The relative age of a rock refers to its age compared to the ages of other rocks or geological features, determining the chronological sequence of events without stating a specific number of years. This method establishes which rock layer or event is older and which is younger, creating a timeline of Earth’s history in a specific location. The process, known as relative dating, relies on fundamental observational principles developed primarily for layered sedimentary rocks. Geologists use these principles to piece together the order in which rock units formed and were subsequently altered.
The Foundational Rules of Rock Layering
The first steps in determining the relative age of rock layers come from principles governing their original deposition, established by Nicolas Steno in the 17th century. The Principle of Superposition states that in an undisturbed sequence of sedimentary strata, the oldest layers are at the bottom, and the layers become progressively younger toward the top. This is a logical consequence of how gravity causes sediment to settle, where a new layer must be deposited on top of an existing one. The Principle of Original Horizontality proposes that layers of sediment are initially deposited in nearly horizontal layers. If rock layers are observed to be tilted, folded, or steeply inclined, it means they were moved into that position by crustal forces after their deposition and hardening.
Sequencing Disruptions and Geological Events
Not all geological features are flat-lying layers; many events, like faults or magma intrusions, cut across existing rock formations. The Principle of Cross-Cutting Relationships states that any feature that cuts across another rock unit or structure must be younger than the feature it cuts. For example, if a volcanic dike of igneous rock slices through several horizontal sedimentary layers, the dike is younger than all the layers it penetrates. This principle also applies to fault lines; the fault must be younger than the rocks it has broken and displaced.
Another rule for sequencing events is the Principle of Inclusions, which deals with fragments of one rock type found within another. Any rock fragment, or inclusion, contained within a larger mass of rock must be older than the rock that encloses it, called the host rock. A common example is a conglomerate rock containing pebbles of granite; the granite pebbles must have existed first, been eroded, and then incorporated into the younger sedimentary conglomerate. Similarly, if a magma body cools and incorporates fragments of the surrounding rock, the surrounding rock is older than the magma intrusion.
Matching Layers Across Landscapes
Applying relative dating principles over wide areas requires methods to match rock layers that are separated by distances or erosion. The Principle of Lateral Continuity assumes that sedimentary layers extend horizontally in all directions until they gradually thin out or hit a barrier. This allows geologists to infer that identical rock layers found on opposite sides of an erosional valley or canyon were once continuous.
A powerful tool for regional and global correlation is the Principle of Faunal Succession, which uses the fossil record to date rock layers. This principle is based on the observation that life forms have evolved over time, appearing, existing for a specific duration, and then going extinct. Therefore, sedimentary layers containing the same distinctive groups of fossils must be roughly the same age, even if they are located thousands of miles apart.
The most useful fossils for this purpose are “index fossils,” which are from organisms that were geographically widespread but only existed for a relatively short span of geological time. The presence of an index fossil allows geologists to quickly assign a rock layer to a specific, narrow window of time. When attempting to correlate layers, geologists sometimes identify unconformities, which are surfaces representing a gap in the geological record, indicating missing time.