Name: What Really Killed the Dinosaurs?
Uploaded: 2021-04-27T03:37:38.000Z
Duration: 10 min 45 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

様態の副詞

So you just clicked on this video, you saw the title - you probably think you already

But as it often turns out, the science is more complicated than what we have been told.

First, let's take a look at the story we've heard, and why we're so sure that an asteroid

impact was a big part of the dinosaurs not existing anymore.

Some time around 66 million years ago, a giant asteroid about the size of Manhattan careened

It smashed into the ocean on the shores of what we now think of as the Gulf of Mexico

The energy of the impact itself vaporized every living thing within a day's walk.

Then huge chunks of the meteorite that had been thrown up into the atmosphere started

The ensuing cloud of ash and dust that was vaulted into the atmosphere obscured the sun

for decades, creating a dim, cold climate plagued by acid rain and firestorms.

Over the millennia that followed, at least 50% of the species on Earth disappeared - 20%

of the world's species of sharks, rays, and skates; nearly 60% of the plants; 98% of the

warm-water corals; and of course, 100% of the dinosaurs.

For most of us, that collision has become a kind of shorthand explanation for why the

dinosaurs disappeared, and because we associate asteroids with extinction, it's a scenario

But as our knowledge expands, we've come to see that what killed the dinosaurs, like all

of the other great extinctions that happened before, was probably a lot more complex than

Because it turns out, there was more than one disaster movie playing at the cineplex

It was such a calamity that it literally, in geological terms, marked the end of an

The dinosaur dominated period of life that we knew as the Cretaceous Era was suddenly

The new normal, marked by the rise of mammals, would be called the Paleogene Era.

The giant die-out that created this transition is now known as the Cretaceous-Palogene Extinction

And like many things in science, there's not a whole lot of consensus about what caused

the K-Pg, but there are a few facts that everyone agrees on.

First, about 66 million years ago, the climate changed, a lot, all over the world.

We can tell by changes in the types of marine fossils from both before and after the event,

along with differences in soil chemistry and other evidence that the climate became much

cooler really fast - by 7 degrees Celsius in some places.

The transition was so abrupt that the layer of rock that separates the before picture

from the after picture is easy to spot with a well-trained eye.

It's known as the Cretaceous-Palogene Boundary, and the chemistry of this boundary of rock

is itself another weird clue, because it shows up all over the world in the same way - a

sudden appearance of a layer of clay that's rich in iridium.

Iridium is a metal, and if you've never heard of it, that's because it's very rare on earth.

We tend to find it most often in meteorites and other celestial deposits.

The last clue that everyone agrees on is simply extinction itself - tons of living things

Tyrannosaurus Rex, the biggest land predator at the time, Bennettitales, a group of tall

plants with thick armored trunks, and whole passel of reptiles that lived alongside the

dinosaurs and are still sometimes mistaken for them.

Aquatic reptiles like mosasaurs and plesiosaurs, and pterosaurs - those freaky flying reptiles

So the reason that the Asteroid Impact Theory is so compelling is that it explains pretty

The theory first came about because of that iridium.

In 1980 geologist Walter Alvarez was studying clay deposits in Italy that dated back about

65 million years and found that they contained hundreds of times more iridium than normal,

and the more of that stratum of the earth that he sampled in different parts of the

Since iridium is rare on earth but plentiful in space-rocks, Alvarez and his father - Nobel

prize winning physicist Luis Alvarez - hypothesized that it was a sign of an enormous asteroid

Based on the amount of iridium they'd found, they figured that the collision was caused

by a giant metallic rock, about 10 kilometers across - big enough create an impact with

the energy of 180,000,000 megatons of TNT.

Alvarez and son theorized that this collision must have been what wiped out the dinosaurs.

Soon, scientists stared finding other signs of a colossal asteroid impact - like shocked

At K-Pg sites around the world, the quartz looked like it had been transformed - its

crystals rearranged by tremendous pressure that you'd never suspect to find on earth's

Then they found tektites - big tear-dropped shaped pieces of glass that form when rocks

are thrown into the atmosphere by an impact and then melt on re-entry.

So all the signs pointed towards a huge, ugly collision.

But there was one thing missing - a crater!

Scientists new about a lot of different impact craters around the world, but none of them

matched the magnitude of the one that the Alvarezes envisioned.

Then researchers started noticing that a lot of the evidence they were studying - the iridium,

the quartz, the glass - was all concentrated in North America, so they focused their search,

using new technology, like satellite imagery and scans that measure the density of rock

beneath the surface, and in 1991 they found what they were looking for - a giant crater

just off the coast of Mexico's Yucatan Peninsula, about 180 kilometers across - about the size

you'd expect a 10 kilometer asteroid to make, and marked by huge disruptions in the layers

It was named after the nearby town of Chicxulub, and that is how the biggest piece of the puzzle

came together - the Asteroid Impact Theory, also known as the Alvarez Hypothesis.

So, like I said, this theory checks off a lot of the boxes when it comes to the evidence

Explains the abundance of iridium in the rocks?

Accounts for the drastic and sudden change in climate?

Was capable of killing 50% of life on earth?

So the rest is history - by which I mean it became accepted as historical fact, and it's

the quick and easy answer to the question of what killed the dinosaurs.