METEOR CRATER: THE FIRST PROVEN IMPACT CRATER

METEOR CRATER, ALSO CALLED BARRINGER CRATER, FORMED IN A METEORITE IMPACT. This realization caused some scientists to change the way they thought about the solar system. After 1909, researchers began to argue that if a meteorite could make a huge crater on Earth, impacts might have cratered the Moon and other planets too.

When mining engineer Daniel Moreau Barringer first saw the crater in 1903, he not only suspected that it formed during a meteorite impact, he also believed he could unearth a huge meteorite mass and market its valuable metal content. He spent the rest of his life exploring these two ideas.

Although Barringer's mining attempts failed-most of the meteorite had been destroyed on impact-his dedicated research on the crater laid a foundation for understanding impact cratering on Earth and other planets.

HOW DO WE KNOW?

HOW DO WE KNOW AN IMPACT FORMED METEOR CRATER?

When Barringer's drilling crews bored into the rock layers under and around the crater, they found convincing evidence that it could not have formed from a volcanic explosion, as some had suggested. A meteorite impact was the only logical explanation.

Around the rim of the crater, the layers of rock debris are deposited in the opposite order of the original rock layers -- as if they had been folded back and laid upside down by the force of the impact (brown layers, to right of crater).

If an explosion of volcanic steam had formed the crater, as some people argued, the blast from below would have disrupted the underlying rock. Instead they found an undisturbed layer of Supai sandstone.

A volcanic explosion would have ejected samples of the Supai sandstone layer beneath the crater, depositing them around the rim. None of the ejected debris contain this sandstone.

METEOR CRATER

Meteor Crater, the best-preserved impact crater in the world, is located in the high desert of northern Arizona. The name Meteor Crater is slightly inaccurate because meteors, by definition, burn up before reaching the ground. Meteorites found at the crater are called Canyon Diablo meteorites, after the nearest post office. Canyon Diablo is also the name of a nearby canyon, shown in the background above.

THIS IS NOT AN IMPACT CRATER

Until the early 1900s, most geologists assumed that all craters, including Meteor Crater, were formed by volcanic explosions. And at first glance, volcanic and impact craters do look very similar. But most volcanic craters have plenty of lava rock in and around them, while impact craters do not.

A violent explosion of volcanic gas or steam 700,000 years ago blew open Zuni Salt Lake Crater, in New Mexico (left). Recent minor eruptions blew out dark cinders, which form a small ring in the center of the crater.

Zuni Salt Lake Crater is about one-third wider than Meteor Crater. Its proximity to Meteor Crater-just 226 kilometers (140 miles) east-led many people to falsely assume the two formed in the same way.

The lake inside the crater is fed by salty springs rising through the cracked volcanic rock below. Four Native American tribes, including the Zuni people, visit the crater each year to collect salt for

THE MAKING OF A CRATER

When an explosive impact crater forms, the impacting meteorite's kinetic energy compresses and heats the rock below. This intense heat and pressure melt, crack and toss out the impacted rock.

CONTACT/COMPRESSION STAGE

Just after the meteorite strikes the ground, most of the meteorite and some surrounding surface rock turn to liquid and vapor. As this molten meteoritic rock and surface rock begin to splash up and out of the widening crater, a wave of pressure travels down into the rocks below.

EXCAVATION STAGE

Next, the pressure wave rebounds back upward, forcing millions of tons of broken rock up out of the crater. The pressure also pushes outward, forming the crater's raised rim. The rock surrounding the crater fractures into a network of cracks hundreds of feet deep.

MODIFICATION STAGE

As dust and smoke fill the air, the ejected debris crashes back to the ground. Much of it falls inside the crater, forming a lens-shaped layer of pulverized, fused and fractured rock. Other debris lands in a ring-shaped blanket around the rim of the crater. A mist of molten meteoritic rock hardens in midair and rains down as tiny rock spheres.

Tens of thousands of Canyon Diablo meteorite fragments were found at Meteor Crater-most were much smaller than this 200 kilogram (440 pound) sample.