Physical Geography

Jaesung Lee
Geography 1
Professor Thomas Orf
Chapter 14 Questions 1) The first evidence Wegner was able to find was the remarkable number of close affinities of geologic features on both sides of the Atlantic Ocean. He found the continental margins of the subequatorial portions of Africa and South America fit together with jigsaw-on-like precision. He also determined that the petrologic records on both sides of the Atlantic show many distributions- such as ancient coal deposits-that would be continuous if the ocean did not intervene. 2) Despite the vast number of distributional coincidences, most scientists felt that two difficulties made the theory improbable if not impossible: (1) Earth’s crust was believed to be too rigid to permit such large-scale motions-after all, how could solid rock plow through solid rock? (2) Further, Wegener did not offer a suitable mechanism that could displace such large masses for a long journey. 3) When any rock containing iron is formed-such as iron-rich ocean floor basalt-it is magnetized so that the iron-rich grains become aligned with Earth’s magnetic field. This orientation then becomes a permanent record of the polarity of Earth’s magnetic field at the time the rock solidifies. In 1963, Fred Vine and D.H. Matthews used paleomagnetism to test the theory of seafloor spreading by studying paleomagnetic data from a portion of the midocean ridge system. If the seafloor has spread laterally by the addition of new crust at the oceanic rides, there should be a relatively symmetrical pattern of magnetic orientation-normal polarity, reversed polarity, normal polarity, and so on-on both sides of the ridges. Another confirmation of seafloor spreading is that several thousand ocean floor cores of sea-bottom sediments were analyzed, and it was evident from this work that, almost invariably, sediment thickness and age increase with increasing distance from the midocean ridges, indicating that sediments farthest from the ridges are oldest. 4) The relationship between earthquakes and plate tectonics are that plate tectonics tells us about the outer shell or also known as the lithosphere. the lithosphere is the thinnest part of the earth. 5) Plate tectonics is a coherent theory of massive crustal rearrangement based on the movement of continent-sized lithospheric plates. The driving mechanism for plate tectonics is thought to be convection within Earth’s mantle. A very sluggish thermal convection system appears to be operating within the planet, bringing deep-seated heated rock slowly to the surface. Plates may be “pushed” away from midocean ridges to a certain extent, but it appears that much of the motion is a result of the plates being “pulled” along by the subduction of dense oceanic lithosphere down into the asthenosphere. 6) Divergent plate boundary is a location where two lithospheric plates spread apart. Convergent plate boundary is a location where two lithospheric plates collide. Transform plate boundary is two plates slipping past one another laterally. 7) A divergent boundary is usually represented by a midocean ridge. Most of the midocean ridges of the world are either active or extinct spreading ridges. Such spreading centers are associated with shallowing-focus earthquakes (meaning that ruptures that generate the earthquakes are within about 70 kilometers [45 miles] of the surface), volcanic activity, and hydrothermal metamorphism. Divergent boundaries can also develop with a continent, resulting in a continental rift valley such as the great East African Rift Valley that extends from Ethiopia southward through Mozambique. 8) Oceanic-continental convergence: Because oceanic lithosphere includes dense basaltic crust, it is denser than continental lithosphere, and so oceanic lithosphere always underrides continental lithosphere when the two collide. The dense oceanic plate slowly and inexorably sinks into the asthenosphere in the process of subduction. The subducting slab pulls on the rest of the plate. Wherever such an oceanic-continental convergent boundary exists, a mountain range is formed (the Andes range of South America is one notable example; the Cascades in northwestern North America is another) and a parallel oceanic trench develops as the seafloor is pulled down by the subducting plate.
