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Chapter 8: Metamorphism: A Process of Change
Feature Articles
The Rest of the Story: Shock Metamorphism
by Stephen MarshakThe compound SiO2 (silica) most typically exists in the form of the common mineral quartz. As we learned in Chapter 5, the arrangement of atoms or ions and chemical bonds in a crystal of quartz looks like scaffolding, or a jungle gym. Researchers found that if you fire a projectile at high speed into sandstone (which consists of quartz sand grains), the grains surrounding the impact site transform from quartz into two other minerals, coesite and stishovite. These minerals have the same chemical composition as quartz (they consist of SiO2), but have a different crystal structure, one in which the silicon and oxygen atoms pack together much more tightly. Evidently, the strong shock wave that passes through the sandstone during impact creates such incredibly high pressures that the quartz recrystallizes; this process is known as shock metamorphism.
Interest in shock metamorphism increased when astronauts first brought regolith back from the Moon, which turned out to contain coesite and stishovite as well. This came as no surprise, because meteorites pummeled the Moon in the past, and each collision would have caused an intense shock. Researchers then began to use the presence of coesite and stishovite as a "litmus test" for determining if strange circular structures on Earth were made as a result of meteor impact. Numerous such structures, defined by circular faults and circular igneous intrusions, had been discovered, but they did not have the bowl-shaped depression and raised rim characteristic of lunar meteor craters, and so their origin remained a subject of debate—some geologists thought they might be a consequence of volcanic explosions. But the discovery of coesite and stishovite at these sites confirmed that they formed as a result of an impact. They lack the bowl shape of lunar craters because they are so old that the bowl has eroded away.
More recently, geologists have started to look for coesite- and stishovite-bearing layers of sediment that have the same age as important extinction events on Earth. Fossil evidence suggests that at specific times in the past, numerous species of life went extinct at the same time. Many geologists have come to favor the hypothesis that major impacts between asteroids or comets and Earth could wipe out life forms, an idea movies and TV have made popular. If it is true, then we should be able to find a widely distributed layer of debris containing coesite and stishovite at the level in a stratigraphic sequence at which many fossil species became extinct. And sure enough, a worldwide layer of clay deposited at the time the dinosaurs disappeared does indeed contain these two minerals.
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