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Earth...
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the only planet orbiting the sun
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that has continents,
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oceans,
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plate tectonics,
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and the sole planet with an atmosphere
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that has sustained life
for billions of years.
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How did Earth get this way?
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What's behind the power, the violence
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that cracks solid rock in an instant,
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that shatters buildings
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and sends tsunamis racing for the shore,
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that erupts seas of lava
and billowing clouds of ash?
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Earth has been making headlines lately.
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Powerful images of destruction
never seen in our lifetime.
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People are asking,
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"has Earth entered a period
of intense geologic upheaval?"
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Scientists have new answers
to questions about our home,
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the third planet from the sun.
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It's a story that, just a few decades ago,
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could not have been told.
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About 10 billion years
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after the big bang created the universe,
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our solar system began to form.
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Scientists calculate that this happened
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4.567 billion years ago,
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and that was when planet Earth was created.
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A long time ago,
about 4 1/2 billion years ago,
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the Earth formed
from this nebula around the sun.
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The sun in the center,
and the nebula swirling around,
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and the bits of the nebula accreting,
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that is, sticking together
and colliding with each other,
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and it was a very turbulent time.
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And the Earth gradually came together,
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and as it became more massive,
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the gravitational force became larger,
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and it could attract
more of the small pieces,
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and the Earth grew and grew.
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It took about 100 million years
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for Earth to reach its present size,
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but before it did,
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our planet experienced its
largest, most dramatic event.
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So after the early Earth formed,
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probably tens of millions
of years afterwards,
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you had a Mars-sized object
that actually hit the Earth.
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The giant planetoid
impactor was called theia.
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The Mars-sized impactor hit the Earth,
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and it spewed off a lot
of the material from the Earth,
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and part of that formed the small...
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The moon that we now have.
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And also we generated a lot
of heat from this impact,
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and so it might have remelted
a lot of the early Earth.
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Scientists only recently learned
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of this massive impact
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by studying rocks collected on the moon.
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It turned out that
the moon rocks were very similar
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in composition to Earth rocks.
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After the impact,
the moon became a satellite,
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while the Earth was left
a molten ball of fire.
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And so the early Earth
was, like, this molten mass.
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That must have looked
liked maybe one giant volcano
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or lava lake.
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It slowly solidified
and formed this outer crust
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that is where we stand today.
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As the crust formed,
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collisions and bombardments died down,
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but the meteorites
continue to fall to this day.
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We start with some breaking news this hour.
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People in the Urals in Russia
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have seen burning objects
raining down from the sky
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after a meteorite exploded above the Earth,
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causing a meteor shower.
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Eyewitnesses reported several blasts.
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Despite the shock
that a meteor shower can cause,
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meteorites are
extremely valuable to scientists
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who study when the Earth was created.
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They are collected and studied worldwide.
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But the best place
to find these rocky aliens
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is one of the Earth's
most unearthly places:
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Antarctica.
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On the sleek, blue ice expanse
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of the central antarctic ice sheet,
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the surface is swept clean
by relentless winds.
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A meteorite sits exposed
for an average of 20,000 years,
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sometimes for several million years,
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before scientists might spot it
and collect it.
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A type of meteorite called a chondrite
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contains the same basic elements
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that make up the sun and planets,
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so they tell US a great deal
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about where we originally came from
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and when.
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To figure out how old
meteorites and the Earth
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actually are,
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scientists use high-precision instruments
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to measure their radioactive properties.
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But why must scientists rely on meteorites
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to figure out how old the Earth really is?
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Why don't they simply study
the oldest rocks
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that were formed on the Earth itself?
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From the Earth's formation
around 4.6 billion years ago
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up to about 3.8 billion years ago,
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is an era called the hadean,
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when the Earth was undergoing
heavy bombardment,
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and there's virtually no rock record
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from that period of time at all
on the Earth remaining today.
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The hadean era,
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Greek for underworld, or hell.
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Nothing survived this inferno...
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Or did it?
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It was a missing link hundreds
of millions of years long
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between Earth's creation
and its oldest rocks,
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like these 3.6 billion-year-old
rocks found in Australia.
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Beautiful.
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But breakthroughs in analyzing these rocks
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finally opened a window into the hadean.
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Interestingly, we do have
some materials from that time.
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They are small zircon grains,
4.4 billion or so of age.
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Zircons are the oldest terrestrial material
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ever found on Earth.
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But rocks containing zircons
that are billions of years old
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have only been discovered
in a handful of places on Earth,
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like Antarctica.
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The rest have been obliterated.
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Studying those rare vestiges
of the early days of Earth,
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scientists were astonished to find evidence
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that very early on,
Earth developed a characteristic
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that it and no other planet
in the solar system possesses:
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Plate tectonics.
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Plate tectonics has been around on Earth
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from the early days,
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at least 2 1/2, 3 billion years ago.
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So more than 3/4 of the life of the planet.
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Mars nearly developed plate tectonics,
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but failed.
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Mars is smaller than Earth.
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It cooled faster.
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It may well have had a style
of plate tectonics
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early in its history,
but it clearly does not today.
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Venus, considered Earth's sister planet,
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is also missing Earth's trait
of plate tectonics.
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Venus doesn't have plate tectonics
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in the style that we recognize
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with very clear, linear
boundaries between rigid plates,
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perhaps because there's no water
on the surface of Venus.
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It is the striking and
distinguishing characteristic
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of Earth's surface,
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and scientists are finding
that this substance is the key
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that started Earth's early crust
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into constant, ever-changing motion.
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One of the reasons we have plate tectonics,
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which we don't see on any other planets,
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is that we have
a lot of water on the Earth.
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So we have water not only in the oceans,
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but there's approximately
an ocean's worth of water
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locked up in the rocks in, say,
the upper 100 kilometers
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of the Earth.
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When you put water into a rock,
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it makes the rock weaker
by maybe a factor of 100
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than it would be without the water.
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Weaker rocks make for plate tectonics
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because you have to have plate boundaries
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where rocks either slide by each other,
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or one goes down underneath the other.
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And so unless you have rocks
that are weak enough
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to be able to slide past each other,
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you're not gonna have plate tectonics.
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But where did Earth's vast amount
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of liquid water come from?
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In the early history of the Earth,
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when there was a lot of volcanism going on,
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volcanoes actually brought gases up
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that created the early atmosphere.
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And magma has water in the form of vapor
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that it can bring up.
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Icy meteorites and comets
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were the other likely source
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that infused Earth's crust with water
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and got it moving.
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And once plate tectonics started,
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it never stopped.
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It has influenced everything
that has happened on the surface
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including life.
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The oxygen, the minerals,
everything that we get
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comes from the interior of the Earth.
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We all have internally within US
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a whole suite of minerals
and things that...
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We're from the Earth.
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Essentially, that's where we...
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Our chemical makeup is from the Earth.
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And if you had no way of getting
that outside of the Earth,
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then essentially you'd have a dead planet.
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Plate tectonics became recognized
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as being a crucial ingredient for life,
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as is water.
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This was an important piece of knowledge
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for scientists who were studying
Earth's ancient rocks.
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If the conditions for life were present
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when Earth's ancient rocks
formed billions of years ago,
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then wasn't it possible
that life also took hold
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in the early dawn of Earth's formation?
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The search was on.
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Some scientists zeroed in on
the 3.5 billion-year-old rocks
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in the outback of western Australia.
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Amazingly, it turns out
that fairly soon after,
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people started discovering
some potential signs of life
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in those rocks,
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and one of the major discoveries
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was structures called stromatolites.
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Stromatolites are peculiar rocky structures
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built by colonies of cyanobacteria,
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or blue-green algae.
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They are extremely rare on Earth today,
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growing in only a few places,
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like laguna Bacalar in Mexico.
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But they were the first
life forms to appear on Earth,
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and they gave rise
to all other forms of life,
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including people.
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It took awhile, though.
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Stromatolites dominate
the first 3 billion years
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of the history of life on Earth.
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So from 3 1/2 billion years ago
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to up to about 1/2 a billion years ago,
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the fossil record consists
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almost entirely of stromatolites.
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Stromatolites lived in oceans and lakes.
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For that period, no fossils have been found
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of creatures that lived on land.
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Does that mean there were no continents
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for the first 3 billion years
of Earth's history?
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00:13:03,070 --> 00:13:04,470
Once again,
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Earth's ancient rocks
yielded clues to this mystery.
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00:13:09,410 --> 00:13:11,530
We don't know for sure
when plate tectonics started,
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00:13:11,580 --> 00:13:16,680
but we do know for sure
when we first had continents.
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We have intact areas
that we can go and map in
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that go back as far as 4 billion years.
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That's nearly 90% of the age of the Earth,
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so we're quite sure about the fact
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that we had continents back that far.
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No one knows what the continents were like
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4 billion years ago
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or why we ended up with seven continents
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that ride around
on a dozen large crustal plates
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and many microplates in between.
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But the question that plagued
scientists for years was this:
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What was the prime force
that made the plates move?
