Plate Tectonics and Crust Oceanic Lithosphere

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Lithosphere
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Ecology (NASC 1093)|

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The Lithosphere
* The lithosphere is the hard shell of the Earth, consisting of the crust and the topmost part of the upper mantle. * It is a relatively thin layer, about 31 to 62 miles (50 to 100 km) thick under the oceans and 93 miles (150 km) thick on the continents. * It contains the minerals, rocks and soils that humans have used for building materials, metals and agriculture. * This layer is composed of the upper crust, about 3 miles (5 km) thick in the oceans and 40.3 mles (65 km) thick on the continents, and the upper mantle, which makes up the remainder.

Two types of Lithosphere
* Oceanic Crust
* Continental Crust

Oceanic Lithosphere
* the outermost layer of Earth’s lithosphere that is found under the oceans and formed at spreading centres on oceanic ridges. The oceanic crust is about 6 km (4 miles) thick. It is composed of several layers, not including the overlying sediment. * The topmost layer, about 500 meters (1,650 feet) thick, includes lavas made of basalt. Oceanic crust differs from continental crust in several ways: it is thinner, denser, younger, of different chemical composition, and formed above the subduction zones. Continental Lithosphere

* The continental crust is 20 to 70 kilometers thick and composed mainly of lighter granite. The density of continental crust is about 2.7 grams per cubic centimeter. It is thinnest in areas like the Rift Valleys of East Africa and in an area known as the Basin and Range Province in the western United States (centered in Nevada this area is about 1500 kilometers wide and runs about 4000 kilometers North/South). Continental crust is thickest beneath mountain ranges and extends into the mantle. Plate Tectonics

* The Earth's surface is made up of a series of large plates (like pieces of a giant jigsaw puzzle). * These plates are in constant motion travelling at a few centimetres per year. * The ocean floors are continually moving, spreading from the centre and sinking at the edges. * Convection currents beneath the plates move the plates in different directions. * The source of heat driving the convection currents is radioactive decay which is happening deep in the Earth. * The edges of these plates, where they move against each other, are sites of intense geologic activity, such as earthquakes, volcanoes, and mountain building. * The Earth’s crust is divided into 12 major plates which are moved in various directions. * This plate motion causes them to collide, pull apart, or scrape against each other. * Each type of interaction causes a characteristic set of Earth structures or “tectonic” features. * The word, tectonic, refers to the deformation of the crust as a consequence of plate interaction Three types of tectonic boundaries:

* Divergent, where plates move apart; and
*  Transform, where plates move sideways in relation to each other; * Convergent, where plates move into one another.
Divergent Boundaries
* magma from deep in the Earth's mantle rises toward the surface and pushes apart two or more plates. Mountains and volcanoes rise along the seam. The process renews the ocean floor and widens the giant basins. A single mid-ocean ridge system connects the world's oceans, making the ridge the longest mountain range in the world. * On land, giant troughs such as the Great Rift Valley in Africa form where plates are tugged apart. If the plates there continue to diverge, millions of years from now eastern Africa will split from the continent to form a new landmass. A mid-ocean ridge would then mark the boundary between the plates. Transform Boundaries

* The San Andreas Fault in California is an example of a transform boundary, where two plates grind past each other along what are called strike-slip faults. These boundaries don't produce spectacular features like mountains or oceans, but the halting motion often...
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