20120529-Volcano eruption Shishaldin_Volcano_eruption_1999.jpg
Shishaldin Volcano in the Aleutian Islands
A volcano is a vent, or opening, in the Earths's crust from which hot rock has been ejected sometime in its history. It is fueled by magma (molten rock found deep in the Earth) that flows upwards in fissures in the Earth’s crust. Most volcanoes are the size of mountains. Materials ejected by them during eruptions include lava (molten rock), ash, steam and a variety of gases. Some eruptions are quite large, with thousands, even millions, of tons of material being ejected. Related to volcanoes are geysers and fumaroles which are small vents from which materials are ejected. [Source: Mostly from USGS and newspaper articles]

Volcanoes are often described as active, dormant or extinct. An active volcano is one that has erupted in some way relatively recently: say, within the last few hundred or few thousand years. A dormant volcano is one that hasn't erupted for some time, perhaps several hundred or thousand years, but is still is considered likely of erupting sometime in the future. Extinct volcanoes are ones that are considered incapable of erupting. Some volcanoes labeled as extinct, however, such as Mt. St. Helens, have erupted quite violently.

Many geological features and phenomena including volcanoes are explained in terms of plate tectonics, a geological theory that gained credibility in the 1970s and now is accepted as fact. According to theory the earth's surface is fragmented into large plates that can range in size from a small country to a continent. These plates float along the mantel under the earth's surface. In places where plates meets faults form and earthquakes occur. At a subduction zones two plates collide head on, with one plate going over he top, forcing the other one down. Where ocean crust, pushed down by the weigh of ocean water, is shoved under the thick crust of continents, the rock is heated, causing water and gases to bubble out. As they rise they melt the rock above it, creating magma that can fuel volcanoes. Rift zones are where the earth's tectonic plates are pulled apart as new material rises from inside the earth, sometimes volcanically, creating millions of tons of new crust every day and causing the plates to spread.. Mountains and volcanoes rise form here as they do in subduction zones but for different reasons. See Separate Article PLATE TECTONICS factsanddetails.com

Volcanoes generally occur in three places: 1) rift zones, where the Earth's tectonic plates are pulled apart; 2) subduction trenches, where two plates collide, with the overlapping plate forcing the other one down; and 3) hot spots, where a plate floats over a source of magma coming up from deep inside the Earth. Rift zone volcanoes, most of which are located on the bottom of the sea floor, and hot spot volcanoes produce basaltic lava which generally flows out without massive explosions and don’t kill anyone. These are the kind of eruptions that take place in Hawaii. Explosive eruptions like those at Mt. St. Helens and Krakatau generally occur along subduction zones.

The word volcano comes from the little island of Vulcano in the Mediterranean Sea off Sicily. Centuries ago, the people living in this area believed that Vulcano was the chimney of the forge of Vulcan — the blacksmith of the Roman gods. They thought that the hot lava fragments and clouds of dust erupting from Vulcano came from Vulcan's forge as he beat out thunderbolts for Jupiter, king of the gods, and weapons for Mars, the god of war. In Polynesia the people attributed eruptive activity to the beautiful but wrathful Pele, Goddess of Volcanoes, whenever she was angry or spiteful. Today we know that volcanic eruptions are not super natural but can be studied and interpreted by scientists.

Websites and Sources on Volcanoes: USGS Volcanoes volcanoes.usgs.gov ; Volcano World volcano.oregonstate.edu ; Volcanoes.com volcanoes.com ; Volcano Tourism volcanolive.com ;Wikipedia Volcano article Wikipedia , Smithsonian Global Volcanism Program volcano.si.edu operated by the Smithsonian has descriptions of volcanoes around the globe and a catalog of over 8,000 eruptions in the last 10,000 years.; Volcano Pictures Volcano Photo Gallery decadevolcano.net/photos ; Archive of Volcano Photos doubledeckerpress.com Book: “Volcanoes in Human History” by Jelle Zeilinga de Boer and Donald Theodore Sanders (Princeton University Press, 2002)

Volcano Structure

Regions of an
erupting volcano
The main parts of a volcano are 1) the crater, a depression at the top of the volcano from which volcanic material has been ejected; 2) the vent, the conduit between crater and the magma; and 3) the cone, the area around the crater at the top of the volcano, made up of material ejected during an eruption. Many volcanoes have several craters, cones and vents.

