No. 49: Jan-Feb 1987
For a long time the Moho (Mohorovicic discontinuity) has been considered a stable plane dividing the crust from the mantle. It is at the Moho that seismic wave velocities change abruptly. There is something there, but no one knows just what. At the recent Second International Symposium on Deep Seismic Reflection Profiling of the Continental Lithosphere, a lot of doubts about the stability and character of the Moho surfaced. Under the North American Cordillera, which runs from Alaska to Mexico, the Moho is flat, continuous and oblivious to the faults, terrane plastering, mountain "roots," and the geological phenomena above it. In other areas, though, several Mohos are stacked up. Some Mohos are discontinuous, jumping from one depth to another. Others are strongly influenced by overhead geological structures. Gone is the neat, so simple Moho figured in all the textbooks.
(Barton, Penny; "Deep Reflections on the Moho," Nature, 323:392, 1986. Also: Weisburd, S.; "The Moho Is Immutable No More," Science News, 130:326, 1986.)