They reached a depth of 12,260 metres (40,220 ft), the world's deepest hole, before abandoning the project. Soviet scientists at the Kola Superdeep Borehole pursued the goal from 1970 until 1992. Reaching the discontinuity by drilling remains an important scientific objective. After initial success in establishing deep-ocean drilling, the project suffered from political and scientific opposition, mismanagement, and cost overruns, and it was cancelled in 1966. In the early 1960s, Project Mohole was an attempt to drill to the Moho from deep-ocean regions. These early studies gave rise to modern seismology. By observing the Moho's refractive nature and how it affects the speed of P-waves, scientists were able to theorize about the earth's composition. The Moho has played a large role in the fields of geology and earth science for well over a century. Using velocity data from the earthquake, he was able to calculate the depth of the Moho to be approximately 54 km, which was supported by subsequent seismological studies. As a result of this information, he theorized that the second set of waves could only be caused by a sharp transition in density in the Earth's crust, which could account for such a dramatic change in wave velocity.

Mohorovičić knew that waves caused by earthquakes travel at velocities proportional to the density of the material carrying them. In 1909, he was examining data from a local earthquake in Zagreb when he observed two distinct sets of P-waves and S-waves propagating out from the focus of the earthquake. History Ĭroatian seismologist Andrija Mohorovičić is credited with first discovering and defining the Moho. Serpentinization of mantle rock below slowly spreading mid-ocean ridges can also increase the depth to the Moho, since serpentinization lowers seismic wave velocities. The Moho may lie well below the crust-mantle boundary and care must be used in interpreting the structure of the crust from seismic data alone. Xenoliths (lower crust and upper mantle rock brought to the surface by volcanic eruptions) and seismic-reflection data showed that, away from continental cratons, the transition between crust and mantle is marked by basaltic intrusions and may be up to 20 km thick. Īs shown in the figure, the Moho maintains a relatively stable average depth of 10 km under the ocean sea floor, but can vary by more than 70 km below continental land masses.īeginning in the 1980s, geologists became aware that the Moho does not always coincide with the crust-mantle boundary defined by composition. The Mohorovičić discontinuity was first identified in 1909 by Mohorovičić, when he observed that seismograms from shallow-focus earthquakes had two sets of P-waves and S-waves, one set that followed a direct path near the Earth's surface and the other refracted by a high-velocity medium.

Named after the pioneering Croatian seismologist Andrija Mohorovičić, the Moho separates both the oceanic crust and continental crust from the underlying mantle. The Mohorovičić discontinuity is 5 to 10 kilometres (3–6 mi) below the ocean floor, and 20 to 90 kilometres (10–60 mi) beneath typical continental crusts, with an average of 35 kilometres (22 mi). Only beneath mid-ocean ridges does it define the lithosphere– asthenosphere boundary (the depth at which the mantle becomes significantly ductile). The Moho lies almost entirely within the lithosphere (the hard outer layer of the Earth, including the crust). It is defined by the distinct change in velocity of seismic waves as they pass through changing densities of rock. The Mohorovičić discontinuity ( / m oʊ h ə ˈ r oʊ v ɪ tʃ ɪ tʃ/ MOH-hə- ROH-vitch-itch, Croatian: ), usually referred to as the Moho discontinuity or the Moho, is the boundary between the Earth's crust and the mantle. Earth's crust and mantle, Moho discontinuity between bottom of crust and solid uppermost mantle

Seismac discontinuity