Himalaya

The Himalayas, or Himalaya, constitute one of the greatest physical features on the earth, containing some of the world's youngest and highest mountains, deepest gorges, and largest glaciers. The complex geology, loftiness, and length of the Himalayan mountain chain affect the climate and life patterns of much of continental Asia.

Formation and Characteristics

Until approximately 3 or 4 million years ago, most of the Himalayan mountain region, including the highlands of the Tibetan plateau, was covered by the broad, shallow Tethys Sea that lay between present-day India and Tibet. The Himalaya formed as the South Asian tectonic plate drifted northward, pushing the sedimentary-rock sea bottom against the inner Asian continent. This process of crustal plate movement, attributed to the theory of continental drift, began during the Jurassic period, about 150 million years ago, and continues today. Such continent-continent collision results in a greatly thickened crust, as neither continent can sink. In the Himalaya, the crust is approximately 60 kilometers thick, almost double the average for continents as a whole, and consists of the underthrust Indian plate along with crust that has been shortened because of compression. The actual collision of the northern Indian shield with Eurasia dates to the Tertiary period, some 20 million years ago, and represents the initial period of tectonic activity associated with mountain building, or orogeny.

Orogenic activity of the Himalaya is conventionally divided into three phases. The first of these occurred at the end of the Eocene epoch and into the Oligocene epoch, between approximately 50 and 35 million years ago, when upheaved crystalline rock and sedimentary rock formed the central axis of the Himalaya. The second phase of folded sediments took place during the Miocene epoch, which lasted until approximately 5 million years ago. The raised central part of the range and the outer foothills formed during the third phase, which coincides with the post-Pliocene, between 2 and 5 million years ago. Activity during this phase gave the range its contemporary morphology. Since the last glacial epoch, about 20,000 years ago, the Himalaya have grown more than 1,500 meters (between 7.5 and 10 centimeters annually).

Geographers and geologists differ in their measurements of the length of the Himalaya. Some extend the chain to include the Koh-i-Baba and Safed Koh Ranges of the Hindu Kush in the west and the Assamese highlands in the east. This makes the Himalayan range more than 4,000 kilometers in length—an arc spanning the entire Indian subcontinent, including the Hindu Kush, Karakoram, and Himalayan physiographic divisions. Other scientists place the dividing line at the Oxus River in Afghanistan and include the Karakoram in northern Pakistan as part of the Himalayan mountain system but omit the Hindu Kush in Afghanistan, thus delimiting the chain to 3,000 kilometers. Finally, some experts separate the Himalaya proper from the Karakoram and ranges farther west by using the Indus River catchment as the boundary. This latter division defines the Himalaya as the mountain chain extending between the Indus and Brahmaputra Rivers, a distance of some 2,700 kilometers.

There are no compelling geological arguments that clearly show where to divide the Himalaya from east to west in genetic or structural terms. Geomorphological divisions use rivers, such as the Oxus and Indus Rivers, to make the divisions. Elevation character is also used to distinguish the main Himalayan range from its western extensions. For example, west of longitude 68 degrees west, the mountains rarely exceed 4,000 meters, while most of the Himalaya average between 6,000 and 7,000 meters elevation. Using elevation as a criterion, some experts extend the Himalaya for 3,000 kilometers in a northwest-southeast strike between longitude 68 degrees east and longitude 96 degrees east.

When the Karakoram is included as part of the Himalayan chain, the system includes all fourteen of the world's peaks over 8,000 meters and hundreds over 7,000 meters. Nine of the fourteen tallest peaks are in Nepal, including the world's highest mountain, Mount Everest, at 8,872 meters. The second-highest peak is K2 at 8,611 meters, located in the Karakoram range. The Nepal Himalaya contain several of the next tallest mountains: Kangchenjunga (8,585 meters), Lhotse (8,501 meters), Makalu (8,470 meters), Dhaulagiri (8,172 meters), Cho Oyu (8,153 meters), Manaslu (8,125 meters), Annapurna (8,091 meters), and Kao-seng-tsan Feng (8,013 meters). The remaining peaks over 8,000 meters are in the Karakoram and include Nanga Parbat (8,125 meters), Hidden Peak (8,068 meters), Gasherbrum (8,060 meters), and Broad Peak (8,047 meters).

