straddles the Mid-Atlantic Ridge. However, fissure volcanism on land is most associated with the past, with the great plateaux to be found on most continents. Plateau basalts, flood basalts, or ignimbrites, as they are called, have formed, among others, the Deccan Plateau of west-central India; the Paraná Basin of southern Brazil, Argentina, and Uruguay; the Columbia Plateau of the north-western United States; the Drakensberg Plateau of South Africa; and the central plateau of the North Island, New Zealand.

Central Volcanoes.  The majority of surface volcanic activity, however, is associated with more-or-less circular vents, or clusters of vents, in the earth's crust, rather than fissures. These vents give rise to central volcanoes, of which there are two basic types. The steep-sided conical volcano mentioned above is occasionally constructed entirely from solid material, or tephra, which ranges in size from ash and cinders to rocks and boulders. The tephra have been ejected explosively in an eruption, or series of eruptions, and have fallen back to the ground in the immediate vicinity of the crater, the external outlet of the vent. A well-known example of such a volcano is Parícutin, in Mexico, which first erupted in a field on February 20, 1943, and within six days had built a cinder cone 150 m (492 ft) high; by the end of the year the cone was more than 336 m (1,100 ft) high.

However, very few conical volcanoes eject only tephra in every eruption, to become cinder-cone volcanoes. Lava is likely to be extruded in some eruptions, in which case the resulting volcanic structure will comprise alternating layers of tephra and lava. Such volcanoes are called composite volcanoes, or strato-volcanoes. Most of the world's largest and best-known volcanoes, including Stromboli and Vesuvius in Italy, Popocatépetl in Mexico, Cotopaxi in Ecuador, and Kilimanjaro in Tanzania, as well as Fuji and Mayon, are of this type. Although most conical and near-conical volcanoes generally have a single central vent, this does not preclude volcanic material sometimes emerging from secondary, often temporary, vents on the flanks of a volcano.

Shield Volcanoes.  The other main type of central volcano is the shield volcano. These are very large structures, which can be up to many tens of kilometres in diameter, and which have relatively gentle slopes, generally of less than 12°. They are usually formed by hundreds of outpourings of fluid, basaltic lava. Shield volcanoes often have more than one vent, as well as fissures along their sides. This is particularly true of the largest shield volcanoes, notably those of the Hawaiian Islands in the North Pacific. The Hawaiian Islands are a complex of shield volcanoes rising from the ocean floor; Mauna Loa, on the island of Hawaii, is one of the most recently formed. It is considered to be the most massive mountain on earth, rising more than 10,000 m (32,800 ft) from the seabed. In Europe, Mount Etna is a shield volcano.

Surface Volcanoes and Plate Tectonics.  Surface volcanoes are often associated with the destructive boundaries formed by tectonic plates, which are moving together. When two plates converge, the leading edge of one plunges down beneath that of the other towards the mantle, the semi-molten layer which underlies the lithosphere. This causes subduction, the re-incorporation of the rocks of the lithosphere into the mantle. Occasionally the leading edges of converging plates are both composed of oceanic lithosphere; more commonly one is composed of oceanic lithosphere, the other of continental crust. Because the latter is thicker and less dense, it is the oceanic lithosphere, which is subducted.

When oceanic crust melts as a result of subduction, the magma formed rises upwards along the plane of subduction to erupt on the earth's surface—usually on the landward side of the destructive boundary, which is normally marked by oceanic trenches. Where the magma erupts on land, it leads to the creation of long mountain chains, notably the Andes of South America and the Cordillera of North America, which includes the Cascade Range and the Rocky Mountains. Where the volcanism caused by subduction occurs in the ocean, long, arc-shaped chains of volcanic islands are produced, such as Japan and the Philippines.

The majority of the earth's subduction zones lie around the edge of the Pacific Ocean, and so do more than three-quarters of active, dormant, or extinct surface volcanoes. They form a belt known as the “Ring of Fire”, along which earthquakes are also common. The Ring of Fire extends through the Andes, the Cordillera, the Aleutian Islands, the Kamchatka Peninsula in eastern Siberia, the Kuril Islands, Japan, the Philippines, Sulawesi, New Guinea, the Solomon Islands, New Caledonia, and New Zealand.

