Discovery of a meteorite fragment in the Steinheim impact crater
The crater of Steinheim am Albuch close to Heidenheim an der Brenz is the “little sister“ of the Nördlinger Ries crater. Both of the craters are supposed to have been formed ca. 14.8 Million years ago by meteorite impacts. Previously, little was known about the impactor forming the Steinheim crater. A new discovery may, however, change our picture about this impact…
Volcano or impact crater?
Steinheim am Albuch is situated in a large bowl-shaped depression with a central uplift, which is easily visible when you enter the bowl-like structure by car from the north.
This setting has caused decades of scientific discussions, because the central peak has long been viewed as the remains of a volcano. Today we know that the basin was formed by a meteorite impact and that the peak originates from a rebound effect, which is the reaction of the ground subsurface after the strong compression by the impact. Further evidence for an impact as the cause of the formation of the basin is the occurrence of shatter cones, which are cone-shaped structures in the hard rock, formed by the high pressure of the impact.
But where is the meteorite?
Although the origin of the Steinheim Basin structure has been known for centuries, the search for the meteorite that hit the Swabian Alb in the Miocene has so far been unsuccessful. This fact is not surprising at all because most meteorites widely vaporize when they hit Earth’s surface.. Therefore, one big question has not yet be answered: how did the double impact of Nördlinger Ries and Steinheim Basin took place? Did one large meteorite break into a larger and a smaller fragment? Or was the smaller projectile a moon of the larger one?
Under this premise, a message from Michael Hölzel from the Steinheim Meteorkratermuseum caught my attention at the end of the year 2016. The exhibition of this Museum presents a large block of Jurassic limestone with shatter cones as a “hands on” exhibit. Visitors can touch the block in order to feel the shatter cone structures. One piece of this rock was loose and removed from the block, surprisingly showing a centimeter-sized metallic fragment. This suspicious fragment was brought to me at the Staatliches Museum of Natural History Stuttgart by Michael Hölzel, and I forwarded it to the impact researcher Elmar Buchner, who has extensive research experience on Nördlinger Ries and Steinheim Basin. Even though I am not an impact researcher on my own, I am highly interested in the development of the Steinheim Basin and its fossil lake sediments, because it comprises a unique evolutionary sequence of snail shells, which is one of my main research areas.
After this, a series of highly sophisticated investigation methods were applied, led by Elmar Buchner. The particle was analysed in Stuttgart, Kiel and Vienna. Further methods are currently being applied at ETH Zurich. The analyses revealed that the fragment is mainly composed of the minerals Kamacite, Taenite, Schreibersite, and Troilite. These minerals are typically present in iron and stony-iron meteorites. Both the trace elements and single olivine crytals furthermore suggest that it is a very rare meteorite variety, namely pallasite.
Open questions remain
At first glance, we were very happy about these results because we were supposed to have found a fragment of the meteorite that has formed the Steinheim crater. And even more interesting: its composition was completely different from the Nördlinger Ries meteorite, which was possibly a stony meteorite. Does this imply that the two impacts may not have been time-equivalent? Or that the Steinheim meteorite was a smaller moon of the Nördlinger Ries meteorite? After a second look, however, open questions appeared, such as: how did the meteorite fragment get into this fissure?
Did the fissure of the shatter cone open slightly during the impact, so that the fragment could be injected and preserved within this fissure? Or is it a completely different story, and a meteorite reached the earth 150 Million years ago, i.e., during the Jurassic, when fragments of it sunk onto the Jurassic sea floor? Such remains from meteorite showers are well known from today’s sea floors.
Baden-Württemberg’s first meteorite
Whether Miocene or Jurassic in age, whatever the results from the current research – which is not yet finished – will be, the Steinheim meteorite is something special because there are only a handful of fossil meteorites known worldwide. Most of the meteorites that we know are very young and have reached the earth surface only recently. Another interesting aspect is that Baden-Württemberg has so far been the only larger German state without a meteorite find – which has now changed.
Heizmann, E.P.J. & Reiff, W. 2002. Der Steinheimer Meteorkrater. Pfeil-Verlag, München.
Rasser, M.W. 2012. Ein wissenschaftliches Kleinod auf der Schwäbischen Alb: Das Steinheimer Becken und seine Schnecken. – Schwäbische Heimat, 63: 56-62.