Fossils and Earth's Past
Science

A fossil from an organism that existed for a short time, it is used to determine the age of other fossils
Index fossil
A fossil from an organism that existed for a short time, it is used to determine the age of other fossils. Index fossil. An index fossil helps scientists determine the age of rock layers.

Comparing the amount of radioactive and nonradioactive isotopes to determine rock age
Radiometric dating
Comparing the amount of radioactive and nonradioactive isotopes to determine rock age. Radiometric dating is a method used to determine the age of rocks and fossils by measuring the decay of radioactive elements within them.

A gap in a rock layer caused by erosion or periods without sediment deposition
Unconformity
A gap in a rock layer caused by erosion or periods without sediment deposition. Unconformity. An unconformity is like a missing page in a history book of Earth's layers due to times when no new layers were added or old ones were worn away.