Friday Rock Blogging: Sedimentary Concretions
The Earth Science Picture of the Day is often light on both “earth” and “science” (it is reliably daily, and almost always a picture) but yesterday’s was a good one: bowling ball sized concretions in mudstone.
Concretions form within a sedimentary rock, after the material has been laid down. Water circulating within the rock will pick up and dissolve chemicals from the surrounding rock (or anywhere, really, ground water is kinda slutty like that); these chemicals then precipitate out around some small nucleus in the rock, like the chip of slate shown here.
Concretions are generally harder than the surrounding material, so are often exposed by erosion. They have historically been mistaken for fossils or alien artifacts – and of course some people never learn, but hey, everything is aliens when it’s on Mars, right?