Moho Discontinuity

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What is Mohorovicic Discontinuity?

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Mohorovicic discontinuity, also known as moho discontinuity is the boundary between Earth's Crust and its Mantle. The moho lies at the depth of 30-50 km below the continents and 5-10 km below the sea level in the ocean. It is defined by the discontinuous changes in the velocity of seismological waves as they pass through the densities of rock. The moho lies almost entirely within the lithosphere.

Named after the pioneering Croatian Seismologist Andrija Mohorovicic, the moho divides both the oceanic crust and continental crust from the underlying mantle. The Mohorovicic discontinuity was first introduced in 1990 by Mohorovicic, when he observed that seismograph from shallow focus earthquakes has 2 sets of P waves and S- waves, one that followed a direct path near the Earth's surface and another refracted by the high velocity medium. 

What is Moho Discontinuity?

The ‘Moho Discontinuity’ is the boundary between the crust and mantle. It has been named so as it was discovered by Croatian Seismologist Andrija Mohorovicic. This boundary marks a change in the seismic waves velocity from the crust to the uppermost mantle within the lithospheric plate. 

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Who Discovered Moho Discontinuity?

Croatian Seismologist Andrija Mohorovicic is first credited with discovering and defining the Moho.  In 1990, the Mohorovicic observed the data from a local earthquake in Zagreb where he observed two different sets of P waves and Q waves propagating out from the focus of the earthquake. Mohorovivis knew that the waves caused by earthquakes travelling at velocities are proportional to the density of the material carrying them. On account of this information, he theorized that the second set of waves could only be caused by a sharp transition in density in the Earth crust, which could account for such a drastic change in the eave velocity. Using the velocity data from the earthquake, Mohorovicic was able to calculate the depth of the Moho to be approximately 54 km, which was backed up by subsequent seismological studies.      


How Deep is the Moho Discontinuity?

The Moho discontinuity marks the lower limit of the Earth crust. The Moho discontinuity is 5-10 kilometres (3-6) mi below the ocean base and 20 - 90 kilometres (10 - 60) mi below the continental surface with an average of 35 kilometres (22 mi). Mohorovicis further used his discovery to study thickness variations of the crust which has a relatively uniform thickness, while continental thickness is thickest under the mountain ranges and thinner under the plains. 

Moho Role

Moho has played a large role in the field of geology and Earth Science well for a century.  By observing the Moho reflective nature, and how it affects the speed of W waves, scientists were able to hypothesize about Earth’s composition. This study gave rise to Earth seismology.

The Project Mohale in the 1960s attempted to drill to the Moho from deep - ocean regions.  After an initial success in establishing deep ocean drilling, the project underwent political and scientific opposition, mismanagement, cost overturn, and it was cancelled in 1966.

Did You Know?

  • Moho discontinuity is defined as the boundary between Earth's crust and the mantle, observed by a sharp increase in the velocity of the seismic waves passing through the Earth.

  • The name Mohorovicic discontinuity was received from Croatian geophysicist Andrija Mohorovičič, in 1909.

  • As per the recent studies, the moho is now realised as a transition zone rather than a sharp discontinuity.

FAQ (Frequently Asked Questions)

1. What Does the Term Moho Discontinuity Mean?

Ans: Moho discontinuity forms the boundary between the Earth crust and asthenosphere ( upper reach of the mantle), where there is a discontinuity in the seismic velocity.

The Moho discontinuity occurs at an average of about 8 km below the ocean basins and 30 kilometres below the Earth surface.

The cause of moho is considered to be the change in rock composition from the rock containing feldspar (above)  to the rock that contains no feldspar ( below).

2. What is Earth Crust?

Ans: Earth crust is a thin shell on the outside of the Earth, accounting for 1% of the total volume. The Earth crust is the top component of the Lithosphere, a division of the Earth's layers that includes the crust and upper part of the mantle. The Earth crust is made up of two parts namely Oceanic crust and Continental crust.

Oceanic crust lies underneath the land of the Earth whereas the continental crust lies underneath the sea. Both the crust are made up of 90% igneous rocks but also contain sedimentary and metamorphic rocks. However, the continental crust consists of less dense rock than the oceanic crust.

3. What Does Discontinuity Mean in Geography?

Ans: In geography, discontinuity is an area in the Earth where the speed of the Earthquake changes. Moho discontinuity is the renowned discontinuity, which is also known as Moho in short. It is the division between the Earth and the Mantle. The moho was discovered in 1909 by the seismologist Andrija Mohorovicics. He observed that the speed of the seismic waves changed in this area and thought that this change must be because of the density of the material.