The correct option is It is thicker in the areas of major mountain systems..
Explanation
The Earth's crust is the outermost solid part of the Earth, known for its brittle nature. Its thickness is not uniform across the globe and varies significantly depending on whether it is oceanic or continental crust.
- It is 70 km in the oceanic region. is Incorrect: The oceanic crust is thinner than the continental crust. Its average thickness is approximately 5 km, whereas 70 km is a characteristic thickness found under major mountain ranges.
- It is 5 km in the Himalayan region. is Incorrect: The continental crust is generally thicker, with an average thickness of about 30 km. In the Himalayan region, the crust is exceptionally thick, reaching up to 70 km, not 5 km.
- It is thicker in the areas of major mountain systems. is Correct: The crust is thicker in the areas of major mountain systems. This increased thickness supports the high elevation of mountain ranges (such as the Himalayas and the Andes) relative to the surrounding terrain.
- It is uniform everywhere. is Incorrect: The crustal thickness is highly variable, ranging from roughly 5 km in the ocean basins to over 70 km in mountainous continental regions.
Key Takeaway: The oceanic crust is thin (approx. 5 km) and composed primarily of basalt, while the continental crust is thicker (avg. 30 km) and can reach up to 70 km under major mountain systems like the Himalayas.