Facts and Details

VOLCANOES: TYPES AND STRUCTURE

Shishaldin Volcano in eastern Russia 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.

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.

Websites and Resources

false color image of plume from
eruption of Eyjafjallajokull in Iceland Links in this Website: VOLCANOES AND JAPAN Factsanddetails.com/Japan ; MAJOR VOLCANOES AND ERUPTIONS IN JAPAN Factsanddetails.com/Japan ; EARTHQUAKES AND JAPAN Factsanddetails.com/Japan ; EARTHQUAKES AND LIFE IN JAPAN Factsanddetails.com/Japan ; LARGE EARTHQUAKES IN JAPAN Factsanddetails.com/Japan ; KOBE EARTHQUAKE OF 1995 Factsanddetails.com/Japan ; LARGE EARTHQUAKES IN JAPAN IN THE 2000s Factsanddetails.com/Japan ; TSUNAMIS IN JAPAN Factsanddetails.com/Japan

Good Websites and Sources on Volcanoes: USGS Volcanoes volcanoes.usgs.gov ; Volcano World volcano.oregonstate.edu ; How Volcanoes Work geology.sdsu.edu ; Volcanoes.com volcanoes.com ; Smithsonian Global Volcanism Program volcano.si.edu Electronic Volcano dartmouth.edu/~volcano ; Volcano Tourism volcanolive.com ; Protection from a Volcano Report from FEMA fema.gov/hazard/volcano ;Wikipedia Volcano article Wikipedia

Volcano Pictures Volcano Photo Gallery decadevolcano.net/photos ; Archive of Volcano Photos doubledeckerpress.com ; Volcano Eruptions: Ancient and Modern from Life Magazine life.com/image/first ; Google Map Look at Active Volcanoes geocodezip.com/v2_activeVolcanoes. ; Volcano Videos Discovery Channel dsc.discovery.com/videos/volcano-video ; National Geographic nationalgeographic.com/video

eruption of Usu volcano
on Hokkaido Volcano Information in Japan: Volcano Research Center at Tokyo University eri.u-tokyo.ac.jp ; Volcano Information from Japan Meteorological Agency Japan Meteorological Agency ; Tectonics and Volcanoes of Japan volcano.oregonstate.edu ; Laboratory of Volcano Physics at the University of Hokkaido uvo.sci.hokudai.ac ; Hayakawa Paleovolcanology Laboratory (last updated in 2000) edu.gunma-u.ac.jp ; Wikipedia List of Volcanoes in Japan Wikipedia ; USGS Volcanoes of Japan vulcan.wr.usgs.gov/Volcanoes/Japan and vulcan.wr.usgs.gov/Volcanoes/Japan/Maps ; Volcano and Earthquake Map homepage3.nifty.com ; U.S. Professor Disappears During Volcano Hike cnn.com/2009 ; Earthquakes in Japan japan-guide.com

Websites: Weekly Volcano Activity Report (www.volcano.si.edu/gvp/usgs/index.htm ); Volcano World www.volcano.und.nodak.edu ), run by the University of North Dakota. Cascade Volcano Observatory of the United States Geological Survey. Global Volcanism Program (www.volcano.si.edu/gvp ), operated by the Smithsonian has a catalog of over 8,000 eruptions in that 10,000 years.

Book: Volcanoes in Human History by Jelle Zeilinga de Boer and Donald Theodore Sanders (Princeton University Press, 2002)

Plate Tectonics

subduction zone Many geological features and phenomena 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. In some cases the motion produces a descending convection current that sucks down the ocean floor. In these places deep seas trenches form; mountain building activity and earthquakes occur; and millions of tons of rock sink into the crust everyday.

Subduction faults are usually angled at about 10 to 15 percent and often located where major oceans and continents meet. 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.

the Earth's plates 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. Most rift zones are in the middle of the oceans. The Mid-Atlantic Ridge, which runs north-to-south the entire length of the Atlantic is one.

What generates all this activity is the slow release of heat from the interior of the earth. This heat is produced by the radioactive decay of elements such as uranium, potassium and thorium. The heats causes rock to liquify into magma and creates pressures that makes the plates move.

