Himalayas

The Himalayas arc across southern Asia between the Tibetan Plateau to the north and the plains of the Indian subcontinent to the south. Encyclopaedia Britannica describes the uninterrupted range as about 2,500 kilometres long from west to east and roughly 200–400 kilometres wide. It crosses borderlands in India, Nepal, Bhutan, Pakistan and China. The exact lines used to define a mountain system can vary; the term Hindu Kush Himalaya (HKH), used in regional research, describes a broader 3,500-kilometre region extending across parts of eight countries, from Afghanistan to Myanmar. The HKH region should not be confused with the Himalayas alone.

The Himalayan system is made up of several roughly parallel belts with distinct landforms and geological histories. From south to north, Britannica groups them as the Outer or Sub-Himalayas (the Siwalik Range), the Lesser Himalayas, the Great Himalaya Range and the Tethys or Tibetan Himalayas. The region's landscape includes steep peaks, deep valleys and gorges, glaciers and high plateaus. Mount Everest is its highest summit and the highest point on Earth above sea level.

The mountains formed as the Indian continental plate moved north and collided with the Eurasian plate, closing the Tethys Ocean between them. The collision began about 50–40 million years ago. Because neither thick continental plate could readily sink beneath the other, compression folded and thickened the crust and pushed rock upward, building the Himalayas and the Tibetan Plateau. The plates continue to converge, so tectonic deformation and earthquakes remain part of the region's geology; uplift, erosion and landslides continually reshape the landscape.

Snowfields and glaciers feed many rivers that descend from the mountains. The Indus and Brahmaputra have extensive catchments in the Himalayan region, while the Ganges is also closely linked to Himalayan waters. These rivers carry water and sediment toward the South Asian plains. Sharp changes in elevation produce a succession of climatic and ecological zones, from lower slopes to alpine terrain and the high snow zone. The broader HKH region is the headwater area for ten major Asian river systems, according to the International Centre for Integrated Mountain Development (ICIMOD).

The Himalayas are inhabited and crossed by many communities whose languages, livelihoods and traditions reflect the region's varied environments. For thousands of years the mountains have held religious and cultural importance in South Asia, appearing in literature, mythologies and religious practice. Their elevation and rugged terrain have also shaped routes and contact between the subcontinent and the Tibetan Plateau. Because the Himalayas form part of a wider, interconnected mountain and river region, their environmental changes can affect communities both in high valleys and far downstream.