The division is based on differences in mechanical properties and in the method for the transfer of heat. The lithosphere is cooler and more rigid, while the asthenosphere is hotter and flows more easily. In terms of heat transfer, the lithosphere loses heat by conductionwhereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient. This division should not be confused with the chemical subdivision of these same layers into the mantle comprising both the asthenosphere and the mantle portion of the lithosphere and the crust:
Plate tectonics has revolutionized virtually every discipline of the Earth sciences since the late s and early s. It has served as a unifying model or paradigm for explaining geologic phenomena that were formerly considered in unrelated fashion.
Plate tectonics describes seismic activity,… The theory of plate tectonics is based on a broad synthesis of geologic and geophysical data. It is now almost universally accepted, and its adoption represents a true scientific revolution, analogous in its consequences to quantum mechanics in physics or the discovery of the genetic code in biology.
For details on the specific effects of plate tectonics, see the articles earthquake and volcano. A detailed treatment of the various land and submarine relief features associated with plate motion is provided in the articles tectonic landform and ocean.
Principles of plate tectonics In essence, plate-tectonic theory is elegantly simple. While the interiors of the plates are presumed to remain essentially undeformed, plate boundaries are the sites of many of the principal processes that shape the terrestrial surface, including earthquakes, volcanismand orogeny that is, formation of mountain ranges.
A cross section of Earth's outer layers, from the crust through the lower mantle. For a deeper discussion of plate-driving mechanisms, see Plate-driving mechanisms and the role of the mantle.
Depending on the material they travel through, the waves may either Plate tectonics theory up, slow down, bend, or even stop if they cannot penetrate the material they encounter.
Collectively, these studies show that Earth can be internally divided into layers on the basis of either gradual or abrupt variations in chemical and physical properties. Chemically, Earth can be divided into three layers. A relatively thin crustwhich typically varies from a few kilometres to 40 km about 25 miles in thickness, sits on top of the mantle.
Beneath the mantle is the core, which extends to the centre of Earth, some 6, km nearly 4, miles below the surface. Geologists maintain that the core is made up primarily of metallic iron accompanied by smaller amounts of nickelcobaltand lighter elements, such as carbon and sulfur.
There are two types of crust, continental and oceanicwhich differ in their composition and thickness. The distribution of these crustal types broadly coincides with the division into continents and ocean basins, although continental shelveswhich are submerged, are underlain by continental crust.
The continents have a crust that is broadly granitic in composition and, with a density of about 2. Continental crust is typically 40 km 25 miles thick, while oceanic crust is much thinner, averaging about 6 km 4 miles in thickness. These crustal rocks both sit on top of the mantle, which is ultramafic in composition i.
The Moho is clearly defined by seismic studies, which detect an acceleration in seismic waves as they pass from the crust into the denser mantle. The boundary between the mantle and the core is also clearly defined by seismic studies, which suggest that the outer part of the core is a liquid.
The four main types of seismic waves are P waves, S waves, Love waves, and Rayleigh waves. The effect of the different densities of lithospheric rock can be seen in the different average elevations of continental and oceanic crust. The less-dense continental crust has greater buoyancy, causing it to float much higher in the mantle.
Its average elevation above sea level is metres 2, feetwhile the average depth of oceanic crust is 3, metres 12, feet. The lithosphere itself includes all the crust as well as the upper part of the mantle i. However, as temperatures increase with depth, the heat causes mantle rocks to lose their rigidity.
This process begins at about km 60 miles below the surface. This change occurs within the mantle and defines the base of the lithosphere and the top of the asthenosphere. This upper portion of the mantle, which is known as the lithospheric mantle, has an average density of about 3.
The asthenosphere, which sits directly below the lithospheric mantle, is thought to be slightly denser at 3. In contrast, the rocks in the asthenosphere are weaker, because they are close to their melting temperatures.
As a result, seismic waves slow as they enter the asthenosphere. 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.The Plate Tectonics Teaching Packet is available for purchase here.
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Over pages of ready-to-run materials covering: The Theory of Plate Tectonics. See also This Dynamic Planet, the map showing the Earth's physiographic features, current plate movements, and locations of volcanoes, earthquakes, and impact craters. The USGS also has created a website for teachers: This Dynamic Planet: A Teaching Companion.
The theory of plate tectonics, which was formulated in the late s, revealed that the configuration and position of the continents and oceans are dynamic, rather than static, features of Earth. Oceans grow and shrink, while continents break into fragments or coalesce into larger masses.
Theory of Plate Tectonics. When the concept of seafloor spreading came along, scientists recognized that it was the mechanism to explain how continents could move around Earth’s surface.
Like the scientists before us, we will now merge the ideas of continental drift and seafloor spreading into the theory of plate tectonics. The progress of the earth sciences and the advancement of technologies associated with the understanding of our planet during the 's and 50's have led geologists to develop a new way of looking at the world and how it works.
Plate tectonics - Development of tectonic theory: The outlines of the continents flanking the Atlantic Ocean are so similar that their correspondence was apparent as soon as accurate maps became available. The earliest references to this similarity were made in by Flemish cartographer Abraham Ortelius and later in by the English philosopher Francis Bacon, in his book Novum Organum.