Investigating Plate Tectonics, Earthquakes, and Volcanoes
PLATE TECTONICs The modern of the theory motions of Earth's outer layers is called plate tectonics It provides a framework for understanding many of Earth's features such as mountains earthquakes and vol canoes as well as the distribution of fossils and the ages of rocks It also helps scientists reconstruct ancient climates and continental configurations BASICS OF THE THEORY In 1912 the German meteorologist Alfred Wegener proposed that the continents had once been joined as a gigantic land mass called Pangea also spelled Pangaea According to this theory called continen tal drift the supercontinent long ago broke into pieces the present continents which have since drifted to their current posi tions At the time many geologists rejected Wegener s ideas partly because he had no convincing explanation why continents would move Io
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
