05 Geologic Time and Dating
Learn how geologists sequence rock and fossil evidence, estimate ages with radioactive isotopes, and combine both approaches in the geologic time scale.
Establishing the order of geological events
Earth’s history spans billions of years, so geologists combine evidence about the order of events with measurements that estimate ages in years. These approaches answer different but connected questions: which event happened first, and how long ago did a particular event occur?
establishes an order without assigning ages in years. estimates an age in years, but the estimate has uncertainty and may refer to a specific event, such as a mineral crystallizing, rather than the entire history of a rock.
Reading rock relationships
Several principles help establish a sequence:
: In an undisturbed sequence of deposited layers, the bottom layer is oldest, and each layer is older than the one above it.
Original horizontality: Sediments are generally deposited in horizontal or nearly horizontal layers. If layers are tilted or folded, that deformation happened after deposition.
: A feature that cuts or disrupts another is younger than the feature it affects.
Inclusions: Fragments enclosed in a rock are older than the rock around them. Pebbles in a conglomerate, for example, existed before the conglomerate cemented.
: Fossils occur in a recognizable order. Widespread, short-lived index fossils can help compare the relative ages of layers in different places.
These principles must be considered alongside rock structures. Folding, faulting, erosion, and overturning can disrupt the apparent sequence.
Takeaway: Relative relationships establish event order; numerical dates can then connect that order to estimated ages in years.
, correlation, and gaps in the record
is the study of layered rocks and their relationships. A single layer, or stratum, records material deposited during an interval of time. By tracing layers and comparing rock features and fossils, geologists correlate strata: they identify layers in separate locations that formed at roughly the same time or under related conditions.
The rock record is not always continuous. An marks a surface where erosion occurred or sediment was not deposited, leaving missing time. For example, tilted older layers overlain by horizontal younger layers record this sequence: deposition of the older layers, deformation, erosion, and renewed deposition.
can establish the sequence even when there is no material suitable for direct . Numerical dates from appropriate rocks or minerals can anchor parts of that sequence to years and help correlate records across regions.
Takeaway: Correlation connects separate rock records, while unconformities warn that part of the history may be missing.
Estimating ages with radioactive isotopes
Many minerals contain radioactive isotopes. A decays into a at a predictable rate. The is the time required for half of the parent atoms in a sample to decay. Measuring parent and daughter isotopes and applying the known decay rate allows scientists to estimate when the mineral’s isotopic clock began—for example, when it crystallized or cooled enough to retain the isotopes.
For a simple illustration, a mineral that starts with a known amount of radioactive has about of that parent remaining after one and about remaining after a second . Real estimates require laboratory analysis and consideration of the sample’s starting composition and later history.
A reliable estimate requires the sample to have preserved its isotope record. Heating, fluids, or weathering can alter that record, so geologists assess whether it remained a sufficiently closed system. Different isotope systems are suited to different materials and age ranges. Carbon-14 is used to date once-living material from the relatively recent past; it is not used to date ancient rocks across most of geologic time.
Sedimentary rocks are often difficult to date directly because their grains may have formed long before deposition. Geologists may instead date volcanic ash layers above and below a sedimentary layer to bracket its age, or use fossils and other evidence to correlate it with dated rocks.
Takeaway: Radiometric dating estimates when an isotope clock began, provided the sample preserves a useful record of radioactive decay.
Organizing Earth history on a time scale
The organizes Earth history into nested intervals: eons, eras, periods, epochs, and ages. It combines stratigraphic, fossil, and numerical-dating evidence so geologists can communicate when events happened and compare records from different locations.
For example, the Cretaceous Period is part of the Mesozoic Era, which is part of the Phanerozoic Eon. Many boundaries are defined by the International Commission on using a designated reference point in a rock section, called a Global Boundary Stratotype Section and Point (GSSP). Numerical ages assigned to boundaries are estimates; they can be revised as measurements improve and do not, by themselves, define every boundary.
On the cited chart, the base of the Cretaceous is about million years ago (); the base of the Paleogene is ; and the base of the Quaternary is . The base of the Hadean is placed at about billion years ago (), near the beginning of Earth’s history. means million years ago, and means billion years ago. Ages are counted backward from the present, so a larger numerical age means an older time. These values are subject to refinement.
Takeaway: The time scale gives shared names and boundaries to Earth history, while numerical ages provide estimates that can improve with new evidence.
Putting the methods together
Geologists reconstruct Earth’s history by combining the relative order of rocks and events with numerical age estimates. Rock relationships and fossils reveal sequences and support correlation; radioactive decay can anchor parts of those sequences to years. The brings this evidence together, and its boundaries and numerical estimates remain open to refinement as new evidence becomes available.