DESCRIPTION:
"Ring of Fire", Plate Tectonics, Sea-Floor Spreading, Subduction Zones, "Hot Spots"

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Map, Active Volcanoes, Plate Tectonics, and the "Ring of Fire"
-- Modified from: Tilling, Heliker, and Wright, 1987, and Hamilton, 1976

Volcanoes are not randomly distributed over the Earth's surface. Most are concentrated on the edges of continents, along island chains, or beneath the sea forming long mountain ranges. More than half of the world's active volcanoes above sea level encircle the Pacific Ocean to form the circum-Pacific "Ring of Fire." In the past 25 years, scientists have developed a theory -- called plate tectonics -- that explains the locations of volcanoes and their relationship to other large-scale geologic features.

The peripheral areas of the Pacific Ocean Basin, containing the boundaries of several plates are dotted by many active volcanoes that form the so-called "Ring of Fire". The "Ring" provides excellent examples of "plate-boundary" volcanoes, including Mount St. Helens.

According to the new, generally accepted "plate-tectonics" theory, scientists believe that the Earth's surface is broken into a number of shifting slabs or plates, which average about 50 miles in thickness. These plates move relative to one another above a hotter, deeper, more mobile zone at average rates as great as a few inches per year. Most of the world's active volcanoes are located along or near the boundaries between shifting plates and are called "plate-boundary" volcanoes. However, some active volcanoes are not associated with plate boundaries, and many of these so-called "intra-plate" volcanoes form roughly linear chains in the interior of some oceanic plates. The Hawaiian Islands provide perhaps the best example of an "intra-plate" volcanic chain, developed by the northwest-moving Pacific Plate passing over an inferred "hot spot" that initiates the magma-generation and volcano-formation process. The peripheral areas of the Pacific Ocean Basin, containing the boundaries of several plates are dotted by many active volcanoes that form the so-called "Ring of Fire". The "Ring" provides excellent examples of "plate-boundary" volcanoes, including Mount St. Helens. ...

In the Pacific Northwest, the Juan de Fuca Plate plunges beneath the North American Plate, locally melting at depth; the magma rises to feed and form the Cascade volcanoes.

Volcanoes are not randomly distributed over the Earth's surface. Most are concentrated on the edges of continents, along island chains, or beneath the sea forming long mountain ranges. More than half of the world's active volcanoes above sea level encircle the Pacific Ocean to form the circum-Pacific "Ring of Fire." In the past 25 years, scientists have developed a theory -- called plate tectonics -- that explains the locations of volcanoes and their relationship to other large-scale geologic features.

According to this theory, the Earth's surface is made up of a patchwork of about a dozen large plates that move relative to one another at speeds from less than one centimeter to about ten centimeters per year (about the speed at which fingernails grow). These rigid plates, whose average thickness is about 80 kilometers, are spreading apart, sliding past each other, or colliding with each other in slow motion on top of the Earth's hot, pliable interior. Volcanoes tend to form where plates collide or spread apart, but they can also grow in the middle of a plate, as for example the Hawaiian volcanoes.

The boundary between the Pacific and Juan de Fuca Plates is marked by a broad submarine mountain chain about 500 kilometers long, known as the Juan de Fuca Ridge. Young volcanoes, lava flows, and hot springs were discovered in a broad valley less than 8 kilometers wide along the crest of the ridge in the 1970's. The ocean floor is spreading apart and forming new ocean crust along this valley or "rift" as hot magma from the Earth's interior is injected into the ridge and erupted at its top.

In the Pacific Northwest, the Juan de Fuca Plate plunges beneath the North American Plate. As the denser plate of oceanic crust if forced deep into the Earth's interior beneath the continental plate, a process known as subduction, it encounters high temperatures and pressures that partially melt solid rock. Some of this newly formed magma rises toward the Earth's surface to erupt, forming a chain of volcanoes above the subduction zone.

Located in the middle of the Pacific Plate, the volcanoes of the Hawaiian Island chain are among the largest on Earth. The volcanoes stretch 2,500 kilometers across the north Pacific Ocean and become progressively older to the northwest. Formed initially above a relatively stationary "hot spot" in the Earth's interior, each volcano was rafted away from the hot spot as the Pacific Plate moves northwestward at about 9 centimeters per year. The island of Hawaii consists of the youngest volcanoes in the chain and is currently located over the hot spot.

In the early 1960's, the related concepts of "sea-floor spreading" and "plate tectonics" emerged as powerful new hypotheses that geologists used to interpret the features and movements of the Earth's surface layer. According to the plate tectonics theory, the Earth's surface consists of about a dozen rigid slabs or plates, each averaging at least 50 miles thick. These plates move relative to one another at average speeds of a few inches per year -- about as fast as human fingernails grow. Scientists recognize three common types of boundaries between these moving plates:

1. Divergent or spreading -- adjacent plates pull apart, such as at the Mid-Atlantic Ridge, which separates the North and South American Plates from the Eurasian and African Plates. This pulling apart causes "sea-floor spreading" as new material is added to the oceanic plates.

2. Convergent -- plates moving in opposite directions meet and one is dragged down (or subducted) beneath the other. Convergent plate boundaries are also called subduction zones and are typified by the Aleutian Trench, where the Pacific Plate is being subducted under the North American Plate.

3. Transform fault -- one plate slides horizontally past another. The best known example is the earthquake-prone San Andreas fault zone of California, which marks the boundary between the Pacific and North American Plates.

