Discontinuities Inside the Earth : Crust, Mantle & Core, Seismic Discontinuities | UPSC Notes

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• The inner part of Earth is made of many different things. Each of these materials is different from the others in terms of their physical and chemical properties, such as their temperature, density, and so on. Inside the earth, there are different layers that are made of different materials with different qualities. A transition zone separates all of these levels from each other. We call these places of change “discontinuities.”

Discontinuities Inside the Earth

• There are five Discontinuities inside the earth.

Conrad Discontinuity: The area between SIAL and SIMA that is in between.

Mohorovicic Discontinuity: The area where the Crust and Mantle change into each other.

Repiti Discontinuity: The area where the outer mantle changes into the inner mantle.

Gutenberg Discontinuity: The area where the mantle and the core meet.

Lehman Discontinuity: The area where the outer core and the inner core change places.

Conrad Discontinuity:

The part of the lithosphere where the upper and bottom parts meet is called the Conrad discontinuity. Vector Conrad, an Austrian geophysicist, came up with the name.

• Until the middle of the 20th century, it was thought that the continental region’s upper crust was made of rocks like granite and the lower crust was made of rocks with more magnesium, like basalt.

• Because of this, seismologists at the time thought that the Conrad discontinuity should be a clear line where the chemically different layers of SIAL and SIMA meet.

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• When longitudinal earthquake waves go through the Conrad discontinuity, their speed suddenly jumps from about 6 to 6.5 km/sec.

Mohorovicic Discontinuity:

• The boundary between the crust and the mantle is called the mohorovicic discontinuity.

• Andrija Mohorovicic found the Mohorovicic gap in the year 1909.

The Moho is 35 km below the continents and 8 km below the oceanic crust. It splits the mantle from both the continental crust and the oceanic crust. The Moho is almost completely in the lithosphere. It marks the boundary between the lithosphere and the asthenosphere only under the Mid Oceanic Ridge.

• The speed of the P wave is 6 km/sec right above the Moho and 8 km/sec right below the Moho.

• Moho is made up of things that are up to 500 km thick.

Repiti Discontinuity

• It is the area where the outer mantle and the inner mantle change into each other.

Gutenberg Discontinuity

Gutenberg discontinuity is the transition zone between the mantle and the core. It was found by Weichert Gutenberg in 1912 at a depth of 2,900 km below the earth’s surface. In this area, the speed of earthquake waves changes quickly.

• At this depth, the speed of the P wave slows down and the S wave goes away totally.

• An S wave breaks up matter and can’t go through liquid. So, it is thought that the part above the break is solid and that the part below the break is liquid or melting.

• This section of liquid rock is thought to be 700°C hot, which is hotter than the mantle above it. It is also thicker, which is likely because it has more iron.It is a small, bumpy area with hills that can be up to 5 to 8 km wide. This wave is affected by the heat-driven convection in the mantle above it. These waves are also affected by the eddies and currents in the iron-rich fluids in the outer core, which are eventually responsible for the earth’s magnetic field.

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• It’s important to note that the border between the mantle and the core does not stay the same. As the heat inside the earth slowly but constantly goes away, the molten core inside the earth solidifies and shrinks. This makes the border between the core and the mantle move slowly deeper and deeper into the earth’s core.

Lehmann discontinuity

It is the boundary between the outer core and the inner core. Seismologist Inge Lehmann found that P-wave and S-wave speeds suddenly increased at a depth of 220–30 km. This is called the Lehmann discontinuity.

It shows up under continents but not usually under oceans, and it doesn’t show up easily in averaged world studies. Several theories have been put forward to explain this, including a lower limit to the flexible asthenosphere, a phase transition, and, most likely, depth variation in the shear wave anisotropy.