How to Find the Relative Age of Rocks

By Tracy Barnhart
Horizontal rock layers, age, the bottom
Rock Formation image by Dawn from Fotolia.com

Geologists determine the relative age of a rock by examining the nearby rocks, minerals or rock inclusions, index fossils and the original position of the rock layers. According to the United States Geological Survey (USGS), the first person to evaluate the relative age of rocks was James Hutton, a geologist from Scotland, in the late 1700's. When geologists discuss relative age, they do not involve methods which determine the absolute age of a sample, such as carbon dating. A relative age determination for an area will tell you which rock layers formed first, second and so forth.

Evaluate the formation and determine if the rock layers are in their original position or if they are deformed. Folding is an example of deformation. For areas showing deformation, draw the original layered formation on a sketch pad or field notebook. Sediment layers form horizontally at the time of deposition. For example, if the rocks are in a "U" shape, sketch the layers with the ends as if they were horizontal.

Label the sketched horizontal layers as oldest on the bottom to youngest on the top. This "law of superposition" gives you the relative age of each layer with respect to the other layers in the formation. The law of superposition says that in horizontal layers, the oldest layers are on the bottom.

Evaluate the rock formation for evidence of dikes. Igneous rocks may form dikes when magma fills a crack in previously-existing rock layers, cools, and forms an igneous rock layer at an angle to the original horizontal layers in the area. The dike is younger than the rock layers that it intersects.

Examine the rock layers for rock and mineral inclusions. Inclusions are rock or mineral pieces embedded in a rock layer that did not originate from that location. The included pieces are older than the surrounding rock. For example, granite inclusions embedded within a layer of sandstone are older than the sandstone. Granite did not form in the sandstone, because granite is an igneous rock and sandstone is a sedimentary rock.

A trilobite, a common index fossil
fossil image by Hubert from Fotolia.com

Identify index fossils found in the rock layers. Index fossils are fossils of animals or plants which lived during a specific time period. Index fossils appear as an impression on the rock surface or as an inclusion of animal or plant parts. The USGS website provides sketches and approximate ages of 24 common index fossils. Once you identify the fossil, you can determine the relative age of the rock.