Gilbert, and other early observers of lunar craters through telescopes, could see that they were of two types: smaller, rounded, bowl-shaped depressions, and larger, more complex structures with central peaks and collapsed rim terraces. Shoemaker, in his study of Meteor Crater, discovered why. When an asteroid or comet traveling at interstellar speeds strikes the earth, two powerful shock waves are created. The first, the explosive wave, travels downward through the target rocks, pushing them down and out. The second, the release wave, moves in the opposite direction. The shock and release waves interact in a complex manner, melting, vaporizing, and ejecting the rocks at ground zero. Fractured rock and crater walls fall back into the crater and mix with melt to form a breccia.
If the impactor is less than a few hundred meters in diameter, a simple crater (Figure 6) like Meteor Crater is formed. Such craters range up to about 4 km in diameter. Larger craters are not just bigger versions of small ones, as Tycho (Figure 7) illustrates. Such large craters start out in the same way as simple ones, but the greater energy released by the larger (or faster) impactor causes the rocks at ground zero to rebound to form the central peak. The crater rim cannot hold and falls in on itself to form terraces.
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