You may have seen the recent news about the near-Earth flyby of a 2.7 km asteroid. Fortunately the rock missed collision with the Earth, and continued its journey around the Solar System. However, in the past Earth has not been so lucky. Asteroid and comet collisions have shaped the surface of the Earth throughout the past 4.5 billion years ago. The scars of these cataclysmic events have often been eroded by active geological processes.
One such event occurred in southern Germany ~14.5 million years ago, when a projectile 1.5 km in diameter slammed into the Earth. The impact created a round shaped crater 24 km in diameter. A smaller impactor, only about 150 m in diameter, hit the ground about 42 km kilometres to the south-west creating a 3.8 km crater. The larger of the two craters is known as Ries, and the smaller crater is called Steinheim.
Both are the unique places on Earth to study the products of impact cratering, and some of the Apollo astronauts even trained here prior to their missions to the Moon. Our BSc. and MSci. Geology with Planetary Science undergraduate students had the opportunity to follow in their footsteps when they visited the Ries crater recently for their third year field trip.
For the excursion we stayed in the medieval town of Nördlingen, which hosts the excellent Ries crater museum. The field class took in a range of quarry sites that revealed the nature of the ejected rocks, known as suevite, that were partially melted during the crater-forming event. These rocks were thrown high up into the air before crashing back down on the land, and were also ejected on ballistic trajectories into the surrounding region. There is a nice diagram of their formation process here. The cathedral in Nördlingen itself is made out of the suevite, and its bricks reveal the complex mixtures of different types of rocks that were mixed and melted together by the force of the impact.
Other quarry sites we visited were formed of huge kilometre sized megablocks that were thrown aside in a matter of seconds, forming a jumbled chaotic rim structure around the crater. The students also looked at rocks that were formed in lakes that infilled the impact craters after they were formed, and collected fossils of snails that once thrived in the waters.
The Ries is an excellent fieldsite to understand comparative geological processes, and we look forward to the next SEAES fieldtrip to Germany study the effects of impact cratering.
SEAES fieldtrip to Germany http://www.seaes.manchester.ac.uk/study/fieldwork/fieldworkingermany/
Ries crater museum http://w09.devweb.mwn.de/Joomla1_5/index.php Many thanks to Dr. Gisela Pösges from the museum and Maya for their expertise!
Ries Crater Park http://www.geopark-ries.de/index.php/en/geopark_ries
Ries crater virtual tour http://www.psi.edu/epo/explorecraters/riestour1.htm