The asthenosphere is a layer of the inner earth where the crust begins to melt into the mantle. This lesson will discuss the important properties of the asthenosphere.
What Is the Asthenosphere?
The asthenosphere is part of the upper mantle located below the crust of the earth.
The asthenosphere boundary is primarily defined by temperature, and it starts when the crust heats up to 1300° C. At this temperature the crust begins to melt and move more as a liquid. The asthenosphere is important in plate tectonics, as convection currents slowly move the tectonic plates that lie above.Seismic waves pass relatively slowly through the asthenosphere. The boundary is characterized by a drop in velocity of earthquake seismic waves. When an earthquake occurs, two types of seismic waves are released: primary and secondary. The speed at which the seismic waves travel depends on the density of the material through which it is passing.
When a primary wave travels from a solid to a liquid two things happen; it slows down, and it refracts.Secondary waves have an even more pronounced changed at the asthenosphere. These are lateral waves, meaning they move from side to side. As a secondary wave moves from a solid to liquid, its velocity drops down to zero. Secondary waves are not able to pass through the asthenosphere.
The density of the asthenosphere is dependent on depth.
The deeper you move through the asthenosphere, the more the density increases. Temperature and density differences in the mantle cause convection currents. Magma deep in the mantle rises due to its increased temperature, bringing hot magma towards the asthenosphere. It is believed that these convection currents are the source of the Mid-Atlantic Ridge.
Let’s review. The asthenosphere is the depth in the earth where heat from the core begins to melt the crust. At around 1300; C, solid crust begins to melt and move in more of a liquid manner. Seismic waves are important in locating the depth of the asthenosphere. Changes in the properties of seismic waves as they move from solid to liquid help geologists determine the properties of the asthenosphere.