Volcanoes have been described as “inherently unstable structures” vulnerable to landslides and collapses. According to an article in Natural History magazine, “They are made of intermixed layers of solid lava and flows and fragmented material, all of which has been weakened by hot gases and fluids and shaken by earthquakes. A host of other factors contribute to their instability: steep slopes, stress that arises from faulting and from the intrusion of hot magma into vertical fractures, or dikes, and weak, sloping foundations.”

Some volcanoes have lava tubes. A lava tube is an unusual environment. They are empty caverns that are not penetrated by rainstorms or roots and thus experience no erosion. Hardened drops of lava hang from the ceiling like stalactites. The floor often looks like solidified bubbling porridge. In places where there is drop off there s a solid cascade.

Volcano Dynamics

Volcanoes are mountains but they are very different from other mountains; they are not formed by folding and crumpling or by uplift and erosion. Instead, volcanoes are built by the accumulation of their own eruptive products — lava, bombs (crusted over ash flows, and tephra (airborne ash and dust). A volcano is most commonly a conical hill or mountain built around a vent that connects with reservoirs of molten rock below the surface of the Earth. The term volcano also refers to the opening or vent through which the molten rock and associated gases are expelled.

A volcano is like a giant container where chemical substances mix, heat and react according to the laws of nature that are not completely understood. Driven by buoyancy and gas pressure the molten rock, which is lighter than the surrounding solid rock forces its way upward and may ultimately break though zones of weaknesses in the Earth's crust. If so, an eruption begins, and the molten rock may pour from the vent as non-explosive lava flows, or if may shoot violently into the air as dense clouds of lava fragments. Larger fragments fall back around the vent, and accumulations of fall-back fragments may move downslope as ash flows under the force of gravity. Some of the finer ejected materiaIs may be carried by the wind only to fall to the ground many miles away. The finest ash particles may be injected miles into the atmosphere and carried many times around the world by stratospheric winds before settling out.

fumarole releases
Heat concentrated in the Earth's upper mantle raises temperatures sufficiently to melt the rock locally by fusing the materials with the lowest melting temperatures, resulting in small, isolated blobs of magma. These blobs then collect, rise through conduits and fractures, and some ultimately may re-collect in larger pockets or reservoirs ("holding tanks") a few miles beneath the Earth's surface. Mounting pressure within the reservoir may drive the magma further upward through structurally weak zones to erupt as lava at the surface. In a continental environment, magmas are generated in the Earth's crust as well as at varying depths in the upper mantle. The variety of molten rocks in the crust, plus the possibility of mixing with molten materials from the underlying mantle, leads to the production of magmas with widely different chemical compositions.

After the eruption, many volcanoes become largely silent for years, decades or centuries. Some volcanoes sort of level off at the stage of fumarolic activity. Fumarola is a gas stream with a temperature of 300 to 800 degrees C. Outputs of vapors and gases with a lower temperature are called solfataram. Fumaroles containing hydrogen sulphide, sulfur dioxide and carbon dioxide in addition to water vapor, change the rocks beyond recognition. Acid rivers enriched with iron and aluminum flow on the ground; sulfur crystallizes on the stones near the fumaroles; zinc, lead, arsenic, and mercury deposits are formed in some places.

Volcano Swelling and the Accumulation of Magma

Swelling indicates an accumulation of magma under a volcano. Crustal movement can be observed using satellites and the Global Positioning System and the depths and amounts of magma can be calculated. In the case of Shinmoedake volcano in Japan, the volcano body began swelling, indicating an accumulation of magma, in December 2009. During a nine-month period from May 2010, the The Geospational Information Authority of Japan calculated about 6 million cubic meters of magma accumulated in a reservoir about six kilometers underground and about 10 kilometers west-northwest of the Shinmoedake crater. During the same period, about 1 million cubic meters of magma accumulated in a chamber about three kilometers underground just beneath the crater.

Generally, a mountain body swells when magma accumulates underground, causing the distance between observation points to become longer. When magma is released through eruptions, the mountain body will contract and observation points move closer together. There are exceptions, however. Eruptions have continued to take place on Sakurajima in Kagoshima, for example, but the mountain's body is swelling because the amount of magma accumulating underground is larger than the volume released through the eruptions.

Two weeks after the first eruption “distances between observation points have already changed from expanding to shrinking. Originally 23 kilometers, the distance between two particular points in the Kirishima mountain system expanded by four centimeters during the approximately one year from December 2009 to just before the eruptions. However, the same distance shrank by one centimeter in three days” after the eruptions.