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There were two contending forces.
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00:14:05,100 --> 00:14:09,200
The first was discovered aboard
seafaring research vessels
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00:14:09,270 --> 00:14:13,540
mapping the sea floor decades ago.
250
00:14:13,610 --> 00:14:15,510
Scientists learned that the sea floor
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was spreading apart
in the middle of the oceans
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at submarine mountain ranges
called mid-ocean ridges.
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00:14:22,750 --> 00:14:25,020
Submersibles were sent down
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and came back with footage
of hot gases welling up
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in the rift zones.
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00:14:30,960 --> 00:14:33,230
Magma was rising from below here,
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00:14:33,290 --> 00:14:35,230
and it was believed that it was pushing
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00:14:35,300 --> 00:14:39,370
the oceanic plates apart.
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00:14:39,430 --> 00:14:43,900
The key to plate tectonics
was at last discovered...
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00:14:43,970 --> 00:14:47,370
Or was it?
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Seismic experiments
of Earth's deep crust and mantle
262
00:14:51,180 --> 00:14:53,910
changed scientists' thinking.
263
00:14:53,980 --> 00:14:56,680
A second force was discovered
264
00:14:56,750 --> 00:14:59,020
where plates collided head-on,
265
00:14:59,090 --> 00:15:02,720
as they do under
the Andes mountains of Ecuador.
266
00:15:02,790 --> 00:15:06,730
One plate was imaged diving
underneath the other.
267
00:15:06,790 --> 00:15:10,330
The was called a subduction zone.
268
00:15:10,400 --> 00:15:13,000
The sinking slab appeared to be pulling
269
00:15:13,070 --> 00:15:14,970
the rest of the plate behind it,
270
00:15:15,040 --> 00:15:18,400
making the entire plate move.
271
00:15:18,470 --> 00:15:22,040
A great debate arose over
which force was more important
272
00:15:22,110 --> 00:15:25,010
in driving plate tectonics:
273
00:15:25,080 --> 00:15:30,920
Pushing from the rising magma
or pulling in subduction zones.
274
00:15:30,990 --> 00:15:34,920
There now appears to be a winner.
275
00:15:34,990 --> 00:15:38,260
When people first recognized
mid-ocean ridge spreading
276
00:15:38,330 --> 00:15:40,860
as the key to understanding
plate tectonics,
277
00:15:40,930 --> 00:15:45,300
the view was that the magma, the mantle,
278
00:15:45,370 --> 00:15:49,370
rose up in the center
and forced the plates apart.
279
00:15:49,440 --> 00:15:51,740
But that view has changed over the years
280
00:15:51,810 --> 00:15:55,440
and has largely been replaced
by the view that it's the pull
281
00:15:55,510 --> 00:15:58,310
of the subduction slab that matters most.
282
00:15:58,380 --> 00:16:01,950
Bob, can you please hand me
the pvc pipe, please?
283
00:16:02,020 --> 00:16:04,350
Sure.
284
00:16:04,420 --> 00:16:07,190
So scientists looked harder at subduction,
285
00:16:07,260 --> 00:16:13,260
and as they did, more geologic
wonders were uncovered.
286
00:16:13,330 --> 00:16:17,100
For one, seismic studies
of subduction zones
287
00:16:17,170 --> 00:16:19,670
revealed that oceanic plates are very thin
288
00:16:19,730 --> 00:16:22,330
compared to continents.
289
00:16:22,400 --> 00:16:25,910
The sea floor is 3 to 6 miles thick,
290
00:16:25,970 --> 00:16:28,810
while continents average 30 miles thick
291
00:16:28,880 --> 00:16:33,610
and swell up to 50 miles
under great mountain belts.
292
00:16:33,680 --> 00:16:35,680
This has a major influence
293
00:16:35,750 --> 00:16:38,250
on how the plates affect each other,
294
00:16:38,320 --> 00:16:41,390
especially in subduction zones.
295
00:16:41,460 --> 00:16:44,390
Where subducting plates meet continents,
296
00:16:44,460 --> 00:16:47,230
Earth's tallest mountains are built:
297
00:16:47,300 --> 00:16:50,360
The Himalaya of Asia,
298
00:16:50,430 --> 00:16:52,730
the Alps of Europe,
299
00:16:52,800 --> 00:16:55,430
the Andes of South America.
300
00:16:55,500 --> 00:16:58,100
Subduction created them all.
301
00:17:00,240 --> 00:17:05,010
But subduction can create havoc
on the surface.
302
00:17:05,080 --> 00:17:08,080
As seismic networks
improved around the world,
303
00:17:08,150 --> 00:17:11,050
it became apparent
that subduction was the culprit
304
00:17:11,120 --> 00:17:14,350
behind Earth's most powerful earthquakes.
305
00:17:16,820 --> 00:17:20,430
First the earthquake itself...
306
00:17:22,400 --> 00:17:25,760
A grocery store in the capital
at the moment of impact.
307
00:17:25,830 --> 00:17:28,300
The tohoku earthquake of 2011
308
00:17:28,370 --> 00:17:30,870
was the most powerful ever known in Japan
309
00:17:30,940 --> 00:17:34,510
at magnitude 9.
310
00:17:34,580 --> 00:17:36,880
It struck where the oceanic pacific plate
311
00:17:36,940 --> 00:17:40,580
subducts under a piece
of the north American plate.
312
00:17:43,020 --> 00:17:45,050
Where subduction is occurring,
313
00:17:45,120 --> 00:17:49,990
you see a very well-defined zone
of earthquakes,
314
00:17:50,060 --> 00:17:53,860
essentially between
the downgoing oceanic crust
315
00:17:53,930 --> 00:17:58,160
and the continental crust,
316
00:17:58,230 --> 00:18:00,470
and it forms a line of earthquakes
317
00:18:00,540 --> 00:18:04,040
where these two crusts
are in contact with each other
318
00:18:04,110 --> 00:18:08,940
and slipping past each other.
319
00:18:12,310 --> 00:18:14,150
Subduction quakes can also generate
320
00:18:14,220 --> 00:18:15,980
devastating tsunamis
321
00:18:16,050 --> 00:18:18,520
because of their proximity to coastlines.
322
00:18:18,590 --> 00:18:20,820
One of the worst earthquakes ever...
323
00:18:20,890 --> 00:18:24,490
The Japanese quake and
the 2004 Indian ocean earthquake
324
00:18:24,560 --> 00:18:27,530
off sumatra are striking examples.
325
00:18:35,400 --> 00:18:38,140
Earthquakes and tsunamis
are not the only action
326
00:18:38,210 --> 00:18:40,870
spawned by subduction.
327
00:18:40,940 --> 00:18:43,640
Subduction also creates volcanoes,
328
00:18:43,710 --> 00:18:48,410
and they, too, can be large and dangerous.
329
00:18:48,480 --> 00:18:52,220
Volcanoes in the subduction
zones are formed
330
00:18:52,290 --> 00:18:57,920
by magmas that are more silicic,
or more viscous,
331
00:18:57,990 --> 00:19:01,690
and they are much more explosive.
332
00:19:01,760 --> 00:19:05,160
So those are the volcanoes
that can really be dangerous
333
00:19:05,230 --> 00:19:07,230
and really create trouble.
334
00:19:11,710 --> 00:19:15,910
Subduction volcanoes
are Earth's most notorious:
335
00:19:15,980 --> 00:19:19,510
Mount Tungurahua in Ecuador,
336
00:19:19,580 --> 00:19:23,020
mount Vesuvius in Italy,
337
00:19:23,080 --> 00:19:25,820
mount Asama in Japan,
338
00:19:25,890 --> 00:19:28,720
mount Saint Helens in the u.S.
339
00:19:28,790 --> 00:19:32,630
All are known for explosive,
destructive eruptions.
340
00:19:34,130 --> 00:19:37,060
They also grow to great heights.
341
00:19:37,130 --> 00:19:38,960
This is chimborazo,
342
00:19:39,030 --> 00:19:42,200
rising 20,700 feet above sea level
343
00:19:42,270 --> 00:19:45,970
in the Andes mountains of Ecuador.
344
00:19:46,040 --> 00:19:48,140
When measured from the center
of the Earth's core
345
00:19:48,210 --> 00:19:50,010
to the volcano's summit,
346
00:19:50,080 --> 00:19:53,080
chimborazo is Earth's tallest mountain,
347
00:19:53,150 --> 00:19:57,380
soaring over 7,000 feet closer
to the sun than mount Everest.
348
00:19:59,150 --> 00:20:02,250
That's because the volcano sits
on the equator
349
00:20:02,320 --> 00:20:05,590
where the Earth bulges outward
from the force of rotation.
350
00:20:10,970 --> 00:20:13,170
Earth scientists have gotten a good handle
351
00:20:13,230 --> 00:20:15,270
on the forces driving plate tectonics,
352
00:20:15,340 --> 00:20:17,800
like subduction,
353
00:20:17,870 --> 00:20:20,470
but questions remain.
