London – “Al-Quds Al-Arabi”: American scientists recently discovered something new about the layer of partially molten rock that lies under the cold outer crust of the Earth, which could help in predicting earthquake zones and risks that may lead to earthquakes in the future.
The detected elastic band of hot material is known as the “astenosphere” or loose envelope, and is generally considered solid, with some liquid material weakening the overall structure. However, the top layer of it appears to be much softer than scientists previously thought.
According to a report published by the “Science Alert” website, which specializes in science news, the study, led by researchers at the University of Texas in the United States, identified distinctive characteristics in the flow and density of a thin part of the “asthenosphere”, which led to resolving the boundaries of regions within the layer that could extend around the world.
Scientists said that having a clear map of the differences in the echoes of seismic waves that pass through the bowels of the Earth can help us identify the activities that drive the movements of the tectonic plates floating on the surface of the planet.
The discovery adds vital detail to the global structure of the upper mantle, allowing geologists to rule out any influence this soft region of the upper mantle might have had on the overall rippling of the asthenosphere.
“We can’t rule out that local melting is not important,” says physicist and geophysicist Thorsten Becker, of the University of Texas. But I think it prompts us to look at these melt observations as a sign of what’s going on in the ground, not necessarily as an active contributor to anything.”
While some previous studies suggested that the asthenosphere was interrupted by occasional bursts of magmatic activity, it was not clear until recently how widespread this phenomenon was.
For the new study, Baker and his colleagues created a global map of the globe using seismic images of the mantle, which were collected from stations around the world.
When seismic waves sent from these stations above the Earth hit the upper part of the asthenosphere, they slowed down significantly, and this indicates that the upper layer is more molten than the other parts.
More liquid materials usually allow for greater flow, but that does not necessarily appear to be the case here. The scientists’ map of the asthenosphere does not correspond to the movement of tectonic plates above. For example, regions where seismic waves move slower do not show more tectonic activity. “When we think of something melting, we intuitively think that melting must play a big role in the viscosity of the material,” says Junlin Hua, who led the research. But what we found is that even when the fraction of melt is very high, its effect on mantle flow is very small.”
Oddly enough, there seem to be several pools of magma scattered throughout the asthenosphere and not just at the top, where hot magma tends to collect at depths of about 100 to 150 kilometers (about 60 to 90 miles).
In the lower part of the asthenosphere, for example, magma usually emerges, perhaps as a result of desiccation melting, which can occur when rocks are not saturated with water.
On the other hand, the middle layer about 260 km deep is not widespread but appears sporadically and could be the result of carbon-assisted melting of the mantle.
Scientists have long suspected that Earth’s tectonic plates move based on streams of molten rock that lie deep below the surface, but the exact dynamics of the rise and sink of gas, liquid or rock are unclear.
Based on the current findings, the University of Texas researchers believe that gradual changes in temperature and pressure in the asthenosphere are driving the deep flow of molten rock. The overall viscosity of this region is not a big factor when it comes to moving the tectonic plates above.