Mohorovicic Discontinuity (Moho)

The Mohorovicic discontinuity, sometimes referred to as "Moho," is the boundary where Earth's crust meets Earth's upper mantle (approximately 31 mi/50 km below the surface), and where seismic waves travel at a different and more rapid rate than the crust or mantle. The Moho is named after Andrija Mohorovicic (1857–1936), a Croatian meteorologist and seismologist who was fascinated with the faults and movements in the earth's infrastructure that result in earthquakes. The discovery of the Moho was most important because it helped scientists discover a second layer, or mantle, inside the earth. It also helped scientists to determine more accurately where this second layer was located in relation to Earth's surface, or crust.

Since the early 1900s, scientists were almost certain that Earth, like an onion, was made up of many layers, but they did not know exactly where the layers started and ended. In 1906, Mohorovicic studied Yugoslavian earthquake records, which revealed the existence of two different sets of earth shock waves from one earthquake. Because the second set of waves exactly mirrored the first set, Mohorovicic discovered that the additional set was actually the first bouncing back from a resistant surface, or a layer of different material inside the earth. This resistant surface, or discontinuity, allowed Mohorovicic to postulate the existence of a second stratum of material under the crust. He did this by gauging the time between the waves, which helped him determine how far this layer resided from the earth's surface.

Mohorovicic also noticed from these experiments that the waves, or tremors, traveled at different speeds depending on the thickness of the material inside Earth. This information helped scientists discover the different types of rocks in areas where drilling was impossible. For example, the lowest level of the crust is composed of basaltic rock, the material that rests next to the mantle. After the Moho was discovered, scientists were able to further plot seismic wave movements on sensitive shock recording devices called seismographs. From this information, we know that the outer crust of Earth is 20–25 mi (32–40 km) thick except under many places in the ocean, where it is only 3 mi (4.8 km) thick. The mantle is only the second interior layer. Deeper within the earth lies the most interior layer, Earth's core. We know from mountains and valleys that Earth's surface has changed and shifted with the ages. Similar to Earth's uneven crust, Earth's mantle is thought to be comparably uneven, mostly caused by enormous pressures inside Earth forcing the weaker areas of the rocky sub-layers out of alignment. When the weaker sub-layers, or plates, give way to pressure or stronger plates, earthquakes result. Ever since the existence of the mantle became certain, scientists sought to probe into the physical nature of the earth's inner layer. And because the Moho is located so much closer to the surface beneath the ocean, there were plans in the late 1950s to drill into the Moho from floating platforms out at sea. After a number of test drillings, and a drop in funding, the project—Project Mohole—was abandoned in the mid-1960s.

See also Crust; Earth (planet); Earth, interior structure