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: Two ways to mark the Moho.
The seismic Moho is where wave speed jumps as crust gives way to mantle rock. The petrologic Moho is where the rock type itself changes to ultramafic mantle.

Step 2: Why they differ.
The rock change and the velocity jump do not sit at the same level. The composition boundary comes first, while the full velocity jump appears a bit deeper.

Step 3: Order the two boundaries.
Because of this, the petrologic Moho lies shallower and the seismic Moho lies deeper. So they do not coincide.

Step 4: Test the options.
The option saying they coincide is wrong. The option saying the seismic one is shallower has the order backwards. In ophiolites the petrologic Moho sits below the layered gabbro, so that option is wrong too.

Step 5: Final choice.
The correct statement is that the petrologic Moho is shallower than the seismic Moho.
\[ \boxed{\text{Petrologic Moho lies at shallower depths relative to the seismic Moho}} \]