Question

Choose the correct statement about Moho.

• A. Seismic and petrologic Moho coincide
• B. Seismic Moho lies at shallower depths relative to the petrologic Moho
• C. Petrologic Moho lies at shallower depths relative to the seismic Moho
• D. Petrologic Moho lies above the layered gabbro in ophiolites

Hint:

In the continental crust, the petrologic Moho is typically shallower than the seismic Moho because seismic waves detect the boundary where velocity changes at a deeper depth.

Answer 1

The Correct Option is C

Step 1: Understanding the Moho.
The Moho (Mohorovičić discontinuity) is the boundary between the Earth's crust and mantle. There are two ways to define it:
- Seismic Moho: Identified by a change in seismic wave velocities, typically caused by the transition from the crust to the mantle.
- Petrologic Moho: Defined by changes in rock composition, particularly the transition from mafic rocks (in the crust) to ultramafic rocks (in the mantle).

Step 2: Analyzing seismic vs. petrologic Moho.
- Seismic Moho is often found at a depth of around 30-40 km in continental crust, but can vary depending on the location.
- Petrologic Moho is generally found at a shallower depth than the seismic Moho in the continental crust. This difference arises because seismic waves detect the boundary at a depth where the rocks change composition, while petrologic boundaries are based on actual changes in mineral composition.

Step 3: Analyzing each option.
- (A) Seismic and petrologic Moho coincide: This is incorrect because the seismic Moho is typically at a deeper depth than the petrologic Moho in continental crust.
- (B) Seismic Moho lies at shallower depths relative to the petrologic Moho: This is also incorrect; the seismic Moho is deeper than the petrologic Moho.
- (C) Petrologic Moho lies at shallower depths relative to the seismic Moho: This is correct. The petrologic Moho is found at shallower depths because seismic waves detect the boundary where velocity changes at a deeper level.
- (D) Petrologic Moho lies above the layered gabbro in ophiolites: This is incorrect. In ophiolites, the petrologic Moho lies beneath the layered gabbro, not above it.

Step 4: Conclusion.
The correct statement is (C), as the petrologic Moho lies at shallower depths relative to the seismic Moho.
\[ \boxed{\text{Petrologic Moho lies at shallower depths relative to the seismic Moho}} \]