8.1.5: Impact Metamorphism

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8.8.png — Figure 8.8: Shatter cones caused by a meterorite impact. Cap-aix-Oies, Quebec

Impact metamorphism, also called shock metamorphism, is related to dynamic metamorphism because it also involves physical changes involving crushing and deformation. This kind of metamorphism occurs when a meteorite impacts Earth. The products may include high-pressure metamorphic minerals such as coesite or stishovite, both polymorphs of quartz. Highly granulated, deformed, and shattered rocks are common, and sometimes intriguing structures called shatter cones develop. Figure 8.8 is a photo of shatter cones created by a meteorite impact in Quebec. Shatter cones are akin to the damage that a pebble does when it strikes the windshield of your car.

Regional and contact metamorphism account for most metamorphic rocks. Dynamic and impact metamorphism are in distant third and fourth places. Some geologists have also described another kind of metamorphism, called burial metamorphism, but it is really just high-temperature diagenesis.