The Ries impact crater lies to the west of Eichstätt, where we were staying and we drove via a quarry at Aumuhle to Nordlingen where we visited the Rieskrater Museum and had some free time, before returning via another quarry at Otting. The two quarries enabled us to see exposures of impact rocks, ejecta breccia, and field relationships whilst the Museum has a superb display including meteorite and Moon rock samples, and St Georgskirche in the town centre is built of suevite, the diamondiferous rock produced by the impact.
Aumuhle quarry (Steinbruch Aumuhle)
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| Aumuhle quarry. There are particularly interesting exposures on the right hand side as you enter |
This quarry lies about one km north of the village of Hainsfarth and is easily accessible via a level track. The impact crater was first recognised as such, rather than a volcanic formation, in the 1960s by Eugene Shoemaker. An information board by the entrance explains how a 1.5 km diameter meteorite, travelling at some 70,000 km/h impacted in this area during the Tertiary Period, creating a 5 km deep crater, 24 km across. Kilometers of underlying rock were reduced to rubble - the Bunte Breccia.
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| The information board |
There are three rock types exposed here:
"Buntetrummernmasse" (Bunte breccia) is a disordered reddish material deposited rapidly on the underlying Triassic/Palaeogene rocks and crystalline basement. It is composed of sedimentary fragments of Keuper sandstone and marl
Polymict crystalline breccia
Both of these are low pressure rocks, there is no evidence of shock mineralisation.
Suevite on the other hand is a high pressure, coesite and stishovite (after Q) bearing rock. It contains glassy fragments, the result of basement gneiss melted at around 600 ⁰C by the impact and then ejected as a plume into the atmosphere. The suevite may have settled on top of the breccias as it fell back to earth, or it may have flowed outwards. The similarities between this and a plinian style eruption are striking and it isn't surprising that for many years the crater was thought to be volcanic in origin.
There are a number of places in the quarry where the contact between the suevite and underlying breccia is exposed.
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| John Nudds explaining the exposure: Bunte breccia below and suevite over |
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| Crystalline breccia |
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| Rock cores... sadly not infilled afterwards |
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| The contact between Bunte Breccia (left0 and Suevite (right). Trying to convince myself that there is a baked rind on the Bunte breccia as a result of hot suevite impacting |
We had time to explore the quarry ourselves before continuing to Nordlingen where we were able to park just outside the town walls and walk to the museum.
Lots of traditional buildings in the historic centre of Nordlingen
The museum. Enormous pitched roofs - two stories in the roofspace!
Inside, there was a lovely relief model of the crater which shows the extent well. We had noticed as we drove to Nordlingen that we were in a flat area, but completely surrounded by low hills
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| Relief model of the Ries crater with the town of Nordlingen in red |
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| An aerial photo shows how the crater rim is picked out by wooded areas and clouds |
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The geological map of the area showing the inner, crystalline ring, where basement rocks were raised up,
and the outer, crater rim. |
https://www.geopark-ries.de/en/entstehung-rieskrater/ the Geopark webpages have lots of useful information about the crater formation, discovery and geology.
As well as Ries crater specific displays there were some beautiful meteorite specimens.
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| A stony iron meteorite from Chile with olivine crystals and widdmanstatten patterning |
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| Iron meteorite, Cape York, Greenland |
From the museum we wandered on into the town centre in search of lunch and found several restaurants in the square around St Georgskirche, so that we had a good view of this suevite church while we ate!
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| St Georgskirche |
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| St Georgskirche |
We were able to have a look around inside the church, accompanied by organ music. The interior is a delight in pale cream and grey with local stone slab flooring.
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| Limestone flooring |
From Nordlingen we drove (separately because of various road closures!) towards Otting, finally meeting up again by the Gemeinde where we found a beautiful boulder of suevite - a super opportunity to examine its fresh surface for some of the interesting clasts to be found.
The meteorite excavated a kms deep crater, reducing crystalline basement material to rubble before ejecting it. This then fell back to Earth, mantling the landscape, and becoming lithified as suevite.
Very like a welded ignimbrite! Easy to see why the volcanic interpretation arose.
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| A clast with a glassy rim where molten material aggregated around it |
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| Another clast with a glassy rim but crystalline basement material in the centre |
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| A vesicular, melted clast with plastic deformation |
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| A vesicular clast, volatiles were liberated from the rock during melting |
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| A clast of unmolten coarse grained crystalline material and molten, then recrystallised, fine grained darker areas |
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| A clast half gneiss, and half vesicular fine-grained mafic. |
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| A clast with a vesicular mafic exterior and an unmolten crystalline gneiss core |
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| Spindle shaped clast with vesicles and a fine grained, paler core |
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| An particularly pale coloured clast, and no apparent glass around the rim |
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| Plastic deformation of a vesicular mafic clast |
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| Plastic deformation of mafic clast, with paler areas |
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| Plastic deformation of vesicular mafic material and paler area |
The gneissose areas in the clasts reminded me of deep basement material that I'd seen in a volcanic neck in Spain, Cerro Hoyazo, where gneissic clasts had been incorporated at depth, though they aren't melted there. It is easy to see how it was initially thought to be volcanic.
There's more about the impactites and the clasts in them here:
http://www.impact-structures.com/impact-germany/the-ries-impact-structure-germany/the-ries-impactites/
Otting quarry - historical site
Our last stop of the day was nearby at a small quarry just outside Otting, famous for being where Eugene Shoemaker first identified the impact crater as such.
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| Otting quarry, now disused and somewhat overgrown |
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| Ted having a look around in the corner! |
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| A close up. Apparently there are some sedimentary clasts in the suevite here but I didn't see any |
Shoemaker and Chao identified coesite in samples from this quarry in 1960, a high pressure polymorph of quartz, which, in unmetamorphosed rocks, would only be found at impact sites.
The quarry is just outside the crater rim, showing that the ejecta behaved as a fluidised flow, forming this example of a rampart crater.
https://en.wikipedia.org/wiki/N%C3%B6rdlinger_Ries
