Moho

The level in the Earth where the velocity of sonic waves first increases rapidly or discontinuously to a value between 4.7 and 5.3 mi/s (7.6 and 8.6 km/s). A. Mohorovičić discovered this boundary while investigating seismograms of the Zagreb (in former Yugoslavia) 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. 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_mP) from the boundary. The Moho is generally assumed to mark the boundary between the crust and mantle, although this need not always be the case. See also Earth interior.

While the definition of Moho in the continents is based solely on seismic refraction observations, the term has been utilized to describe a range of observations pertaining to the oceanic environment. In the strictest sense, Moho refers to the depth where material velocities, as determined from seismic refraction methods, exceed 5 mi/s (8 km/s). From geological studies of ocean crust and ophiolites (ancient oceanic sections subsequently emplaced on continents), oceanic crust is understood to originate from partial melting of mantle that upwells and decompresses in response to sea-floor spreading. The eruption and intrusion of this melt, and the formation of Moho, occurs in a very narrow zone at the axis of sea-floor spreading. Different mantle conditions can lead to different volumes of melting, accounting for most of the observed variations in crustal thickness (depth to the Moho). Very slow spreading centers, while rare, are expected to form somewhat thinner crust, largely reflecting the magma-starred nature of these areas. The Moho is a proxy for the transition from crustal materials (for example, basalts, diabase dikes, or gabbros) formed by mafic melts extracted from the mantle to the ultramafic residual mantle (for example, peridotite) that has remained at depth. See also Basalt; Dolerite; Gabbro; Ophiolite; Plate tectonics.