Geologic Zones

The relief range from high to low spots in the Himalayan chain is unsurpassed by other world mountain systems. Using the most liberal definition of the lateral extent of the chain, it is also one of the world's longest systems of mountains. The interplay of geological forces that contributed to the initial formation of this impressive mountain range is still not fully understood. Early geological research emphasized the study of rock layers, or strata, based on fossil findings. More recent scientific investigations have considered the underlying structure of the Himalayan thrust sheets. These studies geologically divide the Himalaya into three zones.

The first zone is the Outer-Himalayan foothills, which rise out of the lowlands of the Indo-Gangetic plains in the south. These hills form a series of ridges of up to 1,300 meters in elevation that strike northwest-southeast, separated by longitudinal depressions called dun valleys. In India, the Outer Himalayan ranges are called the Siwaliks; in Nepal, they are referred to as the Churia Hills. Composed of detrital rocks, such as clays, sandstones, limestones, and conglomerates, these hills are a series of broad anticlines (upfolds) and synclines (downfolds) formed from the weathered granitic core of the central Himalaya.

North of the foothills of the Outer Himalaya are the middle ranges of the Lesser Himalaya, which reach elevations surpassing 5,000 meters. This band of intermediate hills, averaging 65 kilometers in width, lies between the Great Himalaya central upthrust region and the outer ranges of the Outer Himalaya. This geological zone is composed of compressed or metamorphosed rock of various ages. Combined with the Great Himalaya, it forms the second geological zone. The lofty peaks and snow summits of the Great Himalaya, in numerous places exceeding 8,000 meters, cap the complicated geology of the main thrust zone. The convoluted geology of this region is partially attributed to great rock sheets, called thrust sheets, or nappes, that have moved many kilometers. In this Great Himalayan region is found the oldest crystalline core material of Tethyan sediment origin in thick layers thrust approximately 160 kilometers south over the Lower Himalaya.

The third geological zone is called the Tibetan zone. Unusually thick crust (up to 120 kilometers) and intensive folds and upthrusts characterize this extensive northward-dipping plateau region that lies north of the Great Himalaya. Extensive basins as well as outcroppings of crystalline rock punctuate the expanse of the Tibetan highland plateau zone.

The mountains of the Great Himalaya and the outer foothills are traversed by valleys that cut across the strike of the range, producing the deepest gorges in the world. These erosional valleys are river-cut and differ from the tectonic dun valleys situated in the Outer Himalaya, which are actually synclinal troughs. The rivers that cut these great transverse valleys originate in the Tibetan zone and predate the Himalaya's orogeny; thus, they have been continually eroding while the mountains were uplifting. The main rivers breaching the Himalaya include the Hunza River, at about 1,800 meters in elevation, only 4 kilometers from Mount Rakaposhi (7,788 meters); the Indus River, about 1,200 meters and 22 kilometers from Nanga Parbat (8,125 meters); the Kali Gandaki River, about 1,500 meters and less than 7 kilometers from Dhaulagiri (8,172 meters); and the Trishuli River, about 1,800 meters in elevation and 13 kilometers from Langtang-Lirung Himal (7,245 meters).

The location of the Himalayan mountain range in southern Asia predicts its role in the continental climate and the consequential impact of local weather on surface geomorphology. It presents a barrier to north-south airflow associated with the Asian monsoon, thus producing heavy orographic rainfall along the southern flanks in the summer, when moisture-bearing winds from the southern oceans are forced to rise over the mountains. The north side in Tibet and Central Asia lies in the Himalayan rainshadow. The precipitation in the form of snow at high elevations feeds some of the world's largest glaciers, including the Siachen Glacier (72 kilometers), the Hispar Glacier (61 kilometers), and the Baltoro Glacier (58 kilometers), all located in the Karakoram. Meltwaters from these glaciers and rainfall from lower elevations enter into extensive drainage systems that, in turn, erode the land to contribute to the greatly dissected topography of the mountains. Precipitation receipts, elevation, and slope aspect vary greatly in this highly complex and dynamic natural system and contribute to a multitude of habitats for more diverse assemblages of flora and fauna as well as for human settlement.