Magma, Lava and Composition of Volcanic Materials

20120529-Erupt Hawaii Aa_large.jpg
Aa lava in Hawaii
Molten rock below the surface of the Earth that rises in volcanic vents is known as magma, but after it erupts from a volcano it is called lava. Originating many tens of miles beneath the ground, the ascending magma commonly contains some crystals, fragments of surrounding (unmelted) rocks, and dissolved gases, but it is primarily a liquid composed principally of oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, potassium, titanium, and manganese. Magmas also contain many other chemical elements in trace quantities. Upon cooling, the liquid magma may precipitate crystals of various minerals until solidification is complete to form an igneous or magmatic rock.

If magmas cool rapidly, as might be expected near or on the Earth's surface, they solidify to form igneous rocks that are finely crystalline or glassy with few crystals. Accordingly, lavas, which of course are very rapidly cooled, form volcanic rocks typically characterized by a small percentage of crystals or fragments set in a matrix of glass (quenched or super-cooled magma) or finer grained crystalline materials. If magmas never breach the surface to erupt and remain deep underground, they cool much more slowly and thus allow ample time to sustain crystal precipitation and growth, resulting in the formation of coarser grained, nearly completely crystalline, igneous rocks. Subsequent to final crystallization and solidification, such rocks can be exhumed by erosion many thousands or millions of years later and be exposed as large bodies of so-called granitic rocks, as, for example, those spectacularly displayed in Yosemite National Park and other parts of the majestic Sierra Nevada mountains of California.

Two Polynesian terms are used to identify the surface character of Hawaiian lava flows. Aa, a basalt with a rough, blocky appearance, much like furnace slag, is shown at the left. Pahoehoe, a more fluid variety with a smooth, satiny and sometimes glassy appearance, is shown at the right.

Lava is red hot when it pours or blasts out of a vent but soon changes to dark red, gray, black, or some other color as it cools and solidifies. Very hot, gas-rich lava containing abundant iron and magnesium is fluid and flows like hot tar, whereas cooler, gas-poor lava high in silicon, sodium, and potassium flows sluggishly, like thick honey in some cases or in others like pasty, blocky masses.

All magmas contain dissolved gases, and as they rise to the surface to erupt, the confining pressures are reduced and the dissolved gases are liberated either quietly or explosively. If the lava is a thin fluid (not viscous), the gases may escape easily. But if the lava is thick and pasty (highly viscous), the gases will not move freely but will build up tremendous pressure, and ultimately escape with explosive violence. Gases in lava may be compared with the gas in a bottle of a carbonated soft drink. If you put your thumb over the top of the bottle and shake it vigorously, the gas separates from the drink and forms bubbles. When you remove your thumb abruptly, there is a miniature explosion of gas and liquid. The gases in lava behave in somewhat the same way. Their sudden expansion causes the terrible explosions that throw out great masses of solid rock as well as lava, dust, and ashes.

The violent separation of gas from lava may produce rock froth called pumice. Some of this froth is so light — because of the many gas bubbles — that it floats on water. In many eruptions, the froth is shattered explosively into small fragments that are hurled high into the air in the form of volcanic cinders (red or black), volcanic ash (commonly tan or gray), and volcanic dust.

Volcanic Rocks and Ash

medium volcanic bombs
Among the materials that are released during an eruption are: 1) lava (molten rock), 2) ash (fine particles of volcanic material); 3) bombs (lava rocks), 4) cinders (rough pieces of stone that are formed during an explosion and have hardened very quickly); 5) gases (carbon dioxide and various compounds with chlorine and sulfur); and 6) steam.

The rocks and minerals associated with volcanoes are: 1) basalt, a dark, heavy type of volcano that usually comes from rift zone and hot spot volcanoes; 2) rhyolite, a type of solidified of lava that is usually a pale shade of green, red or gray; 3) pumice, a porous, hole-filled kind of rhyolite that is produced when melted rock contains gas bubbles; and 4) obsidian. a grasslike lava that is produced when certain kinds of lava cool quickly and the individual minerals do not crystalize.

Ash is not rock dust but actually little shreds of silica — glass if you will — that are created when the lava is quickly cooled. Ash ejected by large eruptions can spread a layer of upper-atmosphere dust are the globe that can block sunlight and lower world temperatures. It also poses a serious threat to commercial jets. Since 1980 more than 100 commercial jets have suffered significant damages after unknowingly flying into volcanic ash clouds. In at least 10 cases the aircraft lost power minutes after their jet engines sucked in ash that melted in their turbines. No fatalities have occurred but serious crashes have come very close to happening. One of the main reasons problems is that satellites have difficulty telling the difference between weather clouds and volcano ash clouds.