Power and Attraction of Volcanoes

Krakatoa Volcanoes destroy and volcanoes create. The catastrophic eruption of Mount St. Helens on May 18, 1980, made clear the awesome destructive power of a volcano. Yet, over a time span longer than human memory and record, volcanoes have played a key role in forming and modifying the planet upon which we live. More than 80 percent of the Earth's surface--above and below sea level--is of volcanic origin. Gaseous emissions from volcanic vents over hundreds of millions of years formed the Earth's earliest oceans and atmosphere, which supplied the ingredients vital to evolve and sustain life. Over geologic eons, countless volcanic eruptions have produced mountains, plateaus, and plains, which subsequent erosion and weathering have sculpted into majestic landscapes and formed fertile soils. [Source: Robert I. Tilling, USGS]

Ironically, these volcanic soils and inviting terranes have attracted, and continue to attract, people to live on the flanks of volcanoes. Thus, as population density increases in regions of active or potentially active volcanoes, mankind must become increasingly aware of the hazards and learn not to "crowd" the volcanoes. People living in the shadow of volcanoes must live in harmony with them and expect, and should plan for, periodic violent unleashings of their pent-up energy.

Vesuvius, Mt. St. Helens and the Island of Vulcan

Mt. St. Helens eruption in 1980 On August 24, A.D. 79,Vesuvius Volcano suddenly exploded and destroyed the Roman cities of Pompeii and Herculaneum. Although Vesuvius had shown stir-rings of life when a succession`of earthquakes in A.D. 63 caused some damage, it had been literally quiet for hundreds of years and was considered "extinct." Its surface and crater were green and covered with vegetation, so the eruption was totally unexpected. Yet in a few hours, hot volcanic ash and dust buried the two cities so thoroughly that their ruins were not uncovered for nearly 1,700 years, when the discovery of an outer wall in 1748 started a period of modern archeology. Vesuvius has continued its activity intermittently ever since A.D 79 with numerous minor eruptions and several major eruptions occurring in 1631, 1794, 1872, 1906 and in 1944 in the midst of the Italian campaign of World War II.

In the United States on March 27, 1980, Mount St. Helens Volcano in the Cascade Range, southwestern Washington, reawakened after more than a century of dormancy and provided a dramatic and tragic reminder that there are active volcanoes in the "lower 48" States as well as in Hawaii and Alaska.The catastrophic eruption of Mount St. Helens on May 18, 1980, and related mudflows and flooding caused significant loss of life (57 dead or missing) and property damage lover $1.2 billion). Mount St. Helens is expected to remain intermittently active for months or years, possibly even decades.

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.

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.

Nature of Volcanoes

crater of Tambora 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.

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.

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.

Magma, Lava and Composition of Volcanic Materials

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.

Principal Types of Volcanoes

Cinder cone volcano Geologists generally group volcanoes into four main kinds: 1) cinder cones, 2) composite volcanoes, 3) shield volcanoes, and 4) lava domes.

Cinder cones are the simplest type of volcano. They are built from particles and blobs of congealed lava ejected from a single vent. As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as cinders around the vent to form a circular or oval cone. Most cinder cones have a bowl-shaped crater at the summit and rarely rise more than a thousand feet or so above their surroundings. Cinder cones are numerous in western North America as well as throughout other volcanic terrains of the world.

In 1943 a cinder cone started growing on a farm near the village of Parícutin in Mexico. Explosive eruptions caused by gas rapidly expanding and escaping from molten lava formed cinders that fell back around the vent, building up the cone to a height of 1,200 feet. The last explosive eruption left a funnel-shaped crater at the top of the cone. After the excess gases had largely dissipated, the molten rock quietly poured out on the surrounding surface of the cone and moved downslope as lava flows. This order of events--eruption, formation of cone and crater, lava flow--is a common sequence in the formation of cinder cones.

During 9 years of activity, Parícutin built a prominent cone, covered about 100 square miles with ashes, and destroyed the town of San Juan. Geologists from many parts of the world studied Parícutin during its lifetime and learned a great deal about volcanism, its products, and the modification of a volcanic landform by erosion.