The Earth's crust is broken into moving plates of "lithosphere". ... There are seven very large plates, each consisting of both oceanic and continental portions, and a dozen or more small plates. ... Each plate is about 80 kilometers (50 miles) thick and can be pictured as having a shallow part that deforms by elastic bending or by brittle breaking, and a deeper part that yields plastically, beneath which is a viscous layer on which the entire plate slides. The plates tend to be internally rigid, and they interact mostly at their edges. ...

All plates are moving relative to all others. There are grounds for suggesting that the African plate may now be approximately fixed relative to the deep mantle, but if so it is the only such plate. Velocities of relative motion between adjacent plates range from less than 1 centimeter (a small fraction of an inch) to about 13 centimeters (5 inches) per year. Although these velocities are slow by human standards, they are extremely rapid by geologic ones: a motion of 5 centimeters (2 inches) per year, for example, adds up to 50 kilometers (30 miles) in only 1 million years, and some plate motions have been continuous for 100 million years.

Plates are now pulling apart primarily along the system of great submarine ridges in the world's oceans. Hot material from the deeper mantle wells up into the gap, and some of it melts and is erupted on the surface as lava or is injected near the surface to crystallize as other igneous rocks. The ridge stands high because its material is hot, and hence low in density. As the plates move apart, the ridge material gradually cools and contracts, and its surface sinks. Ridges generally form step-like alternations of spreading centers perpendicular to the direction of motion and of strike-slip faults parallel to that direction. ...

Where plates converge, one tips down and slides beneath the other. Generally, an oceanic plate slides ("subducts") beneath a continental plate (for example, along the west coast of South America) or another oceanic plate (for example, the east side of the Philippine Sea plate). A trench is formed where the under-sliding plate tips down, and the ocean-floor sediment it carries is scraped off against the front of the overriding plate. ... We know much about the mechanics of these junctions from geophysical studies and particularly from seismic-reflection profiles made across them with instruments developed for oil-field exploration. Farther back under the overriding plate, zones of earthquakes, inclined down into the mantle to depths that reach 700 kilometers (450 miles), show the trajectory of the descending plate. Typically, a belt of volcanoes lies above the part of this inclined earthquake zone, which is about 125 kilometers (80 miles) deep. ...

New oceanic-plate (lithosphere) material is generated by the upwelling processes at spreading ridges. Old lithosphere is consumed, and recycled deep into the mantle, at the same rate as the convergent trenches. The balance is global only: the formation of lithosphere at the Mid-Atlantic Ridge is compensated by subduction primarily in the western Pacific.

Plates slide past one another along strike-slip faults, which can be either on land or at sea. The best known of these faults is the San Andreas Fault of California. ...

... Plate motions have dominated tectonic and magmatic processes for the past 2,500 million years. ...

... If present major plate motions continue for another 50 million years, Australia will be crowded against China, and the island complexes of Indonesia and the Philippines will be squashed into a mountain system between the colliding continents. ...

Most volcanoes are products of lithosphere-plate motions. The "ring of fire" around the Pacific represents one type of this volcanism. The chains of volcanoes in the island arcs (such as the Aleutian Islands) and continental margins (such as the Andes) around much of the ocean form above undersliding oceanic plates. The main volcanic axis is typically about 125 kilometers (80 miles) above the inclined zone of earthquakes that marks the descent of the lithosphere plate into the deep mantle ... so processes related to the descent and to that depth must control the melting of the magmas. The melts that arrive at the surface, to erupt in volcanoes, have been profoundly modified by reactions with the mantle and crustal rocks through which they have risen. Lavas formed in this setting have distinctive compositions and systematic variations that relate directly to their height above the subducting plate. These characteristics permit us to recognize rocks formed in similar settings in the geologic past and to estimate the depths to the long-dead seismic zones above which they formed. Where, in ancient terrains, the volcanic rocks have been eroded away, we now see granites and other rocks which crystallized slowly within the crust from similar magmas.

The high volcanoes of the Cascade Range in Oregon and Washington -- Mount Hood and Mount Rainier, for example -- form a short chain of this type, vigorously active until not many thousand years ago but now showing only infrequent activity. The decline in volcanism reflects a plate-boundary change now underway to the west: there was until recently rapid subduction of a small Pacific plate beneath northern California, Oregon, and Washington, but the pattern is presently changing; the San Andreas Fault system is now breaking across the small plate. ...

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Earthquakes, Active Volcanoes, and Plate Tectonics
-- Earthquakes around the world as compared to plate boundaries of the "Ring of Fire"
-- Topinka, USGS/CVO, 1999

Earth scientists believe that most earthquakes are caused by slow movements inside the Earth that push against the Earth's brittle, relatively thin outer layer, causing the rocks to break suddenly. This outer layer is fragmented into a number of pieces, called plates. Most earthquakes occur at the boundaries of these plates. In Washington State, the small Juan de Fuca plate off the coast of Washington, Oregon, and northern California is slowly moving eastward beneath a much larger plate that includes both the North American continent the land beneath part of the Atlantic Ocean. Plate motions in the Pacific Northwest result in shallow earthquakes widely distributed over Washington and deep earthquakes in the western parts of Washington and Oregon. The movement of the Juan de Fuca plate beneath the North America plate is in many respects similar to the movements of plates in South America, Mexico, Japan, and Alaska, where the world's largest earthquakes occur. ...

