B2 中上級 21516 タグ追加 保存
Once upon a time,
South America lived harmoniously alongside Africa
until a crack in the Earth
drove the two continents apart.
This breakup began about 200 million years ago
during the separation of the supercontinent
known as Pangaea.
Their proximity back then
explains why the same plant fossils and reptile fossils,
like the Mesosaurus,
can be found on the South American east coast
and African west coast.
However, this evidence does not account forhow the continents moved apart.
For that, we'll need to take a close look
at the earth below our feet.
Though you may not realize it,
the ground below you is traveling across the Earth
at a rate of about 10 cm/year,
or the speed at which your fingernails grow.
This is due to plate tectonics,
or the large-scale movement of Earth's continents.
The motion occurs within the top two layers of the Earth's mantle,
the lithosphere and asthenosphere.
The lithosphere, which includes the crust and uppermost mantle,
comprises the land around you.
Beneath the lithosphere
is the asthenosphere
the highly viscous but solid rock portion of the upper mantle.
It's between 80 and 200 km below the Earth's surface.
While the asthenosphere wraps around the Earth's core as one connected region,
the lithosphere is separated on top into tectonic plates.
There are seven primary tectonic plates
that compose the shape of the planet we know today.
Like the other smaller tectonic plates,
the primary plates are about 100 km thick
and are composed of one or two layers:
continental crust and oceanic crust.
Continental crust forms the continents
and areas of shallow water close to their shores,
whereas oceanic crust forms the ocean basins.
The transition from the granitic continental crust
to the basaltic oceanic crust
occurs beyond the continental shelf,
in which the shore suddenly slopes down
towards the ocean floor.
The South American Plate is an example
of a tectonic plate made of two crusts:
the continent we know from today's map
and a large region of the Atlantic Ocean around it.
Collectively comprising the lithosphere,
these plates are brittler and stiffer
than the heated, malleable layer of the asthenosphere below.
Because of this,
the tectonic plates float on top of this layer,
independently of one another.
The speed and direction in which these tectonic plates move
depends on the temperature and pressure
of the asthenosphere below.
Scientists are still trying to nail downthe driving forces behind this movement,
with some theories pointing towards mantle convection,
while others are examining
the influence of the Earth's rotation
and gravitational pull.
Though the mechanics have not been sorted out,
the scientific community agrees
that our tectonic plates are moving and have been for billions of years.
Because these plates move independently,
a fair amount of pushing and pulling
between the plates occurs.
The first type of interaction
is a divergent boundary,
in which two plates move away from one another.
We see this in the Mid-Atlantic Ridge
between South America and Africa.
The next interaction is when two plates collide,
known as a convergent boundary.
In this instance, the land is pushed upward
to form large mountain ranges,
like the Himalayas.
In fact, the Indian Plate is still colliding
with the Eurasian Plate,
which is why Mount Everest
grows one cm/year.
Finally, there's the transform boundaries,
where two plates scrape past one another.
The grinding of the transform boundary
leads to many earthquakes,
which is what happens
in the 810 mile-long San Andreas Fault.
The moving Earth is unstoppable,
and, while a shift of 10 cm/year may not seem like a lot,
over millions of years our planet will continue
to dramatically change.
Mountains will rise,
shorelines will recede,
islands will pop up.
In fact, one projected map shows
the cities of Los Angeles and San Francisco
on top of each other.
Maybe South America and Africa
will come together again, too.
Only time will tell.



【TED-Ed】パンゲア超大陸の誕生 ― マイケル・モリーナ (The Pangaea Pop-up - Michael Molina)

21516 タグ追加 保存
Kevin Tan 2018 年 1 月 13 日 に公開
  1. 1. クリック一つで単語を検索


  2. 2. リピート機能


  3. 3. ショートカット


  4. 4. 字幕の表示/非表示


  5. 5. 動画をブログ等でシェア


  6. 6. 全画面再生


  1. クイズ付き動画


  1. クリックしてメモを表示

  1. UrbanDictionary 俚語字典整合查詢。一般字典查詢不到你滿意的解譯,不妨使用「俚語字典」,或許會讓你有滿意的答案喔