METEOR CRATER: PROOF OF IMPACT

SHOCKED, MELTED AND PULVERIZED ROCKS HELPED PROVE THAT METEORITE IMPACTS CAN MAKE CRATERS.

THE FIRST GEOLOGICAL REPORT ON THE GIGANTIC NORTHERN ARIZONA CRATER, in 1891, focused not on the crater itself but on tiny diamonds found in nearby meteorites. Back then, researchers assumed that only an explosive volcanic eruption could make such a large crater. But after more careful study of the meteorites and altered rocks in and around the crater, scientists realized that it must have formed during a massive meteorite impact.

The story of Meteor Crater, also called Barringer Crater, began 50,000 years ago with an asteroid roughly twice as wide as this hall hurtling to Earth at about 50 times the speed of sound. The explosive force of its impact blasted out 175 million tons of limestone and sandstone, sprayed molten rock upward and tossed chunks of meteorite several miles away.

Once the dust and debris settled, the crater sat quietly for tens of thousands of years as the Arizona climate gradually grew drier, helping to preserve evidence of the impact.

The Canyon Diablo meteorite, which formed Meteor Crater, broke apart as it passed through Earth's atmosphere. Tens of thousands of fragments were found strewn around the crater.

This is the second-largest surviving fragment of the Canyon Diablo meteorite that formed Meteor Crater; it probably broke off when the main mass hit the atmosphere.

PUFFED UP AND PULVERIZED

Both the pulverized rock, called rock flour, and the foamy-looking lechatelierite shown below are made of glass. They were originally part of the same layer of gray Coconino sandstone. Sandstone is made of quartz, a crystalline form of silicon dioxide. Under intense heat and pressure, quartz crystals turn into glass.

The presence of these two types of rock is clear evidence that a meteorite impact briefly compressed the sandstone. Only an impact can create such intense pressures on Earth's surface.

The porous lechatelierite was puffed to more than twice its original size by steam that was forced into the rock as the pressure released. The rock is now so light that it can float on water.

The rock flour contains grains of coesite, another type of quartz that forms under even higher pressure.

In 1946, the newly formed Meteor Silica Corporation began mining the millions of tons of pulverized silica glass beneath Meteor Crater as raw material for manufacturing telescope lenses. The venture quickly failed, and left yellowish white pits on the crater's south rim.

RUST NEVER SLEEPS

Many of the thousands of the fragments of Canyon Diablo found at Meteor Crater have been heavily weathered. Over the years, wind, water and drastic temperature changes have combined to slowly break them apart.

A meteorite fragment, such as the sliced sample of Canyon Diablo (bottom), can eventually rust as water seeps into its crystal structure and cracks it. Cracked and rusted meteorites are called shale balls (right and below), for their resemblance to the sedimentary rock shale. Particularly fragile shale balls break apart into fragments (in dish below).

LOOK CLOSELY

More than 80 percent of the Canyon Diablo meteorite that formed Meteor Crater vaporized into a hot mushroom cloud of meteoritic metal, which later condensed to form these spherical bits of iron. Meteorite specialist Harvey Nininger discovered them in 1946 by attaching a magnet to the end of a cane and sweeping it around the crater. Spheres like these help to prove that a given crater formed from an iron meteorite explosion.

COOKING WITH LIMESTONE

Some of the Kaibab limestone that melted during the Meteor Crater impact mixed with solid rock and bits of meteorite. It then solidified in midair, landing as "impactites." The heated limestone released carbon dioxide gas, leaving behind porous calcium oxide. Limestone cooked in a kiln undergoes this same reaction.

HOW DO WE KNOW?

EJECTED EVIDENCE

In only six seconds, the Canyon Diablo meteorite excavated Meteor Crater, lifting up 175 million tons of sandstone and limestone, tossing much of it outside the crater. These pieces of rock, some as large as houses, helped to prove that the crater was formed by a meteorite impact.

When mining engineer Daniel Barringer first argued in 1906 that an impact formed the crater, he noted that the meteorites and other rock debris around the crater were randomly mixed together in one layer. This mixing suggested that the meteorite fell at the same time the crater formed.

In the 1960s, researchers discovered that some pieces of ejected sandstone contained microscopic evidence of the intense impact pressures. On the surfaces of individual quartz grains, researchers saw criss-crossing sets of parallel lines. These lines show that intense pressure passed through the rock in a fraction of a second, altering the grains' three-dimensional crystal structure. Shocked quartz and other shocked minerals have provided crucial evidence that many known craters were formed by meteorite impacts.

This block of limestone, as tall as the height of this hall's ceiling, was thrown up during the explosive formation of the crater and landed several feet from the rim.

The pattern of intersecting parallel lines on the surface of this grain of shocked quartz (magnified using a scanning electron microscope) show how the shock wave disturbed the grain's crystal structure.