Calderas.  The crater, which has emitted volcanic material, often remains a depression, even when the volcano is dormant, as a result of lava sinking back into the volcanic vent. Sometimes the lava sinks so far that the top of the volcano collapses into the vent, forming a much larger depression known as a caldera, which can be several kilometres in diameter. Calderas can also be formed by very violent explosions that “blow the top off” the volcano in question, for example, Krakatau in Indonesia (see below). Over time, calderas of dormant or extinct volcanoes can become filled up with water to form crater lakes. The best known is probably Crater Lake in Oregon, United States. Some 8 km (5 mi) in diameter, it was formed by the collapse of a prehistoric composite volcano, Mount Mazama.

Tephra = Volcanic Materials.  Beneath most active or potentially active volcanoes lies a magma chamber containing molten rock. The magma it contains probably originated in the asthenosphere, the mobile layer immediately below the earth's lithosphere; the chamber is a “staging post” on its way to the surface. When the magma reaches the surface, however, it can be in liquid, solid, or gaseous form.

Most magmas contain dissolved gases, such as carbon dioxide and sulphur dioxide, which are released during the severe pressure reduction that occurs as the magma rises towards the surface. The release can be very sudden, taking place with an explosive force that can shatter the magma and send it skyward as tephra, and molten or semi-molten fragments that cool to a greater or lesser extent as they fall back to the ground. Tephra range in size from very small dust and ash particles, which can be carried vast distances by wind, to boulders weighing over 100 tons. Associated with intense eruptions, these boulders can be thrown several kilometres from the vent. In some less intense eruptions, volcanic fragments are not blown upwards. Instead, mixed in deadly combination with hot gases, they flow along the ground as a nude ardent (French, “glowing cloud”), which smothers and destroys everything in its path.

Some volcanoes never erupt explosively but quietly extrude magma on to the ground. Such eruptions are caused by extremely fluid basaltic magma, which contains little silica or gases. They are mostly associated with fissure volcanism and shield volcanoes, such as those of Hawaii. The more silica that magma contains the more viscous (sticky and slow-moving) it becomes. It is harder for gases to escape from viscous magma, so increasing viscosity is usually associated with more explosive eruptions.

Types of Eruptions.  Any volcano is capable of erupting in several ways, but certain types of eruptions tend to be associated with particular volcanoes. This is reflected in the classification of volcanic eruptions, with each category bearing the name of a typical volcano. Fissure and shield-type eruptions are usually categorized as Icelandic and Hawaiian respectively. More explosive eruptions are categorized, on a scale of increasing viscosity of magma, as Strombolian, Vulcanian (after Vulcano in the Lipari Islands, Italy), Vesuvian, Plinian, and Peléean (after Mount Pelée, Martinique). Vesuvian, Plinian (a more violent form of Vesuvian), and Peléean are the most paroxysmal in character, expelling large amounts of ash as well as lava blocks. Peléean eruptions are particularly associated with nuées ardentes. The May 8, 1902, eruption of Mount Pelée destroyed the city of Saint-Pierre and killed about 30,000 people; the majority died from the effects of nuées ardentes and of asphyxiation.

The most violent eruptions tend to be associated with destructive plate boundaries. The two greatest volcanic explosions in recorded history—Krakatau (or Krakatoa) and Mount Tambora—occurred at the juncture of the Asian and Australian plates. Tambora, on the northern coast of Sumbawa island, erupted in 1815, blowing off its top half and killing an estimated 50,000 islanders. The volcanic island of Krakatau, lying between Java and Sumatra in Indonesia, erupted in 1883, destroying two-thirds of its land area. The resulting tidal waves caused the deaths of tens of thousands of people throughout South East Asia. The noise the explosion created travelled more than 4,830 km (3,000 mi), while the millions of tons of ash it threw into the earth's atmosphere caused spectacular sunsets around the world for more than a year.

