Asthenosphere and Lithosphere Definition and Properties
Our earth has different layers below the topsoil. All these layers are concentric in nature. The two layers that constitute the upper mantle region and the crust above it are the asthenosphere and lithosphere. In this section, we will study the lithosphere and asthenosphere in detail and learn their characteristics. The properties of these layers should be learned properly to answer specific questions asked in the exams. Learn what is the asthenosphere and lithosphere here.
Our Earth: A Brief Description of the Layers
Our earth has four major layers that constitute the entire planet. We are just living on the crust. It goes way down and covers a radius of 6357 km. The core, as we all know, comprises the liquefied lava. The temperature of the core is 6000°C. The temperature decreases as we move to the top. The core is subdivided into the inner core and the outer core. The temperature of the outer core ranges from 4500 to 5500°C.
Above these concentric cores, we find the mantle. The mantle is 2890 km in thickness. It is also called the transition between the core and the crust. It is also the thickest part of the earth’s entire radius. It rests on the core region and has the crust above it. The crust part of the earth has a thickness ranging from 5 to 70 km. It is the thinnest part of the earth’s foundation and constitutes the outer surface of the planet. This is where our ocean floors, tectonic plates, and the entire ecosystem of the planet reside.
About Lithosphere
The planet’s inner part is also subdivided into two different parts. One such part is the lithosphere. As per the lithosphere definition, it is the outer layer that resides on the top of the mantle and underneath the crust. It is the rigid and rocky part of the earth’s inner sections that goes up to 100 km beneath the earth’s crust.
This layer of earth lies on the asthenosphere. It is also responsible to support the tectonic plates and the entire earth crust lying above the fluidic asthenosphere underneath. It is cooler and more solid than the layers below. It is not entirely solid but broken in different regions or blocks. These rigid blocks support the tectonic plates and are found in dozens. If we look a little deeper into what is meant by lithosphere, we will discover that these tectonic plates are further divided into oceanic plates and continental plates. These plates are rather the topmost part of the earth’s crust where life has originated. The oceanic crust is denser but thinner than the continental crust. The continental crust is thicker but lighter in nature. It is dominated by aluminium and silica whereas the oceanic crust is made of magnesium and silica.
As per the lithosphere definition geography, we find the tectonic plates residing on it. It means that the plate tectonic movement is also responsible for the movements underneath. The physical property of the lithosphere residing on the fluidic asthenosphere causes the movement of these plates resulting in the formation of oceans, seas, and continents.
The floating rigid plates and the gravitational force play a major role in causing the movement and collision of these tectonic plates. In fact, these plates are diverging from each other creating different landmasses. The fresh crust comes up and cools down to turn into a new land mass forming ridges in the middle of an ocean. By studying this part, you must have understood what do you mean by lithosphere.
About Asthenosphere
Now that we have understood what lithosphere means, let us define asthenosphere. It is the upper part of the mantle lying beneath the lithosphere. The depth of this layer ranges from 100 to 700 km. It tolerates huge pressure from above and underneath which keeps in the form of tar. It is denser than the lithosphere but constantly moving underneath due to the rotational motion and gravitational force of the planet.
Scientists have learned that this layer witnesses the generation of convectional currents causing the movement of the rigid lithosphere above. As per the asthenosphere meaning, it is denoted as a weak layer that constantly feels the stress of the layers above and below. The upper movement of magma from this layer causes volcanic eruption from the weak fissures of the lithosphere. In fact, the movement also causes tsunamis and earthquakes in the earth’s crust.
If we consider what is the asthenosphere, we will find out that this layer is denser but fluidic in nature. The movement of the continental and oceanic crust happens because of the disturbance in this layer. In fact, the convection current in this layer also causes the formation of new land crust emerging from the oceanic crust.
Upon studying the asthenosphere definition, we will find out that all the landmasses formed for millions of years are because of the magma movements here. The formation of Pangea and then progressing to Angara land and Gondwanaland took millions of years. Further, these landmasses produced the continents supporting the diverse ecosystem.
FAQs on Lithosphere and Asthenosphere: Definition and Brief Description
1. What is the lithosphere in simple terms?
The lithosphere is the rigid, rocky outer layer of the Earth. It includes the entire crust (both continental and oceanic) and the solid, uppermost part of the mantle. Think of it as the Earth's solid, brittle shell that is broken into large pieces called tectonic plates.
2. What is the asthenosphere and what is it made of?
The asthenosphere is a highly viscous, mechanically weak, and ductile layer of the upper mantle, located just below the lithosphere. It is not liquid like water, but rather a solid that can flow very slowly, similar to thick honey or tar. This unique state is due to extreme heat and pressure, which brings the rock close to its melting point, allowing the rigid lithospheric plates to move on top of it.
3. What is the main difference between the lithosphere and the asthenosphere?
The primary difference is their mechanical behaviour in response to stress. The lithosphere is rigid and brittle, meaning it breaks when under pressure. In contrast, the asthenosphere is plastic and ductile, meaning it flows and deforms without breaking. This allows the solid lithospheric plates to float and move on the flowing asthenosphere below.
4. Where are the lithosphere and asthenosphere located within the Earth's layers?
The layers are arranged from the surface downwards:
- Crust: The Earth's thinnest, outermost surface.
- Lithosphere: This layer includes the crust AND the very top, solid part of the mantle.
- Asthenosphere: This layer lies directly beneath the lithosphere and is considered the upper, semi-fluid part of the mantle.
5. How does the interaction between the lithosphere and asthenosphere cause earthquakes and volcanoes?
This interaction is the direct cause of most major geological events. The lithosphere is broken into massive plates that drift on the slowly moving asthenosphere.
- Earthquakes are caused by the sudden release of energy when these rigid lithospheric plates grind against each other, get stuck, and then slip.
- Volcanoes often form at plate boundaries where magma from the hot asthenosphere rises through cracks and weak points in the lithosphere to reach the surface.
6. Why is the asthenosphere referred to as a 'weak' layer if it's solid rock?
The term 'weak' in this context does not mean it lacks strength but refers to its low viscosity and ductility. Because it is so hot and under immense pressure, the rock behaves plastically, deforming and flowing over geological time instead of breaking. This physical property of being able to flow is what makes it 'weaker' compared to the rigid, brittle lithosphere that sits on top of it.
7. How are the lithosphere and asthenosphere essential to the theory of plate tectonics?
The relationship between these two layers is the core concept of plate tectonics. The theory explains that the Earth's surface is not a single piece but is made of several large lithospheric plates. The existence of a flowing, ductile asthenosphere underneath provides a mobile medium that allows these rigid plates to move, collide, and separate, driving processes like continental drift, the formation of mountains, and the creation of ocean basins.
