For example, a xenolith in an igneous rock or a clast in sedimentary rock must be older than the rock that includes it Figure 8. The regular order of the occurrence of fossils in rock layers was discovered around by William Smith. Student Voices. Subsequent layers would follow the same pattern. This is because inclusions can act like "fossils" — trapping and preserving these early melts before they are modified by later igneous processes. The law of superposition states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it. Relative dating tells scientists if a rock layer is "older" or "younger" than another.
8.2 Relative Dating Methods
Extinction of species is common; most of the species that have lived on the earth no longer exist.
A subatomic particle found in the atomic nucleus with a neutral charge and a mass approximately equal to a proton. For example, in the rocks exposed in the walls of the Grand Canyon Figure 1 there are many horizontal layers, which are called strata. By correlating fossils from various parts of the world, scientists are able to give relative ages to particular strata. Accordingly, the oldest rocks in a sequence are at the bottom and the youngest rocks are at the top.