Geologic Time and the Age of Rocks

Geologists use fossils to divide Earth history into named intervals, and they use the rate of radioactive decay to determine the numeric age of those intervals.

This map classifies the geologic materials of the San Francisco Bay region by age, dividing them into eons, eras, periods, and epochs of geologic time. Geologists have long observed that fossils vary from the bottom to the top of sedimentary layers, and that there is a consistent pattern to the variation. This observation, combined with the knowledge that younger sedimentary layers are laid down on top of older layers, led geologists to conclude that the fossils in the lowest sedimentary rock layers were the oldest, and that those in the highest layers were the youngest. They then used this consistent pattern of variation, observed in fossils worldwide, to divide geologic time into the named divisions and subdivisions that are used today. In the oldest eon (Archean), only the most primitive lifeforms were present, and these left very few fossils. In the youngest epoch (Holocene), the fossils are very similar to plants and animals living today.

In the 20th century, geologists developed a way to assign numbers to the ages of each fossil-based division of geologic time. By observing the rate of decay of radioactive elements and then by measuring the amounts of both radioactive elements and their decay products in rocks, geologists can calculate a numeric age (called a radiometric age) for the rocks. By careful study of the relations between the fossil-bearing sedimentary rocks and the rocks that have yielded radiometric ages, geologists have calculated the ages of the divisions of geologic time, from the Archean eon, more than 2.5 billion years ago, to the Holocene epoch, less than 11,500 years ago.

Illustration - description below
This chart shows many of the divisions of geologic time, as well as how the map units fit into these divisions. Although the chart shows all the eons, it shows only the eras, periods, and epochs of the rocks found in the region. The radiometric ages of the boundaries between the divisions also are shown. Notice that geologists have divided the periods of the Cenozoic era in two different ways. Also notice that the oldest rocks in the region are Paleozoic age, and that almost all the rocks are Middle Jurassic age (176-161 million years) or younger.