Composite Volcanoes

Some of the Earth's grandest mountains are composite volcanoes--sometimes called stratovolcanoes. They are typically steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs and may rise as much as 8,000 feet above their bases. Some of the most conspicuous and beautiful mountains in the world are composite volcanoes, including Mount Fuji in Japan, Mount Cotopaxi in Ecuador, Mount Shasta in California, Mount Hood in Oregon, and Mount St. Helens and Mount Rainier in Washington.

Most composite volcanoes have a crater at the summit which contains a central vent or a clustered group of vents. Lavas either flow through breaks in the crater wall or issue from fissures on the flanks of the cone. Lava, solidified within the fissures, forms dikes that act as ribs which greatly strengthen the cone.

The essential feature of a composite volcano is a conduit system through which magma from a reservoir deep in the Earth's crust rises to the surface. The volcano is built up by the accumulation of material erupted through the conduit and increases in size as lava, cinders, ash, etc., are added to its slopes.

When a composite volcano becomes dormant, erosion begins to destroy the cone. As the cone is stripped away, the hardened magma filling the conduit (the volcanic plug) and fissures (the dikes) becomes exposed, and it too is slowly reduced by erosion. Finally, all that remains is the plug and dike complex projecting above the land surface--a telltale remnant of the vanished volcano.

An interesting variation of a composite volcano can be seen at Crater Lake in Oregon. From what geologists can interpret of its past, a high volcano--called Mount Mazama- probably similar in appearance to present-day Mount Rainier was once located at this spot. Following a series of tremendous explosions about 6,800 years ago, the volcano lost its top. Enormous volumes of volcanic ash and dust were expelled and swept down the slopes as ash flows and avalanches. These large-volume explosions rapidly drained the lava beneath the mountain and weakened the upper part. The top then collapsed to form a large depression, which later filled with water and is now completely occupied by beautiful Crater Lake. A last gasp of eruptions produced a small cinder cone, which rises above the water surface as Wizard Island near the rim of the lake. Depressions such as Crater Lake, formed by collapse of volcanoes, are known as calderas. They are usually large, steep-walled, basin-shaped depressions formed by the collapse of a large area over, and around, a volcanic vent or vents. Calderas range in form and size from roughly circular depressions 1 to 15 miles in diameter to huge elongated depressions as much as 60 miles long.

Subduction Zones and Composite Volcanoes

Volcanic Arc system At a subduction zones two tectonic plates collide head on, with one plate going over he top, forcing the other one down. In some cases the motion produces a descending convection current that sucks down the ocean floor. In these places deep seas trenches form; mountain building activity and earthquakes occur; and millions of tons of rock sink into the crust everyday. Subduction faults are usually angled at about 10 to 15 percent and often located where major oceans and continents meet. 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. Subduction zones are also described as convergent boundaries (See Plate tectonics)

Subduction zone volcanoes are fueled by the heat generated from the friction of the two plates colliding together and the release of water and gases, which diffuses upwards and soaks into the mantle rock, causing it to melt and form magma.

Composite volcanoes are found along subduction zones. Potentially dangerous, they are created by successive mountain building eruptions and can erupt on an annual basis or lie dormant for centuries and suddenly explode. Frequently erupting volcanoes are much less dangerous than those that lie quiet and build up pressure and finally explode with catastrophic force.

Subduction zone volcanoes are more explosive than rift zone volcanoes in part because their magma composition is different. The basalt that emerges from rift zone volcanoes is relatively uniform and fluid and tends to flow like a river. Conversely, the magma and lava found in subduction zones is less uniform and fluid. It is formed from the melting and merging of basaltic rock and sediments, mixed with water and goes, and tend to much more viscous. As a result it doesn’t move smoothly out of cracks or flow like a river like basalt does. Rather it congeals in the throats of the volcanoes, causing pressure to build up that can cause a massive explosion.

Evolution of a Composite Volcano

collapse phase of an eruption A. Magma, rising upward through a conduit, erupts at the Earth's surface to form a volcanic cone. Lava flows spread over the surrounding area.

B. As volcanic activity continues, perhaps over spans of hundreds of years, the cone is built to a great height and lava flows form an extensive plateau around its base. During this period, streams enlarge and deepened their valleys.

C. When volcanic activity ceases, erosion starts to destroy the cone. After thousands of years, the great cone is stripped away to expose the hardened "volcanic plug" in the conduit. During this period of inactivity, streams broaden their valleys and dissect the lava plateau to form isolated lava-capped mesas.

