Tuesday, March 20, 2012

Strangers in a Strange Land: Tuff luck, it's all your fault, so don't be an ash about it.

So, what do you do first? You sketch it. Drawing forces you to recognize patterns that you might not otherwise see. Then you stick your nose on it. How many times have I dragged reluctant students on a field trip only to have them sit on the far side of the highway trying to text someone when there was no cell service? It wasn't the case with these students. This group was a bunch of go-getters.You do what you can to identify the rocks in the roadcut, at whatever skill levels you've reached. This trip to a Strange Land has brought together many strangers who have had no classes in geology and some who have had many.

We have to establish some possibilities. We sit down and talk it over. What in the world could that black stuff be? The whole outcrop is layered. Doesn't that make it a sedimentary exposure? Tilted, you think? An example of original horizontality? What if it were volcanic? That makes it basalt in the middle, but what is the red and brown stuff? Is it a flow or an intrusion? A book suggests that this is an intrusive sill. It that possible?

We start to organize our thoughts and questions. Look at the picture below...it was taken in an abandoned pit at the Black Mesa Coal Mine on the Navajo Reservation in Arizona. It is composed of light colored sedimentary layers. Although not visible in this image, there are coal seams between these layers. Could the mysterious black layer be a coal seam?
Or another serious possibility. At the other end of Death Valley I encountered an intrusive sill, a place where molten basalt forced its way between the layers of light-colored limestone. The basalt has weathered deeply to a brownish-red color, but a fresh exposure would be black. Could the Charlie Brown outcrop be a sill? How would the sill affect the color of the surrounding layers? Would it oxidize the iron in the sediments, turning the rocks reddish?
What about a buried lava flow? Maybe sediments were laid down, then a thin lava flow covered them. Then new layers of sediment buried the basalt flow. How would that look?

Armed with new information, the students take a second look. And they come back. "It's not coal, the rocks aren't right, and an intrusion shouldn't have holes and cavities in it. And the rocks on either side of the dark stuff don't look like normal sedimentary layers. We don't think any of your explanations work."

Their closer inspection of the rock reveals that the "layers" aren't really layers at all; all the contacts are gradational, one color slowly merging into another, getting progressively darker and darker until it turns black and shiny. It's obsidian! Or more properly, vitrophyre, a glass-rich volcanic rock. This outcrop is showing us something else entirely. The entire outcrop is a single volcanic deposit that formed in a single vigorous eruption.

A few million years ago a small rhyolite magma chamber broke through the crust and erupted violently, producing a small caldera and coating the surrounding landscape with seething hot ash. The first ash to hit the ground cooled quickly. But the interior of the ash deposit was still so hot that the portion about 10 feet above the base fused into the volcanic glass of the vitrophyre. The uppermost ash layers cooled quickly and did not darken like the rock in the interior. This exposure is an excellent example of a welded tuff.

I really love this outcrop...it encapsulates very well the concept of the scientific method. We see a phenomena that raises questions. We do a preliminary investigation that results in a number of possible explanations (hypotheses). We test each one, assuming that one of the hypotheses will be supported by the evidence, and that the other hypotheses will be shown to be wrong. Like many times in science, all the proposed explanations turn out to be incorrect, and we go back to square one, not yet at an answer, but far more knowledgeable about our mystery. In the end, if we are diligent, and sometimes lucky, we arrive at an answer that fits all the evidence.

At this outcrop we have the added benefit of being able to learn about the various kinds of faults, and use that knowledge to identify the faults found in the same roadcut. The left side of the fault (the headwall) is down relative to the right side (the footwall), making this a fine example of a normal fault, which is generated by extensional forces. The entire region, the basin and range province, has been stretched and broken, so the faults in this one outcrop are a microcosm of the faulting found throughout this strange landscape.
This is the kind of outcrop that shows that diagrams on a chalkboard can never be as powerful a learning tool as standing on the ground staring at and manipulating the rocks.

It was time for a bathroom and a cold drink...we headed down to the village of Shoshone and got ready to see the heart of Death Valley. In a coming post....


Gaelyn said...

You will make a Great Park Ranger.

Randy A. said...

Tuff luck? It's all my fault? Don't be an ash?

Those aren't gneiss things to say! You can't take your readers for granite, you know! And you can't treat us like schist, either!

(Of quartz, I pun-ish my students that way, too!)