The plate tectonics theory is a starting point for understanding the forces within the Earth that cause earthquakes. Plates are thick slabs of rock that make up the outermost 100 kilometers or so of the Earth. Geologists use the term "tectonics" to describe deformation of the Earth's crust, the forces producing such deformation, and the geologic and structural features that result.

Earthquakes occur only in the outer, brittle portions of these plates, where temperatures in the rock are relatively low. Deep in the Earth's interior, convection of the rocks, caused by temperature variations in the Earth, induces stresses that result in movement of the overlying plates. The rates of plate movements range from about 2 to 12 centimeters per year and can now be measured by precise surveying techniques. The stresses from convection can also deform the brittle portions of overlying plates, thereby storing tremendous energy within the plates. If the accumulating stress exceeds the strength of the rocks comprising these brittle zones, the rocks can break suddenly, releasing the stored elastic energy as an earthquake.

Three major types of plate boundaries are recognized. These are called spreading, convergent, or transform, depending on whether the plates move away from, toward, or laterally past one another, respectively. Subduction occurs where one plate converges toward another plate, moves beneath it, and plunges as much as several hundred kilometers into the Earth's interior. The Juan de Fuca plate off the coasts of Washington and Oregon is subducting beneath North America.

Ninety percent of the world's earthquakes occur along plate boundaries where the rocks are usually weaker and yield more readily to stress than do the rocks within a plate. The remaining 10 percent occur in areas away from present plate boundaries -- like the great New Madrid, Missouri, earthquakes of 1811 and 1812, felt over at least 3.2 million square kilometers, which occurred in a region of southeast Missouri that continues to show seismic activity today.

The Cascadia subduction zone off the coast of Washington, Oregon, and northern California is a convergent boundary between the large North America plate and the small Juan de Fuca plate to the west. The Juan de Fuca plate moves northeastward and then plunges (subducts) obliquely beneath the North America plate at a rate of 3 to 4 centimeters per year. ... In sum, the subduction of the Juan de Fuca plate beneath the North America plate is believed to directly or indirectly cause most of the earthquakes and young geologic features in Washington and Oregon.

Earthquakes and Seismicity Menu

There are more than 500 active volcanoes (those that have erupted at least once within recorded history) in the world -- 50 of which are in the United States (Hawaii, Alaska, Washington, Oregon, and California) -- although many more may be hidden under the seas. Most active volcanoes are strung like beads along, or near, the margins of the continents, and more than half encircle the Pacific Ocean as a "Ring of Fire". ...

Some volcanoes crown island areas lying near the continents, and others form chains of islands in the deep ocean basins. Volcanoes tend to cluster along narrow mountainous belts where folding and fracturing of the rocks provide channelways to the surface for the escape of the magma. Significantly, major earthquakes also occur along these belts, indicating that volcanism and seismic activity are often closely related, responding to the same dynamic Earth forces.

Island-Arc Volcanics:

In a typical "island-arc" environment, volcanoes lie along the crest of an arcuate, crustal ridge bounded on its convex side by a deep oceanic trench. The granite or granitelike layer of the continental crust extends beneath the ridge to the vicinity of the trench. Basaltic magmas, generated in the mantle beneath the ridge, rise along fractures through the granitic layer. These magmas commonly will be modified or changed in composition during passage through the granitic layer and erupt on the surface to form volcanoes built largely of nonbasaltic rocks.

Island-Arc Volcanics -- [Graphic,30K,GIF]

Oceanic Volcanics:

In a typical "oceanic" environment, volcanoes are alined along the crest of a broad ridge that marks an active fracture system in the oceanic crust. Basaltic magmas, generated in the upper mantle beneath the ridge, rise along fractures through the basaltic layer. Because the granitic crustal layer is absent, the magmas are not appreciably modified or changed in composition and they erupt on the surface to form basaltic volcanoes.

Oceanic Volcanics -- [Graphic,26K,GIF]

Continental Volcanics:

In the typical "continental" environment, volcanoes are located in unstable, mountainous belts that have thick roots of granite or granitelike rock. Magmas, generated near the base of the mountain root, rise slowly or intermittently along fractures in the crust. During passage through the granite layer, magmas are commonly modified or changed in composition and erupt on the surface to form volcanoes constructed of nonbasaltic rocks.

Continental Volcanics -- [Graphic,33K,GIF]

Most Alaskan volcanoes are in the Aleutian arc which extends approximately 2,500 kilometers along the southern edge of the Bering Sea and Alaskan mainland. This classic volcanic arc contains some 80 Quaternary stratovolcanoes and calderas. Aleutian arc volcanism is the result of subduction of the Pacific Plate beneath the North American Plate. The 3,400-kilometer-long Aleutian trench that extends from the northern end of the Kamchatka trench to the Gulf of Alaska marks the boundary between the two plates.

The great sweep of the Sunda Arc, over 3,000 kilometers from NorthWest Sumatra to the Banda Sea, results from the subduction of the Indian Ocean crust beneath the Asian Plate. This arc includes 76 percent of the region's volcanoes, but those on either end are tectonically more complex. ...

In the Pacific Northwest, the Juan de Fuca Plate plunges beneath the North American Plate. As the denser plate of oceanic crust is forced deep into the Earth's interior beneath the continental plate, a process known as subduction, it encounters high temperatures and pressures that partially melt solid rock. Some of this newly formed magma rises toward the Earth's surface to erupt, forming a chain of volcanoes (the Cascade Range) above the subduction zone.