D. Continued erosion removes all traces of the cone and the land is worn down to a surface of low relief. All that remains is a projecting plug or "volcanic neck," a small lava-capped mesa, and vestiges of the once lofty volcano and its surrounding lava plateau.

Shield Volcanoes

Kilauea eruption Shield volcanoes, the third type of volcano, are built almost entirely of fluid lava flows. Flow after flow pours out in all directions from a central summit vent, or group of vents, building a broad, gently sloping cone of flat, domical shape, with a profile much like that of a warrior's shield. They are built up slowly by the accretion of thousands of highly fluid lava flows called basalt lava that spread widely over great distances, and then cool as thin, gently dipping sheets. Lavas also commonly erupt from vents along fractures (rift zones) that develop on the flanks of the cone. Some of the largest volcanoes in the world are shield volcanoes. In northern California and Oregon, many shield volcanoes have diameters of 3 or 4 miles and heights of 1,500 to 2,000 feet. The Hawaiian Islands are composed of linear chains of these volcanoes including Kilauea and Mauna Loa on the island of Hawaii-- two of the world's most active volcanoes. The floor of the ocean is more than 15,000 feet deep at the bases of the islands. As Mauna Loa, the largest of the shield volcanoes (and also the world's largest active volcano), projects 13,677 feet above sea level, its top is over 28,000 feet above the deep ocean floor.

In some eruptions, basaltic lava pours out quietly from long fissures instead of central vents and floods the surrounding countryside with lava flow upon lava flow, forming broad plateaus. Lava plateaus of this type can be seen in Iceland, southeastern Washington, eastern Oregon, and southern Idaho. Along the Snake River in Idaho, and the Columbia River in Washington and Oregon, these lava flows are beautifully exposed and measure more than a mile in total thickness.

Lava Domes

explanation of pyroclastic
flows and dome collapse
on Unzen
Volcanic or 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." 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.

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.

Submarine Volcanoes

Surtsey eruption Submarine volcanoes and volcanic vents are common features on certain zones of the ocean floor. Some are active at the present time and, in shallow water, disclose their presence by blasting steam and rock-debris high above the surface of the sea. Many others lie at such great depths that the tremendous weight of the water above them results in high, confining pressure and prevents the formation and explosive release of steam and gases. Even very large, deep-water eruptions may not disturb the ocean surface.

The unlimited supply of water surrounding submarine volcanoes can cause them to behave differently from volcanoes on land. Violent, steam-blast eruptions take place when sea water pours into active shallow submarine vents. Lava, erupting onto a shallow sea floor or flowing into the sea from land, may cool so rapidly that it shatters into sand and rubble. The result is the production of huge amounts of fragmental volcanic debris. The famous "black sand" beaches of Hawaii were created virtually instantaneously by the violent interaction between hot lava and sea water. On the other hand, recent observations made from deep-diving submersibles have shown that some submarine eruptions produce flows and other volcanic structures remarkably similar to those formed on land. Recent studies have revealed the presence of spectacular, high temperature hydrothermal plumes and vents (called "smokers") along some parts of the mid-oceanic volcanic rift systems. However, to date, no direct observation has been made of a deep submarine eruption In progress.

During an explosive submarine eruption in the shallow open ocean, enormous piles of debris are built up around the active volcanic vent. Ocean currents rework the debris in shallow water, while other debris slumps from the upper part of the cone and flows into deep water along the sea floor. Fine debris and ash in the eruptive plume are scattered over a wide area in airborne clouds. Coarse debris in the same eruptive plume rains into the sea and settles on the flanks of the cone. Pumice from the eruption floats on the water and drifts with the ocean currents over a large area.

Surtsey, the Creation of an Island

Surtsey eruption On November 14, 1963 a fishing boat saw some smoke rising from the sea. Thinking it was a ship on fire the crew went over to investigate and possibly help out. When they approached they saw a column of smoke as wide as a town shooting thousands of feet into the air. The plume was so large it created its own lightning. [Source: Sigurdur Thorarinsson, National Geographic, May 1965]

The plume, it turned out, was coming from a volcano that was rising up from the seafloor 450 feet below the surface and was trying to establish itself as an island. Named Surtsey after a Norse god who tried to set the world on fire the volcano erupted most violently in the first few days after it was spotted. At times 400,000 tons of ash spewed out every second creating a 50,000 foot column of ash, steam and smoke that could be seen 75 miles away in Reykjavík.

