Only about 2 percent of the asteroids floating around in the inner solar system are made of iron and nickel, whose fragments are fairly easy to recognize as foreign. But other types of meteorites blend in with the rest of the stones on the ground. The easiest place to pick them out is in Antarctica, because few other rocks find their way to the middle of an ice field. Elsewhere, recognizing a meteorite crater requires careful mapping and laboratory work. Geologists look for several distinctive features, which result from the enormous velocities and pressures involved in an impact. Even a volcanic eruption does not subject rocks to quite the same conditions.

--J.C.W.

Shatter cones. These impressions, found in the rocks around a crater, look like cookie-cutter cones or chevrons. Occasionally, you can see them in rock outcroppings if the cones have fractured lengthwise. No shatter cones appear at Wabar because the site formed in loose sand.

High-temperature rock types. Laminated and welded blocks of sand have been seen at Wabar and at nuclear test sites. In addition, tektites‹glassy rocks thought to form when molten rock is splattered into orbit and then solidifies on the way back down‹appear around many large impact sites.

Microscopic rock deformation. The crystal structure of some minerals is transformed by the shock waves during an impact. Quartz, for example, develops striations that are oriented in more than one direction. It can also recrystallize into new minerals, such as coesite and stishovite, detectable only in x-ray diffraction experiments.