plate tectonics Essay

Ch. 3 Outline
*a key to the curious puzzle was found 50 years earlier by Alfred Wegener, a busy German meteorologist and polar explorer
-in 1912 he proposed a theory called continental drift; he suggested all Earth’s land had once been in a single supercontinent surrounded by ocean
-he called the land mass Pangaea (pan/”all”; gaea/”Earth, land”) and the surrounding ocean Panthalassa (thalassa/”ocean”)
*although he wasn’t first to assemble the continents using geology + geometry (Antonio Snider-Pellegrini in 1858) he was first to propose mechanism to account for the “drift” -Wegener died on an expedition across Greenland in 1930, his theory in eclipse
*geologists are convinced that Earth has distinct interior layers, resembling an onion
*the forces that cause earthquakes generate low-frequency waves called seismic waves (seismos/”earthquake”)
-some of these waves radiate through Earth, reflecting + blending as they travel and reappear at the surface
*seismic waves form in two types: surface waves and body waves -surface waves move along Earth’s surface; they cause most of the damage in an earthquake -body waves are less dramatic but useful for analyzing Earth’s interior structure *the P wave (primary wave) is a compressional wave similar to a sound wave *the S wave (secondary wave) is a shear wave like a rope shaken side to side
- P waves travel twice as fast as S waves, so P waves arrive first at a distant seismograph, an instrument that senses and records earthquakes
*in 1906 Oldham made a critical discovery: no S waves survive deep passage through Earth
-he predicted that a shadow zone, a wide band from which S waves were absent, would be on the side of Earth opposite location of the earthquake
-he found that P waves arrived at a seismograph farthest away from an earthquake much more slowly than expected; deflected by the Earth’s core
-working with this info, later researchers calculated the mantle-core boundary is about 2,900 km below the surface
*in 1935, Danish seismologist Inge Lehmann suggested that the faint P waves had sped up as they passed through an inner core, indicating it was solid
*in March of 1964, one of the largest earthquakes ever struck 144 km east of Anchorage, Alaska
-the release of energy tore the surface of Earth for 800 km between the port of Cordova (east) and Kodiak Island (west)
-in some places the vertical movement of the crust was 3.7 m, one small island was lifted 11.6 m; horizontal movement caused the greatest damage: 65,000 km of land abruptly moved west
*the first useful scientific classification of Earth’s interior was based on chemical composition; geologists named the layers crust, mantle, and core
-the crust is the thin outermost layer; accounts for only 0.4% of Earth’s total mass and less than 1% of its volume
*the thin oceanic crust is mostly basalt, a heavy dark rock composed of oxygen, silicon, magnesium, and iron; its density is about 2.9 g/cm^3
*the thicker continental crust is mostly made up of granite, a light colored rock composed mainly of oxygen, silicon, and aluminum; density about 2.7 g/cm^3
-the mantle, layer beneath the crust, composes 68% of Earth’s mass and 83% of its volume; mantle materials are thought to contain mainly silicon and oxygen with iron + magnesium; its average density is about 4.5 g/cm^3
-the core, Earth’s innermost layer, consists mainly of iron (90%) and nickel, along with silicon + sulfur + heavy elements; its average density is about 13 g/cm^3 and its radius is about 3,470 km *the core accounts for about 31.5% of Earth’s mass and about 16% of its volume
*physical properties are more important than the chemical one in determining movement so geologists have devised a classification based on physical properties
-the lithosphere is Earth’s cool, rigid outer layer, 100-200 km thick. It compromises the crust and the uppermost portion of the mantle
-the asthenosphere is the hot, slowly flowing layer of

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