American Museum of Natural History 📤 5 years ago 2012-07-10
There are two basic ways to tell geologic time and determine the age of a dinosaur fossil. One is called relative geologic time, which involves deciding whether one dinosaur fossil is older or younger than another. The other is often called absolute geologic time, which involves estimating how many millions of years old a dinosaur fossil is.
To determine relative age, one must realize that dinosaur fossils are preserved in layers of sedimentary rock, such as sandstone, siltstone, mudstone and limestone, which are deposited in sequence, one on top of the other. Geologists record the sequence of these layers in a drawing called a stratigraphic section. Since lower layers in the sequence were deposited first, they are older than higher layers in the sequence. Thus, fossils contained in a lower rock layer are relatively older than fossils contained in a higher layer.
The sequences of rock layers containing dinosaur fossils often also contain layers of ancient volcanic ash, and the ash is composed of minerals crystals formed at the time of the eruption, which contain radioactive atoms, such as Uranium-238 and Potassium-40. These break apart at a constant rate in a process called radioactive decay. The atoms that break apart are called parent atoms, while those that result are called daughter atoms. Using sophisticated scientific instruments, geologists can measure how long it takes for half of the parent atoms to decay into daughter atoms, a time period called the half-life (4.5 billion years for U-238 and 1.3 billion years for K-40). Geologists then measure the proportion of parent and daughter atoms present in the ash crystals to determine the crystal's age. For example, if half the parent K-40 atoms in a crystal have decayed to Argon-40 daughter atoms, the age of the ash is 1.3 billion years old, and fossils in rock layers near the ash layer are also assumed to be about 1.3 billion years old.