Relative Dating

Looking at the cross-sections of rock formations, we can determine the relative ages of the layers and features in the formation. The study of strata is called stratigraphy. The table below lists five events that can occur in a rock forming environment:

Event Description Tectonic location / process
Sedimentation Nicholas Steno's (1638-1686) laws: the Principle of Original Horizontality states sediments are laid down in horizontal layers. The Principle of Superposition explains that younger layers are above older. Subsidence due to tectonic processes or changes in sea level.
Deformation Layers of rock are folded and distorted. Usually caused by mountain building at convergence boundaries.
Erosion Gaps in the geologic record caused by erosion of rock are called unconformities. Results from uplift at convergence boundaries.
Intrusions Magma can cut across existing rock formations. Such a cross-cutting event must be younger than the formations it crosses. Results from hot spot magma intrusions or magma intrusions due to melting at convergence boundaries.
Faulting Another cross-cutting event. Found at most tectonic boundaries.