However, as temperatures increase with depth, the heat causes mantle rocks to lose their rigidity. The asthenosphere, which sits directly below the lithospheric mantle, is thought to be slightly denser at 3.4–4.4 grams per cubic cm (0.12–0.16 pound per cubic inch). Plate tectonics, theory dealing with the dynamics of Earth ’s outer shell—the lithosphere —that revolutionized Earth sciences by providing a uniform context for understanding mountain-building processes, volcanoes, and earthquakes as well as the evolution of Earth’s surface and reconstructing its past continents and oceans. The Thingvellir fracture lies in the Mid-Atlantic Ridge, which extends through the centre of Iceland. It has generally been accepted that tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere. Its average elevation above sea level is 840 metres (2,750 feet), while the average depth of oceanic crust is 3,790 metres (12,400 feet). Although it has a higher temperature than the outer core, the inner core is solid because of the tremendous pressures that exist near Earth’s centre. Theoretical diagram showing the effects of an advancing tectonic plate on other adjacent, but stationary, tectonic plates. This process begins at about 100 km (60 miles) below the surface.
This density difference creates two principal levels of Earth’s surface. Because new crust is formed, divergent margins are also called constructive margins. Depending on the material they travel through, the waves may either speed up, slow down, bend, or even stop if they cannot penetrate the material they encounter. Announcing our NEW encyclopedia for Kids! Dissipation of heat from the mantleis acknowledged to be the original source of the energy required to drive plate tectonics through convection or large scale upwelling and doming. During the late 20th and early 21st centuries, scientific understanding of the deep mantle was greatly enhanced by high-resolution seismological studies combined with numerical modeling and laboratory experiments that mimicked conditions near the core-mantle boundary. The boundary between the crust (continental or oceanic) and the underlying mantle is known as the Mohorovičić discontinuity (also called Moho), which is named for its discoverer, Croatian seismologist Andrija Mohorovičić. With increasing depth, however, the greater pressure from the weight of the rocks above causes the mantle to become gradually stronger, and seismic waves increase in velocity, a defining characteristic of the lower mantle. Developed from the 1950s through the 1970s, plate tectonics is the modern version of continental drift, a theory first proposed by scientist Alfred Wegener in 1912. The polarity of the iron crystals of the OIC is oriented in a north-south direction, whereas that of the IIC is oriented east-west. Beneath the mantle is the core, which extends to the centre of Earth, some 6,370 km (nearly 4,000 miles) below the surface. As a result, seismic waves slow as they enter the asthenosphere.
Because the partial melt is basaltic in composition, the new crust is oceanic, and an ocean ridge develops along the site of the former continental rift. The Pacific Plate, for example, is entirely oceanic, whereas the North American Plate is capped by continental crust in the west (the North American continent) and by oceanic crust in the east and extends under the Atlantic Ocean as far as the Mid-Atlantic Ridge.
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