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  • The geography of our planet is in flux.

  • Each continent has ricocheted around the globe on one or more tectonic plates, changing quite dramatically with time.

  • Today, we'll focus on North America and how its familiar landscape and features emerged over hundreds of millions of years.

  • Our story begins about 750 million years ago.

  • As the super continent Rodinia becomes unstable, it rifts along what's now the west coast of North America to create the Panthalassa Ocean.

  • You're seeing an ancestral continent called Laurentia, which grows over the next few hundred million years as island chains collide with it and add land mass.

  • We're now at 400 million years ago.

  • Off today's east coast, the massive African plate inches westward, closing the ancient Iapetus Ocean.

  • It finally collides with Laurentia at 250 million years to form another supercontinent Pangea.

  • The immense pressure causes faulting and folding, stacking up rock to form the Appalachian Mountains.

  • Let's fast forward a bit.

  • About 100 million years later, Pangea breaks apart, opening the Southern Atlantic Ocean between the new North American Plate and the African Plate.

  • We forge ahead, and now the eastward-moving Farallon Plate converges with the present-day west coast.

  • The Farallon Plate's greater density makes it sink beneath North America.

  • This is called subduction, and it diffuses water into the magma-filled mantle.

  • That lowers the magma's melting point and makes it rise into the overlying North American plate.

  • From a subterranean chamber, the magma travels upwards and erupts along a chain of volcanos.

  • Magma still deep underground slowly cools, crystallizing to form solid rock, including the granite now found in Yosemite National Park and the Sierra Nevada Mountains.

  • We'll come back to that later.

  • Now, it's 85 million years ago.

  • The Farallon Plate becomes less steep, causing volcanism to stretch eastward and eventually cease.

  • As the Farallon Plate subducts, it compresses North America, thrusting up mountain ranges like the Rockies, which extend over 3,000 miles.

  • Soon after, the Eurasian Plate rifts from North America, opening the North Atlantic Ocean.

  • We'll fast forward again.

  • The Colorado Plateau now uplifts, likely due to a combination of upward mantle flow and a thickened North American Plate.

  • In future millennia, the Colorado River will eventually sculpt the plateau into the epic Grand Canyon.

  • 30 million years ago, the majority of the Farallon Plate sinks into the mantle, leaving behind only small corners still subducting.

  • The Pacific and North American plates converge and a new boundary called the San Andreas Fault forms.

  • Here, North America moves to the south, sliding against the Pacific Plate, which shifts to the north.

  • This plate boundary still exists today, and moves about 30 millimeters per year, capable of causing devastating earthquakes.

  • The San Andreas also pulls apart western North America across a wide rift zone.

  • This extensional region is called the Basin and Range Province, and through uplift and erosion, is responsible for exposing the once deep granite of Yosemite and the Sierra Nevada.

  • Another 15 million years off the clock, and magma from the mantle burns a giant hole into western North America, periodically erupting onto the surface.

  • Today, this hotspot feeds an active supervolcano beneath Yellowstone National Park.

  • It hasn't erupted in the last 174,000 years, but if it did, its sheer force could blanket most of the continent with ash that would blacken the skies and threaten humanity.

  • The Yellowstone supervolcano is just one reminder that the Earth continues to seethe below our feet.

  • Its mobile plates put the planet in a state of constant flux.

  • In another few hundred million years, who knows how the landscape of North America will have changed.

  • As the continent slowly morphs into something unfamiliar, only geological time will tell.

The geography of our planet is in flux.

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B2 中上級

TED-Ed】北米はどのようにして形作られたのか - ピーター・J・ハプロフ (【TED-Ed】How North America got its shape - Peter J. Haproff)

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    Sh, Gang (Aaron) に公開 2021 年 01 月 14 日
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