Moho (Mohorovicic discontinuity)

Article By:

Fountain, David M. Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming.

Tolstoy, Maya Lamont Doherty Earth Observatory, Columbia University, Palisades, New York

Mutter, Carolyn Z. Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York.

Last reviewed:November 2019

DOI:https://doi.org/10.1036/1097-8542.429400

The level in the Earth where the velocity of sonic waves first increases rapidly or discontinuously to a value between 7.6 and 8.6 km/s (4.7 and 5.3 mi/s). A. Mohorovičić discovered this boundary while investigating seismograms of the Zagreb (now capital of Croatia) earthquake of October 8, 1909. He recognized that low-velocity waves traveling directly from the earthquake source were overtaken at large distances by refracted waves traveling through the deeper, high-velocity layer (Fig. 1). Modern determinations of the depth and nature of the Moho are commonly made in seismic refraction studies that use artificial seismic sources, such as explosions, rather than earthquakes. This method allows identification of the wave traveling in the high-velocity medium (P_n) and a wide-angle reflection (P_m P) from the boundary (Fig. 1). The Moho is generally assumed to mark the boundary between the crust and mantle, although this need not always be the case. Despite drilling attempts in the early 1960s, the Moho has not been directly sampled. Knowledge of the oceanic and continental Moho is based on interpretation of geophysical data, as well as geological interpretation of ophiolites (believed to be sections of oceanic crust and mantle uplifted onto land). See also: Ophiolite

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