As with earthquakes, volcanic activity is linked to plate-tectonic processes. Most of the world's active above-sea volcanoes are located near convergent plate boundaries where subduction is occurring, particularly around the Pacific basin. However, much more volcanism -- producing about three quarters of all lava erupted on Earth -- takes place unseen beneath the ocean, mostly along the oceanic spreading centers, such as the Mid-Atlantic Ridge and the East Pacific Rise.

Subduction-zone volcanoes like Mount St. Helens (in Washington State) and Mount Pinatubo (Luzon, Philippines), are called composite cones and typically erupt with explosive force, because the magma is too stiff to allow easy escape of volcanic gases. As a consequence, tremendous internal pressures mount as the trapped gases expand during ascent, before the pent-up pressure is suddenly released in a violent eruption. Such an explosive process can be compared to putting your thumb over an opened bottle of a carbonated drink, shaking it vigorously, and then quickly removing the thumb. The shaking action separates the gases from the liquid to form bubbles, increasing the internal pressure. Quick release of the thumb allows the gases and liquid to gush out with explosive speed and force.

In 1991, two volcanoes on the western edge of the Philippine Plate produced major eruptions. On June 15, Mount Pinatubo spewed ash 40 km into the air and produced huge ash flows (also called pyroclastic flows) and mudflows that devastated a large area around the volcano. Pinatubo, located 90 km from Manila, had been dormant for 600 years before the 1991 eruption, which ranks as one of the largest eruptions in this century. Also in 1991, Japan's Unzen Volcano, located on the Island of Kyushu about 40 km east of Nagasaki, awakened from its 200-year slumber to produce a new lava dome at its summit. Beginning in June, repeated collapses of this active dome generated destructive ash flows that swept down its slopes at speeds as high as 200 km per hour. Unzen is one of more than 75 active volcanoes in Japan; its eruption in 1792 killed more than 15,000 people--the worst volcanic disaster in the country's history.

While the Unzen eruptions have caused deaths and considerable local damage, the impact of the June 1991 eruption of Mount Pinatubo was global. Slightly cooler than usual temperatures recorded worldwide and the brilliant sunsets and sunrises have been attributed to this eruption that sent fine ash and gases high into the stratosphere, forming a large volcanic cloud that drifted around the world. The sulfur dioxide (SO2) in this cloud -- about 22 million tons -- combined with water to form droplets of sulfuric acid, blocking some of the sunlight from reaching the Earth and thereby cooling temperatures in some regions by as much as 0.5 °C. An eruption the size of Mount Pinatubo could affect the weather for a few years. A similar phenomenon occurred in April of 1815 with the cataclysmic eruption of Tambora Volcano in Indonesia, the most powerful eruption in recorded history. Tambora's volcanic cloud lowered global temperatures by as much as 3 °C. Even a year after the eruption, most of the northern hemisphere experienced sharply cooler temperatures during the summer months. In part of Europe and in North America, 1816 was known as "the year without a summer."

Apart from possibly affecting climate, volcanic clouds from explosive eruptions also pose a hazard to aviation safety. During the past two decades, more than 60 airplanes, mostly commercial jetliners, have been damaged by in-flight encounters with volcanic ash. Some of these encounters have resulted in the power loss of all engines, necessitating emergency landings. Luckily, to date no crashes have happened be-cause of jet aircraft flying into volcanic ash.

Since the year A.D. 1600, nearly 300,000 people have been killed by volcanic eruptions. Most deaths were caused by pyroclastic flows and mudflows, deadly hazards which often accompany explosive eruptions of subduction-zone volcanoes. Pyroclastic flows, also called nuées ardentes ("glowing clouds" in French), are fast-moving, avalanche-like, ground-hugging incandescent mixtures of hot volcanic debris, ash, and gases that can travel at speeds in excess of 150 kilometers per hour. Approximately 30,000 people were killed by pyroclastic flows during the 1902 eruption of Mont Pelee on the Island of Martinique in the Caribbean. In March-April 1982, three explosive eruptions of El Chichón Volcano in the State of Chiapas, southeastern Mexico, caused the worst volcanic disaster in that country's history. Villages within 8 km of the volcano were destroyed by pyroclastic flows, killing more than 2,000 people.

Mudflows (also called debris flows or lahars, an Indonesian term for volcanic mudflows) are mixtures of volcanic debris and water. The water usually comes from two sources: rainfall or the melting of snow and ice by hot volcanic debris. Depending on the proportion of water to volcanic material, mudflows can range from soupy floods to thick flows that have the consistency of wet cement. As mudflows sweep down the steep sides of composite volcanoes, they have the strength and speed to flatten or bury everything in their paths. Hot ash and pyroclastic flows from the eruption of the Nevado del Ruiz Volcano in Colombia, South America, melted snow and ice atop the 5,390-m-high Andean peak; the ensuing mudflows buried the city of Armero, killing 25,000 people.

