Correct Option
The correct option isIt is about 35 km on the continental masses and 5 km on the ocean floors.
Explanation
The crust is the outermost solid part of the Earth and is brittle in nature. Its thickness is not uniform across the globe; it varies significantly between the oceanic areas and the continental masses.
Statement-wise Analysis
- It is uniform everywhere. is Incorrect: The thickness of the Earth's crust is not uniform. It varies depending on whether it is under the oceans or the continents.
- It is thicker on the ocean floors than on the continental masses. is Incorrect: The crust is thinner on the ocean floors compared to the continental masses. While oceanic crust is denser (composed largely of basalt), it is physically thinner than the continental crust.
- It is about 35 km on the continental masses and 5 km on the ocean floors. is Correct: The mean thickness of the oceanic crust is approximately 5 km, whereas the mean thickness of the continental crust is approximately 30 to 35 km. In major mountain systems like the Himalayas, the continental crust can be as thick as 70 km.
- It is about 100 km on the continental masses. is Incorrect: The average thickness of the continental crust is around 35 km, not 100 km. The figure of 100 km typically refers to the thickness of the lithosphere (which includes the crust and the uppermost solid mantle), not the crust alone.
Key Takeaway
Oceanic crust is thinner (~5 km) and denser (basaltic), whereas Continental crust is thicker (~35 km) and less dense (granitic).