Introduction

Asia is crossed at its center by the vast mountainous terrain of the Tibetan Plateau, bounded on the south by the Himalayas and on the north by the Kunlun Shan. These mountain belts act as a climatic barrier between arid lands to the north and humid lands to the south. and also a barrier between the people to the north and those to the south. Chains of volcanic islands line much of Asia's eastern coast.

Himalayas

Location (84E 29N) Highest point : Mt Everest 8,848 m Length: 2,500km; average width: 300 km.

The Himalayas are both a mountain system and a set of mountain ranges that arc around the northern part of the Indian sub-continent. The mountain system was formed when the Indian plate crushed into the southern flank of the Asian plate beginning, perhaps, 65 millions years ago.

This young mountain system is still rising by a few centimetres a year and it includes the world's ten highest peaks. Because the region is geologically active, the whole area is subject to frequent earthquakes. The zone of most intense activity is the Indian state of Assam, where tremors are felt quite often and where the last catastrophic earthquake occurred in 1955.

The Himalayan system is the name given to the whole of the mountainous terrain formed by the plate movements described above. Geographically, the system is divided into many parallel ranges, of which the Himalayan Ranges lie only between the Indus River and the Brahmaputra River, effectively the lands of Nepal and Bhutan and northern India.

The continuation of the ranges to the west lead to the Karakoram Range, which begins in northern Kashmir, India. The Karakoram Range meets the Hindu Kush and the Kunlun Shan mountains in a region known as the Pamir Knot. The Hindi Kush run south west from this point, fading out gradually into the Iranian Plateau. The Kunlun Shan makes the northern boundary of the Tibetan Plateau.

The Himalayan Ranges are made up of the Great, or High, Himalayas on the Chinese border, and includes Everest. Here the average height is 6,000m . To the south lie the Lesser Himalayas, averaging 4,000m. Finally, the Outer Himalayas make up the foothills of the mountain system, being about 1,000m high.

The ranges are not all the same age, but are younger to the south. To imagine how they were formed, think of a tablecloth being pushed over a table. One part of the cloth is held in place and the other part is slid towards it, causing the cloth to ruck up in a series of parallel folds that fall over towards the sliding part of the cloth. The equivalent of the fixed part of the cloth is the Tibetan Plateau, and the sliding part is India, with the rucked up folds being the Himalayas.

Himalayan rivers

Many major rivers have their sources in the Himalayan system, of which the Brahmaputra, the Arun, the Ganges and the Indus are especially notable. Each of these rivers rises on the northern flanks of the Himalayan Range and flows into the Tibetan Plateau. Each river then turns to the south and flows in a spectacular gorge right through each of the Himalayan ranges, finally flowing out to the Indian Ocean.

This surprising route, in which the rivers cut the deepest gorges in the world, helps to show how young the mountain are. In fact the rivers are much older than the mountains. As the mountains rose, so the rivers were powerful enough to be able to continue to cut into their beds fast enough to keep pace with the rising mountains. Geographers call such river patterns antecedent drainage patterns (ante means before).

Because the rivers from both the north and south flanks of the mountains drain to the south, huge volumes of sediment have been deposited in the region south of the mountains. Although the North Indian Plain appears to be a low plain, the land conceals thousands of metres of sediment contained in a trench thousands of km long and hundreds of kilometres wide (a geosyncline). In time, the sediment accumulating in this trench will be uplifted to form new mountains.

Climate and vegetation

The Himalayas are so high that they are able to split the flow of air in the lower part of the atmosphere. As air flows east, so part of it moves south of the mountains, while part moves north. In winter, more air moves on the southerly arm, piling up over India to give a high pressure region which is seen as the dry season. As summer progresses, so the world's air flows begin to move north, but the air flowing south of the Himalayas cannot easily escape north. As a result, the patterns of air flow remain in place far longer than in other regions at the same latitude. Then, sometime in June, there is a rapid shift of air and the southerly route is almost abandoned. This allows air to flow in over India from the hot Indian Ocean, and brings with it the start of the monsoon.

