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Geologic Time
Understanding geologic time is essential to understanding geology. By the 17th century, the modern principles underlying geologic time were defined. The first principle is that sedimentary rock layers are laid down in horizontal layers and that each layer represents a specific period of time. The second (the law of superposition) proposed that younger rock units are deposited on top of older rock units.
The image on the right illustrates the geological time scale showing the age of Earth from about 4,500 million years ago to the present time. Major divisions of geologic time shown include the Precambrian from 4,500 to 541 million years ago, the Paleozoic from 541-251 million years ago, the Mesozoic from 251-65 million years ago and the Cenozoic from 66 million years ago to present day.
Shorter time units called Periods begin about 630 million years ago to the present day and include the Ediacaran, Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene and Quaternary.
Although it sounds simple, applying the principles to rocks we see can be a difficult task. Rocks can be changed after they are formed. They are worn down by erosion, tilted or folded by geologic processes. Newly formed igneous rocks can melt their way into older rocks. Rock layers of the same age may look very different, created by varied processes in different environments.
The geologic time scale is a tool used to understand events that occurred during Earth history and the relationships between them. It has developed over more than a century as a system of names that help geologists relate the formation of rocks to time, and it organizes Earth’s geologic history based largely on events in the evolution of the planet and life. We still speak of the Cambrian Period as the ‘Age of Trilobites’, the Devonian Period as the ‘Age of Fishes’, and the Upper Carboniferous Period as the ‘Coal Age’. The divisions between time periods represent major events in Earth history and the evolution of life; such as the first appearance of animals with shells or the extinction of the dinosaurs.
More recently time periods have been refined by absolute age measurements. Radiometric dating uses the rate of decay of radioactive elements in a rock to determine its age. Geologists calculate the Earth to be about 4.570 billion years old, while the Cambrian Period is the time from 542 + 0.3 to 488.3 + 1.7 million years ago. The geologic time scale is used to compare rock layers all over the world. The geologic record in New Brunswick contains rocks that represent about one billion years of Earth history. Our story divides the time periods into ten sections as shown on the time scale on the sidebar.
