SLO 4 - Geologic Time

The difference Between Relative and Absolute Dating and What Purpose They Each Serve

To begin, we first need to remind ourselves what we are dating, and why.

Rocks have been around for far longer than any living species on Earth, and even today we can still find large rocks that have survived for millions, or in the case of Rocklin in Canada, billions of years without becoming completely eroded, broken down, or taken over by continental drift. But how are geologists supposed to know exactly how long a specific rock has been around? This is where dating comes in.

The first method of dating rocks, and perhaps the simplest, is relative. This is done by looking at the relationships between geologic features. Its especially discernible when one remembers the law of superposition. Unless the whole sequence has somehow been overturned, whether by disrupted faulting or tectonic processes, the bottom layers will always be older than the more recent top layers. Even the principle of inclusion states that any rock fragments found in rocks must be older, despite it not being as horizontally layered as sediments.

There is also a very clear way to study rocks that includes watching out for any kind of cross-cutting. What this means is that if any geological feature disrupts another, it evidently came after the original, making it younger.

The second method of dating rocks, and perhaps the more complex one, is absolute. This includes dating that uses Isotopes, which is where the chemistry comes in.

Isotopic dating is when scientists use their knowledge of how certain isotopes decay in certain elements, helping them estimate the age of the fossil or rock they are attempting to give an accurate age to.

In class, we were given an assignment that included playing a radioactive dating game, in which students were able to mess with fossils and their percentage of Carbon-14 and Unranium-238 isotopic elements.

The goal was to get students to learn about how this particular dating process works, and how to work around any issues a scientist might face when not enough isotopes, if any, are present in a particular object.

Below are some of the biggest and most interesting take-aways from the activity:

• Carbon-14 is mainly used to date organic objects as his radioactive element combines with oxygen in the atmosphere to form radioactive carbon dioxide, which plants absorb to make energy and to grow. When animals eat the plants, they absorb the radioactive carbon. When plants and animals die, they cease to take in new carbon-14 and so start the clock running as the carbon-14 decays into nitrogen-14. 

• Uranium-238 is mainly used to date fossils with minerals because it goes through a long decay chain that eventually ends with lead. Through observations and understanding of chemistry, scientists know that lead-and-uranium-containing minerals form with certain proportions of each element. As the uranium decays, the ratios of the elements will change.

• By combining their knowledge of chemistry and physics, scientists can use the rate at which uranium atoms decay and observations of the ratios of the elements today to figure out how old the minerals are.

• If a fossil cannot be dated because of its low percentage of either element, scientists have turned toward finding the closest next-best fossil from the same layer the one they were originally trying to date was found in, in order to get a rough estimate. Bringing back the law of superposition, it is only fair to surmise that if they were fossilized around the same time, they must also be close in age.