(photo courtesy of John David McFarland)
The photo above illustrates surface iridescence. Iridescence is a play of colors caused by the interference of light waves. This phenomenon is also called thin film interference. You have probably seen this effect on soap bubbles and oil sheens. Light reflecting from a thin coating of iron oxide on the piece of novaculite above is producing the play of colors. Light waves are reflected from the top of the iron coating and the base of the iron coating producing multiple waves. A color is seen when the waves interfere constructively. The resultant color is dependent on the thickness of the coating and consequently, streaks and bands of differing color develop since the thickness of the iron oxide coating varies. The colors also change when the angle of reflection is changed.
Imagine you took a stack of ribbons, compressed it till it buckled into bows, and then tilted the whole stack on its side. That pretty much sums up what you can see in this picture of plunging, folded bedrock at Gulpha Gorge Campground, north of Hot Springs, in the Ouachita Mountains of Arkansas. The bedrock of the Ouachitas was buckled and tilted about 200 million years ago when the South American and North American continents collided – part of the incredible process geologists call plate tectonics.
This is just a couple of wee folds that are exposed at the surface because the bedrock at Gulpha Gorge is novaculite – a really hard rock that doesn’t erode away easily. However, if we could strip the vegetation and civilization away in central Arkansas, we would see that pretty much all the rocks in the region are folded and tilted in similar ways. Some of the folds cover many square miles and can be seen from space on a clear day, and others are no bigger than a speed-bump.
This is a picture of a sedimentary rock called novaculite collected in the Ouachita Mountains of central Arkansas. On the surface are thin coatings of a manganese oxide mineral in shapes reminiscent of trees. When a mineral forms in these tree-like patterns, it is called a dendrite (named for the impulse-transmitting nerve component of a similar shape). Manganese oxide minerals commonly form in this peculiar morphology.
For more views of manganese dendrites click here