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EARTH SCIENCE -- CH. 13 Volcanoes 7.1 Volcanoes and Plate Tectonics Scientists have no direct way to measure temperatures deep within the earth. Despite the high temperature in the asthenosphere, most of this zone remains solid because of the great pressure of the surrounding rock. Sometimes areas of the solid rock will melt, forming _________________________. Geologists think that magma forms in areas where the surrounding rock exerts less pressure than normal. ▪The lower pressure allows the rock particles to move more quickly; thus, the rock becomes liquid. Any activity that includes the movement of magma toward or onto the surface of the earth is called ____________________________________. ▸Pockets of magma grow due to melting of some of the surrounding rock. ▸As more rock melts, the magma pockets expand. ▸The magma slowly pushes upward into the crust because it is less dense than solid crustal rock. ▸The magma slowly rises, forcing its way into cracks in the overlying rock. Most magma forms at plate boundaries, where one lithospheric plate, usually of oceanic crust, is ____________________________ beneath another plate, often of continental crust. ▸ The subducting plate moves deep into the hot asthenosphere, where parts of it melt to become magma. ▪Magma that erupts onto the earth's surface is called __________________. The opening through which the molten rock flows onto the surface is called a ______________________________. 1 ▸The vent and the volcanic material that builds up on the earth's surface around the vent is called a ________________________. Major Volcanic Zones Most of these active volcanoes are found in zones near both ____________________ and ____________________ boundaries of the lithospheric plates. Subduction Zones (Review Material) Many volcanoes are located along subduction zones, where one plate moves under another plate. ▸When a plate with oceanic crust meets a plate with continental crust, the oceanic crust, which is more dense, moves beneath the continental crust. ▸A _________________________________ forms on the ocean floor along the edge of the continent where the plate is being subducted. ▸The plate with the continental crust buckles and folds, forming a line of ________________________ along the edge of the continent. A major zone of active volcanoes is caused by subducting plates encircling the Pacific Ocean. ▸ This zone, called the ___________________________, results from plates subducting along the Pacific coasts of North America, South America, Asia, and the islands of the western Pacific Ocean. ▪The Pacific Ring of Fire is also one of earth's three major earthquake zones. ▸As a plate descends into the mantle, it melts. ▸Some of the resulting magma breaks through to the surface along the trench. ▪ In time, a string of volcanic islands, called an _____________________________, forms along the trench. The early stages of this type of subduction produce an arc of small ____________________________________. 2 ▸ Example: _____________________________________, which stretch across the North Pacific Ocean between Alaska and Siberia. As more magma surfaces, more islands appear, become larger, and join, such as the group of volcanic islands that make up __________________________. The greatest amount of magma comes to the surface where plates are moving apart along ____________________________________. ▸ A major zone of volcanoes is the interconnected mid- ocean ridges that circle the earth. ▸ Magma rises through the fractures and comes to the surface along long, narrow cracks called __________________________________. Most volcanic eruptions along the mid-ocean ridges go unnoticed. (Exception: _________________________________) Iceland is a part of the Mid-Atlantic Ridge that _____________________________. ▸ ▸ ▪The middle of Iceland is cut by large fissures, cracks through which lava flows. Areas of volcanism within plates are called ______________________________. ▸ Example: ▪ 7.2 Volcanic Eruptions The lava that erupts from volcanoes provides an opportunity for scientists to study first hand the materials that form deep within the earth's mantle. 3 By analyzing the types of minerals found in hardened lava, geologists have concluded that there are two general types of lava: 1) ▪ 2) ▸ ▪Felsic lava is usually from melted ________________________. Other lavas have a range of compositions that fall between the mafic and the felsic varieties. ▸ Thin, mafic lava usually hardens with a wrinkled surface. ▪This lava is known as _________________________________, (meaning "____________________" in Hawaiian) Rapid cooling on the surface of the lava forms a crust that breaks into jagged chunks as the liquid below continues to flow. ▸The lava deposit that remains is described by its Hawaiian name _________, which refers to the sharp, blocky shapes into which the hardened lava breaks. Sometimes the outer part of a mafic lava flow cools so rapidly that it forms a hardened shell around a liquid interior that flows out, leaving tunnels in the hardened lava shell. When lava flows out of fissures on the ocean