Question

The distinction between Earth's crust & mantle is based on the chemical composition of the rocks. When discussing tectonics, we instead prefer to divide the outer part of the planet into the lithosphere & asthenosphere. How do we define these layers?

a. By depth in the Earth: the lithosphere is everything above 60 km depth, and the asthenosphere is everything below this depth and above the core.
b. By age: the lithosphere is composed of ancient continental rocks while the asthenosphere is made up of younger rocks formed from seafloor spreading
c. By physical properties: the lithosphere is rigid but brittle, while the asthenosphere is weak and ductile.
d. By state of matter: the lithosphere is the solid outer shell of the planet, while the asthenosphere is the liquid interior.

Answer 1

The lithosphere is the rigid outer layer of Earth, including the crust and upper mantle, while the asthenosphere is a weak, ductile layer beneath the lithosphere, allowing tectonic plates to move. The correct answer is option c.

Step-by-step explanation:

The lithosphere and asthenosphere are two layers of Earth defined by their mechanical properties rather than their chemical compositions. The lithosphere includes both the crust and the uppermost mantle, and is characterized by its brittle nature and rigidity. It's roughly 100 kilometers thick and behaves as a rigid solid, which can break under stress, resulting in earthquakes. Contrarily, the asthenosphere is positioned below the lithosphere and is known for its weak and ductile qualities. It's made of partially molten upper mantle material that can flow, accommodating the movement of tectonic plates.

The correct answer to how we define these layers is by physical properties: the lithosphere is rigid but brittle, while the asthenosphere is weak and ductile.