EARTH IMPACTS: ALL CRATERS GREAT AND SMALL

METEORITES COLLIDING WITH EARTH LEAVE CRATERS OF MANY SIZES, WHICH OFTEN PROVIDE CLUES TO HOW THEY FORMED.

DURING EARTH'S 4.5 BILLION YEAR HISTORY, countless meteorites have crashed onto the planet. Many of these impacts initially left behind craters, ranging from small pits to huge cavities dozens of miles across. Yet so far, people have found only about 200 meteorite impact craters.

Most of Earth's craters have been erased by the resurfacing processes of tectonic plates, the massive slabs that make up Earth's crust. Other craters have been buried beneath mud, lava or blowing sand, hidden beneath oceans or weathered away by wind and water.

Craters come in many sizes, just like the meteorites that make them. The largest objects that hit Earth explode upon impact and leave huge craters, while the smallest ones leave no craters at all. But for sizes in between, the results are harder to predict. For example, the 60-ton iron mass called Hoba did not explode and left no crater.

EARTH'S KNOWN IMPACT CRATERS

Meteorite and comet impacts have left more than 165 named craters on Earth. They range in size from Haviland Crater in Kansas, smaller than the width of this hall, to the largest of all—Vredefort Crater, a barely recognizable ring of mountains in South Africa 300 kilometers (186 miles) across.

FLYING GLASS

Tektites are pieces of glass made when rock on Earth's surface is melted by the intense heat and pressure of a large meteorite impact.

The impact blasts the molten rock high into the air, and before it solidifies the air pressure warps and twists it into a bizarre variety of shapes. Dumbbells form when the rapid spinning of an elongated oval blob pulls the two ends apart. Teardrop shapes are simply halves of broken dumbbells. Small button-shaped tektites form when air friction pushes small, thin waves of the molten droplet backward to form a round overhanging skirt.

HARD LUCK CRATER

Although Odessa Crater is unimpressive, and almost completely filled in with sediments, local residents once hoped it might attract tourists. They planned to build an underground gallery to show off the giant meteorite that they believed lay buried deep beneath the crater. But after months of digging, workers found only small meteorite fragments and a fossil of a primitive horse. Researchers later realized the huge meteorite must have exploded on impact.

STRESS FRACTURES

This rock sample, found at Wells Creek Crater, is called a shatter cone. Shatter cones form when the shock wave from a meteorite impact travels down into the rock layers below. The intense pressure cracks the rock in a branching pattern, leaving cone-shaped chunks pointing toward the center of the impact. The tiny ridgelines radiating from the tip of the cone are imprints of the shattering shock wave frozen into the rock's surface.

GIANT OF THE OUTBACK

One of Earth's largest intact meteorite craters, Wolf Creek Crater was long known only to the Djaru Aborigines, who called it Kandimalal. Their legend describes a rainbow snake that emerged from the crater and formed a nearby creek as it slithered away. In 1947, oil company geologists spotted the stunning crater during an airplane survey of some of Australia's most remote desert.

PIECE OF A GIANT

This fragment is a tiny piece of Hoba, the largest intact meteorite ever found on Earth. Hoba weighs roughly 60 tons, almost twice as much as Ahnighito, the world's third-largest meteorite mass displayed at the center of this hall. Despite its large size, Hoba did not make a crater where it fell in Namibia, Africa. Scientists are not sure why.