ROCK & FOSSILS

3. any method of determining whether an event or object is older or younger than other events or objects
7. a principle that states that geologic processes that occurred in the past can be explained by current geologic processes
10. the scientific study of fossils
12. a fossil that is found in the rock layers of only one geologic age and that is used to establish the age of the rocks layers
13. a principle that states that younger rocks lie above older rocks if the layers have not been disturbed
15. an arrangement of rock layers in which the oldest are at the bottom
16. a break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time

1. a method of determining the age of an object by estimating the relative percentages of a radioactive isotope and a stable isotope
2. a principle that states that geologic change occurs suddenly
4. the time needed for half of a sample of radioactive substance to undergo radioactive decay
5. the trace or remains of an organism that lived long ago, most commonly preserved in sedimentary rock
6. a type of fossil that forms when sediments fill in the cavity left by a decomposed organism
8. any method of measuring the age of an event or object in years
9. the process in which a radioactive isotope tends to break down into a stable isotope of the same element or another element
11. an atom that has a different number of neutrons than atoms of the same element
14. a mark or cavity made in a sedimentary surface by a shell or other body
17. a fossilized mark that is formed in soft sediment by the movement of an animal