By Sid Perkins
Most geological processes unfold at less than a snail’s pace. The tectonic plates that cover Earth’s surface slog along, crashing into and sliding over one another at rates of only a few millimeters per year. Over millions of years, however, these unhurried liaisons raise mountain ranges. Wind, rain, and natural chemical erosion gradually rework the mountains into silt, clay, and dissolved minerals. Slowly, this inorganic detritus wends its way to the sea, where it joins a languid rain of dead marine organisms to form thick layers of ocean-floor ooze.
Every now and again, however, things happen in a flash. Asteroids, comets, and smaller objects smack into the planet at clips of thousands of kilometers per hour. When this happens, the impacts can gouge sizable holes in Earth’s outer crust. Within milliseconds, rocks at the impact site vaporize. The rapid expansion of this superheated gas blows melted and pulverized material into the atmosphere or back into space.