In contrast to explosive eruptions, which have killed many thousands of people throughout history, Icelandic and Hawaiian, and to some extent Strombolian, types are seldom a hazard. The lava can flow rapidly, but it often flows slowly enough to enable people to get out of the way; property often suffers, however. Occasionally, it has proved possible to divert flowing lava away from buildings by digging culverts, building retaining walls, or even blowing the lava up, but such methods are seldom completely successful.

Volcanic Deposits.  Magma usually emerges from the earth at temperatures of 800° to 1,200° C (1,472° to 2,192° F). It then cools as it flows, solidifying from the outside inwards until it becomes completely solid in the form of a lava flow. Depending largely on the viscosity of the original magma, lava flows have different forms and surface textures. The three main types are referred to as pahoehoe, aa, and block.

Pahoehoe lava comes from very thin and mobile magma. On reaching the ground the magma quickly forms a thin, plastic surface layer that gets dragged into rope-like folds by the molten lava continuing to flow beneath it. The second type, aa lava, is produced from a less mobile magma, which, as it cools, acquires a thick, hard skin. This skin is broken up by the lava flowing beneath to form a fragmented, jagged surface. Block lava is also fragmented, but its surface is smoother. Not all of the gas within the magma always escapes to the atmosphere during an eruption. Some can remain, trapped in cavities known as vesicles. These vesicles can persist even after the magma has become solid lava. Pumice is a highly vesicular lava; indeed, some has so many vesicles that it is light enough to float on water.

Finally, even tephra can fuse together on the ground, to form what is known as a tuff. The material from a nuée ardente may also consolidate to form an ignimbrite. Tuffs and ignimbrites are therefore composite rocks made up of a wide variety of volcanic fragments.

Igneous Forms.  Rocks formed from magma that has cooled and solidified are known as igneous rocks. A lava flow on the earth's surface is an igneous rock, but there are also other forms. Some magma does not reach the surface at all. Instead it is diverted into natural underground cavities, or breaks off blocks of the surrounding (country) rock to make its own niches. Occasionally, magma may simply be so hot that parts of the country rock melt and flow away.

Magma that enters subsurface openings usually solidifies there to form intrusions, often of great size. A sill is a horizontal ledge-like intrusion lying between beds of layered rock. Examples include the Salisbury Crags in Edinburgh, Scotland, and the Palisades sill along the west bank of the Hudson River, near New York. A laccolith also lies between rock beds. It is formed when the pressure of the original magma pushes the overlying rock upwards to form a central dome, creating a mushroom-shaped intrusion. A lopolith is a saucer-shaped intrusion formed when magma enters between beds of folded, layered rock; a phacolith is shaped like an upturned saucer.

When a volcano becomes extinct, or merely dormant, any magma remaining in the vent may solidify to form a volcanic plug. If the surrounding rock then erodes away, the plug will become exposed to form a conspicuous landscape feature. The Castle Rock in Edinburgh, Scotland, is a volcanic plug. If volcanism takes place through a vertical fissure rather than a cylindrical vent, magma solidifying in the fissure leaves a vertical, sheet-like intrusion known as a dyke. Probably the most impressive example is the mineral-rich Great Dyke in Zimbabwe, which runs some 480 km (298 mi) roughly north-south through the centre of the country.

Hot Spots.  Most volcanic activity occurs along tectonic plate boundaries because that is where the lithosphere is weakest. However, some volcanism occurs away from plate margins, for reasons that are sometimes clear and sometimes not. There are volcanoes in the vicinity of the East African Rift Valley, for example, notably Kilimanjaro. This is understandable because the Rift Valley is a zone in which the continent has begun to split apart and where, in the future, even larger amounts of magma may be expected to rise.

The presence of 10,000 or more undersea volcanoes on the Pacific Ocean floor, however, long defied explanation. Known as seamounts, most, though far from all, are now extinct. The vast majority appears to be scattered randomly around the ocean floor, but some clearly form linear chains, for example, the Hawaiian-Emperor chain. Their presence away from destructive boundaries has now been explained. Within the earth's mantle there are thin vertical plumes of hot magma, probably rising from the core, which remain fixed in position as the tectonic plates move overhead. These plumes create “hot spots” in the lithosphere above, where volcanic activity occurs. The site of such volcanism moves as the plates move. The Hawaiian-Emperor hot spot, for example, is now at the Hawaiian end of the chain; the volcanic islands in the chain get progressively older with distance from the island of Hawaii.

