Relative age dating is a fundamental technique in geology used to determine the sequential order of events in Earth’s history without assigning a specific numerical age. This method allows geologists to establish which rock layer or geological feature is older or younger than another, effectively piecing together a timeline of Earth’s past. Unlike absolute dating methods, such as radiometric dating, relative dating relies on a set of logical, physical, and biological laws to interpret the order in which rocks were formed, deformed, or eroded. These principles form the basis of stratigraphy, the study of rock layers, providing the framework to reconstruct the geological record.
Principles of Initial Layering
The foundation for relative dating rests on three principles governing how sedimentary rocks and lava flows are deposited. These were first articulated by Danish scientist Nicolaus Steno in 1669. The Law of Superposition states that in any undisturbed sequence of rock layers, the oldest layer is at the bottom, and the layers become progressively younger toward the top.
The Principle of Original Horizontality explains that layers of sediment are originally deposited in flat, horizontal sheets. Gravity causes sediment particles to settle evenly across a depositional surface. If rock strata are found tilted, folded, or faulted, this principle dictates that the deformation must have occurred after the layers were deposited.
The Principle of Lateral Continuity suggests that sedimentary layers extend outward in all directions until they thin out, encounter a barrier, or grade into a different type of sediment. If a valley or canyon later erodes a portion of a rock layer, the same rock unit found on either side of the gap is assumed to have once been continuous. This concept aids in correlating rock layers across geographical distances.
Principles of Modification and Intrusion
Once rock layers are deposited, they are often modified by later geological events, such as faulting or the intrusion of molten rock. The Principle of Cross-Cutting Relationships states that any geological feature that cuts across or disrupts another feature must be younger than the feature it cuts. For instance, a fault that displaces several rock layers must have formed after those layers were already in place.
Igneous dikes, which are sheets of magma that cool within existing rock, also follow this rule; the dike is younger than the layers it penetrates. This principle, developed by James Hutton and Charles Lyell, allows geologists to date tectonic events relative to the rock strata they affect. Observing a series of cross-cutting features helps establish a chronological sequence of deformation.
The Principle of Inclusions determines the age relationship between two different rock types. An inclusion is a fragment of one rock unit found within another. The principle states that the fragments contained within the host rock must be older than the rock that contains them.
For example, if sandstone contains pebbles of granite, the granite must have existed and been eroded before the sandstone was deposited around its fragments. Similarly, a xenolith—a piece of surrounding rock found within an igneous intrusion—must be older than the magma that cooled to form the intrusive body. This principle determines the relative ages of different rock formations based on physical evidence of one being broken down to form part of the other.
Principles of Biological Succession
While physical principles address rock formation and deformation, the Principle of Faunal Succession provides a biological means for relative dating and correlation. Developed by William Smith in the early 19th century, this principle observes that fossil organisms succeed one another in a definite and recognizable order through the rock record. Once a species goes extinct, it does not reappear in younger rocks.
This consistent pattern allows geologists to identify and date rock layers by their characteristic fossil content, regardless of the rock type or location. Fossils found in a layer represent the organisms that lived during the time the sediment was deposited. Since the sequence of life is fixed, rocks containing the same assemblage of fossils must be of the same relative age.
Certain organisms, known as index fossils, are useful because they were geographically widespread, abundant, and existed for a relatively short span of geological time. The presence of a specific index fossil, such as trilobites or ammonites, allows geologists to correlate rock layers across continents. Index fossils are the primary tool for creating the global geologic time scale based on biological change.
Identifying Missing Time (Unconformities)
The application of relative dating principles often reveals surfaces where the geological record is incomplete, known as unconformities. An unconformity is a buried erosional or non-depositional surface that represents a significant gap in time. This gap occurs when either no sediment was laid down or existing rock was removed. Recognizing these breaks is essential for accurately interpreting Earth’s history.
One type is the angular unconformity, which separates older, tilted, or folded rock layers below from younger, horizontal layers above. The tilted layers show that a sequence of deposition, deformation, and erosion occurred before the younger layers were deposited.
A disconformity occurs when the rock layers above and below the erosional surface are parallel, representing a period of erosion or non-deposition. A nonconformity is present when sedimentary rock layers are deposited directly on top of much older, eroded igneous or metamorphic rocks. Geologists use these principles to establish the order of visible events and to account for intervals of missing time.