Geologic Time

3. the average weather conditions in an area over a long period of time
7. Method of determining the age of a fossil by comparing its placement with that of fossils in other layers of rock
9. preserved evidence of the activity of an organism
10. a long cylinder of ice drilled from an ice sheet or ice cap that is used to study past climates
11. event during which many species become extinct during a short period of time
12. length of time required for half of the radioactive atoms in a sample to decay
14. the process in which a radioactive isotope tends to break down into a stable isotope of the same element or another element

1. method used to determine the age of rocks using the rate of decay of radioactive isotopes
2. technique used to determine the actual age of a fossil
4. the division of Earth history into blocks of time--eons, eras, periods, and epochs; created using relative dating principles.
5. fossil that is useful for dating or ordering strata of rock in which they are found
6. a principle that states that younger rocks lie above older rocks if the layers have not been disturbed
8. organism with a three-lobed exoskeleton that was abundant in Paleozoic oceans and is considered to be an index fossil
13. Preserved remnant or impression of organism that lived in the past