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Asthenosphere Facts

Asthenosphere Facts
The asthenosphere is the region of the upper earth considered to be mechanically weak. It sits below the lithosphere, reaching from 50 miles to 120 miles below the earth's surface. The boundary where the lithosphere and asthenosphere meet is referred to as the LAB. Most of the asthenosphere is solid but could be melted depending on its location. Because the lower asthenosphere is not well defined it is possible that it may extend as deep as 430 miles in some areas. The asthenosphere is the part of the earth's mantle involved in the tectonic plate movement. The pressure in the asthenosphere is so great that rocks are able to flow as if they were liquid.
Interesting Asthenosphere Facts:
The asthenosphere was named by a British geologist named J. Barrel. His theory divided the Earth's interior into three parts including the lithosphere, asthenosphere, and the centrosphere.
The lithosphere is the layer of the earth that contains the coastal plates. The asthenosphere is the middle layer and the centrosphere is the region from the asthenosphere all the way to the center of the earth.
The rocks located in the asthenosphere are not as dense as the rocks in the lithosphere.
The tectonic plates are located in the lithosphere but because of the movement of the rocks in the asthenosphere they are able to move about as if they were floating on liquid.
Earthquakes are a result of the movement of the tectonic plates. The deep earthquakes are a result of the rocks in the asthenosphere breaking apart.
The seismic waves pass more slowly through the asthenosphere than they do the lithosphere. For this reason the asthenosphere is often referred to as the LVZ (low velocity zone). The speed of the seismic waves decreases as the rigidity decreases in this zone.
The presence of the asthenosphere was suspected as far back as 1926. It wasn't confirmed until after the Great Chilean earthquake of 1960. The earthquake reached 9.5 on the moment magnitude scale. Analysis of the earthquake waves confirmed the presence of the asthenosphere.
The temperature of the asthenosphere is believed to be between 300 and 500 degrees Celsius. By comparison the inner core of the earth reaches 7000 degrees Celsius and is actually solid.
Because the asthenosphere is ductile it can be moved about like silly putty thanks to the heat of the earth.
The rock in the asthenosphere is low density and partially molten.
Underneath the oceans the asthenosphere is closer to the earth's surface.
When crustal plates sink into the earth's mantle deep zone earthquakes can occur in the asthenosphere.
The asthenosphere is sometimes referred to as the plastic zone because the rocks move about in more fluid way that mimics plastic. It has also been compared to the way toothpaste flows out of a tube when squeezed.
The LAB (lithosphere asthenosphere boundary) is determined by a number of factors including the differences in thermal properties, grain size, and chemical composition, as well as the degree of melt. The LAB changes underneath the oceans.


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