The Moho stands for which of the following?

• A. Mohorovicic discontinuity
• B. Gutenberg discontinuity
• C. Lehmann discontinuity
• D. Reid boundary

Answer: (A)

The Moho is the boundary between Earth's crust and mantle, marked by a sharp increase in seismic wave speeds that happens because rock types change—from crustal rocks to mantle peridotite. This velocity jump is what scientists detect with earthquakes and seismic waves, revealing a new layer beneath the surface. The name Mohorovičić discontinuity honors the scientist who first identified it from seismic data, and the term “discontinuity” signals a sudden change in material properties with depth. In practice, the depth to this boundary varies: it sits around roughly 7 km beneath the ocean floor and about 30–50 km beneath continents.

That’s why this option is the best choice—the boundary described is specifically the crust–mantle boundary. The other terms refer to different boundaries in Earth's interior—for example, the Gutenberg discontinuity is associated with the core–mantle boundary, and the Lehmann discontinuity with the outer–inner core boundary—so they do not describe the Moho.