BELL-RINGER On a sheet of paper, write the difference between absolute dating and relative dating and give an example of each.

ANSWER Absolute dating is exact, while relative dating is an estimate. Absolute dating – radioactive dating Relative dating – Law of superposition

Relative Dating Determining the age of a rock by comparing it to the ages of other rocks To find the relative age of a rock, geologists use layers of rock and the law of superposition

Law of Superposition In undisturbed rock layers, the oldest layer is at the bottom and the youngest is at the top.

Law of Superposition The earth is always changing due to rapid and slow changes, and this can disrupt the rock layers. Folding, tilting, faulting, intrusions, and unconformities can change the location of rock layers 

Original Horizontality Sediments are deposited horizontally (thanks to gravity). It is assumed they were tilted by some geologic process AFTER they were deposited A unit of rock is always older than the geological processes that changed it.

Extrusions and Intrusions Intrusions of magma are always younger than the rocks layers around and beneath them

Faulting Faults: BREAKS in the rock layer because of movement Faulting can lead to offset layers and exceptions to the law of superposition Faults are always younger than the rock they cut through

Folding Folds: BENDS in rock layers Folding can lead to offset layers and exceptions to the law of superposition

Index Fossils Used in relative dating by inferring layers with matching fossils are the same age Widely distributed and represent an organism that existed for a short period of time

Absolute Dating Determining the age of a rock, given as a number of years since the rock formed, by measuring the amount of radioactive decay Scientists look at the amount of radioactive elements, such as carbon- 14, to accurately determine the age of a rock

Radioactive Decay Process in which radioactive elements, such as carbon-14, break down over time

Half-Life The amount of time it takes for half of the radioactive atoms to decay