354
00:20:20,540 --> 00:20:24,540
One of the biggest questions
is how plate tectonics is linked
355
00:20:24,610 --> 00:20:27,980
to processes operating
deep below Earth's crust.
356
00:20:30,450 --> 00:20:34,090
The deeper we look, the less we know.
357
00:20:34,160 --> 00:20:36,990
But new research
is turning up big surprises
358
00:20:37,060 --> 00:20:39,860
lurking in the center of the Earth.
359
00:20:39,930 --> 00:20:41,960
Tumble into cascading salt beds,
360
00:20:42,030 --> 00:20:45,360
escape from hissing steam caves,
361
00:20:45,430 --> 00:20:49,030
behold the staggering underground ocean.
362
00:20:49,100 --> 00:20:52,140
You will encounter breathtaking
dangers beyond belief
363
00:20:52,210 --> 00:20:54,770
here at the center of the Earth.
364
00:20:56,740 --> 00:20:58,580
The inside of Earth,
365
00:20:58,650 --> 00:21:01,080
an object of great curiosity,
366
00:21:01,150 --> 00:21:03,920
wild conjecture.
367
00:21:03,980 --> 00:21:06,690
At the center of it all
is the Earth's core,
368
00:21:06,750 --> 00:21:12,490
which was discovered
accidentally only 140 years ago.
369
00:21:12,560 --> 00:21:15,430
A huge earthquake in the 1880s was...
370
00:21:15,500 --> 00:21:17,100
The waves from that earthquake,
371
00:21:17,160 --> 00:21:18,660
traveling through the Earth's core,
372
00:21:18,730 --> 00:21:21,000
bouncing off the Earth's core,
373
00:21:21,070 --> 00:21:23,900
were recognized as arriving
at different times
374
00:21:23,970 --> 00:21:25,500
from the seismic waves
375
00:21:25,570 --> 00:21:27,710
that traveled through
the outer layers of the Earth.
376
00:21:27,780 --> 00:21:30,580
And with that,
the Earth's core was discovered.
377
00:21:30,640 --> 00:21:33,150
It took until the 1920s to recognize
378
00:21:33,210 --> 00:21:35,710
that the Earth had a solid inner core
379
00:21:35,780 --> 00:21:39,250
inside the fluid outer core.
380
00:21:39,320 --> 00:21:41,690
At half the size of the planet,
381
00:21:41,760 --> 00:21:47,330
the Earth's core is peculiar
in that it's all made of iron.
382
00:21:47,400 --> 00:21:52,300
But its inner half is solid,
and its outer half is liquid.
383
00:21:52,370 --> 00:21:54,800
The Earth's outer core is molten
384
00:21:54,870 --> 00:21:57,040
because it's very high
temperature, and it's...
385
00:21:57,100 --> 00:21:58,820
The temperatures are
above the melting temperature
386
00:21:58,840 --> 00:22:00,310
of this iron-rich material,
387
00:22:00,370 --> 00:22:02,540
but the Earth's inner core
is actually... can be solid
388
00:22:02,610 --> 00:22:05,380
because we have even higher pressures,
389
00:22:05,450 --> 00:22:07,880
so get to over 3 million atmospheres,
390
00:22:07,950 --> 00:22:11,020
so 3 million times the pressure
at the surface of the planet.
391
00:22:11,090 --> 00:22:12,580
And at those high pressures,
392
00:22:12,650 --> 00:22:14,890
they cause the material
to actually solidify.
393
00:22:18,430 --> 00:22:21,360
Fortunately for US and all living things,
394
00:22:21,430 --> 00:22:23,830
the Earth's outer core is fluid,
395
00:22:23,900 --> 00:22:28,730
for it generates our planet's
protective magnetic field.
396
00:22:28,800 --> 00:22:31,270
If we didn't have the Earth's
magnetic field today,
397
00:22:31,340 --> 00:22:34,910
we probably couldn't have life
on the surface of the planet.
398
00:22:34,980 --> 00:22:37,780
The sun's always throwing all
these particles at the planet,
399
00:22:37,850 --> 00:22:39,340
but then the Earth's magnetic field
400
00:22:39,410 --> 00:22:41,310
protects this nice, protective coating
401
00:22:41,380 --> 00:22:43,880
that protects US from these damaging rays
402
00:22:43,950 --> 00:22:45,880
and particle... High-energy particles.
403
00:22:48,390 --> 00:22:50,060
The Earth is unique among planets
404
00:22:50,120 --> 00:22:52,860
in having a magnetic field.
405
00:22:52,930 --> 00:22:54,890
Mars had one early on,
406
00:22:54,960 --> 00:22:56,900
but its small core cooled quickly,
407
00:22:56,960 --> 00:22:59,900
and it was lost and, with it,
408
00:22:59,970 --> 00:23:03,470
the chance for life
to take hold on its surface.
409
00:23:03,540 --> 00:23:07,570
Meanwhile, Earth's magnetic
field, for all its importance,
410
00:23:07,640 --> 00:23:11,280
is not well understood.
411
00:23:11,350 --> 00:23:14,350
What is getting clearer
is what is actually going on
412
00:23:14,420 --> 00:23:16,450
in the Earth's core itself.
413
00:23:16,520 --> 00:23:18,350
Okay. Let's put it on there.
414
00:23:18,420 --> 00:23:19,650
That is because
415
00:23:19,720 --> 00:23:21,150
scientists have finally re-created
416
00:23:21,220 --> 00:23:24,690
the core's extreme conditions in the lab.
417
00:23:24,760 --> 00:23:28,760
3 million atmospheres
and 10,000 degrees fahrenheit,
418
00:23:28,830 --> 00:23:31,960
the same temperature
as the surface of the sun.
419
00:23:35,140 --> 00:23:37,270
So you have to reach
millions of atmospheres,
420
00:23:37,340 --> 00:23:39,020
and you have to reach thousands of degrees,
421
00:23:39,040 --> 00:23:42,040
so that's a very challenging
technological problem.
422
00:23:42,110 --> 00:23:43,040
Okay, great.
423
00:23:43,110 --> 00:23:45,940
Looks good.
424
00:23:46,010 --> 00:23:48,310
So one way to reach the very high pressures
425
00:23:48,380 --> 00:23:51,050
and temperatures in the planet
is to compress materials
426
00:23:51,120 --> 00:23:52,920
between the tips of two diamonds,
427
00:23:52,990 --> 00:23:56,220
and since diamond is the hardest
material on the planet,
428
00:23:56,290 --> 00:24:00,190
we can generate very high pressures.
429
00:24:00,260 --> 00:24:01,590
And then, at the same time,
430
00:24:01,660 --> 00:24:03,960
we can use lasers to heat the sample
431
00:24:04,030 --> 00:24:05,800
and to reach the same conditions
432
00:24:05,870 --> 00:24:09,970
we think are inside the planet.
433
00:24:10,040 --> 00:24:14,610
The core's sun-hot
heat is crucial to understand.
434
00:24:14,680 --> 00:24:16,640
It affects everything at the surface,
435
00:24:16,710 --> 00:24:20,150
1,800 miles above.
436
00:24:20,210 --> 00:24:21,450
To get up here,
437
00:24:21,520 --> 00:24:25,520
it travels through two layers
of solid rock.
438
00:24:25,590 --> 00:24:29,760
The first is the mantle,
Earth's biggest layer by far.
439
00:24:31,360 --> 00:24:32,960
And then the crust,
440
00:24:33,030 --> 00:24:36,830
really just a thin veneer on the Earth.
441
00:24:36,900 --> 00:24:40,070
Scientists believe that
the immense heat of the core
442
00:24:40,130 --> 00:24:43,270
makes the mantle so hot
that it acts like a fluid
443
00:24:43,340 --> 00:24:46,610
and rises until it hits the crust.
444
00:24:46,670 --> 00:24:50,680
Some of the heat then circulates
along the crust's underside
445
00:24:50,740 --> 00:24:54,950
until subducting slabs of crust
take it downward.
446
00:24:55,020 --> 00:24:58,920
It's a giant recycling treadmill
called convection.
447
00:25:00,920 --> 00:25:02,320
Through experiments,
448
00:25:02,390 --> 00:25:05,060
scientists have calculated
that heat's round trip
449
00:25:05,130 --> 00:25:08,730
from the core to the crust
and back to the core again
450
00:25:08,800 --> 00:25:12,500
takes 300 million years.
451
00:25:12,570 --> 00:25:15,030
This circulation of heat is kept alive
452
00:25:15,100 --> 00:25:17,040
by the sinking of cold, heavy plates
453
00:25:17,100 --> 00:25:19,770
into the hot mantle below.
454
00:25:19,840 --> 00:25:21,170
In other words,
455
00:25:21,240 --> 00:25:24,710
those slabs cause
quite a stir as they descend.
456
00:25:26,680 --> 00:25:28,910
We can track those slabs with tomography
457
00:25:28,980 --> 00:25:31,480
down at least into the middle mantle.
