![]() ![]() The Japanese Government is testing the Chikyu Hakken (Earth Discovery) drilling ship that should make a stab at the elusive goal of reaching the Moho. In the 1950s, US National Science Foundation made an unsuccessful attempt to reach the Moho discontinuity through the ocean floor in a project named “Project Mohole.” The former USSR scientists at Kola Institute also made an attempt to drill the world’s deepest hole but abandoned the project at slightly below 40,000 feet in 1989. One such strategy fronts rock-melting radionuclide powered capsule with heavy tungsten spike that is self-propelled downwards to the Moho to explore and collect materials. Different strategies have been considered to help reach the discontinuity. ![]() In few places on Earth, crust and mantle boundary rocks and materials have been exposed to the surface through tectonic forces though some are millions of years old. ![]() There has not been any successful drilling exploration of the Mohorovicic through the oceanic or continental crusts to scientifically study its composition and features. Reaching the Moho is an important and strategic objective to scientists and explorers. The thickest areas of the earth lie beneath mountain ranges while the thinnest parts lie beneath deepest ocean floors. Mohorovicic’s research revealed that basaltic oceanic crust and the granite continental crust lay beneath materials with a density close to that of the peridotite rock. He called the lower density compositions as the “Earth’s Crust” and the higher density composition as the “Earth’s Mantle.” Observations proved that seismogram readings from shallow earth tremors had dual sets of P-waves and S-waves, one in a direct path close to the surface of the earth and the other was deflected by a high-velocity medium that was then unknown. He explained that the denser the earth's composition and structure is, the faster the velocity of the waves. DiscoveryĪfter years of observation, Andrija Mohorovicic discovered that the speed of seismic waves is directly dependent on the density of the material the waves are moving through. The term discontinuity is used by geologists to refer to the surface whereby seismic forces change speed. The discontinuity was named after Croatian seismologist and geophysicist Andrija Mohorovicic who pioneered its discovery in 1909 and explained the behavior and origin of earthquakes. This boundary is located approximately 24 miles below the earth’s surface and 6 miles below the oceanic floor, a distance which varies from place to place. The Mohorovicic discontinuity (the Moho) is the boundary lying between the crust and the mantle of the earth across which seismic waves change velocities. Thus, our results indicate that seismic velocities across the 400-km discontinuity are consistent with a transition zone of homogeneous peridotitic composition and do not require chemical stratification.What is the Mohorovicic Discontinuity? The Mohorovicic is in between the Earth's crust and mantle. The magnitude of the predicted velocity increase is in agreement with that observed seismically 9,10 if the transition zone is composed of ~60-70% olivine. By requiring phase relations in the Mg 2SiO 4-Fe 2SiO 4 system to be internally consistent thermodynamically, we find that the α-β transition in olivine of mantle (Mg 0.9Fe 0.1) 2SiO 4 composition is extremely sharp, occurring over a depth interval (isothermal) of ~6 km. It has recently been proposed 7,8 that the observed seismic velocity increase at 400km depth is too abrupt and too small to result from a phase change in olivine but instead requires that the transition zone be chemically distinct in bulk composition from the uppermost mantle. The 400-km seismic discontinuity has traditionally been ascribed to the isochemical transformation of α-olivine to the β-modified-spinel structure in a mantle of peridotitic bulk composition 1-6. ![]()
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