Volcanic Hazards (Part 2)

Volcanic Landslides

Landslides are large masses of rock and soil that fall, slide, or flow very rapidly under the force of gravity. These mixtures of debris move in a wet or dry state, or both. Volcano landslides range in size from small movements of loose debris on the surface of a volcano to massive collapses of the entire summit or sides of a volcano. If the moving rock debris is large enough and contains a large content of water and fine material, the landslide may transform into a lahar and flow down valley more than 100 km from a volcano. Large-scale landslides along coasts or in oceans can also cause tsunamis; the deadliest on record was caused by a landslide in the Unzen volcano in 1792 which killed 16,000 Japanese, due to landslide debris and the resulting tsunami wave.

Landslides are common on volcanoes because their massive cones (1) typically rise hundreds to thousands of meters above the surrounding terrain; and (2) are often weakened by the very process that created them--the rise and eruption of molten rock. Each time magma moves toward the surface, overlying rocks are shouldered aside as the molten rock makes room for itself, often creating internal shear zones or oversteepening one or more sides of the cone. Magma that remains within the cone releases volcanic gases that partially dissolve in groundwater, resulting in a hot acidic hydrothermal system that weakens rock by altering rock minerals to clay. Furthermore, the tremendous mass of thousands of layers lava and loose fragmented rock debris can lead to internal faults and fault zones that move frequently as the cone "settles" under the downward pull of gravity.

A scientist stands on one of the many small hills that form the chaotic surface of a massive landslide deposit in the upper North Fork Toutle River valley below Mount St. Helens volcano (10 km in distance). Before the landslide and eruption on May 18, 1980, a forest grew on this part of the valley floor, and a highway followed the meandering river to Spirit Lake, a popular recreation area.

Source: http://volcanoes.usgs.gov/Imgs/Jpg/MSH/30212265-050_large.jpg

Lahars

Lahar produced as a result of an eruption by Mt. St Helens
Source: http://www.geology.sdsu.edu/how_volcanoes_work/Images/lahars/laharmsh_l.jpg

Lahar is an Indonesian term that describe mudflow or debris flow composed mostly of volcanic materials on the flanks of a volcano. As a lahar rushes downstream from a volcano, its size, speed, and the amount of water and rock debris it carries constantly change. The beginning surge of water and rock debris often erodes rocks and vegetation from the side of a volcano and along the river valley it enters. But as lahars move farther away from a volcano, they will eventually begin to lose its heavy load of sediment and decrease in size.The speed of lahars can ranges 20 to 40 miles per hour and they can travel for more than 50 miles.

Some lahars contain so much rock debris (60 to 90% by weight) that they look like fast-moving rivers of wet concrete. Close to their source, these flows are powerful enough to rip up and carry trees, houses, and huge boulders miles downstream. Farther downstream they entomb everything in their path in mud.

Eruptions may trigger one or more lahars directly by quickly melting snow and ice on a volcano or ejecting water from a crater lake. More often, lahars are formed by intense rainfall during or after an eruption--rainwater can easily erode loose volcanic rock and soil on hillsides and in river valleys

Lahars almost always occur on or near stratovolcanoes because these volcanoes tend to erupt explosively and their tall, steep cones are either snow covered, topped with a crater lake, constructed of weakly consolidated rock debris that is easily eroded, or internally weakened by hot hydrothermal fluids.

Tsunami

Tsunamis are giant sea waves that form when volcanoes erupt in or near large bodies of water, and are a secondary hazard of eruptions, especially very violent caldera-forming events. Volcanic tsunami can be generate in a number of ways, including the eruption of a submarine volcano, the inward collapse of a volcano during or after an eruption, the flow of volcanic debris down the side of a volcano into the sea, large earthquakes associated with the volcano, or by boiling or expelling water out of a hot, collapsed crater.

Volcanic Tremors and Earthquakes

Seismicity is another common accompaniment of volcanic activity. Eruptions are commonly preceded by local earthquakes, which may be caused by the cracking and splitting open of fissures as the magmas chamber inflates. There are three general categories of earthquakes that can occur at a volcano: volcano-tectonic earthquakes, long period earthquakes and volcanic or harmonic tremors.