458
00:25:31,550 --> 00:25:33,620
And probably those slabs reach all the way
459
00:25:33,690 --> 00:25:35,520
to the core-mantle boundary,
460
00:25:35,590 --> 00:25:39,990
and there's a slab graveyard
that sits above the core.
461
00:25:40,060 --> 00:25:44,400
That slab graveyard may in turn help drive
462
00:25:44,470 --> 00:25:47,070
the convection cells in Earth's mantle
463
00:25:47,130 --> 00:25:49,170
that in turn then reach up
464
00:25:49,240 --> 00:25:51,470
and drive the pattern of plate tectonics
465
00:25:51,540 --> 00:25:53,340
and the shifting plates at Earth's surface.
466
00:25:55,440 --> 00:25:58,480
The sinking slabs not
only drive heat in the mantle;
467
00:25:58,550 --> 00:26:01,480
they have radically changed
the makeup of the mantle
468
00:26:01,550 --> 00:26:05,950
over the course of geologic time.
469
00:26:06,020 --> 00:26:10,160
The mantle is a very
complicated mixture of things
470
00:26:10,220 --> 00:26:12,420
that have been stirred internally
471
00:26:12,490 --> 00:26:14,660
and things that have come up
to the surface,
472
00:26:14,730 --> 00:26:18,460
been ocean floor for a while,
and then taken back down.
473
00:26:20,830 --> 00:26:23,640
In the 4.5 billion-year history
of the Earth,
474
00:26:23,700 --> 00:26:28,010
the entire mantle has been
circulated about once.
475
00:26:28,080 --> 00:26:30,240
Virtually nothing at the Earth's surface
476
00:26:30,310 --> 00:26:32,540
or in the interior is permanent.
477
00:26:32,610 --> 00:26:33,930
Things are changing all the time.
478
00:26:38,450 --> 00:26:40,390
Heat rising up from the core
479
00:26:40,450 --> 00:26:43,620
does not always recycle back down.
480
00:26:43,690 --> 00:26:47,060
Sometimes it escapes its earthly confines
481
00:26:47,130 --> 00:26:52,000
as volcanic eruptions.
482
00:26:52,070 --> 00:26:53,770
Earth is by far
483
00:26:53,830 --> 00:26:58,540
the solar system's most
volcanically active planet.
484
00:26:58,610 --> 00:27:03,410
Volcanoes are nature's most
violent and destructive force.
485
00:27:03,480 --> 00:27:05,110
But ironically,
486
00:27:05,180 --> 00:27:08,850
this force emanating from
the Earth's mantle deep below
487
00:27:08,920 --> 00:27:12,320
is at the very root
of the creation and maintenance
488
00:27:12,390 --> 00:27:15,190
of life on Earth's surface.
489
00:27:16,960 --> 00:27:20,060
Volcanoes have certainly
influenced life on Earth.
490
00:27:20,130 --> 00:27:23,130
And that's not just when
they have a big eruption
491
00:27:23,200 --> 00:27:24,430
and people die.
492
00:27:24,500 --> 00:27:29,600
But they also may have created
life on Earth.
493
00:27:29,670 --> 00:27:31,870
In the last few decades,
494
00:27:31,940 --> 00:27:35,510
hydrothermal vents were
discovered deep in the ocean,
495
00:27:35,580 --> 00:27:39,780
and minerals are coming out
that can actually feed life
496
00:27:39,850 --> 00:27:41,480
around those vents,
497
00:27:41,550 --> 00:27:45,380
and a very surprising find
was that life could exist
498
00:27:45,450 --> 00:27:48,490
at such depth in the ocean.
499
00:27:52,630 --> 00:27:55,130
Volcanoes were the original architects
500
00:27:55,200 --> 00:27:56,930
of our atmosphere,
501
00:27:57,000 --> 00:27:59,930
unlocking co2 and water
from the mantle's rocks
502
00:28:00,000 --> 00:28:02,000
and taking them to the surface.
503
00:28:04,070 --> 00:28:05,600
One of the largest concentrations
504
00:28:05,670 --> 00:28:09,110
of active volcanoes on Earth is in Japan...
505
00:28:09,180 --> 00:28:11,180
More than 100.
506
00:28:11,250 --> 00:28:14,710
Mount fuji is Japan's largest volcano,
507
00:28:14,780 --> 00:28:18,850
rising 12,400 feet high.
508
00:28:18,920 --> 00:28:22,950
This graceful peak is
a massive subduction volcano,
509
00:28:23,020 --> 00:28:27,460
but the largest volcanoes
on Earth are 100 times bigger.
510
00:28:29,200 --> 00:28:31,160
They belong to a rare class of volcano
511
00:28:31,230 --> 00:28:35,200
known as a mantle plume volcano,
512
00:28:35,270 --> 00:28:37,770
and the largest of all are on Hawaii.
513
00:28:41,070 --> 00:28:44,010
Lava has been erupting
from this spot in the pacific
514
00:28:44,080 --> 00:28:48,110
for at least 75 million years.
515
00:28:48,180 --> 00:28:53,290
It built Mauna Loa, Earth's
largest volcano by volume.
516
00:28:53,350 --> 00:28:59,090
But Mauna Loa is not the biggest
volcano in the solar system.
517
00:28:59,160 --> 00:29:01,190
The largest volcano in the solar system
518
00:29:01,260 --> 00:29:05,130
is Olympus mons on Mars.
519
00:29:05,200 --> 00:29:08,000
It's about 26 kilometers high
520
00:29:08,070 --> 00:29:13,140
and around 600 kilometers across.
521
00:29:15,310 --> 00:29:16,810
Compared to Hawaii,
522
00:29:16,880 --> 00:29:21,350
Olympus mons is vastly bigger
in all respects.
523
00:29:21,420 --> 00:29:25,920
Why are Earth's volcanoes so much smaller?
524
00:29:25,990 --> 00:29:28,390
On Mars, there's no plate tectonics,
525
00:29:28,460 --> 00:29:33,590
so the same, let's say, hot spot
can build up a volcano
526
00:29:33,660 --> 00:29:37,600
in the same location for very,
very long periods of time.
527
00:29:41,000 --> 00:29:43,100
On the Earth, we can't have that situation
528
00:29:43,170 --> 00:29:46,270
because the plates are moving,
so you get, for example,
529
00:29:46,340 --> 00:29:50,410
hot spot like the one that
formed the Hawaiian volcanoes,
530
00:29:50,480 --> 00:29:54,580
but the plate is moving over it,
so it punches one volcano,
531
00:29:54,650 --> 00:29:56,010
but then the plate moves,
532
00:29:56,080 --> 00:30:00,420
and the volcano moves somewhere else.
533
00:30:00,490 --> 00:30:05,160
So on Earth, we don't really
build those sizes of volcanoes.
534
00:30:09,730 --> 00:30:12,600
The source that feeds
Earth's giant volcanoes
535
00:30:12,670 --> 00:30:15,200
is thought to be
a narrow cylinder of hot rock
536
00:30:15,270 --> 00:30:18,240
that reaches deep into the mantle.
537
00:30:18,310 --> 00:30:21,170
How deep is a matter of debate.
538
00:30:21,240 --> 00:30:23,740
But recent seismic studies
of the Hawaiian chain
539
00:30:23,810 --> 00:30:26,040
imaged for the first time
540
00:30:26,110 --> 00:30:29,310
a hot plume reaching halfway
to the Earth's core.
541
00:30:31,450 --> 00:30:34,850
Chemical analysis of lavas
retrieved in drill cores
542
00:30:34,920 --> 00:30:37,490
traces the plume even deeper,
543
00:30:37,560 --> 00:30:40,960
actually extending
to the Earth's core itself.
544
00:30:43,860 --> 00:30:46,430
The deepest reaches of our planet, then,
545
00:30:46,500 --> 00:30:48,030
are intimately connected
546
00:30:48,100 --> 00:30:51,270
with geological action at the surface.
547
00:30:54,340 --> 00:30:56,410
Without these deep-seated forces,
548
00:30:56,480 --> 00:31:01,910
life would have not gained
a foothold on our planet,
549
00:31:01,980 --> 00:31:03,380
and it wouldn't have flourished
550
00:31:03,450 --> 00:31:06,580
and evolved into countless wondrous forms
551
00:31:06,650 --> 00:31:09,590
without something else:
552
00:31:09,660 --> 00:31:13,020
A remarkably stable,
precisely tuned climate.
553
00:31:18,870 --> 00:31:21,300
For most of Earth's existence,
554
00:31:21,370 --> 00:31:23,900
the temperature at the surface
has rarely wavered.
555
00:31:25,770 --> 00:31:29,170
Why this is is a great mystery.
556
00:31:29,240 --> 00:31:34,010
The global mean temperature
is a comfortable 59 degrees.
557
00:31:34,080 --> 00:31:36,350
Increase that by just 18 degrees,
558
00:31:36,420 --> 00:31:40,790
and ice at the Poles would melt.