For people living in the Himalayas, the climate varies with height and also with distance from the rain-bearing winds in the south. The northern valleys are virtually deserts, while the southern foothills have some of the highest rainfall total in the world. Much more rain falls in the east than the west, the western ranges being semi-arid.

Above 4,500m, the temperature is below freezing throughout the year, and glaciers are able to form. However, in the valleys, the temperature rises to 40°C. In the winter the temperature in the valleys falls to just above freezing.

The slopes of the Himalayas can support forests until above 4,000m. In the humid Outer Himalayas the forest is subtropical, evergreen oak, with pine and cedar. Above the tree line rhododendrons grow. The forests have been home to leopard and bear, although few still survive.

Human impact on the ecosystem of the lower Himalayas is high. People live in the valleys and practise transhumance farming, moving animals to the high pastures in summer and bringing them down for the winter. Many slopes that used to be forested have been cleared as the timber is used for fuelwood. The exposed slopes are much more easily eroded by the torrential monsoon storms, and so erosion is common. Extra water in the rivers also increases the likelihood of flooding. These effects are seen as far away as the river mouths, where increases in the amount of sediment in the rivers is causing the rivers to build up their beds and overtop their levees more often, causing catastrophic flooding where the Ganges and Brahmaputra share a common delta. This delta makes up the country of Bangladesh.

Famous mountains of Asia

Everest

Location: 28N 87E (Nepal/China)Height: 8,848m.

Mount Everest, part of the Himalayas that divide Nepal from China, is the world's highest mountain. Everest is made from limestone, a rock once formed under the sea. The entire mountain system, including Mt Everest, was lifted up during the last major mountain building period that began about 60 million years ago, as the continental plate of India began to collide with the plate of Asia.

Mt Everest is still rising by a few centimetres each year, and its twin-peaked summit is cloaked in many glaciers. It frequently suffers blizzards and this, together with the altitude, makes it notoriously difficult to climb. It was first successfully climbed on May 29, 1953 by Sir Edmund Hillary of New Zealand and Tenzing Norgay of Nepal.

The Western name for the mountain was in honour of Sir George Everest, the Briton who first accurately recorded its height. The Tibetan name Chomolungma means 'goddess mother of the world', the Nepalese name Sagarmatha means 'land of eternal snows'.

Mount Fuji Japanese: Fujiyama

Location: 35N 139E (Japan) Height 3800m

Mount Fuji is a large active volcano, 3,800m high near to Tokyo. At the base there is a ring of lakes dammed up by past lava flows. Mount Fuji last erupted in 1707, but because the slopes are made of loose ash, the main hazard is from mudlfows.

Mt Fuji has many vegetation zones. The lower slopes are farmland, but above this are bamboo and then pine forests. The summit of Mount Fuji is snow covered for most of the year. Mount Fuji is one of many volcanic peaks in the Japanese islands, created because Japan lies on the boundary between the Asian plate and the Pacific Ocean plate. The whole island chain that makes Japan is created by those parts of volcanoes that rise above the sea.

(See volcanoes for picture)

Krakatoa

Krakatoa is an active volcanic island west of Java in Indonesia. It is especially famous for the catastrophic eruption that occurred on August 27, 1883, reducing the once 1,800m volcanic cone to its present height.

The volcano had been erupting since May. The violence of the eruptions wracked the cone, fracturing the rocks that lay beneath it. By August 27 the rocks supporting the weight of the cone must have been weakened so much that one of the immensely powerful explosions that occurred caused the cone to collapse down into the magma chamber that had supplied it with lava.

The explosions that occurred on August 27 were heard over 4,000km away. The collapse of the cone created a caldera which lay open to the sea. The huge inrush of water created a tidal wave (a tsunami) that spread away from Krakatoa. On nearby islands the tsunami reached 3m in height and drowned 36,000 people living in coastal settlements.

The 21 cubic km of ash sent into the stratosphere by the eruptions of Krakatoa (some 20 times more than the eruption of Mt St Helens in 1980) caused red sunsets world-wide for a year, but the ash also decreased the amount of solar radiation reaching the Earth, and the global temperature was reduced for several years afterwards.