Volcano-tectonic earthquakes are earthquakes produced by stress changes in solid rock due to the injection or withdrawal of magma (molten rock). These earthquakes can cause land to subside and can produce large ground cracks. These earthquakes can occur as rock is moving to fill in spaces where magma is no longer present.

Long period earthquakes are produced by the injection of magma into the surrounding rock. These earthquakes are a result of pressure changes during the unsteady transport of the magma. When magma injection is sustained, a lot of earthquakes are produced. This kind of earthquakes can be used to predict future occurrences of volcanic eruptions.

Harmonic tremors largely consist of a more or less continuous, low frequency, rhythmic ground motion. It may be associated with actual movement of magma.


Next: Impact of Volcanic Hazards on DCs and LDCs.

Volcanic Hazards (Part 1)

Introduction

Volcanoes produce a wide variety of hazards that can prove fatal and harmful to people as well as destroy property. Large explosive eruptions can endanger people’s lives and obliterate property hundreds of miles away and even affect global climate.

Various Volcanic Hazards
Source: http://pubs.usgs.gov/fs/fs002-97/

Classification of Volcanic Hazards

• Primary volcanic hazard: occurred as a result of volcanic activity itself.
• Secondary volcanic hazard: caused by a primary effect.
  

List of Volcanic Hazards

• Volcanic gases (primary effect)
• Lava flows and domes (primary effect)
• Pyroclastic flows (primary effect)
• Volcanic landslides (secondary effect)
• Lahars (secondary effect)
• Earthquakes (secondary effect)
• Tsunamis (secondary effect)
  

Some of the deadliest volcanic eruptions in the world

Volcanic Gases

Volcanoes emit gases during eruptions. Even when a volcano is not erupting, cracks in the ground allow gases to reach the surface through small openings called fumaroles. he gaseous portion of magma varies from ~1 to 5% of the total weight. More than 90% of all gas emitted by volcanoes is water vapor (steam), most of which is heated ground water (underground water from rain fall and streams). Other common volcanic gases are carbon dioxide, sulphur dioxide, hydrogen sulfide, hydrogen, and fluorine. Sulphur dioxide gas can react with water droplets in the atmosphere to create acid rain, which causes corrosion and harms vegetation. Carbon dioxide is heavier than air and can be trapped in low areas in concentrations that are deadly to people and animals. Fluorine, which in high concentrations is toxic, can be absorbed onto volcanic ash particles that later fall to the ground. The fluorine on the particles can poison livestock grazing on ash-coated grass and also contaminate domestic water supplies.


Lava Flows and Domes

Lava flow moves through an intersection on the south flank of Kilauea
Source: http://volcanoes.usgs.gov/Hazards/What/Lava/lavaflow.html

Lava flows are streams of molten rock that pour or ooze from an erupting vent. Lava is erupted during either nonexplosive activity or explosive lava fountains. Lava flows destroy everything in their path, but most move slowly enough that people can move out of the way. The speed at which lava moves across the ground depends on several factors, including (1) type of lava erupted and its viscosity; (2) steepness of the ground over which it travels; (3) whether the lava flows as a broad sheet, through a confined channel, or down a lava tube; and (4) rate of lava production at the vent.

Low-silica basalt lava can form fast-moving (10 to 30 miles per hour) streams or can spread out in broad thin sheets up to several miles wide. Since 1983, Kilauea Volcano on the Island of Hawaii has erupted basalt lava flows that have destroyed more than 200 houses and severed the nearby coastal highway.

In contrast, flows of higher-silica andesite and dacite lava tend to be thick and sluggish, traveling only short distances from a vent. Dacite and rhyolite lavas often squeeze out of a vent to form irregular mounds called lava domes. Lava domes often grow by the extrusion of many individual flows >30 m thick over a period of several months or years. Such flows will overlap one another and typically move less than a few meters per hour.


Pyroclastic Flows

Pyroclastic flows descend the south-eastern flank of Mayon Volcano, Philippines.
Source: http://volcanoes.usgs.gov/Imgs/Jpg/Mayon/32923351-020_large.jpg

High-speed avalanches of hot ash, rock fragments, and gas can move down the sides of a volcano during explosive eruptions or when the steep side of a growing lava dome collapses and breaks apart. These pyroclastic flows can be extremely hot and move at speeds of 100 to 150 miles per hour. Such flows tend to follow valleys and are capable of knocking down and burning everything in their paths. Lower-density pyroclastic flows, called pyroclastic surges, can easily overflow ridges hundreds of feet high.

