Age of the Appalachians Part 1: Geology Ground Rules
One of those things that “everybody knows” is that the Appalachians are the “oldest in the world.” That conclusion is usually based on the amount that the Appalachians are eroded- young mountains, sharp topography, old mountains, rounded topography. But the age of the Appalachians is a simple question with no simple answer. A good guide can be downloaded from the United States Geological Survey if you click here.
To get at the answer, you need to know something about the way geologists do business. So this is part 1 of a two-part blog.
Geology is often oversimplified as the study of rocks, but if you look closer, it is actually the study of deep time and the sequence of events. You can build a picture of a sequence of events by crosscutting relationships: Younger events cut across older events.
Sediments stack up on top of each other, so it was fairly easy to see what was older and younger. But when they started correlating strata over larger distances, geologists realized quickly that there were missing pieces in the geological record. For instance, some places would have a complete strata from the Cambrian, while others would have the complete section of the Ordovician plus part of the Cambrian. A stratigraphic section is like a piece of music, where the silence and the rests are as important as the notes. The complete Geologic Time Scale was painstakingly pieced together exactly like a geneticist will reassemble a stretch of genetic material from overlapping pieces.
This practice was augmented by the fact that evolution limited certain kinds of fossil to certain ages of sediments. These are called index fossils. The boundaries between geologic periods, epochs and ages are usually marked by extinctions, so it is possible to correlate layers of sediments, and pieces of geology across long distances. It’s a discipline called “stratigraphy”. This is the basic science at work in exploration for petroleum and natural gas: If you find oil in Eocene sediments on the Gulf Coast of the US, then it’s a good idea to look in similar age sediments in the delta sediments of major rivers around the world. Scientists at Exxon Production Research, led by Dr. Peter Vail, produced on of the first comprehensive, worldwide sea level curves to assist in oil exploration. They also published the microfossils that were used to define each time period, unheard of at the time for a petroleum company.
Crosscutting relationships allow you to build up a sequence of events, but only relative ages in the form of “younger than” or “older than.” With the advent of radiometric dating, the “when” of all of these events could be measured. There are a bunch of radioactive decay scheme that are useful for dating different kinds of rocks. The best tool for this is uranium-lead dating of zircon, which is actually the use of two uranium decay systems at one time. By this method, you either get an answer, or you just get noise. Advances in the past twenty years in vacuum technology and electronics have allowed improved analysis of dissolved zircons. Even more so is the ability to fire a stream of atoms at a zircon to blast out a bit of material for analysis. Over the past twenty years, geologists have looked for volcanic sediments near the stratigraphic boundaries, so that absolute dates could be applied to the Geologic Time Scale.
So, the ground rules for Geology:
- Younger events cut across older events, establishing relative ages.
- Certain fossils are limited to certain ages of sediments.
- Isotopic dating can give absolute ages.
And yes, I’ve waited a loooooong time to get in that pun about “ground rules.”