Eruptions of Hawaiian and most other mid-plate volcanoes differ greatly from those of composite cones. Mauna Loa and Kilauea, on the island of Hawaii, are known as shield volcanoes, because they resemble the wide, rounded shape of an ancient warrior's shield. Shield volcanoes tend to erupt non-explosively, mainly pouring out huge volumes of fluid lava. Hawaiian-type eruptions are rarely life threatening because the lava advances slowly enough to allow safe evacuation of people, but large lava flows can cause considerable economic loss by destroying property and agricultural lands. For example, lava from the ongoing eruption of Kilauea, which began in January 1983, has destroyed more than 200 structures, buried kilometers of highways, and disrupted the daily lives of local residents. Because Hawaiian volcanoes erupt frequently and pose little danger to humans, they provide an ideal natural laboratory to safely study volcanic phenomena at close range. The USGS Hawaiian Volcano Observatory, on the rim of Kilauea, was among the world's first modern volcano observatories, established early in this century.

In recorded history, explosive eruptions at subduction-zone (convergent-boundary) volcanoes have posed the greatest hazard to civilizations. Yet scientists have estimated that about three quarters of the material erupted on Earth each year originates at spreading mid-ocean ridges. However, no deep submarine eruption has yet been observed "live" by scientists. Because the great water depths preclude easy observation, few detailed studies have been made of the numerous possible eruption sites along the tremendous length (50,000 km) of the global mid-oceanic ridge system. Recently however, repeated surveys of specific sites along the Juan de Fuca Ridge, off the coast of the Oregon and Washington, have mapped deposits of fresh lava, which must have been erupted sometime between the surveys. In June 1993, seismic signals typically associated with submarine eruptions -- called T-phases -- were detected along part of the spreading Juan de Fuca Ridge and interpreted as being caused by eruptive activity.

Iceland, where the Mid-Atlantic Ridge is exposed on land, is a different story. It is easy to see many Icelandic volcanoes erupt non-explosively from fissure vents, in similar fashion to typical Hawaiian eruptions; others, like Hekla Volcano, erupt explosively. (After Hekla's catastrophic eruption in 1104, it was thought in the Christian world to be the "Mouth to Hell.") The voluminous, but mostly non-explosive, eruption at Lakagígar (Laki), Iceland, in 1783, resulted in one of the world's worst volcanic disasters. About 9,000 people -- almost 20 percent of the country's population at the time -- died of starvation after the eruption, because their livestock had perished from grazing on grass contaminated by fluorine-rich gases emitted during this eight month-long eruption.

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Graphic, Plate Tectonics and the Cascade Range
-- Modified from: Tilling, 1985

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Graphic, Juan de Fuca Subduction - Juan de Fuca Ridge - Cascade Range
-- Modified from: Brantley, 1994

The Cascade Range has been an active arc for about 36 million years as a result of plate convergence. Volcanic rocks between 55 and 42 million years ago occur in the Cascades, but are probably related to a rather diffuse volcanic episode that created the Challis arc extending southeastward from northern to northwest Wyoming. Convergence between the North American and Juan de Fuca plates continues at about 4 centimeters per year in the direction of North-50-degrees-East, a slowing of 2-3 centimeters per year since 7 million years ago. According to most interpretations, volcanism in the Cascades has been discontinuous in time and space, with the most recent episode of activity beginning about 5 million years ago and resulting in more than 3000 vents.

In Oregon, the young terrane is commonly called the High Cascades and the old terrane the Western Cascades, terms that reflect present physiography and geography. The terms are not useful in Washington, where young vents are scattered across the dominantly middle Miocene and older terrane. ...

In Washington and Oregon, a striking contrast has existed for the past 5 million years in the style of volcanism in the Cascades relative to geography. North of Mount Rainier, young volcanism is concentrated in only a few isolated andesitic and dacitic composite cones (notably Glacier Peak, Mount Baker, and the volcanoes of the Garibaldi belt in British Columbia), whereas south of Mount Hood moderate-sized andesitic and dacitic composite cones are relatively unimportant features of a landscape dominated by small andesite and basalt vents. The area between Mounts Rainier and Hood is transitional; large andesite and dacite composite cones ( Rainier, Adams, St. Helens, Hood, and the extinct Goat Rocks volcano) occur together with fields and scattered vents of olivine basalt ( Indian Heaven, Simcoe Mountains, and the King Mountain fissure zone south of Mount Adams. ...

The southern Washington Cascades are seismically active. Most earthquakes occur along the 100-kilometer-long, north-northwest trending St. Helens seismic zone, where most focal mechanisms show dextral slip parallel to the trend of the zone and consistent with the direction of plate convergence. Other crustal earthquakes concentrate just west of Mount Rainier and in the Portland (Oregon) area. Few earthquakes occur north of Mount Rainier or south of Mount Hood.

From tomography, Rasmussen and Humphreys (1988) interpret the subducted Juan de Fuca plate as a quasi-planar feature dipping about 65 degrees to about 300 kilometers under the southern Washington Cascades. The plate is poorly defined seismically, however, owing to a lack of earthquakes within it. Guffanti and Weaver (1988) show that the present volcanic front of the Washington Cascades, defined by the westernmost young vents, parallels the curved trend of the subducting plate reflected by the 60 kilometer-depth contour. The front trends northwest in northern Washington -- where Glacier Peak, Mount Baker, and the volcanoes of southern British Columbia occur along a virtually straight line -- and northeast in southern Washington. A 90-kilometer gap free of young volcanoes between Mount Rainier and Glacier Peak is landward of that part of the subducting plate with the least average dip to a depth of 60 kilometers. South of Portland, the volcanic front is offset 50 kilometers eastward and extends southward into California, probably still parallel to the trend of the convergent margin.