The climactic eruption of Mount St. Helens on May 18, 1980, generated a series of explosions that formed a huge pyroclastic surge. This so-called "lateral blast" destroyed an area of 230 square miles. Trees 6 feet in diameter were mowed down like blades of grass as far as 15 miles from the volcano.

How collapse of a growing lava dome generates the nuée ardente.
Source: http://www.geology.sdsu.edu/how_volcanoes_work/Images/Diagrams/PFDomeCollaps_crop_med.GIF

Pyroclastic flows erupted by Mount Pinatubo on June 15, 1991,
buried the Marella River valley with pumice, ash, and other volcanic rocks
Source: http://volcanoes.usgs.gov/Imgs/Jpg/PFeffects/3041135-092_large.JPG

Tephra and Ash Fall

Tephra is a general term for fragments of volcanic rock and lava regardless of size that are blasted into the air by explosions or carried upward by hot gases in eruption columns or lava fountains. Such fragments range in size from less than 2 mm (ash) to more than 1 m in diameter. Large-sized tephra typically falls back to the ground on or close to the volcano and progressively smaller fragments are carried away from the vent by wind. Volcanic ash, the smallest tephra fragments, can travel hundreds to thousands of kilometers downwind from a volcano.

Tephra consists of a wide range of rock particles (size, shape, density, and chemical composition), including combinations of pumice, glass shards, crystals from different types of minerals, and shattered rocks of all types (igneous, sedimentary, and metamorphic). A great variety of terms are used to describe the range of rock fragments thrown into the air by volcanoes. The terms classify the fragments according to size, shape, or the way in which they form and travel.

Tephra deposit about 9 cm thick blankets former U.S. Clark Air Base, Philippines, about 25 km east of Mount Pinatubo.
Source: http://volcanoes.usgs.gov/Imgs/Jpg/Pinatubo/16112441-008_large.jpg

Classification of Volcano: Eruption Style Pt 2 (Final)

Peléean eruptions

Pyroclastic flow resulted from the eruption of Mt. Pelée at 1902.
Source: http://volcano.und.edu/vwdocs/volc_images/north_america/XXVI.jpg

These eruptions involve viscous magma and shares characteristics with Vulcanian eruptions. They are usually violent and destructive and hence usually resulting in much of the volcano being blown apart. They occur when the gas is highly sticky magma builds up tremendous pressure. This pressure results in a large quantity of gas, dust, ash, and incandescent lava fragments being blown out of a central crater, fall back, and form tongue-like, glowing avalanches that move downslope ("nuées ardentes") at velocities as great as 100 miles per hour. Some of these eruptions may produce domes or short flows or ash and pumice cones. This type of eruption was first described at Mt. Pelée.

Vulcanian Eruptions

An ash-rich vulcanian eruption plumerises above Sabancaya volcano in northern Perú on April 15, 1991.
Source: http://www.volcano.si.edu/world/tpgallery.cfm?category=Pyroclastic%20Fall&photo=047076

Vulcanian eruptions are characterised by the eruption of solid rock and steam. They initially occur as a series of discrete, canon-like explosions that are short-lived, lasting for only minutes to a few hours, often with high-velocity ejections of bombs and blocks. After which, the subsequent eruptions can be relatively quiet and sustained. The fragments deposited by the eruptions can be from ash to blocks in size and cold to incandescent in temperature.

These eruptions are more explosive as compared to Strombolian eruptions as the eruptive columns are normally within 5-10 km high. The amount of tephra produced is relatively small, but due to the explosive nature of the eruption, it is dispersed over a wide area.

Vulcanian eruptions are often connected to andesitic to dacitic magma. The viscous magma makes it difficult for the gases to escape, this leads to the build up of high gas pressure and results in explosive eruptions.