When the volcano had begun to establish itself as an island about three weeks after the eruption began three daredevil French journalists landed on Surtsey, took some pictures and managed to escape unhurt. Seven Icelanders for Westman Island—a fiercely independent people who didn't like the name Surtsey—landed a week later. They carried a marker that bore the name Vesturey—the name the Westman Islanders had selected— but were greeted with such a barrage of volcanic debris they never had time to plant the marker and barely escaped with their lives. In the end they decided that Surtsey was a fine name. Another group that waded through the surf to get to shore sank in waist deep quick sand and nearly drowned.

Surtsey in 1999 Icelandic volcanologist Gigurdur Thorarinsson arrived at Surtsey in February to plant the Icelandic flag. Although the volcano was relatively quiet when they set off from their mother ship rafts it starting erupting violently when they set foot on the island. Falling bombs of lava caused geyserlike explosions in the sea. To avoid getting hit Thoraninsson said he made an effort to stand still and stare skyward and watch the trajectory of the bombs. The trick he said was not to dodge them until a moment before they seemed about to land on our heads. The largest of these bombs were about a yard across. When they fell to the earth they punctured holes in the sand that filled with water that immediately started to boil from contact with the glowing bombs

The big question was whether or not the island was going to survive. not. While it only erupted ash and pumice—which are easily washed away by the surf—hopes were raised about its future when rivers of lava started spewing forth in a "Hawaiian type" eruption. In 1967 when the eruptions finally stopped Surtsey was three square kilometers in size. Since then strong winds and waves have reduced its size by a third. The chances are it will survive. The ash and pumice near the core of the volcano has already fused into hard rock, a process that scientist used to think took thousands of years. The eruption was a boon not only to geologists but also for biologist, who could study how life plants itself on a piece of land completely devoid of life. Twenty-five years after Surtsey's birth about 25 species of plants were, recorded, their seeds bourn by wind, floatied debris, carried by birds, or even attached to fish eggs that float ashore. Some species have failed to take hold but 18 still grow there, covering between one and two percent of the island. The sea sandwort, which first appeared in 1967, is the most common plant. Six species of seabirds breed on the island. [National Geographic Geographica, October 1980].

Surtsey is now very popular with popular with seabirds. Many even landed on the island while it was erupting to warm their feet. The birds are key to the islands survival. Their excrement provides fertilizer for the plants on the island which in turn help protect the island from eroding away. Birds are attracted to plants and sheltered area to raise their young. Scientists believe that Iceland is a bunch of Surtsey-like volcanos that joined together about 15 million years ago.*

Plugs, Maars and Nonvolcanic Craters

Devil's Tower 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.

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

Igneous structures 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.

Glaciers, Climate and Volcanoes

Pinatubo eruption Among the more prominent theories of events that have triggered global climatic changes and lead to repeated glaciation are: (1) known astronomical variations in the orbital elements of the Earth (the so-called Milankovitch theory); (2) changes in energy output from the Sun; and (3) increases in volcanism that could have thrown more airborne volcanic material into the stratosphere, thereby creating a dust veil and lowered temperatures.

The years 1980, 1981, and 1982, for example, saw several major volcanic eruptions adding large quantities of particulate volcanic material and volatiles to the stratosphere, including the catastrophic eruption of Mount St. Helens, Washington, on May 18, 1980, and a large eruption of Mount Hekla, Iceland, on August 17, 1980. The 1982 series of eruptions from El Chichón volcano, Mexico, caused death and destruction in the populated area around the volcano, but a further reaching impact may result from the effect on Earth's climate because of the enormous ejection of volcanic material into the stratosphere.

The potential climatic effect of the Laki volcanic eruption in Iceland in 1783, the largest effusive (lava) volcanic eruption in historic time, was noted by the diplomat-scientist Benjamin Franklin in 1784, during one of his many sojourns in Paris. Franklin concluded that the introduction of large quantities of volcanic particles into the Earth's upper atmosphere could cause a reduction in surface temperature, because the particles would lessen the amount of solar energy reaching the Earth's surface. The catastrophic eruption of the Tambora volcano, Indonesia, in 1815 was followed by a so-called "year-without-a-summer." In New England, for example, frost occurred during each of the summer months in 1816.

