Question

The average thickness of the crust is __________ km below the continent and _________ km below the ocean.
A. 35, 6
B. 10, 50
C. 80, 60
D. 30, 90

Answer 1

Hint: The top of the crust is the surface layer of the land but much of it is sea-bed and therefore cannot be seen. However, the parts of the surface above sea-level carve out the familiar shapes of continents we recognize today.

Complete Answer: The outermost solid portion of the earth made of rocks is called the crust. An abrupt change in the velocity of seismic waves distinguishes crust from mantle. This corresponds to the abrupt change in the rigidity of the rock from crust to mantle. The change in rigidity, in turn, is due to change in mineral composition or in the physical state of rocks. The P waves near the surface travel at about 6 kilometres per second. This speed increases gradually or abruptly to the base of the crust. Here it is 7 kilometre per second. The surface of the sudden increase in wave speed separates the crust above from the mantle below. It is termed as the Mohorovicic discontinuity, also called Moho-discontinuity or M discontinuity. It is named after the Yugoslav Seismologist, Mohorovicic, who first recognised the discontinuity in 1909.

Only P waves pass through the core beyond 145 degrees and surface waves travelling along the surface are received. The earth’s core is calculated to have a radius of 6370 kilometres. It is calculated from the extent of the Shadow Zone. It is a little more than half of the earth’s total radius. The thickness of the Oceanic crust is 6 kilometre and the thickness of the Continental crust is 35 kilometres.

Thus, the correct answer is option (A).

Note: A solid inner core, more than 5000 kilometres below the surface, is enveloped by a molten iron-nickel outer core. Surrounding the outer core is the Mantle. The uppermost part of the mantle is termed as the asthenosphere. Above the mantle is the Earth’s crust.