Oceanic-oceanic convergence: If the convergent boundary is between two oceanic plates, subduction also takes place. As one of the oceanic plates subducts beneath the other an oceanic trench is formed, shallow- and deep-focus earthquakes occur, and volcanic activity is initiated with volcanoes forming on the ocean floor. With time, a volcanic island arc (such as the Aleutian Islands and Mariana Islands) develops; such an arc may eventually become a more mature island arc system (such as Japan and the islands of Sumatra and Java in Indonesia are today)
Continental-continental convergence: Where there is a convergent boundary between two continental plates, no subduction takes place because continental crust is too buoyant to subduct. Instead, huge mountain ranges, such as Alps, are built up. The most dramatic present-day example of continental collision has resulted in the formation of the Himalayas. 9) Subduction cannot take place in the continental plate collision zone because crust is too buoyant to subduct. 10) Oceanic trench, often known as deep oceanic trench, is a deep linear depression in the ocean floor where subduction is taking place. Midocean ridge is a lengthy system of deep-sea mountain ranges, generally located at some distance from any continent; formed by divergent plate boundaries on the ocean floor. 11) The San Andreas Fault system of California is the major component of the transform boundary between the Pacific Plat and North American Plate. Notice that north of the San Andreas Fault system, subduction of several small oceanic plates I taking place; this subduction is associated with the Cascade volcanoes. 12) If the convergent boundary is between two oceanic plates, subduction also takes place. As one of the oceanic plates subducts beneath the other an oceanic trench is formed, shallow- and deep-focus earthquakes occur, and volcanic activity is initiated with volcanoes forming on the ocean floor. With time, a volcanic island arc develops; such an arc may eventually become a more mature island arc system. 13) Plate boundaries are found all of the way around the Pacific basin-primarily subduction zones, along with segments of transform and divergent boundaries. It is along these quakes take place in what is now called the Pacific Ring of Fire. 14) Hot spot is an area of volcanic activity within the interior of a lithospheric plate associated with magma rising up from the mantle below. One of the present-day example of hot spot is the Hawaiian Hot Spot. 15) Hot spot trail is created when the plate above the hot spot is moving, so the volcanoes or other hot spot features are eventually carried off the plume and become inactive, while in turn new volcanic features develop over the plume, so generating a straight-line. Volcanic island arc is the chain of volcanic islands associated with an oceanic plate-oceanic plate subduction zone. 16) To explain the existence of hot spots, the mantle plume model was proposed in the late 1960s. This explanation suggest that midplate volcanic activity develops over narrow plumes of heated material rising through the mantle-perhaps originating as deep as the core-mantle boundary. Such mantle plumes are believed to be relatively stationary over long periods of time. As the magma rises through the plate above, it creates hot spot volcanoes and/or hydrothermal features on the surface-often after an initial large overpouring of lava known zones as flood basalt. 17) Several mountain ranges in North America and Eurasia are in the middle of plates rather than in boundary Also while convention of heated material within the mantle provides the general mechanism for plate movement, the details of heat flow within Earth and the possible relationships of mantle plumes to these overall patterns are still being worked out. 18) Seismic tomography is a technique that uses earthquake waves to produce a kind of “ultrasound” of Earth-suggests that the magma source of at least some hot spots is quite shallow, while the source for others are mantle plumes originating deep from within the mantle. Further, some researchers city evidence suggesting that several mantle plumes may have changed location in the geologic past. 19) Terrane is a mass of lithosphere, bounded on all sides by faults, that has become accreted to a lithospheric plate margin with different lithologic characteristics from those of the terrane; often comprised of lithosphere that is too buoyant to subduct. 20) Volcanism is a general term that refers to movement of magma from the interior of Earth to or near the surface. 21) Magma is a molten material below Earth’s surface. Lava is a molten magma that is extruded onto the surface of Earth, where it cools and solidifies. Pyroclastics, often known as pyroclastic material, is a solid rock fragments thrown into the air by volcanic explosions. 22) ㅁ 23) Flood basalt is a large-scale outpouring of basaltic lava that may cover an extensive area of Earth's surface. 24) Shield volcanoes are volcanoes built up in a lengthy outpouring of very fluid basaltic lava. Shield volcanoes are broad mountains with gentle slopes. 25) Composite volcanoes are volcanoes with the classic symmetrical, cone-shaped peak, produced by a mixture of lava outpouring and pyroclastic explosion. Lava dome, often known as plug dome, is a dome or bulge formed by the pushing up of viscous magma in a volcanic vent. Cinder cone is a small, common volcano that is composed primarily of pyroclastic material blasted out from a vent in small but intense explosions. The structure of the volcano is usually a conical hill of loose material. 