In which layer is the asthenosphere found? (a) in the crust (b) in the mantle (c) in the core (d) None of these
The correct answer and explanation is:
Correct Answer: (b) in the mantle
The asthenosphere is found in the upper portion of the Earth’s mantle. It lies just below the lithosphere, which includes the crust and the rigid uppermost part of the mantle. The asthenosphere extends from approximately 100 kilometers to about 350 kilometers beneath the Earth’s surface.
This layer is partially molten and behaves in a plastic, or ductile, manner. While it is solid, the high temperature and pressure conditions allow the rocks within it to flow slowly over time. This slow flow is essential because it allows tectonic plates, which make up the lithosphere, to move over the more pliable asthenosphere.
The asthenosphere plays a key role in plate tectonics. The movement of tectonic plates is driven by convection currents within this layer. Hot material rises from deeper in the mantle, while cooler material sinks. This motion helps to drag the rigid lithospheric plates across the Earth’s surface. Earthquakes, mountain formation, and volcanic activity are all influenced by this movement.
The asthenosphere also contributes to isostatic adjustments. When large amounts of weight are added or removed from the lithosphere, such as through glaciation or erosion, the asthenosphere flows to compensate for these changes. For example, when glaciers melt, the reduced pressure allows the asthenosphere to push the crust back up, a process known as isostatic rebound.
The asthenosphere is not found in the crust or the core. The crust is the outermost solid layer, and the core, divided into outer and inner parts, is much deeper and composed mainly of iron and nickel. Therefore, the mantle is the only layer where the asthenosphere exists, making option (b) the correct answer.