Geologists determine the sequence of geological events using two main dating methods: absolute and relative. Absolute dating provides a specific age in years, often through radiometric techniques. Relative dating establishes the chronological order of events, determining which feature is older or younger than another. Establishing this sequence is fundamental to stratigraphy, the study of rock layers, and allows scientists to piece together a coherent timeline of deposition, deformation, and erosion.
The Law of Crosscutting Relationships
The Law of Crosscutting Relationships is a powerful principle used in relative dating. It states that any geologic feature that cuts across or penetrates another feature must be younger than the feature it cuts. The feature being cut must have existed before the cutting event could take place.
This principle applies to various geological structures. For example, an igneous intrusion, such as a dike, is younger than the surrounding rock layers it pushes through. An erosional surface, known as an unconformity, must also be younger than the rock strata it has partially removed. This rule allows geologists to establish the order of events like magma injection, faulting, and weathering.
Using the Law to Date Faults
Applying the Law of Crosscutting Relationships to faults allows geologists to determine the minimum age of the fault movement. If a fault visibly cuts and offsets a sequence of sedimentary rock strata, the fault must be younger than those layers. The rocks had to be in place before they could be broken and displaced.
The observation of offset layers provides direct evidence for this relationship. If a fault cuts through three distinct horizontal rock beds, the faulting event occurred after the deposition and solidification of all three layers. By identifying the youngest rock or structure that the fault has broken, geologists establish the earliest possible time the fault could have formed. This serves as the minimum age for the fault activity.
Features That Limit the Maximum Age of a Fault
To complete the age bracket for a fault, geologists must determine its maximum age, which is the time when the faulting movement ceased. This is achieved by identifying features that are not affected by the fault and must have formed after the movement stopped. These features effectively cap the fault’s activity in the geological record.
A common example is a layer of undisturbed sedimentary rock that lies horizontally across the top of the fault plane. If the fault cuts the layers below but the overlying layer remains intact and unfaulted, the deposition of the top layer occurred after the fault movement ended. The age of this capping layer provides the latest possible time, or maximum age, for the fault’s movement.
Another limiting feature is an igneous intrusion that crosscuts the faulted layers but is not offset or broken by the fault. Since the intrusion is continuous across the fault line, it must have been emplaced after the tectonic forces that caused the fault had stopped. By establishing the relative ages of the features that are cut (minimum age) and those that are not cut (maximum age), geologists place the fault’s movement within a specific window of geological time.