The most characteristic features of the Himalayas are their great height, complex geologic structure, snowcapped peaks, large valley glaciers, deep river gorges, and rich vegetation. From south to north the Himalayan ranges can be grouped into four parallel belts of varying width.
They are the Outer, or Sub-, Himalayas; the Lesser, or Lower, Himalayas; the Great, or Higher, Himalayas; and the Tethys, or Tibetan, Himalayas. The Karakoram Range in the northwest is also sometimes considered part of the Himalayan system. The mountains can be divided broadly into three regions. The backbone of the system is the Great Himalayas, a single range rising above the snow line with nine of the 14 highest peaks in the world, including Mount Everest.
Geologically the Himalayas are relatively young folded mountains and are still undergoing the mountain-building process. Precambrian metamorphic rocks--rocks formed by heat and pressure from 4.6 billion to 570 million years ago--make up much of the structure. The uplift took place in at least three phases. The first phase occurred at the close of the Eocene epoch (about 38 million years ago) when the Great and Tethys Himalayas were uplifted. In the second phase, which occurred in the Miocene epoch (26 million to 7 million years ago), ranges of the Lesser Himalayas were formed. The final mountain-building phase started at the end of the Tertiary period (about 7 million years ago) when the Siwalik Range, the foothills of the Outer Himalayas, were formed.
The Himalayas act as a great divide and influence the climatic conditions of the Indian subcontinent to the south and of the Central Asian highland to the north. The winter season lasts from October to February, the summer from March to June, and the rainy season from June to September. Climate varies considerably with altitude; the snow line generally lies at about 16,000 feet (4,900 meters) in the Great Himalayas. The annual and daily temperature variation is much greater in the foothills.
The mountain ranges obstruct the cold, dry air from the north in winter. They also force the monsoonal winds to give up moisture, causing heavy rain and snow on the southern side but arid conditions in northern Tibet. Rainfall decreases from east to west--120 to 60 inches (300 to 150 centimeters). Cherrapunji in Meghalaya State in northeastern India is noted for the world's second highest average annual rainfall of 450 inches (1,140 centimeters).
The Himalayas are drained by 19 major rivers, of which the Indus and the Brahmaputra are the largest. The Jhelum, Chenab, Ravi, Beas, and Sutlej belong to the Indus system; the Yamuna, Ramganga, Kali, Gandaki, Karnali and Kosi are part of the Ganges system; and the Tista, Raidak, and Manas belong to the Brahmaputra system. Rivers are more numerous and extensive on the southern slopes of the Himalayas. Such major rivers as the Indus, Sutlej, and Brahmaputra have narrow and deep upper valleys that are older than the mountains themselves. Glaciers cover more than 12,700 square miles (32,900 square kilometers). One of the largest is Gangotri glacier in northern India--20 miles (32 kilometers) long. Glaciers feed most of the upper courses of the rivers, while the middle and lower courses are fed by rain. The Himalayan rivers also serve to float logs downstream to sawmills in the foothills.
There are several freshwater lakes.
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