2.2 Geologic time & relative/absolute dating principles

Relative dating: ordering events without numbers

Relative dating establishes the sequence of geologic events without assigning ages in years. It rests on a small set of foundational principles, several first articulated by Nicolas Steno in the seventeenth century and refined by later workers such as James Hutton and William Smith:

• Superposition — in an undeformed sequence of sedimentary or layered volcanic rocks, each bed is younger than the layer beneath it and older than the layer above.
• Original horizontality — sediments settle under gravity into nearly horizontal layers, so strata now tilted, folded, or overturned must have been deformed after deposition.
• Lateral continuity — layers extend outward in all directions until they thin, pinch out, or abut a barrier; matching beds on opposite walls of a canyon were once continuous.
• Cross-cutting relationships — any feature that cuts across rocks, such as a fault, joint, dike, or other intrusion, is younger than the rocks it disrupts.
• Inclusions — rock fragments enclosed within another rock (xenoliths in magma, clasts in sandstone) are older than the host that contains them.
• Faunal succession — fossil species appear, persist, and go extinct in a definite, worldwide, repeatable order, so fossil assemblages identify strata and permit correlation between distant sections (biostratigraphy).

Supporting field criteria help interpret contacts: an intrusion bakes (contact-metamorphoses) the rock around it and may enclose inclusions of it, whereas a buried lava flow bakes only the older rock beneath it and may be weathered on top. A chilled margin of fine crystals marks the rapidly cooled edge of an intrusion.

Unconformities

An unconformity is a buried surface of erosion or non-deposition that represents missing time — a gap in the rock record. Three principal types are tested:

A paraconformity is an obscure disconformity lacking any visible erosion surface, recognized only from a gap in the fossil record. Unconformities matter because they record episodes of uplift, erosion, sea-level change, or non-deposition, and they bound the depositional sequences used in stratigraphy.

Absolute (numerical) dating

Absolute dating assigns ages in years, most powerfully through radiometric dating. An unstable parent isotope decays to a stable daughter isotope at a constant, statistically predictable rate expressed by the half-life — the time required for half of the remaining parent atoms to decay. After one half-life, 50% of the original parent remains; after two, 25%; after three, 12.5%; and so on. By measuring the present parent-to-daughter ratio with a mass spectrometer and knowing the half-life, an age is calculated. Reliable ages require a closed system (no gain or loss of parent or daughter after formation) and a known or negligible initial daughter amount. Its decay rate is unaffected by temperature or pressure, which is what makes radiometric clocks dependable.

Common radiometric systems

Because carbon-14 has a short half-life of about 5,730 years, it dates only geologically young, carbon-bearing material — wood, charcoal, shell, bone — younger than roughly 50,000 years, and is useless for most bedrock. Carbon-14 forms in the atmosphere and begins to decay when an organism dies, making it ideal for archaeology and Quaternary geology. Long-lived systems such as uranium-lead, especially in the mineral zircon, are the most precise tools for deep time and have dated the oldest known minerals at roughly 4.0 to 4.4 billion years. Dating volcanic ash beds interbedded with fossil-bearing strata ties numerical ages to the relative (biostratigraphic) time scale, so the two approaches are complementary rather than competing.

The geologic time scale

The geologic time scale organizes Earth's roughly 4.54-billion-year history into a nested hierarchy of eons, eras, periods, epochs, and ages. The oldest eons — Hadean, Archean, and Proterozoic — are collectively the informal Precambrian, which spans about 88% of Earth history. The Phanerozoic eon ("visible life"), from about 541 million years ago (Ma) to today, contains three eras:

• Paleozoic (~541–252 Ma): Cambrian, Ordovician, Silurian, Devonian, Carboniferous (split into Mississippian and Pennsylvanian in North America), and Permian; it ends with the largest mass extinction in Earth history.
• Mesozoic (~252–66 Ma): Triassic, Jurassic, and Cretaceous; the age of dinosaurs; it ends with the K-Pg extinction.
• Cenozoic (~66 Ma–present): Paleogene, Neogene, and Quaternary; the age of mammals; its epochs include the Pleistocene and Holocene.

Boundaries are defined by significant events — commonly mass extinctions or fossil first-appearances — and are calibrated with radiometric ages. Candidates should memorize the order of the Phanerozoic periods and the approximate ages of the major boundaries (541, 252, and 66 Ma), because both relative principles and absolute ages recur throughout the FG and PG exams. Combining the two — using cross-cutting and superposition to order events and radiometric ages to pin them in years — produces the calibrated geologic history on which resource, hazard, and environmental interpretations depend.