Cascade Range Menu

In East Africa, spreading processes have already torn Saudi Arabia away from the rest of the African continent, forming the Red Sea. The actively splitting African Plate and the Arabian Plate meet in what geologists call a triple junction, where the Red Sea meets the Gulf of Aden. A new spreading center may be developing under Africa along the East African Rift Zone. When the continental crust stretches beyond its limits, tension cracks begin to appear on the Earth's surface. Magma rises and squeezes through the widening cracks, sometimes to erupt and form volcanoes. The rising magma, whether or not it erupts, puts more pressure on the crust to produce additional fractures and, ultimately, the rift zone.

East Africa may be the site of the Earth's next major ocean. Plate interactions in the region provide scientists an opportunity to study first hand how the Atlantic may have begun to form about 200 million years ago. Geologists believe that, if spreading continues, the three plates that meet at the edge of the present-day African continent will separate completely, allowing the Indian Ocean to flood the area and making the easternmost corner of Africa (the Horn of Africa) a large island.

Africa Volcanoes Menu

The great majority of the world's earthquakes and active volcanoes occur near the boundaries of the Earth's shifting plates. Why then are the Hawaiian volcanoes located near the middle of the Pacific Plate, more than 2,000 miles from the nearest plate boundary? In 1963, J. Tuzo Wilson, a Canadian geophysicist, provided an ingenious explanation within the framework of plate tectonics by proposing the "Hot Spot" hypothesis. Wilson's hypothesis has come to be accepted widely, because it agrees well with much of the scientific data on the Pacific Ocean in general, and the Hawaiian Islands in particular.

According to Wilson, the distinctive linear shape of the Hawaiian-Emperor Chain reflects the progressive movement of the Pacific Plate over a deep immobile hot spot. 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.

Over a span of about 70 million years, the combined processes of magma formation, eruption, and continuous movement of the Pacific Plate over the stationary hot spot have left the trail of volcanoes across the ocean floor that we now call the Hawaiian-Emperor Chain. Scientists interpret the sharp bend in the chain, about 2,200 miles northwest of the Big Island, as indicating a change in the direction of plate motion that occurred about 43 million years ago, as suggested by the ages of the volcanoes bracketing the bend.

Part of the Big Island, the southeasternmost and youngest island, presently overlies the hot spot and still taps the magma source to feed its two currently active volcanoes, Kilauea and Mauna Loa. The active submarine volcano, Loihi, off the Big Island's south coast, may mark the beginning of the zone of magma formation at the southeastern edge of the hot spot. The other Hawaiian islands have moved northwestward beyond the hot spot, were successively cut off from the sustaining magma source, and are no longer volcanically active.

The progressive northwesterly drift of the islands from their point of origin over the hot spot is well shown by the ages of the principal lava flows on the various Hawaiian Islands from northwest (oldest) to southeast (youngest), given in millions of years: Kauai, 5.6 to 3.8; Oahu, 3.4 to 2.2; Molokai, 1.8 to 1.3; Maui, 1.3 to 0.8; and Hawaii, less than 0.7 and still growing.

Even on the Big Island alone, the relative ages of its five volcanoes are compatible with the hot-spot theory. Kohala, at the northwestern corner of the island, is the oldest, having ceased eruptive activity about 60,000 years ago. 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.

The size of the Hawaiian hot spot is not know precisely, but it presumably is large enough to encompass the currently active volcanoes of Mauna Loa, Kilauea, Loihi, and, possibly, also Hualalai and Haleakala. Some scientists have estimated the Hawaiian hot spot to be about 200 miles across, with much narrower vertical passageways that feed magma to the individual volcanoes.

In 1963, J. Tuzo Wilson, the Canadian geophysicist who discovered transform faults, came up with an ingenious idea that became known as the "hotspot" theory. Wilson noted that in certain locations around the world, such as Hawaii, volcanism has been active for very long periods of time. This could only happen, he reasoned, if relatively small, long-lasting, and exceptionally hot regions -- called hotspots -- existed below the plates that would provide localized sources of high heat energy (thermal plumes) to sustain volcanism.

Specifically, Wilson hypothesized that the distinctive linear shape of the Hawaiian Island-Emperor Seamounts chain resulted from the Pacific Plate moving over a deep, stationary hotspot in the mantle, located beneath the present-day position of the Island of Hawaii. Heat from this hotspot produced a persistent source of magma by partly melting the overriding Pacific Plate. The magma, which is lighter than the surrounding solid rock, then rises through the mantle and crust to erupt onto the seafloor, forming an active seamount. Over time, countless eruptions cause the seamount to grow until it finally emerges above sea level to form an island volcano. Wilson suggested that continuing plate movement eventually carries the island beyond the hotspot, cutting it off from the magma source, and volcanism ceases. As one island volcano becomes extinct, another develops over the hotspot, and the cycle is repeated. This process of volcano growth and death, over many millions of years, has left a long trail of volcanic islands and seamounts across the Pacific Ocean floor.

According to Wilson's hotspot theory, the volcanoes of the Hawaiian chain should get progressively older and become more eroded the farther they travel beyond the hotspot. The oldest volcanic rocks on Kauai, the northwesternmost inhabited Hawaiian island, are about 5.5 million years old and are deeply eroded. By comparison, on the "Big Island" of Hawaii -- southeasternmost in the chain and presumably still positioned over the hotspot -- the oldest exposed rocks are less than 0.7 million years old and new volcanic rock is continually being formed.

