Investigating Plate Tectonics, Earthquakes, and Volcanoes
INVESTIGATING PLATE TRCTONICs EARTHQUAKES AND VOLCANOES Ocean oceanic crust continental crust 10 lithosphere Mohorovičić discontinuity rigid rocks 100 low velocity zone partial melting of rocks 200 asthenosphere 300 H weaker plastic rocks upper mantle S 400 mechanically strong rocks 500 6004 700 lower mantle 2 900 A cross section shows Earth's outer layers subdivided according to their physical properties The crust and the uppermost part of the mantle form the rigid lithosphere Below that is a partly molten some which overlies a more rigid one Encyclopaedia Britannica Inc I2
PLATE TECTONICs crust effectively floats The mantle extends to a depth of about 1 800 miles 2 900 kilo meters where temperatures reach roughly 6 700 °F 3 700 °C Below the mantle is Earth's core made largely of iron and nickel The outer part of the core is liquid the motions of which are believed to create Earth's magnetic field The innermost core though extremely hot is solid because of the extreme pressure Much of what is known about Earth's internal structure comes from the study of vibrations called seismic waves which result from earthquakes The waves travel through Earth's interior and reflect or bend upon encountering changes in the density of the rock One type of seismic wave is absorbed if it enters liquid From this sci entists have discovered a partially molten zone in the mantle between about 50 and 200 miles 80 and 350 kilometers below the surface This layer called the astheno sphere allows the uppermost mantle and crust together called the lithosphere to slide across the top of it Earth's lithosphere is broken into about a dozen large pieces plus some smaller ones called plates The edges of these plates do not necessarily cor respond to the boundaries of continents or 13
INVESTIGATING PLATE TECTONICs EARTHQUAKES AND VoICANOES ALFRED WEGENER AND CONTINENTAL DRIFT Alfred Wegener the originator of the con tinental drift theory was born in Berlin Germany on Nov 1 1880 The son of a direc tor of an orphanage he received his doctorate in astronomy from the University of Berlin in 1905 During this time he became interested in meteorology and geology Wegener went on four expeditions to Greenland and was considered a specialist on the territory He taught meteorology at Marburg and Hamburg and was a professor of meteorology and geo physics at the University of Graz from 1924 to 1930 He died during his last expedition to Greenland in 1930 Like some other scientists before him Wegener became impressed with the simi larity in the coastlines of eastern South America and western Africa which appear that they would fit together like pieces of a jigsaw puzzle He was not the first to spec ulate that those lands had once been joined together or that all the present day con tinents had formed a single large mass or supercontinent However other scientists had explained the separation of the modern continents as having resulted from the sink ing of large portions of the supercontinent 14
PLATE TECTONICS to form the Atlantic and Indian oceans Wegener in contrast proposed that Pangea broke apart and that its parts had slowly moved thousands of miles apart over long periods of geologic time His term for this movement was continental displacement which gave rise to the term continental drift oceans For example the North American Plate includes the western half of the North Atlantic Ocean's seafloor Scientists are now almost certain that Wegener s Pangea existed It probably started to break apart from about 240 million to 200 million years ago after having been assembled from earlier continents only a few tens of millions of years earlier It now seems that earlier incarnations of Pangea may have occurred as part of a roughly 400 million year cycle of the breakup and reassembly of supercontinents PLATE MOVEMENTS The way the plates interact at their mar gins depends on whether the crust forming the top of the plate at the point of con tact is oceanic or continental Continental 15