Study of the Himalaya

Scientists use a variety of techniques to study the Himalayan mountain system, including geological and climatological surveys, erosional studies, geoecological mapping, and land-cover analysis. Modern geotechniques have enabled scientists to reconstruct evolutionary stages in mountain building. For example, paleobiology, the study of plant and animal fossil remains, and radiocarbon dating methods have contributed to more precise dating of rocks and rock units. The geochronological methods are supplemented with geochemical analyses that enable scientists to date rocks and rock strata, ascertain their origins, and monitor their movements. Geological structures are analyzed through geoseismology readings, field interpretation, and mapping of geophysical transverses. By relating structures to rock ages, experts can reconstruct the evolutionary history of such structures. Field survey instruments such as plane tables, compasses, spotting scopes, and altimeters are commonly employed during geological fieldwork.

Climatologists rely on weather data obtained from climate stations that are distributed throughout the mountains, or they conduct on-site climate studies using an assortment of instruments, including rain gauges, thermometers, barometers, wind gauges, and altimeters. Systematic climatological data are not available for many regions within the Himalaya, however, and the understanding of Himalayan weather is incomplete. During the last few decades of the twentieth century, aerial surveys from low-flying aircraft and electronic images from meteorological satellites began providing a wealth of new information.

The extremely difficult terrain and inhospitable climate at high altitudes make fieldwork difficult in the Himalaya. Energy-sensing satellites such as Landsat and Spot provide previously unobtainable information about surface conditions in the region. By computer processing this electronically obtained information, scientists have learned much about areas not frequently visited or studied.

The conventional earth science study approach has been augmented by multidisciplinary surveys that seek to understand the geoecological integration of geomorphology, vegetation, climate, and land surface processes. Because much of the Himalayan natural environment is inhabited by human populations, these studies tend to be executed with a cultural focus as well. Land-use mapping, agroecological research, natural hazards assessments, and resource management studies complement the field research in the natural and earth sciences. Scientists map and interpret surface processes such as soil erosion, landsliding, and mudflows on a local level. These slope problems are often attributed to human activity in the inhabited regions, but experts remain unconvinced about the degree of the cause-and-effect relationship between human activity and geoecological processes.

Environmental Concerns

The Himalaya comprise the earth's most magnificent mountain system. Their importance is reflected in the desire many people have to visit, climb, and conquer the summits; in the lives of the millions of people who inhabit the Himalaya; and in the powerful role that these mountains play in the geology and climate of one of the world's largest continental landmasses. The study of the Himalayan environment not only stimulates further scientific revelations but also kindles a profound sentiment that people throughout the world have acquired for these mountains.

On a practical level, the Himalayan region experiences distressing levels of environmental degradation, with adverse consequences for the land and for the people who live within and near it. The problems that the Himalaya face with respect to deforestation, water flow, soil erosion, and landslides directly affect the lives of residents and visitors of these mountain regions. The waters contained in the highland drainage system help to ensure agricultural productivity in the densely populated lowlands in Pakistan, India, and Bangladesh. Scientists’ understanding of these highland-lowland interactions is still inadequate to plan properly for the optimal resource usage in the mountains.

While many natural disturbances relate to the fact that these are young and dynamic energy environments, human alterations of the land also contribute in places to surface processes such as soil erosion and landslides. The geoecological linkages between human resource needs and environmental disturbances are important to the people who inhabit these mountains and quite possibly critical to the millions of lowlanders who reside in the lower reaches of the mountain catchments. Only through concise and specific fieldwork by both natural and social scientists can problems be properly identified. The eventual resolution of such problems necessitates scientific understanding and political action on the part of Himalayan countries.

In the late twentieth and early twenty-first centuries, increasing attention was drawn to the effects of climate change on the Himalayas (and the rest of the world), particularly human-induced global warming, or anthropogenic climate change. By 2007, photos compared to identical shots from the 1930s, 50s, and 70s showed significant melting of glaciers in the region, findings backed up into the 2010s by satellite imagery. Data showed steadily rising temperatures year after year, and scientists warned that disappearing glaciers would have major impacts on the water supply and energy security of millions of people in surrounding countries. This trend continued into the 2020s, along with the threat of the negative impact of tourism. Biodiversity is also heavily threatened by the effects of climate change and tourism, including habitat loss and shifting weather patterns.

Principal Terms

continental drift: a hypothesis that attributes the present arrangement of continental shields and ocean floors to the breakup of the original supercontinent, Pangaea

drainage system: a network of stream branches bounded by topographical divides with a common outlet

nappe: an underlying rock sheet overturned forward along a low-angle fault

orogeny: tectonic activity that results in folds and faults of strata; this process is associated with mountain building

transverse valley: a river-cut valley or gorge that runs perpendicular to the main strike direction of a mountain chain

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