Igneous rocks are rare in Arkansas, making up about 0.1% of the surface area of the state. Nonetheless, we find some interesting and unusual rocks and minerals within our igneous rocks. The above picture is a pseudohexagonal, zoned, green biotite crystal collected from Magnet Cove, Arkansas just last week. That’s a mouthful!
Magnet cove is a 100 million year old igneous intrusion, now exposed at the surface 12 miles east of Hot Springs, Arkansas. In that little area, over 100 mineral species have been identified, including some that were first discovered there. Students, researchers, and mineral enthusiasts come from all over to visit Magnet Cove, collect samples, and learn about this geologically fascinating place.
This is a picture of a paleokarst feature from the Upper Buffalo River in Newton County, Arkansas. Paleokarst features, like this one, are ancient caves or sinkholes that have been preserved in the rock record.
In this case, a sinkhole formed when bedrock was exposed above sea level and acidic rainwater dissolved a vertical pit in the bedrock. When sea level rose and covered the area again, more sediment was washed in and the sinkhole was filled with sand. Eventually the sand became sandstone and a cast of the sinkhole is preserved today (center of photo).
All of this happened about 450 million years ago. Paleokarst features are one more clue geologists use to decipher earth’s history. If you didn’t know better, you might float right by and never give it a second thought.