20、 Obama
B2 High-Intermediate US 1927 Folder Collection
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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.
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B2 High-Intermediate US

【TED-Ed】How North America got its shape - Peter J. Haproff

1927 Folder Collection
Sh, Gang (Aaron) published on July 12, 2016    Jenny translated    Mandy Lin reviewed

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