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Look at the Pacific Ocean from space,

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and you can trace a line
around almost all of it.

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Alaska.

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Japan.

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The Philippines.

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New Zealand.

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Chile.

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Mexico.

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The Pacific Northwest.

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A horseshoe, 40,000 kilometers long.

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Along that line sit about three quarters
of the world's active volcanoes.

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And about 90% of its earthquakes.

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People call it the Ring of Fire.

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This is the story of the Ring.

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Why it exists.

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The biggest earthquake ever
measured, which happened on it.

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The volcano that cooled the whole planet.

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And the stretch of it that is
quiet now, but not finished.

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Start with the plates.

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The floor of the Pacific is one enormous
plate, with smaller ones around its edges:

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Nazca, Cocos, Juan de Fuca,
the Philippine Sea plate.

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They are all made of ocean crust.

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And ocean crust is heavy.

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Where a heavy ocean
plate meets a continent,

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or a younger ocean plate, it loses.

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It bends, and dives underneath.

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Geologists call it subduction.

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Off the coast of Chile, the Nazca
plate is sinking under South America,

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about 7 centimeters a year.

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Where it goes down,
the seafloor sags into a trench.

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The deepest water on Earth,
the Mariana Trench,

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11 kilometers deep, is one of these.

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The Ring of Fire is
really a ring of trenches.

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And here's the fire.

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The sinking plate carries
seawater down with it.

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About 100 kilometers deep,

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that water lowers the melting
point of the rock above.

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The rock melts.

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Magma rises.

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And a line of volcanoes grows, 100
to 200 kilometers inland from the trench.

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So every arc of the Ring
is the same machine.

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A trench offshore,
where the ocean floor dives.

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A row of volcanoes behind it,
where the melted rock comes up.

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And earthquakes everywhere in between,
because the plates don't slide.

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They stick, and then they snap.

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There's one famous gap in the pattern.

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California.

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The plates there slide past each
other sideways, along the San Andreas,

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instead of one diving under the other.

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That's why California
has great earthquakes,

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but no chain of smoking volcanoes.

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If you watched our Loma
Prieta story, you've seen it.

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Southern Chile.

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May 22, 1960.

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Just after three in the afternoon.

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The Nazca plate, locked against South
America for more than a century, lets go.

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The fault tears for about
1,000 kilometers.

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From Concepción, past
Valdivia, down to Chiloé.

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The shaking lasts around 10 minutes.

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Seismographs around the world recorded it.

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Its magnitude, later calculated at 9.5,

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makes it the largest
earthquake ever measured.

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In Valdivia, the ground itself dropped.

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Along parts of the coast,
the land sank by as much as 2 meters,

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and the sea moved in to stay.

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Farms became marshes.

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Streets ended in water.

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Then came the tsunami.

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The seafloor had lurched upward, and that
shoved the whole water column above it.

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Within minutes, waves up to 25
meters high hit the Chilean coast.

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And the wave kept going.

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Across the open Pacific, a tsunami
travels at the speed of a jet airliner.

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Fifteen hours after the earthquake,
it reached Hawaii.

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Twenty two hours after, Japan.

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About 17,000 kilometers from Chile.

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In Hilo, Hawaii, sirens
had sounded hours earlier.

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Many people went
to the waterfront to watch.

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The third wave came in at about
35 feet, and 61 people died.

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In Japan, with no warning at all,
the wave killed about 140.

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All told, the earthquake and its tsunami
killed more than 1,600 people in Chile,

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and more across the Pacific.

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And then the Ring showed
the other half of its nature.

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Thirty eight hours after the earthquake,

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a volcano in the Andes called
Cordón Caulle began to erupt.

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The shaking had rattled
the plumbing beneath it.

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Trench, earthquake, volcano: one machine.

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Which brings us to the
volcanoes themselves.

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Twenty years after Chile, the Ring
struck the mainland United States.

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May 18, 1980, 8:32 in the morning.

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A magnitude 5.1 earthquake
shook Mount St. Helens,

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and the whole north face
of the mountain slid away.

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The biggest landslide ever recorded.

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Behind it, the volcano exploded sideways.

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The blast flattened forests
as far as 19 miles away.

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Ash fell across 11 states.

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The mountain lost 1,300
feet of its summit.

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Fifty seven people died,
including a geologist, David Johnston,

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who was watching from a ridge
about 6 miles away.

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His last words on the radio:
Vancouver, Vancouver, this is it.

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Scientists had been
watching for two months.

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They had the area closed.

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Without that, the toll
could have been far higher.

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It was a lesson for the whole Ring:
volcanoes give warning signs,

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if someone is watching.

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Eleven years later, that lesson
saved thousands of lives.

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Mount Pinatubo, in the Philippines,
had been quiet for 500 years.

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In the spring of 1991, it woke up.

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Scientists from the Philippines
and the USGS read the warning signs,

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and convinced the authorities to evacuate.

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On June 15, Pinatubo produced the second
largest eruption of the twentieth century.

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The column rose 35 kilometers,
into the stratosphere.

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Hundreds died when wet ash collapsed
roofs, and hundreds more later,

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from mudflows and disease
in crowded camps.

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But more than 60,000 people had
been moved out of the way in time.

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The evacuation is still the model
for how a volcano should be handled.

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And then Pinatubo did
something to the whole planet.

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It threw about 20 million tons
of sulfur dioxide into the stratosphere.

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That formed a haze that circled
the Earth and reflected sunlight.

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For the next 2 years,

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the average temperature of the entire
globe fell by about half a degree Celsius.

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Now look at the stretch of the Ring that
runs from Northern California to Vancouver

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Island.

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The Cascadia subduction zone.

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It has the same trench,
the same volcanoes,

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and it is the quietest great
fault on the whole Ring.

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That is the problem.

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For a long time, nobody knew Cascadia
could produce a great earthquake.

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Then, in the 1980s, geologists found
ghost forests along the Washington coast.

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Stands of dead cedar,

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killed when the land suddenly
dropped and salt water flooded in.

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Tree rings dated the deaths
to the winter of 1699 to 1700.

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Then came the proof,
from the other side of the ocean.

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Japanese records describe a tsunami that
arrived on the night of January 27, 1700,

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with no earthquake felt in Japan.

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An orphan tsunami.

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Work the travel time backwards,

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and its parent was a magnitude
9 earthquake off Cascadia,

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at about nine in the
evening on January 26.

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That was more than three centuries ago.

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The geologic record shows these great
quakes come every few hundred years.

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The next one will shake Seattle,
Portland and Vancouver,

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and send a tsunami onto
the coast within minutes.

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That is why the region drills for it,

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and why the warning sirens
on the Oregon coast are there.

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The Ring is never quiet for long.

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About 450 volcanoes stand along it.

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In a typical year, Japan alone
feels around 2,000 earthquakes.

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And the Pacific plate keeps moving,
7 to 10 centimeters a year,

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into the trenches around its rim.

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On our live globe, the Ring is always lit.

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Earthquakes around the Pacific
appear within minutes,

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and volcanoes on alert are marked.

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Each one is the same story.

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The ocean floor,
going back into the Earth.

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So when you see a quake on the stream
in Chile, or Alaska, or Japan,

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or an alert at a volcano in the
Philippines, you now know why it's there.

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It's the Ring.

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Forty thousand kilometers of coastline,

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where the Pacific is slowly
disappearing beneath our feet.

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Thanks for watching Earth Explained.

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