Plinian Eruptions

Klyuchevskaya eruption, Kamchatka in 1994
Source: http://www.geology.sdsu.edu/how_volcanoes_work/Images/Eruptions/Klyuchevskaya_crop_l.jpg

Plinian eruptions are explosive and are associated with volatile-rich dacitic to rhyolitic lava, which erupts from stratovolcanoes. The eruptions are highly variable, lasting from several hours to about 4 days. Although Plinian eruptions characteristically involve felsic magma, they can occasionally occur in fundamentally basaltic volcanoes where the magma chambers become differentiated and zoned to create a siliceous top.

Rather than producing the discrete explosions that are typical of Vulcanian and Strombolian eruptions, Plinian eruptions generate sustained eruptive columns. Although they differ markedly from non-explosive Hawaiian eruptions, Plinian eruptions are similar to Hawaiian fire fountaining in that both of these eruption types generate sustained eruption plumes. In both, the eruption plumes are maintained because the growing bubbles rise at about the same rate as the magma moves up through the central vent system.

Plinian eruptions generate large eruptive columns that are powered upward partly by the thrust of expanding gases, and by convective forces with exit velocities of several hundred meters per second. Some reach heights of ~45 km. These eruptive columns produce widespread dispersals of tephra which cover large areas with an even thickness of pumice and ash. The region of pyroclastic fall accumulation is generally asymmetric around the volcano as the eruptive column is carried in the direction of the prevailing wind.

Most of the composite volcanoes tend to erupt in this manner. Fast-moving deadly pyroclastic flows, also known as nuées ardentes, are also commonly associated with Plinian eruptions.

Classification of Volcano: Eruption Style Pt 1

The eruptive style of a volcano is based on:

• Physical nature of the magma
• Character of explosive activity
• Nature of effusive activity
• Nature of dominant ejecta
• Structures built around vent

The five eruptive style are

• Hawaiian Eruptions
• Strombolian Eruptions
• Peléean Eruptions
• Plinian Eruptions
• Vulcanian Eruptions

Hawaiian Eruption

1984 eruption of Mauna Loa
Source: http://hvo.wr.usgs.gov/maunaloa/history/4305078_L.jpg

Hawaiin eruptions are the calmest of the eruption styles. They are characterized by the effusive emission of highly fluid and basaltic lavas with low gas contents. The volume of the ejected pyroclastic material is less than that of all the other eruption types. The characteristic of Hawaiian eruptions is the steady lava fountaining and the production of thin lava flows which eventually builds up to form large, broad shield volcanoes. Most of the eruption starts from fissures which come together to one or more central vents. The lava flows down away from the source vents in lava channels and lava tubes.

Strombolian Eruption

Strombolian activity on Mt. Etna in October 2002
Source: http://www.geology.sdsu.edu/how_volcanoes_work/Images/Pfeiffer/etna_strom02.jpg

Strombolian eruptions are named from the small volcanic-island of Stromboli, located between Sicily and Italy. The term “strombolian” has been used to describe a variety of volcanic eruptions that varies from small volcanic blasts, to kilometer-high eruptive columns. Strombolian activity is characterized by short-lived, explosive outbursts of pasty lava ejected a few tens or hundreds of meters into the air.

Strombolian eruptions never develop a sustained eruption column, unlike Hawaiian eruptions. The basaltic lava flows are relatively viscous. The gas pressure is also high as it is required to fragment the pasty lava, therefore resulting in periodic explosions with booming blasts. Although they are noisier as compared to Hawaiian eruptions, they are not more dangerous that it. Strombolian eruptions eject bomb sized and lapilli sized fragments that travels in a parabolic manner before it accumulates around the vent to construct the volcanic structure.

Classification of Volcano: Shape Pt 4 (final)

Acid or Dome Volcanoes

Internal structure of a typical lava dome
Source: http://pubs.usgs.gov/gip/volc/fig18.gif

Acid or dome volcanoes are steep-sided, convex cone because of the viscous (high silica content) lava which quickly cools and solidifies near the crater on exposure to air. The volcano grows largely by expansion from within, as indicated in the internal structure of the volcano by the layers of lava fanning upward and outward from the center. As it grows its outer surface cools and hardens. When part of a dome volcano collapses while it still contains molten rock and gases, it produces pyroclastic flow, one of the most lethal forms of volcanic event. Ultimately, many volcanic domes are destroyed by large explosive eruptions. 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.

