Changes

While met with initial controversy, over time much of Hutton's conclusions were borne out by observations collected from around the world during the rise of the British Empire and in the [[Royal Geographic Society]]. Based upon mounting evidence in 1830, Sir [[Charles Lyell]] published the first of what would eventually grow into twelve editions of ''Principles of Geology''. Charles Lyell reported geological processes occurring every day. He documented geological process which have occurred during the recorded history of man. By inference he concluded that geological processes must have occurred before the recorded history of man.<ref>http://www.britannica.com/eb/article-9049503/Sir-Charles-Lyell-Baronet</ref><ref>http://www.mnsu.edu/emuseum/information/biography/klmno/lyell_charles.html</ref> In time, most geologists came to agree with his theory of [[uniformitarianism]].  

While met with initial controversy, over time much of Hutton's conclusions were borne out by observations collected from around the world during the rise of the British Empire and in the [[Royal Geographic Society]]. Based upon mounting evidence in 1830, Sir [[Charles Lyell]] published the first of what would eventually grow into twelve editions of ''Principles of Geology''. Charles Lyell reported geological processes occurring every day. He documented geological process which have occurred during the recorded history of man. By inference he concluded that geological processes must have occurred before the recorded history of man.<ref>http://www.britannica.com/eb/article-9049503/Sir-Charles-Lyell-Baronet</ref><ref>http://www.mnsu.edu/emuseum/information/biography/klmno/lyell_charles.html</ref> In time, most geologists came to agree with his theory of [[uniformitarianism]].  

Modern geology has learned much about the Earth and geologic processes. Volcanism and deformation bring rocks to the surface of the Earth from depths as great as 50 to 100 km. Scientists can make inferences about some of the properties of the Earth at these depths by studying these rocks. But far more information has been provided through the use of seismic waves created by natural earthquakes, and by controlled explosions designed to learn more about the composition of the Earth, including underground nuclear explosions. This data has revealed that the Earth is composed of three main layers, the crust, the mantle and the core. The crust, or outermost layer, varies in thickness from about 5 km under oceans to about 40 km under continents. The mantle consists of an outermost zone of about 100 km thick named the lithosphere. The layer below is a partially fluid solid named the athenosphere, which ends at a depth of about 200 km. As seismic waves penetrate deeper into the mantle they indicate that between 400 km and about 2900 km atoms are packed closer and closer together by extreme pressures, creating a crystalline structure. The Earth's core is 2900 km from the surface. Seismic waves slow down in this region until a depth of 5100 km, indicating that the core consists of a liquid layer surrounding a solid core at the center.<ref name="press">Press, Frank and Siever, Raymond.1998."Understanding Earth", 2nd Ed. New York: W. H. Freeman and Company</ref>

Modern geology has learned much about the Earth and geologic processes. Volcanism and [[deformation]] bring rocks to the surface of the Earth from depths as great as 50 to 100 km. Scientists can make inferences about some of the properties of the Earth at these depths by studying these rocks. But far more information has been provided through the use of seismic waves created by natural earthquakes, and by controlled explosions designed to learn more about the composition of the Earth, including underground nuclear explosions. This data has revealed that the Earth is composed of three main layers, the crust, the mantle and the core. The crust, or outermost layer, varies in thickness from about 5 km under oceans to about 40 km under continents. The mantle consists of an outermost zone of about 100 km thick named the lithosphere. The layer below is a partially fluid solid named the athenosphere, which ends at a depth of about 200 km. As seismic waves penetrate deeper into the mantle they indicate that between 400 km and about 2900 km atoms are packed closer and closer together by extreme pressures, creating a crystalline structure. The Earth's core is 2900 km from the surface. Seismic waves slow down in this region until a depth of 5100 km, indicating that the core consists of a liquid layer surrounding a solid core at the center.<ref name="press">Press, Frank and Siever, Raymond.1998."Understanding Earth", 2nd Ed. New York: W. H. Freeman and Company</ref>

Extensive exploration and mapping of the seafloor after [[World War II]] led to the discovery of a deep rift running down the center of the Mid-Atlantic Ridge. In the early 1960s [[Harry Hess]] of [[Princeton University]] and [[Robert Dietz]] of the [[University of California]] suggested that the seafloor separates along the rifts in mid-0ceanic ridges and that new seafloor forms by upwelling of hot mantle materials in these cracks, followed by lateral spreading. By 1967 separate lithospheric plates had been identified, which explained phenomena such as high levels of volcanic and [[earthquake]] activity that take place between the plates. By the end of the 1960s the theory of [[plate tectonics]] proved to be a unifying concept that pulled together diverse theories and explained a the large body of observations in the field. <ref name="press" />

Extensive exploration and mapping of the seafloor after [[World War II]] led to the discovery of a deep rift running down the center of the Mid-Atlantic Ridge. In the early 1960s [[Harry Hess]] of [[Princeton University]] and [[Robert Dietz]] of the [[University of California]] suggested that the seafloor separates along the rifts in mid-0ceanic ridges and that new seafloor forms by upwelling of hot mantle materials in these cracks, followed by lateral spreading. By 1967 separate lithospheric plates had been identified, which explained phenomena such as high levels of volcanic and [[earthquake]] activity that take place between the plates. By the end of the 1960s the theory of [[plate tectonics]] proved to be a unifying concept that pulled together diverse theories and explained a the large body of observations in the field. <ref name="press" />