The Barringer
Meteorite Crater (also known as "Meteor
Crater") is a gigantic hole in the middle of
the arid sandstone of the Arizona desert. A rim of
smashed and jumbled boulders, some of them the
size of small houses, rises 150 feet above the
level of the surrounding plain. The crater itself
is nearly a mile wide, and 570 feet deep. When
Europeans first discovered the crater, the plain
around it was covered with chunks of meteoritic
iron - over 30 tons of it, scattered over an area
8 to 10 miles in diameter. |
 |
The process of scientific
discovery involves the development of hypotheses,
tentative explanations which may or may not account for
the observable facts. A good scientific hypothesis will
generate a number of logical consequences or
predictions, which are capable of being tested directly.
Ultimately, the hypothesis will be accepted as valid
only if: repeated tests of these predictions, by
different investigators, tend to confirm it; it is
consistent with other well-confirmed hypotheses; it is
more useful than other hypotheses in accounting for a
broad range of observed facts; and it is more economical
or "elegant" than other hypotheses, in the
sense of requiring fewer additional assumptions.
Although meteorite falls had been reported for thousands
of years, until this century no one had ever identified
a crater created by such a fall. Even a meteorite as
large as the 66-ton Hoba, the largest ever discovered,
may be slowed so much by the Earth's atmosphere that it
lands without making a significant hole. In 1891 Grove
Karl Gilbert, then chief geologist for the U.S.
Geological Survey, decided to test two conflicting
hypotheses about the crater. The first was that the
crater was created by the impact of a giant meteorite;
the second, that it was the result of an explosion of
superheated steam, caused by volcanic activity far below
the surface. If an iron meteorite had created the
crater, Gilbert assumed that it would have had to be
nearly as big as the crater itself. So what predictions
could he test? First, the meteorite should be taking up
a lot of space in the hollow of the crater. The volume
of the hollow would therefore be less than the volume of
the ejected material in the crater rim. Second, the
presence of a large mass of buried iron should affect
the behavior of magnets and compass needles. Neither
prediction was confirmed. Gilbert concluded that a steam
explosion was the only surviving hypothesis, in spite of
the fact that no volcanic rocks had ever been found in
the area. The meteorites around the crater were simply a
coincidence.
 |
Ten
years later a very different sort of explorer came
along. In 1902 Daniel Moreau Barringer, a
successful mining engineer, heard about the
crater. When he learned that small balls of
meteoritic iron were randomly mixed with the
ejected rocks of the crater rim, Barringer
immediately concluded that the crater had resulted
from a meteorite impact. If the meteorites had
fallen at a different time from the time at which
the crater was formed, they would have appeared in
separate layers from the ejected rock. Like
Gilbert, Barringer assumed that the meteorite
which made the crater would have to be extremely
large - large enough, in fact, for a major mining
bonanza. |
Rather than testing his impact
hypothesis, Barringer set out to assemble the evidence
in support of it. In 1906, and again in 1909, he
presented his arguments for the impact origin of the
crater to the Academy of Natural Sciences in
Philadelphia. The evidence included: A. The presence of
millions of tons of finely pulverized silica, which
could only have been created by enormous pressure. B.
The large quantities of meteoritic iron, in the form of
globular "shale balls", scattered around the
rim and surrounding plain. C. The random mixture of
meteoritic material and ejected rocks. D. The fact that
the different types of rocks in the rim and on the
surrounding plain appeared to have been deposited in the
opposite order from their order in the underlying rock
beds. E. The absence of any naturally occurring volcanic
rock in the vicinity of the crater. In 1908, these
conclusions were championed by geologist George P.
Merrill. Merrill analyzed a new type of rock discovered
by Barringer at the crater, which Barringer called
"Variety B". He concluded that it was a type
of quartz glass which could only be produced by intense
heat, similar to the heat generated by a lightning
strike on sand. Merrill also pointed to the undisturbed
rock beds below the crater, which proved that the force
which created the crater did not come from below.
During the same years, a debate
was raging among astronomers about the origin of the
craters on the moon. As with the Barringer crater, most
astronomers initially assumed that those craters were
volcanic. Gilbert himself, ironically, was one of the
first to argue for an impact origin, in a paper
published in 1893. In 1909, a German geologist advanced
the same theory, based in part on the evidence presented
by Barringer for the Arizona crater. One objection to
the idea of an impact origin for the lunar craters was
the fact that all lunar craters are round. Astronomers
assumed that most meteorites would have struck the moon
at oblique angles, producing elongated craters.
Barringer, however, had experimented by firing rifle
bullets into rocks and mud, and had discovered that a
projectile arriving at an oblique angle would
nevertheless make a round hole. In 1923, Barringer's
12-year-old son Richard published an article in Popular
Astronomy, using his father's rifle experiments to argue
for the impact origin of the lunar craters; Barringer
himself repeated the arguments a short time later in the
Scientific American. The conclusive arguments in the
lunar debate were provided by astronomers such as A. C.
Gifford, who demonstrated that the force of an impact at
astronomical speeds would result in the explosion of the
meteorite. Whatever the original angle of impact, the
result would be a circular crater.
