Thursday, August 19, 2004

Open Thread. Subject: Age of Dinosaurs

Time for a little back and forth.

1 comment:

Kendall Miller said...

Back in the years from 1785 to 1800 geologists were looking at layers of rocks in outcroppings. The rocks contained fossils of animals that similar to modern marine animals like clams and barnacles. The chemical composition of the rock was similar to what they knew about the chemical composition of modern sea-beds. So it made sense that these rocks in the Scottish Highlands were once the floor of an ocean. In modern oceans they could observe the slow accumulation of sediments and they could see the same kind of layering in recent sediments that they saw in their rocks. This was further evidence that the rocks had, at one time, been under an ocean. And that older rock layers were under the younger layers.

Some observers found a deposit that had lots of crocodile fossils in it scattered through many layers. Looking at the fossils it was clear that those in the upper layers were much more similar to modern crocodiles than those in the bottom layers. This is consistent with the bottom layers being from an earlier time that the top layers.

By looking at similar layers across large areas of geography it was found that the sequence of the different layers did not vary. Sometimes a layer would be added or omitted in different places but if you found layer A below layer B in one place, it would always be below layer B whenever they both were present. Without any clue to absolute time scale they were able to construct a relative time scale with different eras named after their distinctive formations.

Such was the state of geology until the 1930s. Given that the processes that moved rocks from oceans to mountains were either catastrophic over huge areas or slow with a very long time scale, they tended to choose the later explanation. No one had ever observed such massive catastrophic changes in recorded history but they were all familiar with the hour hand on a clock that moved so slowly that it didn't seem to move at all. But in order to explain the distance the rocks must have moved the earth had to be much older they anyone ever thought before.

In the 30s we discovered radioactive decay in certain elements, or rather, isotopes of elements. We learned that isotopes decayed into daughter elements through predictable pathways. We also learned that isotopes that were more naturally unstable were more likely to have decay events at a higher rate than more stable isotopes. These rates were so consistent that you could make a clock out of a decaying isotope that was the most accurate that had ever been seen. By comparing the amount of parent isotope to the amount of the daughter isotope in a sample and putting that up against the consistent rate of decay it was easy to compute how long that rock had been sitting there as a solid rock.

Interestly enough, when this absolute time scale of rocks was placed next to the relative time scale that had been in place for 100 years, the absolute times did not call the relative time relationships into question. Two independent sources of information leading to the same result. This held true not only in selected locations but everywhere isotopes could be found which was pretty much...everywhere.

Furthermore, the size of that time scale in millions and millions of years gave the kind of times needed for Darwin's "descent with modification" process to work. I see it as a triumph of independent branches of investigation confirming a consistent objective reality. And because of this, I can say that dinosaurs died out 65 million years ago with complete confidence. Anything that contradicts that has to also contradict an enormous body of evidence.