26) During a major eruption about 7700 years ago, the walls of Mount Mazama weakened and collapsed as enormous volumes of pyroclastic material were ejected from the volcano. The partial emptying of magma chamber below Mount Mazama may have contributed to this collapse. The final, cataclysmic eruption removed-by explosion and collapse-the upper 1220 meters (4000 feet) of the peak and produced a caldera whose bottom is 1220 meters (4000 feet) below the crest of the remaining rim. Later, half this depth filled with water, creating one of the deepest lakes in North America. A subsidiary volcanic cone has subsequently built up from the bottom of the caldera and now breaks the surface of the lake as Wizard Island. 27) Pyroclastic flow is a high-speed avalanche of hot gases, ash, and rock fragments emitted from a volcano during an explosive eruption; also known as nuee ardente. Volcanic mudflow is a fast-moving, muddy flow of volcanic ash and rock fragments; also called a lahar. 28) Igneous intrusion is features formed by the emplacement and cooling of magma below the surface. Pluton is a large, intrusive igneous body. Batholith is the largest and most amorphous of igneous intrusions. Dike is a vertical or nearly vertical sheet of magma that is thrust upward into preexisting rock. 29) Folding is the bending of crustal rocks by compression and/or uplift. Faulting is a fracture or zone of fracture where the rock structure is frocefully broken and one side is displaced relative to the other. The movement can be horizontal or vertical, or a combination of both. 30) Syncline is a simple downfold in the rock structure. Anticline is a simple symmetrical upfold in the rock structure. 31) Where a layer is arched over an upfold, tension cracks can form and provide easy footholds for erosional forces to remove materials and incise downward into the underlying strata. Conversely, the compression that acts on the downfolded beds increases their density and therefore their resistance to erosion. Thus, over a long period of time, the upfolds may be eroded away faster than the downfolds, producing anticlinal valleys and synclinal ridges. 32) Normal fault is the result of tension (extension) producing a steeply inclined fault plane, with the block of land on one side being pushed up, or upthrown, in relation to the block on the other side, which is downthrown. Reverse fault is a fault produced from compression, with the upthrown block rising steeply above the downthrown block. Thrust fault is a fault created by compression forcing the upthrown block to override the downthrown block at a relatively low angle; also called an overthrust fault. Strike-slip fault is a fault produced by shearing, with adjacent blocks being displaced laterally with respect to one another. The movement is mostly or entirely horizontal. 33) Fault scarp is a cliff formed by faulting. 34) Graben is a block of land bounded by parallel faults in which the block has been downthrown, producing a distinctive structural valley with a straight, steep-sided fault scarp on either side. Horst is a relatively uplifted block of land between two parallel faults. Tilted fault block mountains is a mountain fourface block is faulted and relatively upthrown on one side without any faulting or uplift on the other side. The block is tilted asymmetrically, producing a steep slope along the fault scarp and a relatively gentle slope on the other side of the block. 35) Earthquake is vibrations generated by abrupt movement of Earth's crust. 36) The energy released in an earthquake moves through Earth in several different kinds of seismic waves that originate at the center of fault motion is called the focus of the earthquakes. Epicenter of the earthquake is the location on the surface directly above the center of fault rupture during an earthquake. 37) Magnitude of an earthquake is a scale used to describe the relative amount of energy released during an earthquake. Several different magnitude scales are in current use, such as the moment magnitude and the Richter scale. While each earthquakes can be assigned a single magnitude number to describe its relative size, every earthquake generates a wide range of local ground shaking intensities- and it is the strength of local ground shaking that directly influences the amount of damage that results from an earthquake. 38) A magnitude 4 earthquake releases about 23 times more energy than a magnitude 3; a magnitude 5 releases 1000 times more energy than a magnitude 3; and a magnitude 7 releases 1,000,000 times more energy than a magnitude 3. 39) Modified Mercalli intensity scale is a qualitative scale from I to XII used to describe the relative strength of ground shaking during an earthquake. 40) The fastest moving earthquake waves, and the first to be felt during an earthquake, are known as P or primary waves. P waves move through Earth in the same fashion as sound waves, by alternately compressing and relaxing the medium through which they are traveling. The initial jolt of the P waves is followed by the strong side-to-side and up-and-down shearing motion of the slower-moving S or secondary waves. Both P waves and S waves travel through the body of Earth, while the motion of a third type of earthquake is limited to the surface. These surface waves typically arrive side movement as well as the up-and-down "rolling" motion often experienced during a large earthquake. 41) Liquefaction is a phenomenon observed during an earthquake when water-saturated soil or sediments become soft or even fluid during the time of strong ground shaking.