Plate Tectonics and Crust Oceanic Lithosphere

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.

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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 oundary, 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 triggers large earthquakes, such as the 1906 one that devastated San Francisco. Convergent Boundaries * Occur where a plate of ocean dives, in a process called subduction, under a landmass. As the overlying plate lifts up, it also forms mountain ranges. In addition, the diving plate melts and is often spewed out in volcanic eruptions such as those that formed some of the mountains in the Andes of South America. At ocean-ocean convergences, one plate usually dives beneath the other, forming deep trenches like the Mariana Trench in the North Pacific Ocean, the deepest point on Earth.

These types of collisions can also lead to underwater volcanoes that eventually build up into island arcs. Three styles of convergent plate boundaries * Continent-continent collision * Continent-oceanic crust collision * Ocean-ocean collision Continent-continent collision * Forms mountains, e. g. European Alps, Himalayas Continent-ocean collision * Called SUBDUCTION Subduction Oceanic lithosphere subducts underneath the continental lithosphere * Oceanic lithosphere heats and dehydrates as it subsides * The melt rises forming volcanism * E. g. The Andes Ocean-ocean collision * When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone. * The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench. * The worlds deepest parts of the ocean are found along trenches. * Ex. The Mariana Trench is 11 km deep! Volcanoes and Earthquakes

Volcanoes * A volcano is a mountain that opens downward to a pool of molten rock below the surface of the earth. * When pressure builds up, eruptions occur. Gases and rock shoot up through the opening and spill over or fill the air with lava fragments. * Eruptions can cause lateral blasts, lava flows, hot ash flows, mudslides, avalanches, falling ash and floods. * Volcano eruptions have been known to knock down entire forests. * An erupting volcano can trigger tsunamis, flash floods, earthquakes, mudflows and rockfalls. Volcanoes are formed by: – Subduction – Rifting Hotspots What are Hotspot Volcanoes? * Hot mantle plumes breaching the surface in the middle of a tectonic plate

* A hotspot is a location on the Earth’s surface that has experienced active volcanism for a long period of time. * The source of this volcanism is a mantle plume of hot mantle material rising up from near the core-mantle boundary through the crust to the surface * The Hawaiian island chain are examples of hotspot volcanoes. * The tectonic plate moves over a fixed hotspot forming a chain of volcanoes. * The volcanoes get younger from one end to the other.

Earthquakes * A sudden movement of the Earth’s lithosphere (its crust and upper mantle). Earthquakes are caused by the release of built-up stress within rocks along geologic faults or by the movement of magma in volcanic areas. * As with volcanoes, earthquakes are not randomly distributed over the globe * At the boundaries between plates, friction causes them to stick together. When built up energy causes them to break, earthquakes occur. Where do earthquakes form? We know there are three types of plate boundaries: Divergent, Convergent and Transform.

Movement and slipping along each of these types of boundaries can form an earthquake. * Depending on the type of movement, the earthquakes occur in either a shallow or deep level in the crust. * The majority of tectonic earthquakes originate at depths not exceeding tens of kilometers. * In subduction zones, where old and cold oceanic crust descends beneath another tectonic plate, “Deep Focus Earthquakes” may occur at much greater depths (up to seven hundred kilometers! ). * These earthquakes occur at a depth at which the subducted crust should no longer be brittle, due to the high temperature and pressure.

A possible mechanism for the generation of deep focus earthquakes is faulting. * Earthquakes may also occur in volcanic regions and are caused there both by tectonic faults and by the movement of magma (hot molten rock) within the volcano. Such earthquakes can be an early warning of volcanic eruptions. * Massive earthquake can form tsunamis. Importance of Lithosphere The lithosphere is very important because it is a large reservoir of resources, which are so useful to man. The majority of the material objects which man requires are supplied by lithosphere. The lithosphere serves as a source of minerals. The minerals supply the basic materials required for making a variety of commodities, which man uses daily. * The lithosphere is also the major source of fuels such as coal, petroleum and a natural gas. Without these fuels human life, as we know it today, would have been impossible.

* The lithosphere in combination with the hydrosphere and the atmosphere plays a vital role in the growth of plants and animals. It provides nutrients to the plants. The plants are the source of food for man and all other animals. How is lithosphere is important? The lithosphere is important to humans, as it shields us, and all other organisms on Earth, from the immense heat of the Earth’s inner core. The consequence of its formation is that water can exist in its liquid form, allowing for the carbon-based life forms that exist today. * the lithosphere is also important for the stability of the Earth. The inner layers of the Earth are subjected to pressure by the lithosphere as it is affected by gravity, forcing the innermost elements into the Earth’s radioactive core, continuing the nuclear fusion that maintains the planet’s constant state of balance.

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Plate Tectonics and Crust Oceanic Lithosphere. (2016, Oct 16). Retrieved from