Tag Archives: Igneous Intrusion

Geopic of the week: Asymmetrical anticline with intrusion

Angela's anticline with igneous dike

This photo is of an asymmetrical anticline in the Stanley Formation.  It’s asymmetrical because the right limb of the fold is dipping at a steeper angle than the left limb.  This type of fold is common in the Ouachita Mountains, however, this one has a small igneous intrusion on the left limb (lower left, dark gray).  The intrusion consists of a dike, which split several of the lower beds at nearly a right angle, and a sill emplaced parallel to the bedding.

From this picture, and basic geologic principals, we can tell the history of these rocks.  Sediment was first deposited in horizontal layers (principal of horizontality).  Later, the layers cemented to form solid rock – the layers must have been firm before they were deformed because they maintained their shape.   Next, tectonic forces in the earth bent the rock into an anticline and, after it was folded, the igneous intrusion was forced into the rock. We know the intrusion was last because it cut across the rock layers and the fold (principal of cross-cutting relationships).

One of the most challenging aspects of geology is interpreting a lot from a little information.  It’s also part of what makes it so interesting!

Geopic of the week: Biotite crystal from Magnet Cove

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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.