Clift Final.Indd

http://oceanusmag.whoi.edu/v42n2/clift.html Moving Earth and Heaven Colliding continents, the rise of the Himalayas, and the birth of the monsoons By Peter Clift, Associate Scientist, clues to reconstruct when and how fast out of trees and upright onto two feet. Geology and Geophysics Department, their mountain sources rose to great All of these developments in recent Woods Hole Oceanographic Institution heights millions of years ago, and how Earth history ultimately may be attributed the climate and other environmental con- to the land masses now known as India Therefore will not we fear, though the earth ditions may have changed in response. and Arabia, which began moving north be removed, and though the mountains be carried into the midst of the sea. —Psalm 46 some 100 million years ago, on a collision Linking mountains and monsoons course with what is now Eurasia. Accord- s a geologist, I do not fear the pro- Tens of millions of years ago, a geo- ing to plate tectonic theory, Earth’s crust is Acesses that carry earth and mountain logical process was set in motion that composed of interlocking, moving oceanic into the sea. I rejoice in them. changed the planet. It produced some of and continental plates. Scientists consider The mountains rise, are lashed by the world’s most dramatic and extensive the collision of the Indian and Eurasian wind and weather, and erode. The rivers mountain ranges. It probably created one Plates the classic example of how plate carry mud and debris from the moun- of the planet’s most intense and important tectonics can alter the circulation of the tains into the ocean, where they settle climate phenomena—the Asian mon- oceans and atmosphere. Here’s the hypo- onto the relatively tranquil seafloor and soons—which today pace and undergird thetical sequence of events: are preserved. The sediments bear evi- the health and welfare of billions of people dence about where they came from, what in South and East Asia, two-thirds of the The birth of the monsoons happened to them, and when. By analyz- total population on the planet. And it may Before the Indian and Eurasian Plates ing, measuring, and dating these seafloor have provoked large-scale environmental collided, an ancient ocean, called the Te- sediments, scientists can piece together changes in the past that brought hominids thys Ocean, existed between Eurasia and Peter Clift Peter A view of Pangong Lake in the Ladakh region of northern India, taken at an altitude of 18,000 feet, shows the great expanse of the Tibetan Plateau extending high and flat in the background, as far as the eye can see. 2 Oceanus Magazine • Vol. 42, No.2 • 2004 Woods Hole Oceanographic Institution • oceanusmag.whoi.edu Colliding Continents Tethys Ocean India Gondwana 120 million years ago 60 million years ago Today The Indian subcontinent was part of a The Indian subcontinent, moving toward The India-Asia collision has closed supercontinent called Gondwana. The Asia at a speed of 10 centimeters per year, the ancient Tethys Ocean, created the ancient Tethys Ocean existed between heads toward a collision about 50 million Himalayan, Karakorum, and Hindu Kush the South American/African and Eurasian years ago. mountain ranges, and uplifted the great supercontinents. Tibetan Plateau. Africa. By about 55 million years ago, the the size of the continental United States. Removing CO2 from the atmosphere continents squeezed out the ocean, and The creation of this dramatic continental As the mountains rose upward, the some research suggests that the resulting topography launched a cascade of plan- land became more exposed to the forces of rearrangement of ocean currents may have etary changes. weather and gravity. Rainwater contains ac- provoked the strong global warming that The Tibetan Plateau acts like a gigan- ids that chemically react with rocks. In the came shortly after. tic exposed brick, absorbing summer heat process, called chemical weathering, car- As India smashed into Asia, the world’s and heating the atmosphere above it. Hot bon dioxide is drawn out of the atmosphere tallest mountain ranges were thrust up air rises, and cool, moist air—drawn in and converted into carbonate in rocks. like the hood of a car in a head-on colli- from over surrounding oceans—rushes in As the monsoons strengthened, chemical sion. On the Indian Plate, the Himalaya to replace it. That moist air is the source of weathering increased correspondingly. Mountains were formed, spanning Paki- monsoon rains. As the monsoon rains increased and stan, India, Nepal, and Bhutan. The Indian New evidence suggests that between 22 the mountains rose, rivers also swelled Plate was shoved under the Eurasian Plate, and 15 million years ago, the Asian mon- and cut more deeply into the mountains, uplifting the Karakorum and Hindu Kush soons may have begun to strengthen. The increasing erosion and carrying more sed- Mountains in Afghanistan and Pakistan, onset of the monsoons may have been trig- iments into the oceans. To give a sense of as well as the great Tibetan Plateau—an gered when the Tibetan Plateau reached a scale, the Indus River today�� deposits about expanse about 4.5 kilometers high and half threshold height of 2 to 3 kilometers. 