Lava Domes

Lava domes are formed by relatively small, bulbous masses of lava too viscous to flow any great distance; consequently, on extrusion, the lava piles over and around its vent. A dome grows largely by expansion from within. As it grows its outer surface cools and hardens, then shatters, spilling loose fragments down its sides. Some domes form craggy knobs or spines over the volcanic vent, whereas others form short, steep-sided lava flows known as "coulees."

explanation of pyroclastic flows and
dome collapse on Unzen in Japan
Volcanic domes commonly occur within the craters or on the flanks of large composite volcanoes. The nearly circular Novarupta Dome that formed during the 1912 eruption of Katmai Volcano, Alaska, measures 800 feet across and 200 feet high. The internal structure of this dome — defined by layering of lava fanning upward and outward from the center — indicates that it grew largely by expansion from within.

A lava dome forms as magma rises to the crater and stops there. A lava dome sometimes grows larger and larger due to a continuous supply of magma, although there are exceptions. For example, on Mt. Showa Shinzan in Sobetsucho, Hokkaido, lava domes cooled as soon as they formed, raising the height of the peak from 1944 to 1945.

Viscid magma and lava domes are often associated with pyroclastic flows. The magma was viscid at Fugendake peak in the Unzen mountain range in Nagasaki Prefecture, where large pyroclastic flows occurred in the 1990s. A large lava dome was observed in the crater at that time, and was the source of pyroclastic flows for a long time. Sometimes the worst explosions occur after the initial eruption when a solidified lava domes plugs the volcano’s vent like the lid of a pressure cooker, causing pressure to build up in the vent and produce a huge explosion when it builds up to a critical level.

Mont Pelée in Martinique, Lesser Antilles, and Lassen Peak and Mono domes in California are examples of lava domes. An extremely destructive eruption accompanied the growth of a dome at Mont Pelée in 1902. The coastal town of St. Pierre, about 4 miles downslope to the south, was demolished and nearly 30,000 inhabitants were killed by an incandescent, high-velocity ash flow and associated hot gases and volcanic dust. Only two men survived; one because he was in a poorly ventilated, dungeon-like jail cell and the other who somehow made his way safely through the burning city.

Plugs, Maars and Nonvolcanic Craters

Congealed magma, along with fragmental volcanic and wallrock materials, can be preserved in the feeding conduits of a volcano upon cessation of activity. These preserved rocks form crudely cylindrical masses, from which project radiating dikes; they may be visualized as the fossil remains of the innards of a volcano (the so-called "volcanic plumbing system") and are referred to as volcanic plugs or necks. The igneous material in a plug may have a range of composition similar to that of associated lavas or ash, but may also include fragments and blocks of denser, coarser grained rocks — higher in iron and magnesium, lower in silicon — thought to be samples of the Earth's deep crust or upper mantle plucked and transported by the ascending magma. Many plugs and necks are largely or wholly composed of fragmental volcanic material and of fragments of wallrock, which can be of any type. Plugs that bear a particularly strong imprint of explosive eruption of highly gas-charged magma are called diatremes or tuff-breccia.

Devil's Tower
Volcanic plugs are believed to overlie a body of magma which could be either still largely liquid or completely solid depending on the state of activity of the volcano. Plugs are known, or postulated, to be commonly funnel shaped and to taper downward into bodies increasingly elliptical in plan or elongated to dike-like forms. Typically, volcanic plugs and necks tend to be more resistant to erosion than their enclosing rock formations. Thus, after the volcano becomes inactive and deeply eroded, the exhumed plug may stand up in bold relief as an irregular, columnar structure. One of the best known and most spectacular diatremes in the United States is Ship Rock in New Mexico, which towers some 1,700 feet above the more deeply eroded surrounding plains. Volcanic plugs, including diatremes, are found elsewhere in the western United States and also in Germany, South Africa, Tanzania, and Siberia.

Also called "tuff cones," maars are shallow, flat-floored craters that scientists interpret have formed above diatremes as a result of a violent expansion of magmatic gas or steam; deep erosion of a maar presumably would expose a diatreme. Maars range in size from 200 to 6,500 feet across and from 30 to 650 feet deep, and most are commonly filled with water to form natural lakes. Most maars have low rims composed of a mixture of loose fragments of volcanic rock and rocks torn from the walls of the diatreme.

