|Did You Know?
Both the names, lithosphere and asthenosphere, have origins in the Greek language. Lithosphere means a rocky sphere, while asthenopshere means a weak sphere. These names are indicative of the distinct physical constitution of each of these two layers.
'To boldly go where no man has gone before...', these were the introductory words spoken at the beginning of many Star Trek episodes, which have throughout the decades captivated the imaginations of both, the young and old alike. Cruising through the universe, looking for brand new worlds; that's what adventures are made of!
But one doesn't need to travel to the far reaches of the universe looking for the unexplored. Right here on Earth itself you will find that there are entire worlds where, until now, no man has ever been before. To see them, all you have to do is steal your gaze from the stars above and peer down deep under the Earth's crust. There, you will find layers and layers of uncharted regions, ripe for exploration.
Among these layers are the lithosphere and asthenosphere. Together, they comprise the upper layers of planet Earth. Today, we shall go on a theoretical exploration of both these sub-earthen layers, and find out how they differ from one another.
The Earth can be imagined to be like an onion. An onion has several different layers all the way down to its central core. Similarly, the Earth too has many layers. However, as far as distinct composition is concerned, there are three important layers.
Starting from the top, these are the Earth's crust, the Earth's mantle, and finally the Earth's core. The Earth's crust is mostly solid, while the mantle is part solid and part liquid. The Earth's core, which is the innermost part of the Earth, due to the immense pressure upon it, is solid.
The Earth's crust, along with the top solid portion of the mantle, is what is known as the lithosphere. Thus, our exploration of the lithosphere begins right from the place that we are standing at. This uppermost part of the lithosphere is the one that interacts with the biosphere, hydrosphere, and the atmosphere. All the different familiar land forms, including mountains, valleys, plains, plateaus, etc., are found here. Planet Earth, as we know it, including all the continents, oceans, and living creatures, are present on the upper portion of the lithosphere, that is, the crust.
But what we are seeing is literally just the tip of the iceberg. As a matter of fact, the lithosphere extends to an average depth of nearly 100 km from the Earth's surface in most places. Since the Earth's surface is divided into land mass and water mass, the lithosphere too is divided into continental lithosphere and oceanic lithosphere. The thickness of oceanic lithosphere ranges from 50 to 140 km, while that of continental lithosphere ranges between 40 to 280 km.
At its lowest part, the lithosphere traverses the boundary between the crust and mantle, and encompasses all the brittle, yet solid upper part of the mantle.
It is interesting to note that the lithosphere is always moving, albeit too slowly for us to notice. The crust part of the lithosphere is rigid, and due to the constant motion, it has developed large cracks, and has been broken down into huge individual sections, known as tectonic plates. These tectonic plates move relative to each other.
The mantle part of the lithosphere is closer to the Earth's furnace, that is, its core. Therefore, it is very, very hot. Just like how heat makes a metal more bendable, this part too has become bendable due to this immense heat. This bendability of the mantle part greatly assists the movement of the tectonic plates above it.
It is thought that the movement of tectonic plates is what caused the different continents to be formed. Scientists believe that originally, the Earth had just one huge mass of land, known as the Pangaea. This mass stood on top of many different tectonic plates. As the plates moved away from each other, they tore apart the Pangaea, forming all the different continents that we see today. An evidence of this can be found in the shape of the western coast of Africa and the eastern coast of South America, which look like the torn pieces of one single landmass. Also, even though they are separated by an ocean, similar plants and animals are found in both these places.
Tectonic activity continues to take place even today. Often, when large tectonic plates either collide or move beneath or above each other, it results in large-scale earthquakes and even volcanoes.
The asthenosphere is the layer of the Earth that is found just below the lithosphere. Thus, our exploration of the asthenosphere begins from the point where the lithosphere ends. Now, during our exploration of the lithosphere, we learned that the depth of the lithosphere is different at different places on the Earth. Hence, accordingly, the asthenosphere too is found at variable depths. Typically, beneath the thin oceanic lithosphere, the asthenosphere is located much closer to the surface, which in this case is the ocean floor.
The asthenosphere is a part of the Earth's mantle. It is closer to the Earth's core than the lithosphere, making it extremely hot. Also, since it bears the entire weight of the lithosphere, the pressure upon it is immense. This combination of both, heat and pressure, is enough to cause even the rocks to become so plastic that they begin to flow like liquid. As such, the asthenosphere can be imagined to be in a molten fluid state. The lithosphere is thought to float over this molten layer.
Considering its great depth, heat, and pressure, it is practically impossible to reach and study this layer and the ones below it. Scientists therefore use the speed and direction of seismic waves brought on by earthquakes to learn about the Earth's structure below the lithosphere. Now, the asthenosphere exhibits plasticity, and also has a lower density than the overlying lithospheric layer. Hence, seismic waves travel at a much slower rate through the asthenopshere as compared to the lithosphere. Owing to this, it is also called the low-velocity zone (LVZ). This change in the speed of seismic waves was what first indicated towards the existence of this layer, along with its typical physical characteristics.
The asthenospheric layer usually starts at the lower boundary of the lithosphere, 100 to 200 km below the Earth's surface, and is believed to extend all the way to a depth of 700 km. The upper part of this layer is the zone upon which the lithospheric plates move. As the depth of the layer increases, the rising heat and pressure makes the rocks increasingly ductile, which get deformed at a rate measured in centimeters per year. Thus, the asthenosphere flows at a very slow rate carrying the lithospheric layer on top of it.
Lithosphere Vs Asthenosphere: Comparison
Now that we have explored the differences between the lithosphere and asthenosphere, it's time to recap all that we have learned. The following is a listing of the major differences between the lithosphere and asthenosphere.
► The primary difference between the lithosphere and asthenosphere is the depth below the Earth's surface to which each of these layers extend. The lithosphere starts from the surface, and on average, extends to a depth of 100 km below the surface. The asthenosphere, on the other hand, starts from the lower boundary of the lithosphere (100 - 200 km), and is thought to extend deep down to 700 km below the surface.
► The lithosphere extends from the Earth's crust down to the solid part of the upper mantle. The asthenosphere is entirely located in the mantle itself.
► The lithosphere interacts with the biosphere, hydrosphere, and atmosphere, while the asthenosphere, being located much deeper within the Earth, does not.
► The asthenosphere is characterized by much higher temperature and pressure than the lithosphere.
► Though both these layers comprise chemically similar elements, the extreme conditions in the asthenosphere causes rocks to flow like liquids, whereas, the same rocks behave like solids in the lithosphere.
► Due to the liquid-like behavior of its comprising rocks, seismic waves travel at a much slower rate through the asthenosphere, as opposed to the rigid and solid lithosphere where their speeds are much higher.
Here ends our exploration of the lithosphere and asthenosphere. Our journey has shown us how these two layers differ from each other based on their own distinct physical characteristics. There is still much we don't know about the sub-earthen layers. As scientific studies continue and newer discoveries are made, we shall once more be in a position to resume our theoretical journey into the unexplored underworld.