Fifteen million years ago, a meteorite a mile wide collided with southern Germany. The collision created Ries Crater, 15 miles wide, at the present site of the city of Nordlingen. On impact, sandstone melted and was propelled out of the melt zone. Silicon in sandstone quartz cooled to form glass "tektites." In 1836, Franz Zippe, the minerals curator for the Vlastenecke Museum in Prague, named this glass "moldavite."
Composition
Moldavite is harder than manufactured glass and melts at a higher temperature. When fused (melted) and cooled again, it remains clear. Ordinary glass is 50 percent silicon, almost no aluminum, 20 percent lime and magnesium, with 20 to 25 percent potash and soda. Moldavite is 79 to 88 percent silicon, five to 13 percentaluminum, 2.6 percent iron, 3 percent lime, 0.22 percent magnesia, 2.6 percent potash, 0.26 percent soda and 0.1 percent water.
Study
In 1788, Mayer described moldavite as manufactured glass from an ancient glassworks. In 1900, the journal of the Mineralogical Society of Great Britain noted that moldavite is common among late tertiary or early quaternary sediments in Bohemia and Moravia. In 1933, Leonard Spencer of the Natural History Museum in London suggested tektites were extraterrestrial, because of their hardness and their surfaces, which are deeply pitted like meteorites. In the 1960s, impact tektites from the moon were studied, and geologists recognized moldavite as a terrestrial tektite. In 2003, a Russian study funded by NASA suggested that the impact object was a binary asteroid (two asteroids orbiting each other).
Ries Crater
The city of Nordlingen, Germany is at the center of Ries Crater. The crater rim is 660 feet higher than the floor. An uplifted central impact ring is 7 miles wide. Geologists first thought Ries Crater was volcanic. St. George Church in Nordlingen is built of locally quarried suevite, a gray breccia (a rock composed of fragments of minerals in a matrix) that was thought to be of volcanic origin. Nordlingen suevite is more than 50 percent heat-fused glass.
Strewn Field
A "strewn field" is an area where tektites of the same age, petrology (rock type), chemical properties and physical characters are found. Tektites are not found in the source crater. The Central European Strewn Field is a relic of the Ries Crater impact, and is the smallest of four fields identified world wide. Mapping of the strewn field supported chemical evidence that moldavite is not extraterrestrial.
Impact
At impact, tektites averaging an inch in diameter were ejected at about 1 mile per second behind the meteorite and about 5 miles per second in front. Cooling was not instant, because atmospheric gases were heated also. Some particles fell to Earth within 10 minutes. Others were propelled to high altitude and were airborne for hours, so that the area of the strewn field was influenced by weather and the Earth's rotation.
Formation
Tektites from the center of Ries Crater were strewn four times further than those from the rim. The meteorite approached at an oblique angle from the west-southwest, creating a strewn field to the east-northeast, in the Czech Republic. Moldavites formed from the upper 130 feet of sandstone. At a distance of 3 miles from the impact, pressure and heat were below the melting point of quartz, so no moldavite formed there.
Tags: Ries Crater, center Ries, center Ries Crater, city Nordlingen, impact tektites, manufactured glass, miles wide