Volcanic dome atop Novarupta vent, Valley of Ten Thousand Smokes, Katmai National Park and Preserve, Alaska
Source: http://volcanoes.usgs.gov/Imgs/Jpg/Katmai/dds40-015_large.jpg

Next entry: Classification of Volcano by eruption style

Classification of Volcano: Shape Pt 3

Tephra Cone

Internal View of Tephra Cone volcano
Source: http://pubs.usgs.gov/gip/volc/fig8.gif

Cinder cones are the simplest type of volcano. They tend to be mainly explosive volcanoes, but they can also issue lavas. They are small volume cones consisting predominantly of tephra that result from eruptions consisting of rhyolitic and andesitic materials. They are actually fall deposits that are built surrounding the eruptive vent. They show an internal layered structure due to the varying intensities of the explosions that deposit different sizes of pyroclastics. They grow rapidly and soon reach their maximum size, not exceeding 250 meters in height and 500 meters in diameter, although some may rise to as high as 650 meters or more. Cinder cones can occur alone or in small to large groups or fields and most of them have a bowl-shaped crater at the summit. The gradual decrease in volume of the fallout materials (ash, lapilli) at greater distances from the vent leads to gentler slopes at the base of the cone. Cinder cones are commonly found in western North America as well as throughout other volcanic terrains of the world.

Pu`u ka Pele on the flanks of Mauna Kea, Hawaii
Source: http://volcanoes.usgs.gov/Imgs/Jpg/Photoglossary/30424305-084_large.JPG


Mt. Veniaminof in Alaska in its final stages of eruption in 1983-1984
Source: http://volcanoes.usgs.gov/Imgs/Jpg/Veniaminof/dds40-057_large.jpg

Classification of Volcano: Shape Pt 2

Composite Volcano or Stratovolcano

Cross-section of a typical Stratovolcano showing composite,
stratified nature with alternating layers of lava and tephra.
Source: http://pubs.usgs.gov/gip/volc/fig10.gif

Some of the Earth’s grandest mountains are composite volcanoes (aka stratovolcanoes). Stratovolcanoes show alternate layering of lava flows, tephra and volcanic ash, which is why they are called composite volcanoes. They are usually steep-sided, symmetrical cones of large dimension built of alternating layers of viscous andesitic lava flows, volcanic ash, cinders, blocks, and bombs and may rise as much as 8,000 feet above their bases. The high silica content of the magma results in it being viscous. It also makes gases trapped in it harder to escape, which frequently results in explosive eruptions. The steep slope near the summit is partially due to the thick, short viscous lava flows that are unable to travel far down the slope. The gentler base is due to the accumulation of material from the volcano and also pyroclastic material.

Most of the composite volcanoes have a crater at the summit which is formed by the explosive ejection of material from the central vent. Sometimes the craters have been filled in by lava flows or domes, or with glacial ice and less commonly they are filled with water.

The sequence of events for a violent eruption of Mount St. Helens, a composite volcano, in 1980. In the first photograph taken at 8:32:47, the original cone is intact, with some steam rising from the vent. The last photograph taken at 8:33:18,
shows the original cone blowing up.
Source: http://erg.usgs.gov/isb/pubs/teachers-packets/volcanoes/poster/graphics/posterfig6-7.jpg

Some of the most beautiful mountains in the world are composite volcanoes, including Mount Fuji in Japan (shown in the picture below), Mount Mayon in the Phillipines, Mount Cotopaxi in Ecuador , Mount Shasta in California, Mount Hood in Oregon and Mount Rainier in Washington.

Mount Fuji, Japan
Source: http://www.mt-fuji.co.jp/MMF/07/07feb.jpeg

Classification of Volcano: Shape Pt 1

Volcanoes can be classified based on their shape, their eruptive style and the tectonic environment. This entry will begin on classification of volcano through their shapes.

