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. Crater Lake, Oregon; Wizard Island, a cinder cone, rises above the lake surface. The Evolution of a Composite Volcano 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. 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 deepend their valleys. 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.
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 The internal structure of a typical shield volcano S hield 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. The exact composition of a cinder cone depends on the composition of the lava ejected from the volcano. Cinder cones usually have a crater at the summit. Cinder cones are often found near larger volcanoes. Supervolcano eruptions are extremely rare in Earth history.
A supervolcano must erupt more than 1, cubic km cubic miles of material, compared with 1. Helens or 25 km3 for Mount Pinatubo, a large eruption in the Philippines in Not surprisingly, supervolcanoes are the most dangerous type of volcano. Supervolcanoes are a fairly new idea in volcanology. The exact cause of supervolcano eruptions is still debated, however, scientists think that a very large magma chamber erupts entirely in one catastrophic explosion.
This creates a huge hole or caldera into which the surface collapses. Yellowstone sits above a hotspot that has erupted catastrophically three times: 2. Yellowstone has produced many smaller but still enormous eruptions more recently. Long Valley had an extremely hot and explosive rhyolite explosion about , years ago.
An earthquake swarm in alerted geologists to the possibility of a future eruption, but the quakes have since calmed down. Resurgent calderas are the largest volcanic structures on Earth. They are the result of catastrophic eruptions that dwarf any eruptions ever recorded by human beings. Yellowstone caldera, sometimes called a "super volcano," is one example. Volcanic plugs : When magma solidifies in the fissure of a volcano the hard dense rock may form a " neck " that remains when softer surrounding rock has been eroded away, according to the U.
Tuff cones : also known as maars, tuff cones are shallow, flat-floored craters that scientists think formed as a result of a violent expansion of magmatic gas or steam, according to the U.
Maars range in size from to 6, feet 60 to 1, meters across and from 30 to feet 9 to meters deep, and most are commonly filled with water to form natural lakes. Maars occur geologically young volcanic regions of the world such as the western United States and the Eifel region of Germany.
Lava plateaus : Shield volcanoes may erupt along lines of fissures rather than a central vent spilling liquid lava in successive layers. Over time as these layers form broad plateaus such as the Columbia Plateau, according to the Encyclopedia Britannica. These plateaus are often cut by deep canyons that expose the layers of rock. It has erupted dozens of times in the past 2, years, according to the Encyclopedia Britannica.
The A. The volcano sent a cloud ejecta into the atmosphere that was more than four times the amount ejected by Mount Pinatubo in , leading to the " Year Without a Summer " of in Europe and North America, according to a paper in Wiley Interdisciplinary Reviews: Climate Change.
Seventy-pound boulders landed on islands 50 miles away, and a foot tsunami devastated hundreds of villages, including Java and Sumatra, according to San Diego State University. About 36, people died. Dust high in the atmosphere caused the moon to appear blue , and sometimes green, for two years, according to NASA. Helens in Washington state blew 1, feet off its top, killing 57 people and causing a midday darkness in towns 85 miles away. The cataclysmic ejected more than 1 cubic mile 5 cubic kilometers of material and buried a U.
Nearly every bridge within 18 miles 30 km of Mount Pinatubo was destroyed , according to the U. Pinatubo's cloud of sulfuric acid, some 20 million tons of it, climbed to more than 12 miles in the stratosphere. On young cones, a depression at the top of the cone, called a crater, is evident, and represents the area above the vent from which material was explosively ejected.
Craters are usually eroded away on older cones. If lava flows are emitted from tephra cones, they are usually emitted from vents on the flank or near the base of the cone during the later stages of eruption. Cinder and tephra cones usually occur around summit vents and flank vents of stratovolcanoes.
This volcano was born in a farmers corn field in and erupted for the next 9 years. Lava flows erupted from the base of the cone eventually covered two towns.
Cinder cones often occur in groups, where tens to hundreds of cones are found in one area. Maars Maars result from phreatic or phreatomagmatic activity, wherein magma heats up groundwater, pressure builds as the water to turns to steam, and then the water and preexisting rock and some new magma if the eruption is phreatomagmatic are blasted out of the ground to form a tephra cone with gentle slopes.
Volcanic Domes also called Lava Domes Volcanic Domes result from the extrusion of highly viscous, gas poor andesitic and rhyolitic lava. Since the viscosity is so high, the lava does not flow away from the vent, but instead piles up over the vent.
Most dome eruptions are preceded by explosive eruptions of more gas rich magma, producing a tephra cone into which the dome is extruded. Volcanic domes can be extremely dangerous. Craters and Calderas Craters are circular depressions, usually less than 1 km in diameter, that form as a result of explosions that emit gases and tephra.
In stratovolcanoes the collapse and formation of a caldera results from rapid evacuation of the underlying magma chamber by voluminous explosive eruptions that form extensive fall deposits and pyroclastic flows. Calderas are often enclosed depressions that collect rain water and snow melt, and thus lakes often form within a caldera.
Crater Lake Caldera in southern Oregon is an 8 km diameter caldera containing a lake The caldera formed about years ago as a result of the eruption of about 75 km 3 of rhyolite magma in the form of tephra, found as far away as Canada, accompanied by pyroclastic flows that left thick deposits of tuff on the flanks of the volcano. Subsequent eruptions have built a cinder cone on the floor of the caldera, which now forms an island called Wizard Island.
Larger calderas have formed within the past million years in the western United States. The Yellowstone caldera is an important example, as it illustrates the amount of repose time that might be expected from large rhyolitic systems, and the devastating effect caldera forming eruptions can have on widespread areas. Yellowstone Caldera which occupies most of Yellowstone National Park, is actually the third caldera to form in the area within the past 2 million years.
The three calderas formed at 2. Thus the repose time is on the average about , years. Tephra fall deposits from the latest eruption are found in Louisiana and into the Gulf of Mexico, and covered much of the Western part of the United States. The eruption , years ago produced about km 3 of rhyolite in comparison, the eruption of Mt.
Helens in May of produced only 0. Magma still underlies Yellowstone caldera, as evidenced by the large number of hot springs and geysers in the area. Resurgent Domes After the formation of a caldera by collapse, magma is sometimes re-injected into the area below the caldera. This can result in uplift of one or more areas within the caldera to form a resurgent dome. Two such resurgent domes formed in the Yellowstone caldera, as shown above. Geysers, Fumaroles and Hot Springs. Plateau Basalts or Flood Basalts Plateau or Flood basalts are extremely large volume outpourings of low viscosity basaltic magma from fissure vents.
In Oregon and Washington of the northwestern U. One of the basalt flows, the Roza flow, was erupted over a period of a few weeks traveled about km and has a volume of about km 3. Volcanoes and Plate Tectonics Global Distribution of Volcanoes In the discussion we had last lecture about how magmas form, we pointed out that since the upper parts of the Earth are solid, special conditions are necessary to form magmas.
Volcanism also occurs in continental areas that are undergoing episodes of extensional deformation. The extensional deformation occurs because the underlying mantle is rising from below and stretching the overlying continental crust.
Upwelling mantle may melt to produce magmas, which then rise to the surface, often along normal faults produced by the extensional deformation.
Basaltic and rhyolitic volcanism is common in these areas.
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