#asthenosphere

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)

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

A 3D computer model of the asthenosphere by Rice University geophysicists finds that the convective cycling and pressure-driven flow can sometimes cause the asthenosphere to move even faster than the tectonic plates riding atop it. This 2D slice of data from the model shows stronger, faster moving sections of the asthenosphere (yellow) bracketed above and below by slower, more fluid regions (orange). 

    New simulations of Earth's asthenosphere find that convective cycling and pressure-driven flow can sometimes cause the planet's most fluid layer of mantle to move even faster than the tectonic plates that ride atop it.

That's one conclusion from a new study by Rice University geophysicists who modeled flow in the 100-mile-thick layer of mantle that begins at the base of Earth's tectonic plates, or lithosphere.

The study, which is available online in the journal Earth and Planetary Science Letters, takes aim at a much-debated question in geophysics: What drives the movement of Earth's tectonic plates, the 57 interlocking slabs of the lithosphere that slip, grind and bump against one another in a seismic dance that causes earthquakes, builds continents and gradually reshapes the planet's surface every few million years?

Earthquakes and volcanic eruptions are a result of the movement of large swaths of Earth’s crust, but while the theory of plate tectonics has been widely accepted as a fundamental law of geology, there are still things to be discovered. New research from the University of Texas Austin points to the presence of a… Read more…

EARTH’S INNER CORE IS ONLY A BILLION YEARS OLD…WAY YOUNGER THAN MOST OF US THOUGHT

Abillion years isn’t the impossible time frame that we people—who are truly around for the squint of the universe’s famous eye—in vast terms. It isn’t even much for our planet.

Earth is believed to be about 4.6 billion years of age. Up to this point, its center was frequently assessed to be nearer to that age, yet reproducing the states of the inward center in a lab changed all that. The…

Flow in the asthenosphere drags tectonic plates along

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