Link to: Hawaiian Volcano Observatory (HVO) for MORE Information

The Mid-Atlantic plate boundary passes through Iceland and is reflected in two zones of spreading and volcanism -- an eastern zone, the site of most historical eruptions, and a western zone.

Iceland, where the Mid-Atlantic Ridge is exposed on land, is a different story. It is easy to see many Icelandic volcanoes erupt non-explosively from fissure vents, in similar fashion to typical Hawaiian eruptions; others, like Hekla Volcano, erupt explosively. (After Hekla's catastrophic eruption in 1104, it was thought in the Christian world to be the "Mouth to Hell.") The voluminous, but mostly non-explosive, eruption at Lakagígar (Laki), Iceland, in 1783, resulted in one of the world's worst volcanic disasters. About 9,000 people -- almost 20 percent of the country's population at the time -- died of starvation after the eruption, because their livestock had perished from grazing on grass contaminated by fluorine-rich gases emitted during this eight month-long eruption.

Iceland Volcanoes Menu

... In the past 25 years, scientists have developed a theory -- called plate tectonics -- that explains the locations of volcanoes and their relationship to other large-scale geologic features. ...

According to this theory, the Earth's surface is made up of a patchwork of about a dozen large plates that move relative to one another at speeds from less than one centimeter to about ten centimeters per year (about the speed at which fingernails grow). These rigid plates, whose average thickness is about 80 kilometers, are spreading apart, sliding past each other, or colliding with each other in slow motion on top of the Earth's hot, pliable interior. Volcanoes tend to form where plates collide or spread apart, but they can also grow in the middle of a plate, as for example the Hawaiian volcanoes.

The boundary between the Pacific and Juan de Fuca Plates is marked by a broad submarine mountain chain about 500 kilometers long, known as the Juan de Fuca Ridge. Young volcanoes, lava flows, and hot springs were discovered in a broad valley less than 8 kilometers wide along the crest of the ridge in the 1970's. The ocean floor is spreading apart and forming new ocean crust along this valley or "rift" as hot magma from the Earth's interior is injected into the ridge and erupted at its top.

In the Pacific Northwest, the Juan de Fuca Plate plunges beneath the North American Plate. As the denser plate of oceanic crust is forced deep into the Earth's interior beneath the continental plate, a process known as subduction, it encounters high temperatures and pressures that partially melt solid rock. Some of this newly formed magma rises toward the Earth's surface to erupt, forming a chain of volcanoes (the Cascade Range) above the subduction zone.

The remaining part of the Pacific Plate currently converging with the American Northwest is the Juan de Fuca Plate, with small platelets at its northern (Explorer Plate) and southern Gorda Plate) terminations. The Explorer Plate separated from the Juan de Fuca approximately 4 million years ago and is apparently no longer being subducted (Hyndman, et.al., 1979); the Gorda split away between 18 and 5 million years ago (Riddihough, 1984). The present slow rate of convergence (3-4 centimeters per year) of the Juan de Fuca Plate is only about half its value at 7 million years (Riddihough, 1984), which probably explains the reduced seismicity, lack of a trench, and debatable decline in volcanic activity. ...

Juan De Fuca Ridge Menu

[Map,27K,InlineGIF]
Map, Major Volcanoes of the Mariana Islands
-- includes location of a section of the Marianas Trench

As with oceanic-continental convergence, when two oceanic plates converge, one is usually subducted under the other, and in the process a trench is formed. The Marianas Trench (paralleling the Mariana Islands), for example, marks where the fast-moving Pacific Plate converges against the slower moving Philippine Plate. The Challenger Deep, at the southern end of the Marianas Trench, plunges deeper into the Earth's interior (nearly 11,000 meters) than Mount Everest, the world's tallest mountain, rises above sea level (about 8,854 meters).

Subduction processes in oceanic-oceanic plate convergence also result in the formation of volcanoes. Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano. Such volcanoes are typically strung out in chains called island arcs. As the name implies, volcanic island arcs, which closely parallel the trenches, are generally curved. The trenches are the key to understanding how island arcs such as the Marianas and the Aleutian Islands have formed and why they experience numerous strong earthquakes. Magmas that form island arcs are produced by the partial melting of the descending plate and/or the overlying oceanic lithosphere. The descending plate also provides a source of stress as the two plates interact, leading to frequent moderate to strong earthquakes.

North Pacific Volcanic Islands Menu

The Mid-Atlantic plate boundary passes through Iceland and is reflected in two zones of spreading and volcanism -- an eastern zone, the site of most historical eruptions, and a western zone.

Divergent boundaries occur along spreading centers where plates are moving apart and new crust is created by magma pushing up from the mantle. Picture two giant conveyor belts, facing each other but slowly moving in opposite directions as they transport newly formed oceanic crust away from the ridge crest.

Perhaps the best known of the divergent boundaries is the Mid-Atlantic Ridge. This submerged mountain range, which extends from the Arctic Ocean to beyond the southern tip of Africa, is but one segment of the global mid-ocean ridge system that encircles the Earth. The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters per year (cm/yr), or 25 km in a million years. This rate may seem slow by human standards, but because this process has been going on for millions of years, it has resulted in plate movement of thousands of kilometers. Seafloor spreading over the past 100 to 200 million years has caused the Atlantic Ocean to grow from a tiny inlet of water between the continents of Europe, Africa, and the Americas into the vast ocean that exists today.

