Science Olympiad Dynamic Planet Practice Test

What represents the boundary between the crust and the mantle?

1. Asthenosphere

2. Lithosphere

3. Mohorovičić discontinuity (Moho)

4. Core-mantle boundary

The correct answer is: Mohorovičić discontinuity (Moho)

The correct answer is the Mohorovičić discontinuity, commonly known as the Moho. This boundary marks the transition between the Earth's crust and the underlying mantle. The significance of the Moho lies in the fact that it indicates a change in the composition and physical properties of the Earth’s materials. Above the Moho, in the crust, rocks are typically lighter and more silicate-rich, whereas below this boundary, the mantle consists primarily of denser, magnesium-iron silicates. The Moho was identified due to the change in seismic wave velocities that occur at this boundary, as waves travel faster through the denser materials of the mantle compared to the crust. In contrast, the asthenosphere is the semi-fluid layer of the upper mantle that lies just beneath the lithosphere and is involved in tectonic movements but is not the boundary itself. The lithosphere refers to the brittle outer layer of the Earth that encompasses both the crust and the uppermost part of the mantle, but again, it does not represent the boundary. Lastly, the core-mantle boundary is located much deeper in the Earth, separating the mantle from the outer core, and is not relevant when identifying the boundary between the crust and mantle.