Asthenosphere

Astenosfera and convection currents

The asthenosphere or asthenosphere (from the Greek ἀσθενός, 'without force' + σφαῖρα, 'sphere') is the upper zone of the Earth's mantle that is below the lithosphere, approximately between 30 and 50 kilometers deep to 660 km. continental and isostasy. The tectonic plates move on it.

Constitution

The lithosphere, which is an extension of the notion of the earth's crust, encompassing a relatively rigid part of the upper mantle, has an average thickness of 101 km under the oceans and around 150 to 250 km under the continents and older cratons.

In the asthenosphere there are slow convection movements that explain continental drift. In addition, basalt from the asthenosphere flows by extrusion along mid-ocean ridges, constantly renewing and expanding the ocean floor. While where the expansion finds an obstacle represented by a continent, it sinks under it, thus returning the bottom matter to melt within the asthenosphere and deeper mantle, a phenomenon known as subduction.

Due to its lower part, the asthenosphere loses its properties below 350 km and progressively acquires the rigidity of the lower mantle towards a depth of 850 km.

History

In 1899, Dutton defined a rigid zone (crust or lithosphere up to 100 kilometers deep) and another weak zone below it which he called the asthenosphere.

To explain the phenomenon of isostasy, in 1914 Joseph Barrell established the hypothesis of the existence of the asthenosphere, with no lower limit, since the existence of the nucleus was unknown.

In 1926, Beno Gutenberg discovered that the speed of seismic waves is reduced by around 6% at a depth of approximately 100 to 200 km, the cause being attributed to a lower density of the material.

In 1962 Don Anderson did not find a convincing enough theory about Gutenberg's discovery, since that level is very heterogeneous. In addition, notable differences in its composition were detected at levels below the oceanic basins with respect to continental cratons, so it cannot be considered a universal layer. Through the study of the seismic waves originated in the tests of the nuclear bombs, he determined that the zone of low speed would be between 60 and 250 km of depth.

Attempts were made to attribute this property to the asthenosphere itself. But since the discovery of paleomagnetism and deep seismicity, both theories have been differentiated.

However, in 1968 John Tuzo Wilson mixed these concepts again, giving them a theoretical body of little scientific validity by basing said theory on the existence of a layer that slows down the speed of seismic waves called the asthenosphere, answered by Vladimirovich, who He replied that it should not be based solely and exclusively on evidence found in the geology of the ocean basins.

Between 1972 and 1981 Anton Halles tried to justify the existence of the asthenosphere more out of necessity than by reliable scientific evidence.

Between 1975 and 1979 Sengör and Burke debate again and the existence of a notorious difference between the deep structure of the continents and that of the oceanic crust is demonstrated, according to Sipkin and Jordan. Under the oldest cratons, they reach a depth of up to 400 km and the zone of attenuation of the velocity of seismic waves is greater. It follows that the continents can be anchored through keels (in English, keels) directly to the nucleus, as if they were roots of more compact material, but this does not prevent their movement on a large scale. It was then that the asthenosphere was defined to a depth of 660 km below the surface.

Doubts about the existence of the asthenosphere

When Alfred Wegener defined their continental drift, the engine of the displacement of the continental plates would be a convective flow in the mantle (later in the asthenosphere). But today there is not enough evidence for the existence of this convective flow.

Since the 1990s, the theory about the existence of this layer was called into question, basic to explain continental drift and isostasy according to new data discovered, proposing a system in which the crust moved in solidarity with the underlying mantle until the earth's core. It was also proposed that the isostatic equilibrium would occur between the lower part of the mantle (solid) and the outer part of the Earth's core (liquid). However, this last proposal does not have sufficient data in this regard to define the situation.