Asthenosphere is :

Question

Asthenosphere is :

Accepted Answer

Asthenosphere is:

1. **Explanation for Option 2 (Correct Answer): Upper portion of Mantle**

The asthenosphere is a highly viscous, mechanically weak, and ductile region of the Earth's upper mantle. It lies just below the lithosphere, which includes the crust and the uppermost part of the mantle. The asthenosphere extends from about 100 km to 200 km below the Earth's surface, although its depth can vary. This layer is crucial for plate tectonics because it allows the rigid lithospheric plates to move over it. The asthenosphere's semi-fluid nature is due to the high temperatures and pressures at this depth, which cause rocks to become partially molten and behave plastically. Therefore, the asthenosphere is correctly identified as the upper portion of the mantle.

2. **Explanation for why other options are incorrect:**

- **Option 1: Lower portion of Mantle**
     The lower portion of the mantle is known as the mesosphere, which is more rigid and extends from the bottom of the asthenosphere to the core-mantle boundary. It does not exhibit the same ductile properties as the asthenosphere and is not involved in the movement of tectonic plates. Therefore, the asthenosphere cannot be the lower portion of the mantle.

- **Option 3: Lower portion of Crust**
     The crust is the outermost layer of the Earth, and it is solid and brittle compared to the underlying mantle. The asthenosphere is not part of the crust at all; it is located beneath the crust within the mantle. Thus, it cannot be the lower portion of the crust.

- **Option 4: Upper portion of Crust**
     Similar to Option 3, the upper portion of the crust is part of the Earth's lithosphere, which is rigid and solid. The asthenosphere lies beneath the lithosphere and is not part of the crust. Therefore, it cannot be the upper portion of the crust.

In conclusion, the asthenosphere is best described as the upper portion of the mantle, which is why Option 2 is the correct answer.