Discussion
Choose the correct statement about Moho.
Answer: B
The correct statement is that the seismic Moho lies at shallower depths relative to the petrologic Moho.
The seismic Moho is where P wave velocity jumps to mantle values of about 8 km/s. The petrologic Moho is the base of the rocks that crystallised from crustal magma. In ophiolites the lowest crustal rocks are ultramafic cumulates, dunite and pyroxenite settled out of the magma chamber; they are crust by origin but have mantle-like seismic velocities. So the velocity jump comes at the top of these cumulates, while the petrologic Moho lies below them, at the contact with tectonized mantle harzburgite.
That rules out the statements that the two coincide or that the petrologic Moho is the shallower one. The layered gabbro sits above the ultramafic cumulates, so the petrologic Moho lies below the layered gabbro, not above it.
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