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Plate Tectonics

PostPosted: September 23rd, 2014, 8:29 pm
by Jessica
In the last century we have gathered much evidence to support the idea that Earth's surface is broken up into "lithospheric" plates that slowly move over the top of the mantle. The map below shows the major plates.

Plates are large pieces of the upper few hundred kilometers of Earth that move more or less as a single unit. It is easier to think of plates as rigid "rafts" floating on the mantle, but some plates also have some internal deformation. However, it is clear that the most active deformation of the plates occurs along their boundaries, where they interact with other plates.

The precise thickness of a plate varies from place to place, but away from plate margins, plates are usually on the order of 100-200 km thick.

Plates are defined not on chemical differences, but using rock strength, and they are composed of the crust and the uppermost part of the mantle.

The precise lower boundary of a plate depends on the temperature of the mantle material. At about 1300°C typical mantle material begins to melt, and softens dramatically. We call that part of the mantle asthenosphere, to indicate that it is a weak zone, that "decouples" the plate from the overlying mantle (actually, there is undoubtedly some "drag" forces that act between the two, but the lithosphere can move independently of the deeper mantle.

Since the construction of the first good maps of the continents, people have puzzled over the close match between the coastlines of South America and Africa.
The first to seriously investigate the connection was Alfred Wegener, a German meteorologist who proposed the continental drift hypothesis (between 1919-1929) to explain the observed shape of the coastlines and several other geologic observations regarding the observation of fossils and rocks on opposite sides of the ocean. Wegener proposed that at one time, all the present-day continents actually were combined into a "super-continent" which he called Pangaea (or Pangea).

The Geomagnetic Field

The "geomagnetic" field is generated by motions of the iron in the outer core. One property of a moving conductor (such as the flowing iron in the outer core) is that it produces a magnetic field. That same magnetic field allows us to use a compass to navigate around Earth's surface. The compass needle is a small magnetic that aligns itself with Earth's magnetic "flow lines" (a magnetic field is a vector quantity, it has a direction and magnitude). In a way, compasses are instruments for measuring the direction of Earth's magnetic field.