Shield Volcano

The Internal Structure of a Typical Shield Volcano
Source: http://pubs.usgs.gov/gip/volc/fig15.gif

Shield volcanoes form the largest volcanoes on Earth. They have gentle upper slopes and somewhat steeper lower slopes (somehow resembling a warrior’s shield, thus its name). It is built up slowly by the accretion of thousands of flows of low viscosity basalitic magma (which is very fluid) that easily spreads over great distances from the summit vent, then cooling as thin gently sloped sheets. The viscosity of magma is dependent on its temperature and composition. Shield volcanoes erupt magma as hot as 1,200 °C, compared with 850 °C for most continental volcanoes, which are usually composed of acidic lava. Because of the fluidity of the lava, major explosive eruptions do not occur in shield volcanoes. Lava also erupts from the vents along fractures that develop on the flanks of the cone. This gives the shield volcanoes the circular/oval shape with nearly flat summits.

The largest shield volcano, Mauna Loa volcano on the Island of Hawaii
Source: http://volcanoes.usgs.gov/Imgs/Jpg/Photoglossary/shieldvolcano1_large.jpg

Newberry Volcano, Oregon
USGS photo by Lyn Topinka

The Hawaiian Islands are composed of linear chains of the shield volcanoes. The islands are more than 15,000 feet above the ocean floor. For the Mauna Loa, the largest shield volcano, it is about 13,677 feet above sea level and its summit is 28,000 feet above the ocean floor. In Northern California and Oregon, many of the shield volcanoes have diameters of 3 or 4 miles and heights of 1,500 to 2,000 feet.

Formation- Case Study: Hawaiian Islands

The Hawaiian Islands form an archipelago of nineteen islands and atolls, numerous smaller islets, and undersea seamounts trending northwest by southeast in the North Pacific Ocean. The archipelago takes its name from the largest island in the group and extends some 1500 miles (2400 km) from the Island of Hawaii in the south to northernmost Kure Atoll.

The principal Hawaiian islands (all capital letters) are the exposed tops of volcanoes that rise tens of thousands of feet above the ocean floor.
Source: http://pubs.usgs.gov/gip/hawaii/fig02.gif

The chain of islands or archipelago formed as the Pacific plate moved slowly northwestward over a hotspot in the Earth's mantle at about 52 km (32 miles) per million years. This hot spot partly melts the region just below the overriding Pacific Plate, producing small, isolated blobs of magma. Less dense than the surrounding solid rock, the magma rises buoyantly through structurally weak zones and ultimately erupts as lava onto the ocean floor to form volcanoes.

Hawaii (the Big Island) is the largest and youngest island in the chain, built from five different volcanoes. Kohala, at the northwestern corner of the island, is the oldest and the smallest, having ceased eruptive activity about 60,000 years ago and go through longest exposure to erosion. The second oldest is Mauna Kea, which last erupted about 3,000 years ago; next is Hualalai, which has had only one historic eruption (1800-1801), and, lastly, both Mauna Loa and Kilauea have been vigorously and repeatedly active in historic times. Because it is growing on the southeastern flank of Mauna Loa, Kilauea is believed to be younger than its huge neighbor. Mauna Loa, comprising over half of the Big Island, is the largest shield volcano on the planet. The measurement from the base locally depressing the sea floor in the Hawaiian Trough to its peak is about 17 km.

Mauna Loa Volcano towers nearly 3,000 m above the much smaller Kilauea Volcano
(caldera in left center).
Source: http://hvo.wr.usgs.gov/maunaloa/4303062_L.jpg

Almost all magma created in the hotspot has the composition of basalt, and so the Hawaiian volcanoes are constructed almost entirely of this igneous rock and its coarse-grained equivalents, gabbro and diabase. The majority of eruptions in Hawaiʻi are Hawaiian-type eruptions because basaltic magma is relatively fluid compared with magmas typically involved in more explosive eruptions, such as the andesitic magmas that produce some of the spectacular and dangerous eruptions around the margins of the Pacific basin.

Formation of Volcanoes

How Do Volcanoes Form?

Volcanic activity is closely linked with plate tectonics (except for the theory of hot spot). As the Earth's huge plates move and interact in the lithosphere, or the planet's outer shell, they trigger a very gradual chain of events that can end with explosions.

