When trying to understand the history recorded in the ground, whether in geological rock formations or archaeological sites, scientists must first establish a timeline for the layers they uncover. These layers, known as strata, are like pages in a book, and determining their sequence is fundamental to reconstructing Earth’s past. Establishing this chronological sequence without determining a precise numerical age is called relative dating. Geologists and archaeologists rely on observational rules to determine the age order of these layered materials, the most basic of which addresses the question of which layer is the oldest.
The Law of Superposition
The Law of Superposition, formulated by Nicolas Steno in the 17th century, provides the answer to which layer is oldest. This law states that in any undisturbed sequence of sedimentary rock layers, the oldest layer is at the bottom, and the layers above it are progressively younger. The logic is straightforward: material must exist before new material can be deposited on top of it.
Sedimentary layers form through sedimentation, where particles settle out of water or air. New layers are continuously deposited onto the surface of existing, older layers. Over time, the weight of the overlying material compresses and cements the lower sediments into solid rock.
This stacking process means the base layer was laid down first, making it the oldest in the sequence. Each subsequent layer was deposited sequentially, with the most recently formed layer resting at the top. This concept allows scientists to assign relative ages to fossils and rock types found within the layers; a fossil found deeper is chronologically older than one closer to the surface.
The principle applies to any material deposited sequentially, including lava flows and ash beds from volcanic eruptions. However, the Law of Superposition is strictly valid only when the strata remain in their original, undisturbed positions. It provides a baseline assumption for all stratigraphic studies.
When the Oldest Layer Isn’t on the Bottom
Earth is a dynamic planet, and geological forces frequently disrupt the original horizontal layering, meaning the oldest layer is not always found on the bottom. Tectonic forces can bend and deform rock layers through folding. Extreme folding can even overturn a sequence entirely, placing the original oldest layer on top of the youngest layer.
Another common disruption is faulting, where layers are broken and shifted along a fracture in the rock. Igneous intrusions, such as dikes or sills, also complicate the sequence. Since they cut across existing layers, they must be younger than the rocks they penetrate, regardless of their position.
Gaps in the rock record, known as unconformities, also challenge simple interpretation. An unconformity represents a period of missing time, typically caused by erosion or a halt in deposition. Scientists must use additional principles, like the Principle of Cross-Cutting Relationships—where the feature that cuts across another is the younger one—to re-establish the relative timeline after these disturbances.
Determining the Numerical Age
While the Law of Superposition establishes the order of events—which layer is older than which—it does not provide an absolute age in years. To assign a specific numerical age, scientists must turn to radiometric dating. This process shifts the focus from relative dating to absolute dating.
Radiometric dating measures the decay of naturally occurring radioactive isotopes found within rocks and minerals. These unstable parent isotopes decay into stable daughter isotopes at a constant, known rate, which is expressed as a half-life. By measuring the ratio of the parent isotope to the daughter product in a rock sample, scientists calculate the time passed since the rock was formed.
Different isotopes are used for different materials and age ranges. For instance, Carbon-14 dating is effective for dating once-living materials up to about 50,000 years old, while potassium-argon dating is used for much older volcanic rock layers. When a volcanically deposited layer is found within a sequence of sedimentary strata, radiometric dating of the volcanic rock provides a numerical age for that layer, thereby bracketing the age of the sedimentary layers above and below it. Combining the relative timeline from superposition with the numerical ages from radiometric dating allows for a comprehensive reconstruction of Earth’s history.