Himalayas, Geology of
Himalayas, Geology of
The Himalayan Mountains are 1,550 mi (2,500 km) long from west to east, encompassing all of Nepal and Bhutan and parts of Afghanistan, India, Pakistan, and China. The north-south width varies from 125-250 mi (200-400 km), and the range cover 229,500 sq mi (594,400 sq km) of Earth’s area. In height, the range rises to the top of Everest at 29,028 ft (8,848 m) and much of the area is at an elevation of 2.5 mi (4 km) above sea level. The Himalayas are part of a band of mountain ranges that cross the globe from North Africa to the Pacific coast of Asia. They are bordered by the Karakoram and Hindu Kush ranges as well as the Tibetan Plateau.
The Himalayas consist of four distinct ranges: the northernmost Trans-Himalayas, the Greater or Tibetan Himalayas, the Lesser or Lower Himalayas, and the southernmost Outer Himalayas. The four ranges parallel each other in long belts from west to east, but each has a different geologic history.
Before the Jurassic period (180 million years ago), India, South America, Africa, Australia, and Antarctica were united as one supercontinent called Gondwanaland or Gondwana. In Jurassic times, this supercontinent began to break into fragments that moved away from each other. India began to move northward toward Eurasia, but between the Eurasian Plate and the Indian Plate, was the Tethys Trench containing a deep ocean. The Indian Plate moved to the north over the course of 130 million years. In the Tertiary period (50 million years ago), it finally collided with Eurasia. Collisions like this between continents typically take millions of years and involve volcanic eruptions, earthquakes, metamorphism of rocks as the result of intense pressure, and episodes of mountain-building. Geologists have been able to use the metamorphic rocks in the Himalayas to date these events by measuring radioactivity remaining in the rocks.
As India pushed toward Eurasia, sedimentary rocks deposited the Tethys Trench were compressed, folded, and faulted. Zones of weaknesses in the sedimentary rocks allowed basalt and granite to intrude upward from Earth’s mantle. When the Indian plate encountered the Tethys trench, the plate was subducted into the trench and uplift of adjacent rocks created the Tibetan Plateau. The Trans-Himalayan Range formed the southern edge of the Plateau. As the mountains rose, new rivers were created, and their drainage changed the climate and the downslope topography. At this point in the development of the Himalayas, the mountains were impressive but had not reached the monumental elevations we know now.
From about 50-23 million years ago, the subduction began to slow (rocks of the Indian plate were too buoyant to be drawn down into the mantle), and the plate corner collided with Asia and began to slide under Asia. During the Miocene epoch (23 million years ago), the compression of the plates intensified and continued into the Pliocene epoch (1.6 million years ago). As the Indian plate slid under the Asian plate, its upper layers were stripped off and thrust back onto the subcontinent. These layers, called nappes, were older metamorphic rock from Gondwana. As the mountains rose, the rivers steepened, the runoff and erosion increased, deposition of sediments similarly increased, and the weight of the sediments forced receiving basins downward so they could hold still more alluvium.
The creation of the nappes left a core zone. In the middle of the Pleistocene epoch about 600,000 years ago, the most significant stage of mountain building began. As Mount Everest itself was uplifted, older sedimentary rocks, including marine limestone from the Tethys Basin, were uplifted and now occupy the summit. These uplift episodes again changed the climate and blocked rains from moving to the north. The mountains on the north (the Trans-Himalayas) and the Tibetan Plateau became deserts. Heavy rains to the south changed the line of the crest and shifted the direction of rivers to create a high midland region between the Greater Himalayas and the Lesser Himalayas to the south. High valleys filled with sediment to form lush valleys like the Kathmandu Valley. The Outer Himalayas including the Siwalik Hills form the southern line of the Himalayas, and the Gangetic Plain (draining toward the Ganges River) lies below it for the full extent of the Indian subcontinent and Bangladesh.
The northeastern end of the Gangetic Plain has experienced four great earthquakes with a Richter magnitude exceeding 8.0 in the past 100 years, beginning with the Assam earthquake in 1897. Over 30,000 people perished in these quakes. The origin of the earthquakes is the same tectonic, plate-moving action that welded the Indian subcontinent to Eurasia and formed the Himalayan Mountain Range as a kind of massive suture. Along the line where the outer Himalayas border the Gangetic Plain, seismic gaps store the strain from tectonic movement. One of these, called the Central Seismic Gap, has not released its strain in the form of an earthquake in an estimated 745 years (since a great earthquake that may have killed the king of Nepal in 1255). The Central Seismic Gap is about 500 mi (800 km) long and lies between the regions struck by great earthquakes in 1905 and 1934. The Muzaffarabad earthquake of October 8, 2005, with a magnitude of 7.6 and an epicenter in eastern Pakistan, killed some 75,000 people in the Kashmir regions of Pakistan and India.
The faults along which earthquakes occur were created by the pressure of the Indian subcontinent. After the period when it was subducted under the Eurasian plate, the plate’s direction changed and it pushed toward Tibet, compressing the edge of Tibet and creating faults. Geologists know plate movement is continuing from the earthquake activity but also from the continuing formation of hills along the southern limits of the Himalayas. The longest fault, called the Main Detachment fault, is as long as the Himalayan Range from west to east. If the fault does
Alluvium —Particles of soil and rock that are moved as sediments by flowing water.
Gondwanaland —An ancestral supercontinent that broke into the present continents of Africa, South America, Antarctica, and Australia as well as the Indian subcontinent.
Metamorphism —The process of changing existing rock by increased temperature or pressure without melting.
Nappes —Folds of rock strata that become flat lying.
Plate tectonics —The motion of large sections of Earth’s crust.
Seismic gap —A length of a fault, known to be historically active, that has not experienced an earthquake recently and may be storing strains that will be released as earthquake energy.
Seismology —The study of earthquakes.
Subduction —In plate tectonics, the movement of one plate down into the mantle where the rock melts and becomes magma source material for new rock.
Trilobites —Extinct crustaceans that lived from the Cambrian through the Permian Periods. Their extensive fossil evidence helps geologists and paleontologists understand early rocks and early life on Earth.
rupture, it is most likely to occur where the greatest strain has accumulated at the Central Seismic Gap.
In analyzing geologic processes and earthquake hazards, geologists have used technology to measure movements in areas that are remote, frigid, and nearly inaccessible. By routinely using global positioning system (GPS) data to survey a line of reference points, scientists are understanding geophysics, geomechanics, and the convergence of continents. They have found that India is moving to the northeast toward Asia at a velocity of about 2.5 in (6 cm) per year. Studies of paleontology (fossils) in the Himalayas have contributed to the current understanding of the range’s geologic and seismic history. Comparison of fossilized trilobites (ancient crustaceans) found in different locations in the Himalayas and the places where they were known to have lived helps superimpose the geologic timetable on the components of the comparatively young Himalayas.
Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, New Jersey: Prentice Hall, 2004.
Gillian S. Holmes