Your goal is to study the smooth, parallel layers of rock to learn how the land built up over geologic time.

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Once we assume that all rock layers were originally horizontal, we can make another assumption: that the oldest rock layers are furthest toward the bottom, and the youngest rock layers are closest to the top. The forest layer is younger than the mud layer, right? When scientists look at sedimentary rock strata, they essentially see a timeline stretching backwards through history.

The highest layers tell them what happened more recently, and the lowest layers tell them what happened longer ago.

Let's say, in this set of rock strata, that we found a single intrusion of igneous rock punching through the sedimentary layers.

We could assume that this igneous intrusion must have happened after the formation of the strata.

Geologists establish the age of rocks in two ways: numerical dating and relative dating.

Numerical dating determines the actual ages of rocks through the study of radioactive decay.

There may be a layer missing in the strata, or a set of sedimentary rock on top of metamorphic rock.

These interfaces between discontinuous layers of rock are called unconformities.

These items are called inclusions - foreign bodies of rock or mineral enclosed within another rock.