The volcanic country of Iceland, which straddles the Mid-Atlantic Ridge, offers scientists a natural laboratory for studying on land the processes also occurring along the submerged parts of a spreading ridge. Iceland is splitting along the spreading center between the North American and Eurasian Plates, as North America moves westward relative to Eurasia.

The consequences of plate movement are easy to see around Krafla Volcano, in the northeastern part of Iceland. Here, existing ground cracks have widened and new ones appear every few months. From 1975 to 1984, numerous episodes of rifting (surface cracking) took place along the Krafla fissure zone. Some of these rifting events were accompanied by volcanic activity; the ground would gradually rise 1-2 m before abruptly dropping, signaling an impending eruption. Between 1975 and 1984, the displacements caused by rifting totaled about 7 m.

Iceland, where the Mid-Atlantic Ridge is exposed on land, is a different story. It is easy to see many Icelandic volcanoes erupt non-explosively from fissure vents, in similar fashion to typical Hawaiian eruptions; others, like Hekla Volcano, erupt explosively. (After Hekla's catastrophic eruption in 1104, it was thought in the Christian world to be the "Mouth to Hell.") The voluminous, but mostly non-explosive, eruption at Lakagígar (Laki), Iceland, in 1783, resulted in one of the world's worst volcanic disasters. About 9,000 people -- almost 20 percent of the country's population at the time -- died of starvation after the eruption, because their livestock had perished from grazing on grass contaminated by fluorine-rich gases emitted during this eight month-long eruption.

Iceland Volcanoes Menu

[Map,27K,InlineGIF]
Map, South America and Plate Tectonics

South America spans the greatest length of any continental volcanic region. Subduction of the eastern Pacific's Nazca Plate beneath South America has produced one of the Earth's highest mountain ranges, and its highest volcano Nevados Ojos del Salado (Argentina). Three distinct volcanic belts are separated by volcanically inactive gaps, where subduction is at such a shallow angle that magma is not generated by the process.

South America Volcanoes Menu

Scientists have traced Yellowstone's origin to a hot spot in the mantle, one of a few dozen such hot spots on Earth. Buoyant material from a hot spot rises through the upper mantle, bringing heat from the Earth's interior closer to the surface. The Yellowstone hot spot impinges on the base of the North American plate, one of several rigid plates that make up the Earth's crust. These plates move a few inches per year with respect to the stationary hot spots and each other, sometimes causing great earthquakes as the plates collide, grind past one another, or split apart.

The Yellowstone hot spot has interacted with the North American plate for perhaps as long as 17 million years, causing widespread outpourings of basalt that bury about 200,000 square miles in Washington, Oregon, California, Nevada, and Idaho under stacks of lava flows half a mile or more thick. Some of the basaltic melt, or magma, produced by the hot spot accumulates near the base of the plate, where its heat melts rocks from the Earth's lower crust. These melts, in turn, rise closer to the surface to form large reservoirs of potentially explosive rhyolite magma. Catastrophic eruptions have partly emptied some of these reservoirs, causing their roofs to collapse. The resulting craters, some of which are more than 30 miles (50 kilometers) across, are known as volcanic calderas. Because the plate was moving an inch or so per year southwestward over the hot spot for millions of years as the calderas formed, groups of calderas are strung out like beads on a string across parts of Idaho and Wyoming.

Yellowstone lies at the intersection of the Basin and Range tectonic province, dominated by E-W extension, and the eastern Snake River Plain, a linear downwarp or graben that has been a locus for basaltic volcanism since middle Miocene time. According to one popular model, the rhyolitic Yellowstone Plateau marks the current location of a "hotspot" or melting anomaly in the upper mantle, and the basaltic Snake River Plain records the hotspot's northeastward track across the mobile North American Plate. ...

Although Hawaii is perhaps the best known hotspot, others are thought to exist beneath the oceans and continents. More than a hundred hotspots beneath the Earth's crust have been active during the past 10 million years. Most of these are located under plate interiors (for example, the African Plate), but some occur near diverging plate boundaries. Some are concentrated near the mid-oceanic ridge system, such as beneath Iceland, the Azores, and the Galapagos Islands.

A few hotspots are thought to exist below the North American Plate. Perhaps the best known is the hotspot presumed to exist under the continental crust in the region of Yellowstone National Park in northwestern Wyoming. Here are several calderas (large craters formed by the ground collapse accompanying explosive volcanism) that were produced by three gigantic eruptions during the past two million years, the most recent of which occurred about 600,000 years ago. Ash deposits from these powerful eruptions have been mapped as far away as Iowa, Missouri, Texas, and even northern Mexico. The thermal energy of the presumed Yellowstone hotspot fuels more than 10,000 hot pools and springs, geysers (like Old Faithful), and bubbling mudpots (pools of boiling mud). A large body of magma, capped by a hydrothermal system (a zone of pressurized steam and hot water), still exists beneath the caldera. Recent surveys demonstrate that parts of the Yellowstone region rise and fall by as much as 1 cm each year, indicating the area is still geologically restless. However, these measurable ground movements, which most likely reflect hydrothermal pressure changes, do not necessarily signal renewed volcanic activity in the area.

Yellowstone Menu

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