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The tectonic plates of the world were mapped in the second half of the 20th century.Plate tectonics (from the Greek word for "one who constructs", τεκτων (tekton)) is a theory of geology developed to explain the phenomenon of continental drift. In the theory of plate tectonics the outermost part of the Earth's interior is made up of two layers, the outer lithosphere and the inner asthenosphere. Plate tectonic theory arose out of two separate geological observations: seafloor spreading and continental drift.
1 Key principles
The division of the Earth's interior into lithospheric and asthenospheric components is based on their mechanical differences. The lithosphere is cooler and more rigid, whilst the asthenosphere is hotter and mechanically weaker. This division should not be confused with the chemical subdivision of the Earth into (from innermost to outermost) core, mantle, and crust. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which "float" on the fluid-like asthenosphere. The relative fluidity of the asthenosphere allows the tectonic plates to undergo motion in different directions.
One plate meets another along a plate boundary, and plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features like mountains, volcanoThis article is about volcanoes geology. For the action movie see: Volcano (movie). A volcano (plural, volcanoes) is a geological landform (usually a mountain) where magma (rock of the earth's interior made molten or liquid by high pressure and temperatures and oceanic trenchThe oceanic trenches are several hundred kilometres long but narrow topographic depressions of the sea floor. They also are the deepest parts of the ocean floor. A trench marks the position at which the flexed, subducting slab begins to go under, on the ces. The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of FireRing of Fire. Click image to see large version. Courtesy USGS The Ring of Fire is a zone of frequent earthquakes and volcanic eruptions that encircles the basin of the Pacific Ocean. It is associated with a nearly continuous series of oceanic trenches and being most active and famous. These boundaries are discussed in further detail below.
Tectonic plates are comprised of two types of lithosphere: continentalThe Continental Crust is the layer of granitic and sedimentary rock which forms the continents and the areas of shallow seabed close to their shores, known as continental shelves. It is less dense than the material of the mantle and thus "floats" on top o and oceanicOceanic crust is the part of Earth's lithosphere which underlies the ocean basins. It is thinner (generally less than 10 km thick) but more dense than continental crust, about 3. 3 g/cc (grams per cubic centimeter). Oceanic crust is composed of mafic basa lithospheres; for example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans. The distinction is based on the density of constituent materials; oceanic lithospheres are denser than continental ones due to their greater mafic mineral content. As a result, the oceanic lithospheres generally lie below sea level (for example the entire Pacific Plate, which carries no continent), while the continental ones project above sea level (see isostasy for explanation of this principle, which is essentially a large-scale version of Archimedes' Bath).
2 Types of plate boundary
There are three types of plate boundary, characterised by the way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundary are:
- Transform boundaries occur where plates slide, or perhaps more accurately grind, past each other along transform-faults. The relative motion of the two plates is therefore either sinistral or dextral.
- Divergent boundaries occur where two plates slide apart from each other.
- Convergent boundaries (or active margins) occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or an orogenic belt (if the two simply collide and compress).
Plate boundary zones occur in more complex situations where three or more plates meet and exhibit a mixture of the above three boundary types.
