Magazine article Oceanus

Moving Earth and Heaven: Colliding Continents, the Rise of the Himalayas, and the Birth of the Monsoons

Magazine article Oceanus

Moving Earth and Heaven: Colliding Continents, the Rise of the Himalayas, and the Birth of the Monsoons

Article excerpt

Therefore will not we fear, though the Earth be removed, and though the mountains be carried into the midst of the sea.--Psalm 46

As a geologist, I do not fear the processes that carry Earth and mountain into the sea. I rejoice in them.

The mountains rise, are lashed by wind and weather, and erode. The rivers carry mud and debris from the mountains into the ocean, where they settle onto the relatively tranquil seafloor and are preserved. The sediments bear evidence about where they came from, what happened to them, and when. By analyzing and dating these seafloor sediments, scientists can piece together clues to reconstruct when and how fast their mountain sources rose to great heights millions of years ago, and how the climate and other environmental conditions may have changed in response.

Linking mountains and monsoons

Tens of millions of years ago, a geological process was set in motion that changed the planet. It produced some of the world's most dramatic and extensive mountain ranges. It probably created one of the planet's most intense and important climate phenomena--the Asian monsoons--which today pace and undergird the health and welfare of billions of people in South and East Asia, two-thirds of the total population on the planet. And it may have provoked large-scale environmental changes in the past that brought hominids out of trees and upright onto two feet.

All of these developments in recent Earth history ultimately may be attributed to the land masses now known as India and Arabia, which began moving north some 100 million years ago, on a collision course with what is now Eurasia. According to plate tectonic theory, Earth's crust is composed of interlocking, moving oceanic and continental plates. Scientists consider the collision of the Indian and Eurasian Plates the classic example of how plate tectonics can alter the circulation of the oceans and atmosphere. Here's the hypothetical sequence of events:

The birth of the monsoons

Before the Indian and Eurasian Plates collided, an ancient ocean called the Tethys, lay between Eurasia and Africa. By about 55 million years ago, the continents squeezed out the ocean, and some research suggests that the resulting rearrangement of ocean currents may have provoked the strong global warming that came shortly after.

As India smashed into Asia, the world's tallest mountain ranges were thrust up like the hood of a car in a head-on collision. On the Indian Plate, the Himalaya Mountains were formed, spanning Pakistan, India, Nepal, and Bhutan. The Indian Plate was shoved under the Eurasian Plate, uplifting the Karakoram and Hindu Kush Mountains in Afghanistan and Pakistan, as well as the great Tibetan Plateau--an expanse about 4.5 kilometers high and half the size of the continental United States. The creation of this dramatic continental topography launched a cascade of planetary changes.

The Tibetan Plateau acts like a gigantic exposed brick, absorbing summer heat and heating the atmosphere above it. Hot air rises, and cool, moist air--drawn in from over surrounding oceans--rushes in to replace it. That moist air is the source of monsoon rains.

New evidence suggests that between 22 million and 15 million years ago, the Asian monsoons may have begun to strengthen. The onset of the monsoons may have been triggered when the Tibetan Plateau reached a threshold height of 2 to 3 kilometers (1.2 to 1.8 miles).

Removing C[O.sub.2] from the atmosphere

As the mountains rose upward, the land became more exposed to the forces of weather and gravity. Rainwater contains acids that chemically react with rocks. In the process, called chemical weathering, carbon dioxide is drawn out of the atmosphere and converted into carbonate in rocks. As the monsoons strengthened, chemical weathering increased.

As the mountains rose and monsoon rains increased, rivers also swelled and cut more deeply into the mountains, increasing erosion and carrying more sediments into the oceans. …

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