Ammonites, index fossils, went extinct at the end of the Cretaceous period.
Geologic history is studied primarily through stratigraphic columns along with the help of the geologic time scale. Stratigraphy has allowed geologists to discover the information necessary to the construction of the geologic time scale and, in turn, the geologic time scale helps correlate dates of rock between stratigraphic columns. Together, they define the major events of geologic history that allow geologists to compile the record of the Earth.
Geologic Time Scale
The geologic time scale is a time scale of the age of the Earth based on a combination of discoveries in stratigraphic columns, fossils of prehistoric flora and fauna and radiometric dating. This time scale covers the events that occurred on Earth from 4.6 billion years ago to the present. It is a device by which time is segmented on the basis of major events in geologic history.
Index fossils are fossils of flora and fauna that have known dates of existence and are a great aid in dividing geologic time. First appearance datum (FAD) and last appearance datum (LAD) are used for fossils in stratigraphic columns to determine their time of existence and extinction. Many of the major events in the geologic time scale are extinction events, such as the Cretaceous-Tertiary boundary. This was a large-scale extinction event of many species, notably ammonites and the dinosaurs.
Two types of dating are available to geologists: radiometric dating and relative dating. Radiometric dating is considered an absolute form because it gives a precise date of time. This is achieved by radioactive decay. Radionuclides decay in half-lives, meaning that after a certain amount of time — one half life — half of a parent element will decay into a more stable daughter product, and this decrease of the parent element by half will occur each time a half life passes. An age for a rock by radiometric dating is found by a comparison of the amount of parent element to daughter element and the length of a half life. Relative dating is simply stating that the rock layers on the bottom are older than the rock layers on top. This is a quick means to correlate occurrences that don’t require a precise date.
A geologic rock column, or stratigraphic column, is a sequence of rocks in a specific order that represents depositional environments and the types of rock deposited by these environments over time. The age of rocks in a stratigraphic column goes from oldest on the bottom to youngest on the top. Stratigraphic columns often contain unconformities, which are considered gaps in the geologic record. These are caused by erosion or a lack of sediment deposition. Columns in different areas can be correlated by age and type of rock. A single stratigraphic column for the entire age of the Earth does not exist due to the large amounts of weathering and tectonic movement over the Earth’s history. But stratigraphic columns pieced together from around the world give clues about the geologic history of the Earth.