#lithosphere

Geologists often refer to plates as lithospheric plates. This is because the plates are not just pieces of the crust broken up but pieces of the rigid upper mantle as well.

What is the lithosphere? The lithosphere is the crust and the uppermost part of the mantle.

How do we know where the lithosphere ends and the asthenosphere begins? That boundary is defined by a difference in a response to stress. The lithosphere is rigid and deforms elastically and brittley (is that even a word?). The asthenosphere deforms plastically. (I will define those words in a later post)

Random line art of random characters doing random things.

Couldn't be bothered to search for pictures to color it.

The Lithosphere-Asthenosphere Boundary

Seismic waves can be used to detect layers inside the earth. After earthquakes, or when humans release energy using explosives, seismic waves head out and travel through the Earth. When they hit boundaries inside the earth, the waves bounce off of them. When those waves come back to the surface, they can be detected by seismic instruments. The amount of time in-between the release of energy and the arrival of the wave back at the surface is a measurement of how deep the layer is in the earth.  These layers occur when the composition of material changes (such as at the bottom of the crust) or when the structure of the mantle minerals change due to increasing pressure. However, one boundary in the Earth has neither of these - the boundary between the lithosphere and the asthenosphere. The Asthenosphere is the part of the planet hot enough to flow, while the lithosphere is the colder part of the planet - too cold to convect with the rest of the mantle. But, because the boundary is only caused by a temperature increase over a broad zone - not a  sudden change in composition or in mineral structure, this boundary is difficult to detect by seismic methods.

While using explosives to study the boundary between the Indo-Australian and Pacific plates, a team of scientists realized they were getting seismic reflections from an surprising source ~100 km away - the base of the Pacific plate. These waves were recorded by over 1,000 seismographs and generated a high-resolution picture.    According to the Nature paper - lead-authored by Prof. Tim Stern of Victoria University - a series of reflectors at ~100 km indicated there was a channel ~10km thick which has been interpreted (based on wave velocity) as a water- or melt-rich shear zone which facilitates a “gliding" movement of the lithosphere  over the asthenosphere. The diagram shows a simplified version of the zone and it’s location.

In this case, the top of the asthenosphere has a change in the phase present that makes it differ in its seismic velocity. Either this area is extra-water rich, or the temperature is such that the top of the lithosphere has begun to melt, or a combination of both.

YK