1,000 million tons of mud and sand each �� The Rise of the Himalayas and the Tibetan Plateau �� In the India-Asia collision, the Eurasian Plate was compressed and thickened to uplift the Tibetan Plateau. The bulk of the Indian Plate continues to be thrust under�� the Eurasian Plate, further uplifting Tibet. Slices of the Indian Plate were scraped off to form the Himalayas. �� Doucette Jayne 3 Oceanus Magazine • Vol. 42, No.2 • 2004 ��Woods Hole Oceanographic Institution • oceanusmag.whoi.edu How to Make a Monsoon 3 Cool moist air is drawn in from over sur- 2 Hot air rises high rounding oceans to replace the rising hot air. above the Tibetan The moist air is the source of monsoon rains. Plateau. 4 Monsoon rains en- 1 The Tibetan Plateau gorge rivers fl owing acts like a gigantic from high mountains, exposed brick, which carry tons of absorbing summer sediments to the heat and heating ocean. the atmosphere above it. Jayne Doucette Jayne year onto the Indus Submarine Fan in the shows an abundance of preserved micro- About 8 million years ago, paleonto- Arabian Sea. Relieved of such massive scopic shells of plankton (and, by infer- logical evidence shows that the great apes sedimentary weight, the mountains could ence, a strengthening of the monsoons) became extinct in Europe and Asia—vic- be thrust up higher, in a reinforcing cycle beginning about 8.5 million years ago in tims of a colder, drier climate. They main- that continued to increase monsoons, ero- the Arabian Sea, though the situation in tained populations only in Southeast Asia sion, and uplift. other parts of Asia is presently less clear. and Africa. About 7 to 8 million years ago, humans Evolving climates Evolving humans began to diverge from great apes in Africa, Climatically, research suggests that the Curiously, that same time period marks which, though still equatorially warm, be- increasing rates of weathering and erosion crucial events in human evolutionary his- gan to dry out. Jungles and forests turned of the mountains converted large volumes tory. The cumulative effects of decreasing into grasslands and deserts. The climate of carbon dioxide from the atmosphere atmospheric carbon dioxide reduced the shift provided evolutionary advantages into carbonate sediments that eventu- global greenhouse effect, creating a much for bipedal hominids with larger brains to ally were deposited on the ocean bottom. colder and drier Earth. cope with environmental changes. As carbon dioxide was drawn out of the atmosphere, the global greenhouse effect was reduced, setting the stage for long- term cooling of the planet that culminated in the ice ages of the last 2.7 million years. In addition, an influx of chemical nu- trients into the ocean may have sparked blooms of phytoplankton. Microscopic ma- rine plants also extract atmospheric carbon dioxide via photosynthesis and convert it into carbonate organic matter that settles to the seafloor when the plankton die. As monsoon winds strengthened, they blew waters laterally across the ocean surface. To replace these waters, cooler, nutrient-rich waters upwelled from the depths to the sunlit surface, providing all NASA the ingredients plankton need to thrive. An aerial view of the Ganges River Delta shows tons of sediments being poured into the Evidence from seafloor sediments cores northern Bay of Bengal off India. 4 Oceanus Magazine • Vol. 42, No.2 • 2004 Woods Hole Oceanographic Institution • oceanusmag.whoi.edu �� The modern Indus River system drains sediments from the high peaks of the Karakoram, Hindu Kush, and western Ti- �� bet. It has created the 10-kilometer-thick �� Indus Fan, which extends 1,000 to 1,500 �� kilometers into the Arabian Sea. Located between the land masses of � �� Arabia and the Indian subcontinent, the �� Arabian Sea is ideally placed to record the �� effects of India’s collision with the Asian mainland. New data now suggest that the Indus River and Fan system was initiated �� shortly after India-Asia collision about 55 million years ago, probably in response to the initial uplift of Tibet. This long history makes it a natural storehouse of informa- tion on how the mountains developed over long time periods. �� An archive buried on the seafloor Studying this rich store of deep-sea sediments, I have been able to estimate that erosion increased around 16 million years ago, somewhat earlier than the start of the monsoon pattern. That pulse of Great rivers (the Ganges, Brahmaputra, and Indus) transport large volumes of sediments from erosion appears to have been the result of great mountains (the Himalayas, the Hindu Kush and the Karakoram) into the ocean.

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