Benefits of Volcanoes and Volcanic Soils

Pinatubo dust layer Volcanic materials ultimately break down to form some of the most fertile soils on Earth, cultivation of which fostered and sustained civilizations. People use volcanic products as construction materials, as abrasive and cleaning agents, and as raw materials for many chemical and industrial uses. The internal heat associated with some young volcanic systems has been harnessed to produce geothermal energy. For example, the electrical energy generated from The Geysers geothermal field in northern California can meet the present power consumption of the city of San Francisco.

Why are volcanic soils fertile? Volcanic materials make soil fertile when they have had the chance to weather and start to break down and release their nutrients. Apart from water and carbon dioxide, plants need three essential nutrients to grow: nitrogen, potassium and phosphorous. They also need some iron to create chlorophyll, which primary function is to absorb sunlight for photosynthesis to occur within the plant. A process which is possible in the presence of radiant energy (light) where carbon dioxide and water are converted into oxygen and organic materials that can be used within the plant. Volcanic materials can be sources of nitrogen, potassium and phosphorous. Furthermore, volcanic soil can also supply in small quantities a number of trace elements that may be scarce, but are very important and necessary to allow plants to make the right proteins and other molecules necessary for life. [Source: Yahoo Answers]

The fertile soil is a result of the breakdown of various minerals - such as olivine, pyroxene, amphibole, and feldspar (the essential ingredients of volcanic ash and lava) which releases iron, magnesium, potassium and other nutrients into to the soil. Lavas and ash are rich in potassium and iron, which is often a limiting nutrient. Certain types of crystallized lava can be very porous, and vesicular, meaning that they can hold large amounts of water, especially if they've been given time to weather and erode. Certain lavas, dependent on local magma chemistry, can include significant amounts of magnesium, silica, aluminum, sodium, and chlorine.

Volcanic soils often carry many non-crystalline (amorphous) minerals, such as allophone and imogolite, which form strong bonds with organic materials, which, in addition to the elemental chemistry in the volcanic soils, allows enormous amounts of plant life to take root. New lavas aren't very fertile. They need time to weather and release their nutrients, and to open up those pores. This is why in Hawaii Kauai'i (an older island where lava has had time to weather and break down) is the Garden Isle and the Big Island (a younger, volcanically-active island where lava has not had time to weather and break down) has surprisingly few places that are actually lush with vegetation.

However, the dispersal of volcanic ash can also be devastating to soil and organics. When Mt. St. Helens went off, its ash covered enormous areas. Its particular ash acted as somewhat of a clay mineral when it gathered, especially when it bonded with precipitation, forming a fairly impermeable layer that hurt not only soils, but also stuck directly to plants themselves, causes physical trauma and chemical trauma as well, blocking out light and gas intake/release for photosynthesis.

Extraterrestrial Volcanism

Volcanoes and volcanism are not restricted to the planet Earth. Manned and unmanned planetary explorations, beginning in the late 1960's, have furnished graphic evidence of past volcanism and its products on the Moon, Mars, Venus and other planetary bodies. Many pounds of volcanic rocks were collected by astronauts during the various Apollo lunar landing missions. Only a small fraction of these samples have been subjected to exhaustive study by scientists. The bulk of the material is stored under controlled-environment conditions at NASA's Lunar Receiving Laboratory in Houston, Tex., for future study by scientists.

From the 1976-1979 Viking mission, scientists have been able to study the volcanoes on Mars, and their studies are very revealing when compared with those of volcanoes on Earth. For example, Martian and Hawaiian volcanoes closely resemble each other in form. Both are shield volcanoes, have gently sloping flanks, large multiple collapse pits at their centers, and appear to be built of fluid lavas that have left numerous flow features on their flanks. The most obvious difference between the two is size. The Martian shields are enormous. They can grow to over 17 miles in height and more than 350 miles across, in contrast to a maximum height of about 6 miles and width of 74 miles for the Hawaiian shields.

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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© 2008 Jeffrey Hays

Last updated January 2012