geologic time

5. the principle that states that layers of rock extend in all directions until the environment in which the sediment was deposited changes
6. the change in species over time
7. a rock or sediment layer that serves as a time marker in the rock record
8. the process used to determine the age of relatively young objects that contain organic material
10. the process of placing geologic events in the order in which they occurred without using numerical ages

1. in an undisturbed sequence of rocks, the oldest rocks are at the bottom and each successive layer is younger than the layer beneath it
2. the period of time it takes for a radioactive isotope, such as potassium-40, to decay to one-half of its original amount
3. the methods that enable scientists to determine the actual age of certain rocks and other objects
4. the theory that geologic processes occurring today have been occurring since Earth formed
9. remains of plants or animals that were abundant, widely distributed, and existed for a relatively short period of time