559
00:31:40,850 --> 00:31:43,250
Decrease it by 18 degrees,
560
00:31:43,320 --> 00:31:46,060
and the entire world would freeze over.
561
00:31:47,760 --> 00:31:50,860
Other planets are far from habitable.
562
00:31:50,930 --> 00:31:54,270
Venus, with 200,000 times more co2
563
00:31:54,330 --> 00:31:56,200
in its atmosphere than Earth,
564
00:31:56,270 --> 00:32:00,270
is ravaged by temperatures
reaching 900 degrees fahrenheit.
565
00:32:03,010 --> 00:32:07,050
Mars, long ago stripped of its
atmosphere and magnetic shield,
566
00:32:07,110 --> 00:32:12,280
is deeply frozen to an average
of 70 degrees below zero.
567
00:32:19,830 --> 00:32:23,160
Earth's climate is kept moderate
by a little understood interplay
568
00:32:23,230 --> 00:32:27,500
of geologic and biologic forces.
569
00:32:27,570 --> 00:32:31,300
Volcanic eruptions spew co2 into the air.
570
00:32:31,370 --> 00:32:34,270
Plate tectonics builds mountains
571
00:32:34,340 --> 00:32:37,810
and rifts apart ocean basins.
572
00:32:37,880 --> 00:32:40,350
Then weather erodes the land,
573
00:32:40,410 --> 00:32:42,110
capturing CO2
574
00:32:42,180 --> 00:32:45,320
and locking it away in sediments.
575
00:32:45,390 --> 00:32:49,950
Living organisms add
their alterations into the mix.
576
00:32:50,020 --> 00:32:53,930
All is built into a system
of global checks and balances.
577
00:32:58,000 --> 00:32:59,700
There have been only a few times
578
00:32:59,770 --> 00:33:03,900
when Earth's climate tipped
well beyond its comfort zone.
579
00:33:03,970 --> 00:33:09,410
Earth's coldest extreme happened
about 750 million years ago
580
00:33:09,480 --> 00:33:13,080
and probably several times before that.
581
00:33:13,150 --> 00:33:17,180
It was called snowball Earth.
582
00:33:17,250 --> 00:33:19,850
Snowball Earth refers
to a time in the Earth's history
583
00:33:19,920 --> 00:33:23,290
when the Earth
was entirely covered with ice.
584
00:33:23,360 --> 00:33:25,960
That means glaciers on the continents
585
00:33:26,030 --> 00:33:28,290
and sea ice covering most of the oceans
586
00:33:28,360 --> 00:33:31,760
all the way down to the equator.
587
00:33:31,830 --> 00:33:33,470
During snowball Earth,
588
00:33:33,530 --> 00:33:35,400
the temperature at the equator plunged
589
00:33:35,470 --> 00:33:38,300
to minus 4 degrees fahrenheit.
590
00:33:38,370 --> 00:33:41,240
Life, still in
its primitive bacterial stage,
591
00:33:41,310 --> 00:33:44,810
survived the deep freeze as it does today:
592
00:33:44,880 --> 00:33:47,980
Below ice shelves and even below glaciers.
593
00:33:50,680 --> 00:33:53,820
Scientists think that
the trigger for snowball Earth
594
00:33:53,890 --> 00:33:59,160
was the bunching
of continents at the equator.
595
00:33:59,230 --> 00:34:01,690
The giant land mass attracted moisture,
596
00:34:01,760 --> 00:34:04,700
and great storms ensued.
597
00:34:04,760 --> 00:34:07,870
Rain stripped co2 from the atmosphere
598
00:34:07,930 --> 00:34:12,340
and locked it in eroding sediments.
599
00:34:12,410 --> 00:34:15,710
As co2 levels dropped,
so did the temperature.
600
00:34:22,020 --> 00:34:25,820
Volcanic eruptions eventually
overpowered snowball Earth
601
00:34:25,890 --> 00:34:30,520
by injecting co2 back into the atmosphere.
602
00:34:30,590 --> 00:34:32,420
As the climate restabilized,
603
00:34:32,490 --> 00:34:36,030
the surviving bacterial
life forms multiplied,
604
00:34:36,100 --> 00:34:37,760
photosynthesized,
605
00:34:37,830 --> 00:34:40,130
and filled the air with oxygen,
606
00:34:40,200 --> 00:34:43,170
a requirement for more complex life.
607
00:34:44,840 --> 00:34:48,810
Finally, 550 million years ago,
608
00:34:48,880 --> 00:34:51,810
life branched out and began
its extraordinary journey
609
00:34:51,880 --> 00:34:54,810
of diversification and evolution.
610
00:34:57,820 --> 00:35:01,450
Why this happened is still a mystery,
611
00:35:01,520 --> 00:35:04,060
but the extreme hit that life took
612
00:35:04,120 --> 00:35:05,590
during snowball Earth
613
00:35:05,660 --> 00:35:10,200
may have actually been part of the reason.
614
00:35:10,260 --> 00:35:12,100
Once life was established on the Earth,
615
00:35:12,170 --> 00:35:15,100
it continued to show evidence
that it was extremely adaptable,
616
00:35:15,170 --> 00:35:18,500
that whatever harsh conditions
were thrown at it,
617
00:35:18,570 --> 00:35:22,710
that it was able to adapt
to find a way to survive.
618
00:35:22,780 --> 00:35:25,580
So even through periods
like snowball Earth,
619
00:35:25,650 --> 00:35:28,680
life would adapt to those
conditions and reemerge.
620
00:35:28,750 --> 00:35:31,920
And in fact, those sorts of
conditions were major factors
621
00:35:31,990 --> 00:35:33,950
driving the evolution of life on Earth.
622
00:35:38,330 --> 00:35:41,490
The next time Earth's
climate was knocked off balance
623
00:35:41,560 --> 00:35:45,130
was at a time when Earth
experienced its biggest upheaval
624
00:35:45,200 --> 00:35:48,870
since the early days of its formation.
625
00:35:48,940 --> 00:35:52,000
About 250 million years ago,
626
00:35:52,070 --> 00:35:55,310
95% of life on Earth disappeared.
627
00:35:57,940 --> 00:36:00,610
We think that's because
of very large eruptions
628
00:36:00,680 --> 00:36:03,820
in the siberian traps.
629
00:36:07,190 --> 00:36:11,160
Earth scientists have
mapped 800,000 square miles
630
00:36:11,220 --> 00:36:14,090
of Siberia that were covered by lava flows
631
00:36:14,160 --> 00:36:17,560
up to 12,000 feet thick.
632
00:36:17,630 --> 00:36:21,630
They call this province the siberian traps.
633
00:36:21,700 --> 00:36:25,000
Those eruptions
would have put a lot of dust,
634
00:36:25,070 --> 00:36:28,170
a lot of sulfur in the atmosphere,
635
00:36:28,240 --> 00:36:30,810
cooled the temperature.
636
00:36:30,880 --> 00:36:34,950
So this killed a lot of life on Earth.
637
00:36:35,020 --> 00:36:39,480
The climate recovered
eventually, and so did life.
638
00:36:39,550 --> 00:36:41,050
But 90 million years ago,
639
00:36:41,120 --> 00:36:45,690
the hottest period ever known
enveloped the Earth.
640
00:36:45,760 --> 00:36:49,130
Dinosaurs roamed
what are now the icy realms
641
00:36:49,200 --> 00:36:51,130
of the arctic circle.
642
00:36:51,200 --> 00:36:56,270
Around them were fellow
swamp-dwellers like alligators.
643
00:36:56,340 --> 00:37:00,100
What were they doing at the Earth's Poles?
644
00:37:00,170 --> 00:37:04,280
Scientists figured out that
plate tectonics was the culprit.
645
00:37:05,750 --> 00:37:07,510
100 million years ago,
646
00:37:07,580 --> 00:37:09,350
the continents had wrenched apart,
647
00:37:09,420 --> 00:37:12,350
and sea level rose.
648
00:37:12,420 --> 00:37:14,650
Mountain building slowed down,
649
00:37:14,720 --> 00:37:18,390
and so did rainfall and erosion.
650
00:37:18,460 --> 00:37:20,860
But co2-spewing volcanic eruptions,
651
00:37:20,930 --> 00:37:26,530
especially in the mid-ocean
rift zones, did not.
652
00:37:26,600 --> 00:37:28,300
When continents are splitting apart
653
00:37:28,370 --> 00:37:30,340
and you have more sea floor spreading,
654
00:37:30,400 --> 00:37:32,770
you're adding more co2 to the atmosphere,
655
00:37:32,840 --> 00:37:35,570
and it's not being removed as quickly
656
00:37:35,640 --> 00:37:39,140
by weathering of the continents,
657
00:37:39,210 --> 00:37:41,680
so it's warmer during those periods.
658
00:37:43,750 --> 00:37:46,280
Temperatures rose to a balmy 54 degrees
659
00:37:46,350 --> 00:37:49,990
at the north and south Poles.
