A 15-Million-Year-Old Event
Around 14.8 million years ago, a stony asteroid approximately 1.5 kilometers in diameter struck the Earth in what is now southern Germany. The impact released energy equivalent to several hundred thousand atomic bombs, creating the Nördlinger Ries crater, a depression roughly 24 kilometers across. The event was catastrophic, vaporizing the asteroid and about 3 cubic kilometers of terrestrial rock almost instantly. A fireball of pulverized and molten material erupted, and as this glowing cloud collapsed, it formed a new type of rock: Suevite.
Suevite is an impact breccia, a rock composed of angular, broken fragments cemented together. Its name derives from "Suevia," the Latin name for the Swabian region. It is a polymict breccia, meaning its fragments come from many different parent rocks, torn from various geologic layers during the impact. The material includes pieces of the underlying crystalline basement (granite and gneiss) and overlying Mesozoic sedimentary rocks. This chaotic mixture was deposited as a hot layer, in some places hundreds of meters thick, both inside the crater and as patchy deposits on the surrounding ejecta blanket. For decades, the Ries structure was mistaken for a volcanic caldera until the 1960s, when scientists Eugene Shoemaker and Edward Chao identified minerals that could only have formed under the extreme pressures of an impact.
A Fingerprint of Cosmic Violence
The composition of Suevite shows its violent origin. Embedded within its clastic matrix are distinctive "glass bombs," pieces of basement rock that were completely melted and then rapidly cooled. The most critical evidence comes from shock metamorphism, the alteration of minerals by high-pressure shockwaves. Suevite contains shocked quartz, a form of the mineral etched with microscopic parallel lines called planar deformation features.
Suevite also contains coesite and stishovite, high-pressure polymorphs of silicon dioxide. These minerals form at pressures far exceeding those found in normal terrestrial processes like volcanism, identifying a meteorite impact. In some cases, the impact pressure on local graphite deposits was so intense—estimated at 60 GPa—that it created an estimated 72,000 tons of microscopic diamonds, which are now embedded within the rock. The study of Suevite and the Ries crater geology provides such an example of impact processes that it is used as a training ground for astronauts. Astronauts from the Apollo 14 and 17 missions trained here in 1970 to learn how to identify impact rocks on the Moon. Today, the European Space Agency continues to use the site for astronaut training.