Consider the following statements regarding the Earth's crust: I. The continental crust is thinner under fold mountains. II. The mean thickness of ocean crust is 30 km. III. The Earth's crust is brittle in nature. Which of the statements given above is/are correct?

Statement I is incorrect: the continental crust is actually THICKER (up to 70 km) under fold mountains because of crustal roots, not thinner. Statement II is incorrect: the mean thickness of oceanic crust is about 5–10 km (around 7 km), while 30 km is approximately the mean thickness of continental crust. Statement III is correct: the Earth's crust, being cool and rigid lithosphere, is brittle and tends to fracture under stress, producing earthquakes. Hence only statement III is correct.