660
00:37:50,060 --> 00:37:52,490
The climate so agreed with the dinosaurs
661
00:37:52,560 --> 00:37:56,490
that they dominated life
on Earth for millions of years.
662
00:37:58,430 --> 00:38:01,630
Then, 65 million years ago,
663
00:38:01,700 --> 00:38:05,400
a massive meteorite impact
suddenly ended their reign.
664
00:38:12,480 --> 00:38:14,980
The meteorite gouged out a huge crater
665
00:38:15,050 --> 00:38:16,680
over 100 miles across
666
00:38:16,750 --> 00:38:20,320
with the town of Chicxulub,
Mexico, at its center.
667
00:38:23,560 --> 00:38:27,060
The city of Merida
also lies within the crater,
668
00:38:27,130 --> 00:38:30,900
which was buried by sediment
millions of years ago
669
00:38:30,960 --> 00:38:34,300
and only discovered by chance
as geologists explored for oil
670
00:38:34,370 --> 00:38:36,200
in the 1970s.
671
00:38:41,140 --> 00:38:43,880
The crater and its surroundings
are also the site
672
00:38:43,940 --> 00:38:47,310
of the great cities
built by the ancient Maya.
673
00:38:47,380 --> 00:38:50,180
But none of the human drama
playing out in this region
674
00:38:50,250 --> 00:38:51,780
would have been possible
675
00:38:51,850 --> 00:38:53,820
if it weren't for the killer meteorite
676
00:38:53,890 --> 00:38:55,550
that struck here,
677
00:38:55,620 --> 00:38:58,860
wiping out the dinosaurs.
678
00:38:58,930 --> 00:39:01,630
Their extinction opened
an evolutionary door
679
00:39:01,690 --> 00:39:04,360
for mammals like US to evolve.
680
00:39:07,500 --> 00:39:09,730
But 65 million years ago,
681
00:39:09,800 --> 00:39:11,800
the earthly stage was far from set
682
00:39:11,870 --> 00:39:14,840
for the entrance of people.
683
00:39:14,910 --> 00:39:19,280
Co2 levels and temperatures
were much too high.
684
00:39:19,350 --> 00:39:23,610
Luckily for US, plate tectonics
built the most massive structure
685
00:39:23,680 --> 00:39:27,590
on Earth to make
the climate human-friendly:
686
00:39:27,650 --> 00:39:32,220
The great Himalaya range.
687
00:39:32,290 --> 00:39:36,590
About 60 million years ago,
India, once a massive island,
688
00:39:36,660 --> 00:39:40,260
rammed Asia, and the building
of the Himalaya began.
689
00:39:42,940 --> 00:39:44,840
As the himalayas grew,
690
00:39:44,900 --> 00:39:49,210
a great block of land
rose up behind the range.
691
00:39:49,280 --> 00:39:51,340
It formed the Tibetan plateau
692
00:39:51,410 --> 00:39:55,280
and even raised the land
as far to the north as Mongolia,
693
00:39:55,350 --> 00:39:58,750
more than 1,000 miles away.
694
00:39:58,820 --> 00:40:02,490
Since then, the Earth's climate
has never been the same.
695
00:40:06,360 --> 00:40:09,230
There's a real belief amongst geologists
696
00:40:09,300 --> 00:40:12,500
that the growth of the Himalaya in Tibet
697
00:40:12,570 --> 00:40:15,570
over the last 60 million years
698
00:40:15,640 --> 00:40:18,270
has led to major changes
in the global climate.
699
00:40:20,670 --> 00:40:23,570
The Indian monsoon is created
700
00:40:23,640 --> 00:40:26,540
when warm air from the Indian ocean
701
00:40:26,610 --> 00:40:28,610
meets the mountains of the Himalaya
702
00:40:28,680 --> 00:40:32,680
and dumps the rain on that mountain front.
703
00:40:32,750 --> 00:40:35,790
Prior to the formation
of the Himalaya in Tibet,
704
00:40:35,860 --> 00:40:37,460
there was no monsoon.
705
00:40:37,520 --> 00:40:40,190
There was no barrier
to the northward progress
706
00:40:40,260 --> 00:40:42,330
of the moist Indian ocean air.
707
00:40:45,900 --> 00:40:48,570
The new monsoon began dumping rain and snow
708
00:40:48,640 --> 00:40:52,240
on the growing mountains of Asia.
709
00:40:52,310 --> 00:40:55,170
This eroded the rocks
and changed the chemical balance
710
00:40:55,240 --> 00:40:58,580
in the atmosphere.
711
00:40:58,650 --> 00:41:02,180
Massive amounts of co2
were removed from the air,
712
00:41:02,250 --> 00:41:05,880
so much that the global climate
began to cool.
713
00:41:08,520 --> 00:41:11,590
It was the first time in
hundreds of millions of years
714
00:41:11,660 --> 00:41:16,130
that the Earth's climate cooled so deeply.
715
00:41:16,200 --> 00:41:19,160
But before we humans made our appearance,
716
00:41:19,230 --> 00:41:22,600
plate tectonics had a few more
modifications to make.
717
00:41:25,440 --> 00:41:27,100
All right, here. Here we go.
718
00:41:27,170 --> 00:41:28,640
In Antarctica,
719
00:41:28,710 --> 00:41:32,010
scientists made the startling discovery
720
00:41:32,080 --> 00:41:34,550
that this lonely continent
at the bottom of the world
721
00:41:34,610 --> 00:41:38,650
was once connected to Australia.
722
00:41:38,720 --> 00:41:41,590
Then 40 million years ago,
723
00:41:41,650 --> 00:41:44,260
the land masses rifted apart,
724
00:41:44,320 --> 00:41:49,060
completely changing
ocean currents in the region.
725
00:41:49,130 --> 00:41:50,830
Over the last 40 million years,
726
00:41:50,900 --> 00:41:54,000
Antarctica became an isolated
continent down there,
727
00:41:54,070 --> 00:41:58,900
and so the circum-antarctic current formed,
728
00:41:58,970 --> 00:42:01,970
which isolated that continent
so it could cool
729
00:42:02,040 --> 00:42:03,710
and start to grow an ice sheet.
730
00:42:07,350 --> 00:42:08,480
For the first time,
731
00:42:08,550 --> 00:42:10,650
Antarctica became frozen with ice,
732
00:42:10,720 --> 00:42:13,280
up to 15,000 feet thick,
733
00:42:13,350 --> 00:42:17,820
dropping global temperatures even more.
734
00:42:17,890 --> 00:42:20,560
And then, 3 million years ago,
735
00:42:20,630 --> 00:42:24,460
a new land bridge welded south
and North America together.
736
00:42:26,700 --> 00:42:29,130
Before that, the two continents
were separated
737
00:42:29,200 --> 00:42:32,970
by the ancient Panama seaway.
738
00:42:33,040 --> 00:42:35,710
As plate tectonics joined them together,
739
00:42:35,780 --> 00:42:40,240
ocean currents were radically
rerouted once again.
740
00:42:40,310 --> 00:42:43,750
The Gulf stream, instead of
going into the pacific ocean,
741
00:42:43,820 --> 00:42:47,950
it actually was then redirected
upward in the Atlantic,
742
00:42:48,020 --> 00:42:53,620
bringing more moisture
up into the north Atlantic,
743
00:42:53,690 --> 00:42:56,030
and that's a source of moisture for ice
744
00:42:56,100 --> 00:43:00,300
on the continents up there.
745
00:43:00,370 --> 00:43:02,900
By 2 1/2 million years ago,
746
00:43:02,970 --> 00:43:07,140
temperatures plunged so much
that the pleistocene ice ages
747
00:43:07,210 --> 00:43:10,240
were triggered,
748
00:43:10,310 --> 00:43:12,240
and in this ice age world,
749
00:43:12,310 --> 00:43:14,780
when freezing glacial periods alternated
750
00:43:14,850 --> 00:43:17,720
with much warmer interglacials,
751
00:43:17,780 --> 00:43:20,120
the story of human evolution unfolded
752
00:43:20,190 --> 00:43:22,190
in the east African rift.
753
00:43:28,290 --> 00:43:30,730
Humans have lived
through eight major ice ages
754
00:43:30,800 --> 00:43:33,500
over the past million years.
755
00:43:33,570 --> 00:43:37,230
The last glacial period
ended 10,000 years ago,
756
00:43:37,300 --> 00:43:42,140
and we are now at the tail end
of an interglacial.
757
00:43:42,210 --> 00:43:45,140
The climate has been warm on average,
758
00:43:45,210 --> 00:43:48,210
but with enough variation
to profoundly affect people
759
00:43:48,280 --> 00:43:52,450
and even civilizations.
