Geologic time, absolute ages, radiogenic isotopes

I. How old is the earth? Answer: 4.6 billion years old

A. As late as the 1700's most geologists thought the earth was several thousands of years old.

B. James Hutton, a Scottish geologist, applied the principle of uniformitarianism to decide that the earth was much older than his contemporaries assumed. Hutton theorized that the earth was possibly 100's of thousands of years old, rather than a few thousand years old. Even though the earth is much older than even Hutton predicted, he was radical for his time. He is credited for introducing the notion that geologic time spans well beyond what humans can really comprehend.

1. Uniformitarianism is best stated in the phrase, "The present is the key to the past." Paraphrased, it says that processes that operate today are those that shaped the geology of the past. It suggests a very slow, gradual progress to geologic process that are found in the rocks.

2. Compared to uniformitarianism, catastrophism, is the idea that sudden and cataclysmic episodes are responsible for affecting geologic history. In Hutton's time, catastrophists tended to explain geologic phenomena through the Biblical flood. Catastrophism is still used as a philosophy today. The theory of a meteor (or bollide) impact to cause dinosaur extinction is very much in the spirit of catastrophist thinking.

II. The geologic time scale describes time in terms of millions of years, which is often the time required to depict geologic processes

III. Relative age tells us whether one thing is older or younger than another thing. It does not tell us what age it is.

IV. Absolute age tells us how old a thing is (how many years old). Absolute ages are much harder to determine than relative ages. (example of telling how old two people are) How are isotopes used to date the Earth?

A. What are isotopes? Isotopes are atoms with the same atomic number, but different atomic weights. Example: hydrogen, deuterium

B. Some isotopes are inherently unstable and decay with time. For instance, uranium 238 undergoes beta decay (losing 2 protons and 2 neutrons) and turns into thorium 234. In this example, uranium 238 is the parent isotope, and thorium 234 is the daughter isotope.

C. Radioactive (or unstable) isotopes tend to decay at a constant rate. This rate is reflected in the half-life of a radioactive isotope. The half-life is the length of time for half of a quantity of radioactive isotope to decay to its daughter product.

D. A radioactive mineral that is incorporated into a mineral will decay into daughter elements that may not naturally reside in that mineral. One can assume that all of that daughter element found in the mineral must have been derived from the parent isotope. By measuring the quantity of parent isotope, daughter isotope, and knowing the half-life of the parent isotope, geochronologists (geologists who measure time in rocks) can calculate the number of half-lives that have since the mineral was formed.