G6- The Grand Canyon: A Window to Earth’s History 800-900L -03

Relative Dating

The process of relative dating relies on the Law of Superposition. This process states that older layers will be deeper in a site than more recent layers. In this way, a sequence of events can be constructed. This method does not give absolute ages of the rock strata. However, it does provide the scientists a geologic storyline of events.

Principle of Cross-cutting

A related principle to the Law of Superposition is the principle of cross-cutting. This principle focuses on events that have affected the rock strata. For example, a fault that cuts through a rock layer is younger than the rock it cuts through. What if a fault is found that cuts through some layers, but not others? This knowledge can be used to place this fault into the sequence of geologic events. Clearly, the rock formations that the fault runs through are older than the fault. The layers that are not cut by the fault must be younger than the fault episode.

Radiometric Dating

Sometimes, scientist need to find an absolute age of a rock. In that case, they turn to radiometric or radioactive dating. This technique uses the trace radioactive impurities found in the rock. Scientists compare the amount of these radioactive isotopes to the amount of its decay products. This ratio provides scientists with a fairly accurate estimate of the age of the rock. This process assumes the rate of decay has stayed the same over time. Together with principles of relative dating, geologists are able to create geologic timelines and timescales. In the Grand Canyon, scientists have used these techniques to learn about the region’s past. It has also provided clues to how it was formed.

Grand Canyon Formation

The deposition in the area was not continuous. There are at least 14 known unconformities in the geologic record. Unconformities occur when deposited material has been weathered and eroded. These unconformities represent a gap in the geologic record. Unconformities are not uncommon. It is why scientists must study numerous locations. The added data helps in the quest to piece together Earth’s geologic history.