Saturday, March 26, 2011

Mohorovičić Discontinuity...in detail !!

What is the Mohorovičić Discontinuity?



The Mohorovicic Discontinuity, or "Moho", is the boundary between the crust and the mantle. 

http://upload.wikimedia.org/wikipedia/commons/thumb/2/2a/Refraction_of_P-wave.PNG/250px-Refraction_of_P-wave.PNG

In geology the word "discontinuity" is used for a surface at which seismic waves change velocity. One of these surfaces exists at an average depth of 8 kilometers beneath the ocean basin and at an average depth of about 32 kilometers beneath the continents. At this discontinuity, seismic waves accelerate. This surface is known as the Mohorovicic Discontinuity or often simply referred to as the "Moho".



How Was the Moho Discovered?



The Mohorovicic Discontinuity was discovered in 1909 by Andrija Mohorovicic , a Croatian seismologist. Mohorovicic 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 seismic waves observed within Earth's outer shell as a compositional change within the earth. The acceleration must be caused by a higher density material being present at depth.

The lower density material immediately beneath the surface is now commonly referred to as "Earth's crust". The higher density below the crust became known as "Earth's mantle". Through careful density calculations Mohorovicic determined that the basaltic oceanic crust and the granitic continental crust are underlain by a material which has a density similar to an olivine-rich rock such as peridotite.


How Deep is the Moho?



The Mohorovicic Discontinuity marks the lower limit of Earth's crust.

As stated above it occurs at an average depth of about 8 kilometers beneath the ocean basins and 32 kilometers beneath continental surfaces. 
http://forum.shankariasacademy.com/download/file.php?id=2&sid=d03435d08d8accac99bbe88f42120630

Mohorovičić was able to use his discovery to study thickness variations of the crust. He discovered that the oceanic crust has a relatively uniform thickness while continental crust is thickest under mountain ranges and thinner under plains.



 

Has Anyone Ever Seen the Moho?



No one has ever been deep enough into the earth to see the Moho and no wells have ever been drilled deep enough to penetrate it. Drilling wells to that depth is very expensive and very difficult because of the extreme temperature and pressure conditions. The deepest well that has been drilled to date was located on the Kola Peninsula of the Soviet Union. It was drilled to a depth of about 12 kilometers. Drilling to the Moho through oceanic crust has also been unsuccessful.

OPERATION MOHOLE : To be started again

Drilling deep into the earth surface till Moho discontinuity i.e discontinuity between crust and mantle...
The deepest hole ever drilled, and the deepest artificial point on the earth is on KOLA peninsula,Russia(NEAR FINLAND) which is 12,262 metres (40,230 ft) and was drilled in 1989...

Discontinuities in the Earth’s structure


Discontinuity
Depth
Boundary
Other notes
Mohorovicic discontinuity
30-50 km (continents)
7 km (ocean floor)
crust-mantle
Observed by abrupt change in seismic wave velocity
Identified by Andrija Mohorovicic (Croatia) in 1909




Gutenberg discontinuity
2900 km
Core-mantle
Observed by difference in seismic wave velocity
The P waves make abrupt drop in velocity at the mantle-core boundary, whereas S wave terminates at the mantle-core boundary. Thus making a plane of discontinuous surface between the core and the mantle known as Gutenberg discontinuity. 




Lehmann discontinuity
220 km

Appears beneath continents but not oceans



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