Partially Molten Rock Layer Discovered In Earth's Mantle, What Are The Implications?
There is more molten rock than expected in the layer of the Earth's mantle called the asthenosphere: is this why it is so plastic that it allows the lithospheric plates to move?
The lithospheric plates , i.e. the huge rocky blocks into which the outer shell of the Earth is fragmented, move on the underlying layer, the asthenosphere . These are able to deform, so much so that they are "scrambled" by continuous circular movements fueled by the internal heat of the Earth - the so-called convective motions .
Thanks to the study of seismic waves at a depth of about 160 km in the asthenosphere, a "new" layer of partially molten rock has been identified . Why is this recent discovery, published in Nature Geoscience , so important?
The characteristics of the asthenosphere
We know that the Earth consists of four main layers , which starting from the earth's surface are: crust , mantle and core , divided into outer and inner . The upper mantle is rigid and solid and forms, together with the overlying crust, the lithosphere . Below the lithosphere, starting about 100 km deep, is the asthenosphere .
The asthenosphere has very different physical properties compared to those of the lithosphere: despite being solid it is not rigid , but its behavior is ductile, i.e. the rocks of which it is made can easily deform. Thanks to this feature, it acts as a "conveyor belt" that drags the lithospheric plates.
But how do we know so well the internal structure of our planet? The information has been obtained for the most part by studying the speed of seismic waves . Earthquakes, in fact, generate seismic waves that propagate deep into the Earth. They don't do it with a constant speed, but slow down or speed up according to the characteristics of the rock layers they pass through. In general, the speed is higher in more rigid and compact materials: for example, the waves slow down dramatically when they encounter the plastic asthenosphere.
Partially molten rocks
When researchers at the University of Texas , led by Junlin Hua, analyzed the speed of seismic waves in the asthenosphere below Turkey, they noticed that in some places the slowdown was even more pronounced. They deduced that here the rocks are not only plastic , but even partially melted . Texan researchers, analyzing a large amount of data from seismographs around the world, have discovered that in the asthenosphere the partially molten rock is not limited to a few restricted areas. Instead, it constitutes a real stratum , extended globally.
The importance of discovery
The presence of partially molten rocks in the asthenosphere was already known, but their extent and distribution were unknown . Furthermore, it was hypothesized that these rocks could influence the plastic behavior of the asthenosphere and thus the movements of the plates. But that's not the case: the researchers found no link between the distribution of the "new" layer and the movements of the plates.
It is not yet known exactly why the asthenosphere is plastic, but it is thought that its behavior is determined by gradual variations in temperature and pressure with depth. The temperature differences between the superficial and deep zones fuel the convective motions that mix this layer. To better understand what it is, just think of the circular movements that originate in a pot of water boiling on a stove (the stove corresponds to the internal heat of the Earth).
In this case, however, the movements are much slower because they don't involve a fluid. In the asthenosphere, the hot and therefore lighter material rises towards the surface, where it cools, becomes heavier and then descends in depth dragging the lithospheric plates.