760
00:43:52,520 --> 00:43:56,020
Once we entered
the warm period we're in now
761
00:43:56,090 --> 00:43:58,960
that started about 10,000 years ago,
762
00:43:59,030 --> 00:44:02,090
the changes in climate
have been relatively smaller
763
00:44:02,160 --> 00:44:04,400
than the ice ages,
764
00:44:04,460 --> 00:44:06,060
but they've been big enough
765
00:44:06,130 --> 00:44:10,400
that there were droughts
and floods and cold periods
766
00:44:10,470 --> 00:44:12,500
and little mini ice ages.
767
00:44:12,570 --> 00:44:14,540
And that people were still migrating
768
00:44:14,610 --> 00:44:17,480
and still having to adapt.
769
00:44:19,780 --> 00:44:23,480
Researchers studying past climate changes
770
00:44:23,550 --> 00:44:26,050
in Guatemala found
that long droughts occurred
771
00:44:26,120 --> 00:44:28,650
at the height of
the ancient Maya civilization,
772
00:44:28,720 --> 00:44:33,760
from 3,000 to 1,000 years ago.
773
00:44:33,830 --> 00:44:35,560
This may have been a big factor
774
00:44:35,630 --> 00:44:38,700
in the Maya civilization's
mysterious collapse.
775
00:44:44,070 --> 00:44:47,610
Because global warming is
again creating serious problems
776
00:44:47,670 --> 00:44:49,740
for people and ecosystems,
777
00:44:49,810 --> 00:44:52,940
scientists are engaged in an urgent effort
778
00:44:53,010 --> 00:44:55,310
to study past climate changes
779
00:44:55,380 --> 00:45:00,450
in order to predict
what might happen in the future.
780
00:45:00,520 --> 00:45:03,450
One of their best tools
is the study of gases
781
00:45:03,520 --> 00:45:05,620
that cause global warming,
782
00:45:05,690 --> 00:45:10,360
especially carbon dioxide, or co2.
783
00:45:10,430 --> 00:45:14,030
These gases are detected
in ancient glacial ice
784
00:45:14,100 --> 00:45:17,170
hundreds of thousands
of years old in Antarctica.
785
00:45:19,510 --> 00:45:22,310
Atmospheric gases get trapped in the ice,
786
00:45:22,380 --> 00:45:25,210
and you get these trapped
bubbles of atmosphere
787
00:45:25,280 --> 00:45:27,080
that are preserved,
788
00:45:27,150 --> 00:45:29,780
and so when you take the ice core,
789
00:45:29,850 --> 00:45:34,820
you have this sort of ancient atmosphere.
790
00:45:34,890 --> 00:45:38,420
And so they can release that
and measure the amount of co2
791
00:45:38,490 --> 00:45:40,060
that was around.
792
00:45:40,130 --> 00:45:44,030
And also how it changed
since the last ice age.
793
00:45:45,830 --> 00:45:49,870
So we know from the ice cores
that the natural level of co2
794
00:45:49,940 --> 00:45:52,440
should be about 280 parts per million,
795
00:45:52,510 --> 00:45:55,770
and today it's about 390 parts per million,
796
00:45:55,840 --> 00:45:59,710
as humans have been burning
fossil fuels and releasing co2.
797
00:46:03,580 --> 00:46:05,220
That's a really big rise.
798
00:46:05,280 --> 00:46:09,190
We've never seen this rapid a rise of co2
799
00:46:09,260 --> 00:46:10,820
in the atmosphere
800
00:46:10,890 --> 00:46:13,560
over, you know, 100 million years.
801
00:46:17,530 --> 00:46:20,930
So it appears
that one natural Earth system,
802
00:46:21,000 --> 00:46:22,330
the climate,
803
00:46:22,400 --> 00:46:25,440
may indeed be entering
a period of intense upheaval.
804
00:46:33,380 --> 00:46:35,850
What about the other natural Earth systems,
805
00:46:35,920 --> 00:46:39,780
like those that cause
earthquakes and volcanoes?
806
00:46:39,850 --> 00:46:43,320
Iceland's most active volcano
has started erupting.
807
00:46:43,390 --> 00:46:44,560
Have any of these
808
00:46:44,620 --> 00:46:46,960
begun a period of increased activity?
809
00:46:47,030 --> 00:46:50,460
Has been spewing ash and smoke
12 miles into the air.
810
00:46:52,160 --> 00:46:54,060
Looking back 100 years,
811
00:46:54,130 --> 00:46:56,530
the number of volcanic eruptions each year
812
00:46:56,600 --> 00:46:59,640
has actually held steady.
813
00:46:59,710 --> 00:47:03,170
In general, the public only
hears about volcanic eruptions
814
00:47:03,240 --> 00:47:07,480
when they're particularly big,
when they kill people,
815
00:47:07,550 --> 00:47:11,750
but, in fact, there are usually
couple dozen volcanic eruptions
816
00:47:11,820 --> 00:47:14,650
going on every month.
817
00:47:14,720 --> 00:47:18,620
And some volcanoes have been
erupting for dozens of years,
818
00:47:18,690 --> 00:47:20,290
like kilauea...
819
00:47:20,360 --> 00:47:24,560
Or even hundreds or thousands of years
820
00:47:24,630 --> 00:47:27,600
like Aetna in Italy.
821
00:47:27,670 --> 00:47:29,800
So there is a lot of variation
822
00:47:29,870 --> 00:47:34,310
in terms of sometimes we have
really big volcanic eruptions,
823
00:47:34,370 --> 00:47:39,380
and then we might go for months
or years without one of those.
824
00:47:39,450 --> 00:47:41,510
But, in fact, volcanoes are erupting
825
00:47:41,580 --> 00:47:43,580
around the Earth all the time.
826
00:47:49,260 --> 00:47:52,220
A similar scenario holds for earthquakes.
827
00:47:56,260 --> 00:47:59,300
The number of big quakes
greater than magnitude 7
828
00:47:59,370 --> 00:48:01,130
averages 18 per year.
829
00:48:01,200 --> 00:48:03,300
At least 250 people have died
830
00:48:03,370 --> 00:48:06,170
after a major earthquake
hit southwestern Pakistan.
831
00:48:06,240 --> 00:48:07,400
One of these quakes
832
00:48:07,470 --> 00:48:10,040
is higher than magnitude 8 each year.
833
00:48:12,210 --> 00:48:17,720
This number has held steady
since the year 1900.
834
00:48:17,780 --> 00:48:20,680
But it may seem that earthquakes
have become more frequent
835
00:48:20,750 --> 00:48:22,220
for several reasons.
836
00:48:22,290 --> 00:48:23,620
Yo!
837
00:48:23,690 --> 00:48:25,090
Excuse me!
838
00:48:25,160 --> 00:48:29,030
First, more are being
recorded and pinpointed.
839
00:48:29,100 --> 00:48:34,030
More than 8,000 stations
listen for quakes worldwide.
840
00:48:34,100 --> 00:48:39,540
In 1931, there were just 350.
841
00:48:39,610 --> 00:48:41,670
Earthquakes also seem more numerous
842
00:48:41,740 --> 00:48:43,710
because of the soaring population
843
00:48:43,780 --> 00:48:47,210
and near-instant reporting
of news worldwide.
844
00:48:50,420 --> 00:48:55,250
The earthquake that killed
1/4 million people in Haiti
845
00:48:55,320 --> 00:48:58,920
would only have killed
1/10 that number 100 years ago,
846
00:48:58,990 --> 00:49:00,430
and we wouldn't have known about it.
847
00:49:00,490 --> 00:49:02,960
This is some of what I saw on the streets.
848
00:49:03,030 --> 00:49:04,960
Now we hear about these things instantly.
849
00:49:05,030 --> 00:49:07,560
We have the live footage of the tsunamis
850
00:49:07,630 --> 00:49:09,570
arriving on the Japanese coast,
851
00:49:09,640 --> 00:49:11,040
and the whole world knows about it.
852
00:49:14,370 --> 00:49:16,840
The recorded number
of tsunamis has increased
853
00:49:16,910 --> 00:49:19,010
in the past 100 years,
854
00:49:19,080 --> 00:49:24,420
but scientists attribute this
to ever-improving detection.
855
00:49:24,480 --> 00:49:27,050
And with better warning systems
being installed,
856
00:49:27,120 --> 00:49:31,360
the loss of life is decreasing.
857
00:49:31,420 --> 00:49:33,820
So do scientists see an uptick
858
00:49:33,890 --> 00:49:36,360
in increased geological activity?
859
00:49:39,270 --> 00:49:41,430
No, the Earth's doing the same thing
860
00:49:41,500 --> 00:49:43,670
it's been doing for billions of years.
861
00:49:43,740 --> 00:49:45,800
We notice the events more
862
00:49:45,870 --> 00:49:48,710
because our more complex society
863
00:49:48,770 --> 00:49:52,710
and our far more populated planet
864
00:49:52,780 --> 00:49:55,180
are much more vulnerable
to these disasters.
865
00:49:58,280 --> 00:50:00,600
There's no real evidence that
we're living through a period
866
00:50:00,650 --> 00:50:02,050
of enhanced activity.
