PHYSICAL PATTERNS

Many of the landforms and even climates of South Asia are the result of huge tectonic forces. These forces have positioned the Indian subcontinent along the southern edge of the Eurasian continent, where it is surrounded by the warm Indian Ocean and shielded from cold airflows from the north by the massive mountains of the Himalayas (see Figure 8.1).

The Indian subcontinent and the territory surrounding it are a dramatic illustration of what can happen when two tectonic plates collide. The Indian-Australian Plate, which carries India, broke free from the eastern edge of the African continent millions of years ago and drifted to the northeast (see Figure 1.8). When this plate collided with the Eurasian Plate about 60 million years ago, it left a giant peninsula jutting into the Indian Ocean. As the relentless pushing from the south continued, both the leading (northern) edge of what became South Asia and the southern edge of Eurasia crumpled and buckled. The result is the world’s highest mountains—the Himalayas, which rise more than 29,000 feet (8800 meters)—as well as other mountain ranges that curve away from the central impact zone—in the west, the Hindu Kush and Karakoram Range and in the east, the lesser-known ranges that border China and Burma (see Figure 8.1B and the figure map). The continuous compression also lifted the Plateau of Tibet, which rose up behind the Himalayas to an elevation of more than 15,000 feet (4500 meters) in some places. The compression and mountain-building process continues into the present.

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The three great rivers of South Asia—the Indus, the Ganga, and the Brahmaputra—all begin within 100 miles (160 kilometers) of one another in the Himalayan highlands near the Tibet, Nepal, and India borders; they are largely fed by glacial meltwater (see the Figure 8.1 map and Figure 8.5). The main river basins of the Indus and Ganga lie to the southwest and south of the Himalayas in what is called the Indo-Gangetic Plain (see Figure 8.1C, F). The Indus flows southwest through Pakistan and ultimately joins the Arabian Sea. The Ganga flows southeast to the Bay of Bengal. The Brahmaputra flows east through Tibet and then turns south through Arunachal Pradesh and Assam into Bangladesh. The mouths of the Ganga and Brahmaputra are blended together in a giant, ever-changing delta.

Lying south of the Indo-Gangetic Plain is the Deccan Plateau, an area of modest uplands (1000–2000 feet [300–600 meters] in elevation) interspersed with river valleys (see Figure 8.1E). This upland region is bounded on the east and west by two moderately high mountain ranges, the Eastern and Western Ghats (see Figure 8.1D). The Ghat mountain ranges descend to long but narrow coastlines interrupted by extensive river deltas and floodplains (see Figure 8.1A). The river valleys and coastal zones are densely occupied; the uplands are only slightly less so.

Because of its continual high degree of tectonic activity and deep crustal fractures, South Asia is prone to devastating earthquakes, such as the magnitude 7.7 quake that shook the state of Gujarat in western India in 2001; the 7.6 quake that hit the India–Pakistan border region in 2005; and the 6.4 quake in Quetta Province, Pakistan, in 2008 that left 120,000 people homeless. Coastal areas are also vulnerable to tidal waves, or tsunamis, that are caused by undersea earthquakes. In 2004, a massive tsunami originating off Sumatra in Southeast Asia wrecked much of coastal Sri Lanka and southern India, killing tens of thousands of people there and many more in Southeast Asia.

The climate of South Asia is characterized by a seasonal reversal of winds known as monsoons (Figure 8.4). These monsoon winds are affected by the intertropical convergence zone (ITCZ), which, as the name suggests, is a zone created when air masses moving south from the Northern Hemisphere and north from the Southern Hemisphere converge near the equator. As the warm air rises and cools, it drops copious amounts of precipitation. As described in Chapter 7, the ITCZ shifts north and south seasonally. The intense rains of South Asia’s summer monsoons are likely caused by the ITCZ being sucked onto the land by a vacuum created when huge air masses over the Eurasian continent heat up and rise into the upper troposphere.

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The summer monsoon (see Figure 8.4A) begins in early June, when the warm, moist ITCZ air first reaches the mountainous Western Ghats. The rising air mass cools as it moves over the mountains, releasing rain that nurtures patches of dense tropical rain forests and tropical crops on the upper slopes of the Western and Eastern Ghats and in the central uplands. Once on the other side of India, the monsoon gathers additional moisture and power in its northward sweep up the Bay of Bengal, sometimes turning into tropical cyclones.

As the monsoon system reaches the hot plains of Bangladesh and the Indian state of West Bengal in late June, columns of warm, rising air create massive, thunderous cumulonimbus clouds that drench the parched countryside. Monsoon rains sweep east to west, parallel to the Himalaya Mountains, moving across northern India to Pakistan and finally petering out over the Kabul Valley in eastern Afghanistan by July. Rainfall is especially intense in the east, north of the Bay of Bengal, where the town of Darjeeling holds the world record for annual rainfall—about 35 feet, even though no rain falls for half the year. These patterns of rainfall are reflected in the varying climate zones (Figure 8.5) and agricultural zones of South Asia (see Figure 8.17). Although sufficient rain falls in central India to support forests, most land has long been cleared of forest and planted in crops. Much of the remaining forests are small and so separated from each other that they no longer provide suitable habitat for India’s wildlife. Forest vegetation is more common in the Himalayan highlands and foothills, but even in the mountains there is widespread deforestation, often leaving only patchy areas of formerly richly forested land.

Periodically, the monsoon seasonal pattern is interrupted and serious drought ensues. This happened in July and August of 2009, when the worst drought in 40 years struck much of South Asia (see the discussion below). Then in September of 2009, heavy rains came to central south India, causing crop-damaging floods that killed hundreds of people. Increasingly, scientists are concluding that the extreme droughts and floods of recent years are not an anomaly but instead are part of global climate change.

The winter monsoon (see Figure 8.4B) is underway by November each year, when the cooling Eurasian landmass sends the cooler, drier, heavier air over South Asia. This cool, dry air from the north sinks down to the lower elevations and pushes the warm, wet air back south to the Indian Ocean. Very little rain falls in most of the region during this winter monsoon. However, as the ITCZ retreats southward across the Bay of Bengal, it picks up moisture that is then released as early winter rains over parts of southeastern India and Sri Lanka.

The monsoon rains deposit large amounts of moisture over the Himalayas, much of it in the form of snow and ice that add to the existing mass of glaciers (see Chapter 1). Meltwater from these glaciers feeds the headwaters of the Indus, the Ganga, and the Brahmaputra. These rivers carry enormous loads of sediment, especially during the rainy season. When the rivers reach the lowlands, their velocity slows and much of the sediment settles out as silt which is then repeatedly picked up and redeposited by successive floods. As illustrated in the diagram of the Brahmaputra River in Figure 8.6, the movement of silt constantly rearranges the floodplain landscape, complicating human settlement and agricultural efforts. However, the seasonal deposit of silt nourishes much of the agricultural production in the densely occupied plains of Bangladesh. The same is true on the Ganga and Indus plains.