Fossil meteorites
This rock is pretty darn awesome.
Thorsburg quarry in Sweden is, for the most part, a standard limestone quarry. It digs into a big unit of limestone, deposited about 480 million years ago when the land that is now Scandanavia was submerged beneath a shallow, tropical ocean. The unit is called the Orthoceratite limestone; it contains some fossils and is used as ingredients for building materials. But, decades ago, several strange rocks began being found within that unit. You’re looking at one.
It took a few decades to figure out what they were, but eventually it was realized that these are meteorites. Buried within this nearly-half-billion-year old limestone, you can find rocks that came to Earth from space. Legitimate fossil meteorites.
The rocks are all of a type called an L-Chondrite. The type itself might not be that interesting, it says things about their chemistry, but the weird thing is, they’re all the same rock type.
When we look at meteorites today, all sorts of different varieties are found. A single meteorite could have originated on one of hundreds of large bodies that formed early in the solar system; having all the meteorites have the same chemistry testifies to them all coming from the same body.
The only way this could happen is if there was a single asteroid that broke up at around this time, probably due to a giant collision within the asteroid belt. The breakup of the L-Chondrite parent body, 480 million years ago, caused a meteor shower on Earth big enough to leave remnants that we’re finding in limestone hundreds of millions of years later. I think that’s pretty awesome.
This particular specimen is owned by Chicago’s Field Museum. The rock is a few cm across, slightly larger than a golf ball (Hey Field Museum, you gotta include a scale bar in press photos!).
-JBB
Image credit: Field Museum http://www.nbcchicago.com/news/local/Field-Museum-adds-rare-meteorites-to-collection--263854301.html