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Plate Tectonics • Theory of plate tectonics describes the forces within the Earth that create continents, ocean basins, mountain ranges, earthquake belts, volcanoes, etc. • Lithosphere divided into 8 major plates • Major interactions of the plate boundaries results in most of the Earth’s seismic activity Folds • Compressive forces cause rocks to buckle and fold • Anticline folds upward • Syncline folds downward Folding – formed the Appalachians In central Pennsylvania, the segment known as the Valley and Ridge has very distinctive topography: the ridges are curved, some shaped almost like an arrowhead, and have generally uniform elevations (not pointed or irregular like many western mountains). Faults • A fault is a large crack in the Earth's crust where one part of the crust has moved against another part. • A fault can be described in terms of its dip and strike. The strike is the direction of its movement on surface of the earth. The dip is a measure of how steeply the fault plane slopes. Dip-slip faults • include both normal and reverse faults • The “slip” occurs along the dip (fault plane) direction. The slip is either a movement upwards (reverse fault) or downwards (normal fault). Reverse Fault • Caused by compression forces • Hanging wall pushes up over footwall Reverse faults • The Sierra Madre fault zone of southern California is an example of reverse-fault movement. There the rocks of the San Gabriel Mountains are being pushed up and over the rocks of the San Fernando and San Gabriel valleys. Movement on the Sierra Madre fault zone is part of the process that created the San Gabriel Mountains. Thrust Fault • Reverse fault with low angle • Eastern flank of the Sierra Nevada, California Normal Faults • Sierra Nevada fault zones are examples of highangle normal faults where the hanging wall slips down relative to the footwall The Caldera. Long Valley Caldera a 15- by 30-km oval-shaped depression located 20 km south of Mono Lake along the east side of the Sierra Nevada in east-central California. Transform Faults • Transform faults are also called strike-slip faults. Here the “slip” or movement occurs along the strike (lateral direction). The fault plane is usually vertical, so there is no true hanging wall or footwall • The lateral or horizontal forces carry the walls alongside each other Horizontal (Transform) Fault Photo of the San Andreas Fault near Gorman California, showing rocks of the Pacific Plate on the left and the North American Plate on the right. Photograph copyright by David Lynch. The San Andreas fault, with a length of more than 800 miles (1,200 kilometers), is the longest fault in California, and one of the longest in North America. On January 9, 1857 at 8:20 am, an earthquake with a estimated magnitude of 8.0 occurred just north of Carrizo Plain. This quake caused nearly 30 feet (9 m) of lateral offset within Carrizo Plain, and ruptured the surface along the trace of the fault for about 220 miles (350 km). It was one of the greatest earthquakes ever recorded in the United States. Interesting facts about the San Andreas fault: The waters of Tulare Lake were thrown upon its shores, stranding fish miles from the original lake bed. The waters of the Mokelumne River rose well over its banks, reportedly leaving the bed dry in places. The Los Angeles River was reportedly flung out of its bed, too. Since that event, this portion of the San Andreas fault has been locked in place seismically quiet for decades, however it is obvious a similar magnitude quake today would cause billions of dollars of damage to Los Angeles, the southern San Joaquin Valley, and Antelope Valley. Retrieved from U.S. Department of the Interior – Bureau of Land Management. 5-02-07 http://www.blm.gov/ca/st/en/fo/bakersfield/Programs/carrizo/geology.html Plate Boundaries • Where lithospheric formation and destruction occur • Plates move due to thermal convection currents in plastic-like asthenosphere • Three types: convergent, divergent, transform fault Divergent Boundary • Tensional forces cause plates to move apart • Hot molten rock from asthenosphere is less dense and rises, creating new lithosphere The Mid-Atlantic Ridge marks where two continents once were one. Continental divergence creates oceans. Also the location of sea floor spreading. Mid-Atlantic Ridge Rifts and Rift Valleys Convergent Boundaries • Plates moving towards each other due to compressional forces • One plate drops below other plate, or folding/faulting occur • Regions of great mountain building • Three subgroups: oceanic-oceanic, oceanic-continental, continentalcontinental Oceanic-oceanic convergent zones are mostly hidden under the sea. Only arcs of volcanic islands mark them, made of dark and heavy basaltic lavas. The western Pacific Ocean is full of these—from north to south they include the Aleutian, Kuril, Japanese, Ryukyu, Izu-Bonin, Philippine, Mariana, Solomon and Tonga-Kermadec island arcs. In the Atlantic are the Caribbean and South Sandwich island arcs. In the Indian Ocean is the tangle of arcs that makes up the Indonesian archipelago. Characterized by earthquake and volcanic activity. Aleutian Islands Marianis Trench Oceanic-continental convergent zones are the classic case. The oceanic plate undergoes subduction and the volcanic arcs arise on land …. The west coast of the Americas is the major example, with volcanic zones in Alaska, the Pacific Northwest states, and a continuous stretch from Central America to Tierra del Fuego ("land of fire"). Italy, Greece, Kamchatka and New Guinea also fit this type. Characterized by earthquake and volcanic activity. • In Northern California, the collision of the Pacific and Juan de Fuca plates have given rise to the Cascade mountains and Mount Saint Helens eruption in 1980 • the subduction of the Pacific plate under the Eurasian plate has created the Japanese islands and volcanoes Cascade Mountains • The "Ring of Fire" is an arc stretching from New Zealand, along the eastern edge of Asia, north across the Aleutian Islands of Alaska, and south along the coast of North and South America. The Ring of Fire is composed over 75% of the world's active and dormant volcanoes. • Ring of Fire is located at the borders of the Pacific Plate and other major tectonic plates. • As the Pacific Plate collides with another tectonic plate, a great amount of energy is produced. This energy is transformed into heat, and can heat the rock into magma. This rises to the surface through volcanoes. When it reaches the surface the magma is called lava. • In addition to creating heat, the Pacific Plate is actually sliding down below the other tectonic plate, creating what is called a subduction zone • About 10 percent of the world’s active volcanoes are located in the United States Andes Mountains • The subduction of the Nazca Plate below the South American Plate has led to the creation of the Andes mountain range • There are also many active volcanoes in this area Andes Mountains (Western side of South America) Continent-continent convergent zones are tectonic sumo wrestling matches in which neither plate is subducted, because continental rock is too light to be carried very far into the dense mantle (about 150 km down at most). Instead the continental crust crumples into thick knots—tectonic mountain ranges—exposing deep-seated granites and gneisses with relatively little volcanism. The continental crust may also be cracked in pieces and shoved aside. Both of these responses are seen to perfection in the great foldbelt that stretches from Turkey to China. There the African, Arabian and Indian plates are moving northward into the Eurasian plate, respectively raising the Anatolian, Makran (Iranian) and Himalayan/Tibetan highlands. Continental-Continental Collision • No drastic difference in density so no subduction occurs • Preceded by an oceanic-continental collision • Compression causes plates to buckle and fold • Earthquakes are characteristic, but volcanoes are NOT Himalayas (due to India pushing up against Asia) Himalayas SOURCES • http://geography.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=geography&cdn=education&tm=9&gps=115_28_ 1020_568&f=00&su=p897.1.336.ip_&tt=2&bt=0&bts=1&zu=http%3A//vulcan.wr.usgs.gov/Glossary/PlateTectonics/ Maps/map_plate_tectonics_world.html • • • http://geography.about.com/cs/earthquakes/a/ringoffire.htm http://geology.about.com/library/bl/maps/blplatetypesehem.htm http://geology.about.com/library/bl/blnutshell_convergence.htm