Correct Option
The correct option is (a) 1, 2 and 3 only.
Explanation
The Himalayas represent a system of geologically young and structurally fold mountains that form the northern borders of India. They were formed due to the collision between the Indo-Australian Plate and the Eurasian Plate during the Tertiary period.
Statement-wise Analysis:
- Statement 1 is Correct: The Himalayas are geologically young (formed relatively recently in geological time) and structurally fold mountains. They consist of uplifted sedimentary rocks and are unstable zones prone to earthquakes and landslides.
- Statement 2 is Correct: These mountain ranges run in a west-east direction from the Indus river in the west to the Brahmaputra river in the east. They form an arc covering a distance of about 2,400 km.
- Statement 3 is Correct: The altitudinal variations are greater in the eastern half than in the western half. The Eastern Himalayas rise abruptly from the plains, resulting in sharp changes in altitude, whereas the Western Himalayas rise more gradually and are broader.
- Statement 4 is Incorrect: The width of the Himalayas is not constant. It varies from approximately 400 km in Kashmir (Western side) to 150 km in Arunachal Pradesh (Eastern side). The compressive forces were stronger in the east, leading to a narrower width and greater height.
Key Takeaway:
The Himalayas are young fold mountains stretching from the Indus to the Brahmaputra. While their length is approximately 2,400 km, their width decreases from west to east, while the altitudinal variation increases in the eastern sector.