867
00:50:02,120 --> 00:50:03,690
In any one area,
868
00:50:03,760 --> 00:50:08,490
there may be regions where
we have a few more earthquakes
869
00:50:08,560 --> 00:50:12,500
for a few hundred years,
and then after that clustering,
870
00:50:12,570 --> 00:50:14,700
seismicity will die away.
871
00:50:14,770 --> 00:50:17,570
But on a planet-wide scale,
there's no evidence for that.
872
00:50:20,810 --> 00:50:23,810
Looking deeper into Earth's geologic past,
873
00:50:23,880 --> 00:50:25,710
far bigger upheavals have occurred
874
00:50:25,780 --> 00:50:29,050
than anything known by humans.
875
00:50:29,120 --> 00:50:32,080
A supervolcano thousands of times bigger
876
00:50:32,150 --> 00:50:35,250
than the huge mount Saint Helens
eruption of 1980
877
00:50:35,320 --> 00:50:39,390
went off at yellowstone 640,000 years ago.
878
00:50:41,460 --> 00:50:45,930
Supervolcanoes are exceedingly
rare in human terms.
879
00:50:46,000 --> 00:50:48,770
None have erupted for 26,000 years.
880
00:50:52,200 --> 00:50:55,870
Earth's history book chronicles
other massive upheavals
881
00:50:55,940 --> 00:50:59,540
like the siberian traps.
882
00:50:59,610 --> 00:51:04,150
There was snowball Earth,
when the whole world froze over.
883
00:51:04,220 --> 00:51:05,950
In more recent times,
884
00:51:06,020 --> 00:51:09,020
our distant ancestors
experienced the bitter cold
885
00:51:09,090 --> 00:51:10,990
of the ice ages,
886
00:51:11,060 --> 00:51:14,390
when Chicago lay
under a mile-thick sheet of ice.
887
00:51:16,860 --> 00:51:20,660
If planet Earth were acting up
more than usual right now,
888
00:51:20,730 --> 00:51:22,200
it would barely register
889
00:51:22,270 --> 00:51:24,700
in the annals of major earthly events.
890
00:51:26,340 --> 00:51:28,670
The exception is the climate,
891
00:51:28,740 --> 00:51:31,540
which is changing in ways never before seen
892
00:51:31,610 --> 00:51:34,380
in the geologic record.
893
00:51:34,450 --> 00:51:38,080
People may have some influence
over this process,
894
00:51:38,150 --> 00:51:40,050
but deep Earth processes
895
00:51:40,120 --> 00:51:42,920
like earthquakes and volcanic eruptions
896
00:51:42,990 --> 00:51:46,960
are beyond anyone's control.
897
00:51:47,030 --> 00:51:48,660
So scientists are focusing on
898
00:51:48,730 --> 00:51:51,360
ways to live with these powerful forces
899
00:51:51,430 --> 00:51:54,630
and reduce their threats.
900
00:51:54,700 --> 00:51:56,420
The public would love it
if we could provide
901
00:51:56,440 --> 00:51:58,900
short-term predictions of hours to days
902
00:51:58,970 --> 00:52:01,610
before an earthquake, and we can't do that.
903
00:52:01,670 --> 00:52:05,740
But what we can do is provide
long-term estimates...
904
00:52:05,810 --> 00:52:07,810
Decadal estimates, perhaps.
905
00:52:07,880 --> 00:52:10,750
That should be important to US,
to our society,
906
00:52:10,820 --> 00:52:12,450
to plan for these events.
907
00:52:14,620 --> 00:52:17,250
In my lifetime, there's a 3/4 chance
908
00:52:17,320 --> 00:52:21,060
there will be a major earthquake
in the bay area where I live,
909
00:52:21,130 --> 00:52:23,560
and that should be enough to encourage me
910
00:52:23,630 --> 00:52:25,900
to plan wisely for that earthquake
911
00:52:25,970 --> 00:52:27,760
that I need to expect.
912
00:52:33,570 --> 00:52:36,610
With earthquake
predictions still in the future,
913
00:52:36,680 --> 00:52:38,610
earthquake-prone Japan has developed
914
00:52:38,680 --> 00:52:43,350
the world's first earthquake and
Tsunami early warning system.
915
00:52:43,420 --> 00:52:47,990
Its first major test came with
the massive earthquake in 2011.
916
00:52:50,190 --> 00:52:52,290
Residents of Tokyo were warned
917
00:52:52,360 --> 00:52:55,060
eight to nine seconds before the quake hit.
918
00:52:55,130 --> 00:52:57,460
At 2:46 in the afternoon.
919
00:52:57,530 --> 00:53:00,030
The shaking went on
for more than 2 minutes.
920
00:53:00,100 --> 00:53:01,700
The closest major city, sendai...
921
00:53:01,770 --> 00:53:04,770
Residents of the
nearest coastal city of sendai
922
00:53:04,840 --> 00:53:07,440
heard Tsunami warnings eight minutes before
923
00:53:07,510 --> 00:53:09,770
the 33-foot Tsunami came ashore.
924
00:53:12,850 --> 00:53:14,950
Japan's advanced state of preparedness
925
00:53:15,010 --> 00:53:18,280
and strong building codes
were the critical factors
926
00:53:18,350 --> 00:53:22,650
in saving countless lives
during this disaster.
927
00:53:22,720 --> 00:53:24,120
And the early warning system
928
00:53:24,190 --> 00:53:28,230
saved an untold number of lives as well.
929
00:53:31,200 --> 00:53:34,730
The Japan meteorological
agencies early warning system
930
00:53:34,800 --> 00:53:38,270
also doubles as Japan's volcanic
eruption warning system.
931
00:53:40,370 --> 00:53:44,270
Eruptions are easier to predict
than earthquakes.
932
00:53:44,340 --> 00:53:47,140
We are getting better all the time
933
00:53:47,210 --> 00:53:49,110
at predicting volcanic eruptions,
934
00:53:49,180 --> 00:53:52,920
but it's by no means perfect.
935
00:53:52,990 --> 00:53:57,350
And it's still a very difficult
thing to do.
936
00:53:57,420 --> 00:53:59,860
We have much better instrumentation.
937
00:53:59,930 --> 00:54:03,390
We can measure the inflation of the ground.
938
00:54:03,460 --> 00:54:06,900
Seismographs see how magma is moving up.
939
00:54:06,970 --> 00:54:11,130
Even temperatures of fumaroles,
changes in the gases.
940
00:54:11,200 --> 00:54:13,440
So we are getting better all the time.
941
00:54:22,180 --> 00:54:24,680
Meanwhile Earth's inner and outer forces
942
00:54:24,750 --> 00:54:28,420
will keep reworking the planet,
943
00:54:28,490 --> 00:54:31,320
from the center of the inner core
944
00:54:31,390 --> 00:54:34,890
to the highest peaks on the surface.
945
00:54:34,960 --> 00:54:37,430
No other planet known to US
946
00:54:37,500 --> 00:54:41,800
has so many different
natural forces at work.
947
00:54:41,870 --> 00:54:43,630
The Earth we see today
948
00:54:43,700 --> 00:54:47,440
has been 4 1/2 billion years in the making
949
00:54:47,510 --> 00:54:51,340
and has never looked quite this way before,
950
00:54:51,410 --> 00:54:55,950
and it will never look the same again.
951
00:54:56,010 --> 00:54:59,150
I think the way the Earth
looks now as a planet
952
00:54:59,220 --> 00:55:02,620
is unique in history,
953
00:55:02,690 --> 00:55:05,320
as is almost every period
in geologic history.
954
00:55:05,390 --> 00:55:08,390
The Earth is constantly undergoing change.
955
00:55:08,460 --> 00:55:09,460
What are the Greens?
956
00:55:09,530 --> 00:55:10,770
The Greens are country roads,
957
00:55:10,800 --> 00:55:12,300
and the yellows are local roads.
958
00:55:12,360 --> 00:55:14,430
From a geologist's point of view,
959
00:55:14,500 --> 00:55:17,530
the Earth is really, really interesting
960
00:55:17,600 --> 00:55:22,570
because, on Earth,
we see all the major processes
961
00:55:22,640 --> 00:55:27,410
that form the surface
of a planet actually going on.
962
00:55:27,480 --> 00:55:30,510
We have volcanism.
963
00:55:30,580 --> 00:55:34,550
We have impact craters formed
not that long ago.
964
00:55:34,620 --> 00:55:36,550
We have an atmosphere,
965
00:55:36,620 --> 00:55:40,520
which means we have liquid water.
966
00:55:40,590 --> 00:55:44,130
We have wind. We have erosion.
967
00:55:44,200 --> 00:55:46,500
We have plate tectonics on Earth
968
00:55:46,570 --> 00:55:50,600
which appears to be unique
in the solar system.
969
00:55:50,670 --> 00:55:55,840
So it's very geologically
diverse and very dynamic.
970
00:55:55,910 --> 00:55:58,440
So I think we are lucky,
actually, to live here.
76399
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