Mohorovičić Discontinuity
- erudite .
- Oct 1, 2022
- 2 min read
By Rita Dwivedi
The Mohorovičić Discontinuity, colloquially known as the “Moho,” is the boundary between the crust and the mantle. It is defined as the discontinuous change in the velocity of seismological waves, such as P and S waves, as they pass through the varying densities in rock. Named after the Croatian meteorologist and seismologist Andrija Mohorovičić, the Mohorovičić discontinuity divides both the oceanic crust and continental crust from the underlying mantle. It lies 30 - 50 kilometers below the continents and 5 - 10 kilometers below sea level and is used by geologists to describe the surface at which seismic forces change speed (1). At the Mohorovičić discontinuity, seismic waves accelerate.
Mohorovičić’s biggest contribution to science was the famous Mohorovičić Discontinuity. One day in 1909, Mohorovičić noticed some P-waves and S-waves were coming later than normal when he was looking at seismograms of the Pokupsko earthquake near Zagreb. For context, P-waves and S-waves are the two kinds of waves that are generated by earthquakes. P-waves travel the fastest and S-waves are slower waves that will always arrive after the P-wave. After this incident, Mohorovičić realized that the velocity of a seismic wave is related to the density of the material that it is moving through. He interpreted the acceleration of the seismic waves within the Earth’s outer shell as a compositional change in material within the Earth’s interior. As a result of these interpretations, Mohorovičić determined that the basaltic oceanic crust and granitic continental crust were underlain by a material that is similar to peridotite rock. Later investigations confirmed this boundary to be where the Earth’s mantle begins at around a temperature of 600°C. Mohorovičić’s work was only truly understood many years later after detailed observations of the effects of earthquakes on various structures, earthquake epicenters, seismographs, and other research into various geoscience topics (2).
Mohorovičić was able to use his discovery to study the thickness of the Earth’s crust. He learned that the oceanic crust has a relatively uniform thickness whereas the continental crust is thickest under areas such as mountain ranges and thinner under areas like the deepest ocean floors and plains. Furthermore, Mohorovičić learned that the denser that the earth’s composition and structure is, the faster the velocity of the seismic waves. Lower density compositions were called the Earth’s “crust” and higher density compositions were called the Earth’s “mantle.” Mohorovičić’s discovery explains that the earth’s composition is heterogeneous in that there must be a boundary at a given depth that separates matter of different densities, pressures, temperatures, and elastic properties (3).
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