However, not all the volcanic hot spots resulting from mantle plumes lie beneath the oceans. An example of a continental hot spot is Yellowstone National Park in the United States. There are no volcanic eruptions at Yellowstone today, but there is abundant heat, which has generated hot water and rising water-jets known as geysers.

Volcanism as Hazard.  Many millions of people in the world are at risk from volcanic eruptions, especially explosive ones. Some of those millions actually live on volcanic slopes. Why do they do it, when the danger is so great? The primary reason is that the soils generated by the breakdown of volcanic products from previous eruptions are highly fertile, and have thus long attracted high populations. Many volcanic danger zones are ancient centres of civilization and continue to be areas of dense settlement. Volcanoes will therefore continue to take their toll, as, for example, Mount Pinatubo did in 1991. Located north of Manila, it erupted in June and July of that year, throwing millions of tons of ash into the air, which combined with tropical rainfall to produce massive mudslides. An estimated 550 people died and 650,000 lost their livelihood. The Pinatubo eruption also highlights the dangers of thinking that a volcano is inactive or extinct. Mount Pinatubo had not previously erupted for more than 600 years. More than three million people continue to live in the Naples area, although it is known that Vesuvius will certainly erupt again suddenly one day. The last significant eruption was in 1906, but by the mid-1990s there were early indications that Vesuvius is reawakening.

Volcano.  A volcano is a fissure or vent through which molten rock material, or magma, and gases from the interior of the earth erupt on to its surface, and the landform, which is produced as a result of this eruption. The word “volcano” derives from Vulcano, one of the volcanic Lipari Islands in the Mediterranean Sea, and the place where, according to Roman mythology, Vulcan, the god of fire, kept his forge. The processes that create volcanoes and other volcanic structures are called volcanism.

As landforms, volcanoes are formed by the deposition of the magma that flows or is ejected, normally from one or several circular vents, as molten or solid material. Molten magma is known as lava when it reaches the earth's surface; the solid material—classified as dust, ash, cinders, and bombs depending on size and shape, is called tephra. Volcanoes which form round, circular vents are known as central volcanoes; the basin-like mouth of the vent is known as the crater. Most volcanoes tend to be conical in shape; some, however, are much larger structures with very gentle slopes. Often covering many square kilometres, they are known as shield volcanoes.

Some volcanoes are much more active than others. A few may be said to be in a state of permanent eruption, at least during the geological present. Stromboli, in the Lipari Islands, has been constantly active since ancient times; Izalco, in El Salvador, has been active since it first erupted in 1770. Other constantly active volcanoes are found in a belt, called the Ring of Fire, that encircles the Pacific Ocean.

Many other volcanoes, such as Vesuvius, in Italy, continue in a state of moderate activity and then become quiescent, or dormant, for periods ranging from months to centuries. The eruption that succeeds prolonged dormancy is usually violent. This was the case with the 1980 eruption, after 123 years of quiescence, of Mount St Helens in Washington State, United States. The massive eruption of Mount Pinatubo in the Philippines during June 1991 came after more than 600 years of dormancy.

For a long period after it has ceased to erupt either lava or tephra, a volcano continues to emit acid gases and vapour in what is called the fumarolic stage. After this phase, hot springs may arise from the volcano. Examples of this type of activity include the geysers of Yellowstone National Park in the United States, and of the central area of the North Island of New Zealand. Eventually, the last traces of volcanic heat may disappear; springs of cold water may issue from the volcano and from the ground in its vicinity.

After becoming inactive, a volcano is progressively reduced in size as a result of weathering and erosion. Finally, the cone may be obliterated, leaving only a volcanic pipe—a chimney filled with lava or tephra, and extending from the earth's surface down to the former magma reservoir under the volcano. The diamond-rich mines of South Africa are found in volcanic pipes.