How Can We Correlate Rock Outcrops? Using Rocks to Tell us a Story About History

Correlation of Rock Outcrops How can we correlate rock outcrops that are located in different parts of the world? Correlation is the process by which geologists try to match similar rock strata in different locations to see if they formed at the same time or under similar conditions. There are several methods that scientists use to correlate rock strata and outcrops.

1. Walking the Outcrop- following a rock layer to make sure it continues to have the same characteristics from one end of the outcrop to the other.

II. Comparing Properties of the Rock:  If two rock outcrops look the same but are found in two separate locations, we would want to know if they were once attached and crustal motions broke them apart.  We could figure this out by looking at the texture, mineral composition, color, thickness, etc. of the strata making up the two outcrops.

III. Index Fossils  Index Fossils are special fossils of organisms that lived for only a short amount of time, but they lived everywhere on the Earth.  As a result, you find them in only a few rock layers (limited vertical distribution), but you find them all over the Earth (widespread horizontal distribution)

IV. Volcanic Ash Layers  A volcanic eruption is a big event that can leave a layer of igneous rock and volcanic ash over a widespread area.  If we are studying two outcrops from two different locations and find the same volcanic ash layer in both, we can tell that those rock outcrops formed at the same time in geologic history.

Correlation is very important when crustal motions have deformed rocks.

Igneous intrusions can also help you correlate rock outcrops