Maars occur in the western United States, in the Eifel region of Germany, and in other geologically young volcanic regions of the world. An excellent example of a maar is Zuni Salt Lake in New Mexico, a shallow saline lake that occupies a flat-floored crater about 6,500 feet across and 400 feet deep. Its low rim is composed of loose pieces of basaltic lava and wallrocks (sandstone, shale, limestone) of the underlying diatreme, as well as random chunks of ancient crystalline rocks blasted upward from great depths.

Some well-exposed, nearly circular areas of intensely deformed sedimentary rocks, in which a central vent-like feature is surrounded by a ring-shaped depression, resemble volcanic structures in gross form. As no clear evidence of volcanic origin could be found in or near these structures, scientists initially described them as "cryptovolcanic," a term now rarely used. Recent studies have shown that not all craters are of volcanic origin. Impact craters, formed by collisions with the Earth of large meteorites, asteroids, or comets, share with volcanoes the imprints of violent origin, as evidenced by severe disruption, and even local melting, of rock. Fragments of meteorites or chemically detectable traces of extraterrestrial materials and indications of strong forces acting from above, rather than from below, distinguish impact from volcanic features.

Other possible explanations for these nonvolcanic craters include subsurface salt-dome intrusion (and subsequent dissolution and collapse caused by subsurface limestone dissolution and/or ground-water withdrawal; and collapse related to melting of glacial ice. An impressive example of an impact structure is Meteor Crater, Ariz., which is visited by thousands of tourists each year. This impact crater, 4,000 feet in diameter and 600 feet deep, was formed in the geologic past (probably 30,00050,000 years before present) by a meteorite striking the Earth at a speed of many thousands of miles per hour.

In addition to Meteor Crater, very fresh, morphologically distinct, impact craters are found at three sites near Odessa, Tex., as well as 10 or 12 other locations in the world. Of the more deeply eroded, less obvious, postulated impact structures, there are about ten well-established sites in the United States and perhaps 80 or 90 elsewhere in the world.

Stocks, Dikes, and Sills

Some types of igneous intrusions typically form at shallower crustal depths; these include stocks, dikes, and sills A stock is smaller than a batholith and typically represents the subsurface passage that fed molten material to a volcano or field of volcanoes over time. Sills and dikes are layers of igneous rock that typically form along fault zones, fractures, or between and parallel to sedimentary layers.

A dikes is a sheetlike body of igneous rock that cuts across layering or contacts in the rock into which it intrudes. Dikes form when magma rises into an existing fracture, or creates a new crack by forcing its way through existing rock, and then solidifies. Hundreds of dikes can invade the cone and inner core of a volcano, sometimes preferentially along zones of structural weakness.

A sill is A tabular body of intrusive igneous rock, parallel to the layering of the rocks into which it intrudes. Stocks, sills, dikes, laccoliths and other intrusions are remnants of past igneous activity and are exposed at the surface long after erosion has stripped away any ancient volcanoes and other overlying rocks and sediments that may have existed in an area.

Last updated April 2022

Structures in the picture above: : A) active magma chamber (called pluton when cooled and entirely crystallized; a batholith is a large rock body composed of several plutonic intrusions) B) old magmatic dykes/dikes C) emerging laccolith D) old pegmatite (late-magmatic dyke formed by aggressive and highly mobile residual melts of a magma chamber) E) old and emerging magmatic sills F) stratovolcano

Accompanied processes and phenomena in the picture above: 1) young, emerging subvolcanic intrusion cutting through older one 2) xenolith (solid rock of high melting temperature which has been transported within the magma from deep below) or roof pendant (fragment of the roof of the magma chamber that has detached from the roof and sunk into the melt) 3) contact metamorphism in the country rock adjacent to the magma chamber (caused by the heat of the magma) 4) uplift at the surface due to laccolith emplacement in the near subground.

Image Sources: Wikimedia Commons, United States Geological Survey (USGS); Volcano Research Center University of Tokyo (the Japan pictures); San Diego State University, U.S. Geological Survey (non-Japan pictures)

Text Sources: United States Geological Survey (USGS), New York Times, Washington Post, Los Angeles Times, Times of London, Yomiuri Shimbun, The Guardian, National Geographic, The New Yorker, Time, Newsweek, Reuters, AP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

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