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?

The correct answer is C, III only. Only the third statement is right: the crust is the outermost solid shell of the Earth and it is brittle in nature, which is why it cracks and faults during earthquakes. The thickness of the crust differs between land and sea. Under the oceans it is thin, about 5 km on average, and made mostly of dense basaltic rock. Under the continents it averages about 30 km, and under major fold mountain systems it is much thicker; beneath the Himalayas it is around 70 km, because the mountains have deep roots. Statement I is wrong because the continental crust is thicker, not thinner, under fold mountains. Statement II is wrong because 30 km is the mean thickness of the continental crust; the oceanic crust averages only about 5 km. Therefore option A (I only), option B (I and II) and option D (I and III) are all wrong, as each includes a false statement. Exam tip: oceanic crust about 5 km, continental crust about 30 km, Himalayan region about 70 km; the crust is brittle.