floor, it cools rapidly, often in rounded shapes. ▸ This is called _______________________________________. ▪ 4 ▸____________________________________________, both those that erupt on the ocean floor and those that erupt on oceanic islands, usually are produced _______________________________________. ▪ Mafic lava is very ___________________________________________________. Because gases can easily escape from mafic lava, eruptions from oceanic volcanoes like those in Hawaii are usually quiet. ◦___________________________________ flows are produced The ___________________________________________________________, such as ______________________________________, tend to be cooler and thicker. ▸ ▸When a vent or fissure opens up, the dissolved gases within the lava boil out explosively, sending molten and solid particles shooting into the air. Unlike mafic lava, which tends to flow quietly, _____________________________, throwing _________________________ into the air. ▸____________________________, sometimes called _________________________, is rock fragments ejected from a volcano. ▪ ◦ Tephra particles less than Particles ▸Some of the smallest dust particles may travel completely around the earth. Larger tephra particles, less than __________________in diameter, are called ___________________________ (Latin word meaning "_____________________") ▸Lapilli generally fall ______________________________. 5 Large clots of lava are thrown out of an erupting volcano while they are red-hot. As the clots spin through the air, they cool and develop a round or spindle shape. ▸ These tephra particles are called _________________________________. The largest tephra, formed from solid rock blasted from the fissure, is known as ___________________________________________. Volcanic cones that are broad at the base and have gently sloping sides are called __________________________________________. ▸A shield cone covers a wide area and generally results from quiet lava eruptions. ▸Layers of hot __________________________________ out around the vent, harden, and slowly build up to form the cone. ▸EXAMPLE: The _______________________________ are a chain of shield cones. Some explosive eruptions form __________________________________. ▸ A cinder cone is made of solid fragments ejected from the volcano. Most cinder cones have very steep slopes, often close to ________, and are rarely more than a few hundred meters high. ▪During a quiet eruption, the cone is __________________________________. Then an explosive eruption occurs, depositing large amounts of tephra around the vent. The explosive eruption is followed again by quiet lava flows. The resulting cone is formed of alternating layers of hardened lava flows and tephra. Composite cones, also known as ______________________________________, often develop into high volcanic mountains. ▸ EXAMPLES: ▪ 6 The funnel-shaped pit at the top of a volcanic vent is known as the ____________________________. ▸The crater is formed when material is blown out of the volcano by explosions. ▸A crater usually becomes wider as magma melts and breaks down the walls of the crater, allowing loose materials to collapse back into the vent. ▸When the magma chamber below a volcano is emptied, ________________________________________________. Explosions may also completely destroy the upper part of the cone, leaving a large, basin-shaped depression called a _________________________________ ▸ EXAMPLE: ▪___________________________________, a volcanic island in Indonesia 7.3 Extraterrestrial Volcanism Indications are that many of the planets and moons in the solar system, including the earth's moon, were volcanically active in the past. ▸ The earth's moon is covered with _____________________________________. ▸This is evidence that sometime in the past, active volcanoes dotted the surface of the moon. ▸Most of the craters on the lunar surface result from ___________________________________________. ▸Some of them have smooth, lava flow interiors and gently sloping, channeled exteriors. These features suggest that the craters were once active volcanoes. Spacecraft have orbited Mars and have sent back photographs that show numerous volcanoes and volcanic features on the Martian surface. ▸ The largest of these is the shield volcano ________________________________. ▪It rises nearly 7 ▪Its base measures ▪Its caldera, Olympus Mons is the __________________________________________________ ▸It is thought that the volcano has grown to such a tremendous size because, unlike the earth's crust, ________________________________________. __________, one of Jupiter’s _______moons, is the first planetary body on which active volcanoes have been sighted. ▸ Io, ▸ Scientists calculate that volcanoes on Io eject several thousand metric tons of material each second. ▪ Volcanoes on lo appear to be much more powerful than those on earth. ▸ Io moves inward and outward in its orbit around Jupiter because of the gravitational pull of the other moons of Jupiter. ▪As ▪Heat from the friction caused by this surface movement probably results in the melting of the interior of Io and leads to volcanism. 8