How the Earth Was Made

>> Earth, a 4.5 billion year old

planet, still evolving.

As continents shift and clash,

volcanoes erupt, and glaciers

grow and recede, the Earth's

crust is carved in countless

fascinating ways, leaving a

trail of geological mysteries

behind. This episode

investigates one awesome force

that shapes the Earth.

It's extraterrestrial, and it

doesn't happen over millions of

years, but in seconds.

It's a force caused by an

immense impact from asteroids -

giant rocks from space.

The investigation of asteroid

strikes has given scientists

insight into the formation of

the universe, providing a

window into planet Earth's

ancient past.

S01x10 Asteroids Original Air Date on April 21, 2009

== sync, corrected by elderman ==

There is a giant hole in the

Arizona desert, 35 miles east of

Flagstaff. It's huge, three

quarters of a mile wide and 550

feet deep.

The Washington monument could

fit inside it. The mystery

confronting geologists in the

late 19th century was, how did

this happen? People agreed that

only a massive force could have

created such a huge chasm.

But what was this force?

The most likely theory was that

a huge volcanic explosion had

ruptured the rugged landscape.

Scientists had discovered

similar sized craters in

volcanic areas before.

But Grove Gilbert, chief

geologist for the US Geological

Survey, had another idea, one

that came to him after

observing craters on the moon

through his telescope.

He saw similarities between the

moon's craters and the

mysterious hole in the Arizona

desert, leading him to

speculate that the Arizona

crater might have been caused

by an asteroid impact.

But this was just a theory.

At the time, no one had proven

that any crater on Earth had

been caused by an asteroid.

So, in 1892, Gilbert decided to

travel to the mysterious

Arizona crater.

>> He wondered if this could

have an impact origin, or

alternatively, a volcanic

origin. And so he had these two

competing hypotheses that he

wanted to test.

>> Gilbert assumed that if the

crater was caused by an

asteroid, he should find a

giant, alien of rock in the

middle of it. But there was

none. But he did see what he

thought were signs that a

volcano might be the cause.

>> KRING: When Gilbert arrived,

he realized that this hole in

the ground was associated with

some volcanic peaks in the

distance, you can see them in

the background over the rim, or

beyond the rim of the crater,

and so, immediately, I think, he

was prejudiced, if you will,

towards a volcanic, as opposed

to a meteoritic origin.

>> Not far from the mysterious

crater, Gilbert found another,

similar, giant hole. He declared

this one as an unusual volcanic

crater, called a maar.

He knew that four years before,

in Japan, scientists had

witnessed the formation of a

maar after a huge underground

explosion of steam. The

resulting crater resembled the

giant hole Gilbert came

across in the Arizona desert.

>> They are produced when

basaltic magma rises through

the Earth's crust, encounters

groundwater, creating a steam

explosion, which causes a blast

that produces, uh, craters like

the one, uh, over my shoulder.

>> Intriguingly, the two almost

identical craters were only 50

miles apart in the same desert.

One was known to have been

caused by an underground steam

explosion. Because of their

close proximity, Gilbert

concluded that the mysterious

crater was also caused by

volcanic activity. In 1896, he

published his findings in an

influential report and, for the

wider geological community,

the debate was resolved.

>> But, six years after

Gilbert's findings, American

entrepreneur and mining engineer

Daniel Barringer arrived on the

same scene. He was intrigued by

mysterious small iron rocks

shepherds had found around the

crater while grazing the

herds. Barringer was convinced

that Gilbert was wrong about the

crater's origins.

>> I'm holding in my hands a

fragment of what started it all.

It is not the type of material

that one finds in any other

geological terrain or created by

any other terrestrial geologic

process.

>> Iron in rock is usually mixed

with other minerals. But at the

mysterious crater, the iron was

almost pure. And there were

large amounts, normally not

found on the earth's surface,

spread over a huge area

surrounding the crater.

Barringer believed the pieces

found here were meteorites,

small space rocks that form when

big asteroids break apart.

He had a hunch that this huge

hole in the desert was formed

by a giant asteroid made largely

from iron. Barringer immediately

saw the commercial

opportunities. From the crater's

size, he calculated that the

asteroid must have weighed ten

million tons.

With iron then at 80 dollars a

ton, Barringer was convinced he

could become a rich man from

mining the iron.

So in 1903, Barringer bought the

crater site of over 1,200 acres

and hired crews to begin

digging. Convinced it was an

impact site, he named it Meteor

Crater.

>> KRING: We're in one of the

remnants of Barringer's mining

camp. This is a place where his

miners lived, ate, slept, while

they looked for the buried

meteoritic mass that they

thought was beneath the floor of

the crater.

>> For years, Barringer and his

men found only small fragments

of iron. Undeterred by this,

they kept digging deeper shafts

into the earth.

>> KRING: The largest of which,

the main shaft, you can see is a

white island of debris in the

centre of the crater. In fact,

one of those holes close to me

here reached a depth of nearly

1,400 feet beneath the surface

of the Earth. In all of these

cases, or most of these cases,

they found telltale hints of the

impacting meteoritic body, but

no giant mass.

>> Barringer did stumble across

some clues, however. Strange and

unique rock formations such as