In 1928, $200,000 was raised for a
final assault on the meteorite. Barringer's directors,
however, were growing nervous. When the new mine shaft
hit water in such great quantities that it could not be
pumped out, they consulted the astronomer F. R. Moulton
for his opinion on the size of the meteorite. Moulton
calculated the amount of energy which would be produced
by an impact at the enormous speed typical of a
meteorite arriving from space. He concluded that an
object big enough to create the crater would probably
weigh only 300,000 tons - 3% of the amount estimated by
Barringer, and too small to justify any further
drilling. In addition, Moulton argued that the explosion
caused by the impact would result in the total
vaporization of the meteorite. In 1929, work was halted
at the crater. By November of that year, it had become
clear that other prominent scientists agreed with
Moulton. Within weeks, Barringer was dead of a massive
heart attack.
Scientists now believe that the
crater was created approximately 50,000 years ago. The
meteorite which made it was composed almost entirely of
nickel-iron, suggesting that it may have originated in
the interior of a small planet. It was 150 feet across,
weighed roughly 300,000 tons, and was traveling at a
speed of 40,000 miles per hour. The force generated by
its impact was equal to the explosion of 20 million tons
of TNT. In 1946, meteorite collector Harvey H. Nininger
analyzed the tiny metallic particles mixed into the soil
around the crater, along with the small
"bombs" of melted rock within it. He concluded
that both types of particles were solidified droplets,
which must have condensed from a cloud of rock and metal
vaporized by the impact. Here, he believed, was proof
that the crater was created by explosion.
 |
In
1963, geologist Eugene Shoemaker published his
landmark paper analyzing the similarities between
the Barringer crater and craters created by
nuclear test explosions in Nevada. Carefully
mapping the sequence of layers of the underlying
rock, and the layers of the ejecta blanket, where
those rocks were deposited in reverse order, he
demonstrated that the nuclear craters and the
Barringer crater were structurally similar in
nearly all respects. His paper provided the
clinching arguments in favor of an impact, finally
convincing the last doubters. Three years earlier,
Shoemaker, Edward Chao and David Milton had also
collaborated in the discovery of a new mineral at
the Barringer crater. This mineral, a form of
silica called "coesite", was first
created in a laboratory in 1953 by chemist Loring
Coes. Its formation requires pressures of at least
20,000 atmospheres (20 kilobars) and temperatures
of at least 700 degrees Celsius - greater than any
occurring naturally on earth. Coesite and a
similar material called "stishovite"
have since been identified at numerous other
suspected impact sites, and are now accepted as
indicators for the impact origin of a geologic
structure. Another indicator is the presence of
rock structures known as "shattercones".
These structures, which can be anywhere from less
than an inch to more than six feet tall, can only
be created by a sudden intense pressure on
existing rock. During the 40's and 50's,
investigations by Robert S. Dietz and others
revealed the existence of shattercones at many
suspected impact sites, although not at the
Barringer crater. Deitz was able to demonstrate
that the apexes of the cones at most of these
sites all pointed upwards, indicating that the
force which created them had come from above. |
 |
Using these methods,
meteoriticists have now identified over 150 proven
impact sites. Evidence suggests that there have been
many thousands of other impacts over the course of the
earth's history. Meteorites weighing a quarter of a
pound or more hit the earth thousands of times a year.
One large enough to form the Barringer crater may arrive
as often as once every thousand years. The mysterious
Tunguska explosion of 1908, which devastated an area of
Siberian forest the size of Rhode Island, may have been
our most recent encounter with a visitor of this size.
In 1980, a new hypothesis emerged. Scientists Walter and
Luis Alvarez discovered that a layer of soil containing
extremely high levels of the mineral iridium - rare on
earth, but abundant in meteorites - had been deposited
all over the earth about 65 million years ago. That date
marks the end of the Cretaceous period, a time when not
only the dinosaurs, but thousands of other plant and
animal species suddenly became extinct. The Alvarezes
theorized that the mass extinctions had been caused by
the impact of a giant meteorite, perhaps six miles in
diameter. Such an impact would throw up a cloud of dust
thick enough to obscure the sun for several years,
disrupting the planetary food chain and causing the
disappearance of vast numbers of species. The recent
discovery of two giant craters, roughly 65 million years
old, underneath the Yucatan peninsula, plus the
worldwide distribution of coesite in the Cretaceous
boundary layer, have made the Alvarez hypothesis seem
more and more convincing.
Meteoroids, Meteors and
Meteorites
When drifting through space,
smaller pieces of space rock are called meteoroids.Most
of them are the size of sand or gravel. Eventually, some
of these meteoroids cross the path of the Earth. They
may enter our atmosphere at a velocity of several miles
per second.
Because of their tremendous speed,
meteoroids are heated by friction with the air to incandescence,
which means they glow red hot or even white hot. They
are now called Meteors. The air around the meteor
also glows, making it possible to see the meteor from
the earth. Most small meteors burn up in the atmosphere,
leaving only microscopic fragments to drift down to the
earth as dust.
Larger meteors are often able to
survive their burning path through our atmosphere,
however, and strike the earth. We call these meteorites.
Because much of the earth is covered by water, most
meteorites are never found. When meteorites manage to
fall onto land, it is possible for us to find and study
them. This is one way to study our solar system.
|