Formation of Volcanoes
Taken from http://www.nationalgeographic.com/forcesofnature/forces/volcanoes.html

1. Island Arc

When two plates come together, one of the plates may slide under another in the process called subduction. Heat from deep in the Earth melts rock in the descending plate. An island-arc volcano is a type of subduction-zone volcano. In subduction zone, either island arc or volcanic arc can be formed. Island arc volcanoes occur when one oceanic plate subducts, or descends, under another oceanic plate. The crustal portion of the subducting slab contains a significant amount of surface water, as well as water contained in hydrated minerals within the seafloor basalt. As the subducting slab descends to greater and greater depths, it progressively encounters greater temperatures and greater pressures which cause the slab to release water into the mantle wedge overlying the descending plate. Water has the effect of lowering the melting temperature of the mantle, thus causing it to melt. The magma produced by this mechanism is usually andesite in composition. It buoyantly rises upward to produce a linear belt of volcanoes parallel to the oceanic trench. This action can then create a chain of volcanic islands.

Island arc formed by oceanic-oceanic collision
Taken from http://www.geology.sdsu.edu/how_volcanoes_work/Images/Diagrams/osubduction.gif

2. Hot Spots
About 5 percent of all known volcanoes form in the middle of plates, not at their edges. These intraplate volcanoes are caused by hot spot, unusually hot areas deep within the Earth. Magma rises from the hot spots and erupts as lava through crack in the Earth's surface, forming volcanoes. Mantle plumes appear to be largely unaffected by plate motions. As plates move across stationary hotspots, volcanism will generate volcanic islands that are active above the mantle plume, but become inactive and progressively older as they move away from the mantle plume in the direction of plate movement. Thus, a linear belt of inactive volcanic islands will be produced. A classic example of this mechanism is demonstrated by the Hawaiian seamount chain.

Hawaiian Seamount Chain model
Taken from http://www.geology.sdsu.edu/how_volcanoes_work/Images/Diagrams/hotspot_diagram_usgs_s.gif

3. Spreading Centres
Volcanoes called rift volcanoes arise in areas called spreading centres. In these zones plates are moving away from each other, spreading or splitting the Earth's crust. Basaltic magma, derived from the partial melting of the mantle, is injected into the fissures or extruded as fissure eruptions.

4. Continental Rift Zones
When spreading centres develop within continents, they form new plate boundaries and trigger volcanic activity. Spreading may have created East Africa's volcanic Great Rift Valley.

Oldoinyo Lengai, an active volcano in the East African Rift Zone.
Taken from http://pubs.usgs.gov/gip/dynamic/graphics/Oldoinyo_erupts.gif

5. Volcanic Arc
Volcanic arc occurs when one denser oceanic plate subducts under a less dense continental plate plate, leading to partial melting of the descending water-rich oceanic plate and some of the overlying mantle. This process produces low-density magma which rises onto the overlying continental crust, through faults and cracks in the lithosphere, where it will cool and crystallise at depth. The accumulation of magma on the continental plate over time would lead to the formation of volcanic arc.

Volcanic arc formed by oceanic-continental subduction
Taken from http://www.geology.sdsu.edu/how_volcanoes_work/Images/Diagrams/csubduction.gif

Andes Mountain, a volcanic arc formed by the convergence of the oceanic Nazca plate
and continental part of South American plate.
Taken from http://en.wikipedia.org/wiki/Andes

Volcano: Introduction

Volcan Pacaya, Guatemala

Volcano is a mountain or hill, which is formed by the accumulation of materials such as hot, molten rock, and ash erupted through one or more openings, which is called volcanic vents, in the earth's surface. Their formation can be attributed to the movement of plates and the existence of mantle plumes.

Cross section of basic components of a volcano
Taken from http://www.bbc.co.uk/schools/gcsebitesize/geography/images/g_lwpt_v_1.gif

The word volcano comes from the little island of Vulcano in the Mediterranean Sea off Sicily. Centuries ago, the people living in this are 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.

About 1,900 volcanoes are active today or known to have been active in historical times. Almost 90 percent of the volcanoes are found in the Ring of Fire, a band of Volcanoes circling the edges of the Pacific Ocean.

Pacific Ring of Fire
Taken from http://pubs.usgs.gov/gip/dynamic/fire.html

Volcanic eruptions bring both hazards and benefits to people. For example, while lava flows can be highly destructive, they also provide fertile soils for agriculture. They can also build new land, as they have in Hawaii.

Next entry: Formation of Volcanoes