Geologic time scale

The geologic time scale is a chronological framework that represents the entirety of Earth’s 4.54-billion-year history. It is constructed from the geological record preserved in rocks, fossils, and geomagnetic signatures, and it provides a universal system for describing the timing, sequence, and duration of events in Earth history. The scale underpins research across geology, palaeontology, geophysics, geochemistry, and related fields, supporting interpretations of environmental change, biological evolution, and planetary processes. Standardisation of geological time units is overseen by the International Commission on Stratigraphy, which maintains the International Chronostratigraphic Chart.
The geologic time scale originates from the principles of stratigraphy—observations about how layers of rock form, accumulate, and relate to each other. These principles allow geologists to determine the relative order of events, while radiometric dating and other absolute dating methods provide numerical ages. Together these methods offer a structured timeline enabling comparison of rock sequences from across the world.

Principles Underlying Stratigraphic Interpretation

The construction of the geologic time scale depends on several foundational principles that establish the relative order of rock layers:

• Law of superposition: in undisturbed sequences, older layers lie beneath younger ones.
• Original horizontality: sedimentary layers are initially deposited horizontally under gravity.
• Lateral continuity: sedimentary layers extend laterally until they thin or meet a barrier.
• Cross-cutting relationships: any feature that cuts across another is younger than the rock it disrupts.
• Law of included fragments: rock fragments enclosed within another rock must be older than the enclosing medium.
• Unconformities: breaks in the rock record represent intervals of erosion or non-deposition.
• Faunal succession: fossil assemblages follow a consistent vertical progression, allowing correlation between stratigraphic sequences worldwide.

These principles permit the recognition of key boundaries in Earth history, such as the Cretaceous–Palaeogene boundary defined by a global extinction event. In earlier parts of Earth history, where fossils or distinctive boundaries are absent, Global Standard Stratigraphic Ages provide numerical markers established by international agreement.

Development and International Standardisation

Regional differences in rock types and fossil distributions historically led to multiple local time scales. The International Commission on Stratigraphy has worked to reconcile these variations by defining global reference points based on identifiable stratigraphic horizons. These reference points, known as Global Boundary Stratotype Sections and Points (GSSPs), serve as physical markers embedding time boundaries within specific rock sequences. The result is a standardised global system enabling consistent communication and comparison across nations and geological disciplines.
Stratigraphic units are distinguished from geochronological units. The former refer to bodies of rock representing intervals of time, whereas the latter refer to the time spans themselves. Revisions to geochronological units occur when new dating techniques refine numerical ages, while the corresponding stratigraphic units remain fixed because they are defined physically in the rock record. An example is the refinement of the Ediacaran–Cambrian boundary from 541 million to approximately 538.8 million years without changing the designated GSSP.

Hierarchy of Geological Time Units

The geologic time scale is organised hierarchically. Each level reflects a distinct scope of geological processes and evolutionary change:

• Eons are the largest units. Four eons are formally recognised: the Hadean, Archean, Proterozoic, and Phanerozoic.
• Eras subdivide eons. Ten recognised eras range from early Archean divisions to the Cenozoic.
• Periods subdivide eras. There are twenty-two defined periods, including the Quaternary, which is the current period.
• Epochs subdivide periods. Thirty-seven epochs are formally recognised, with the Holocene being the present epoch. Eleven subepochs occur within the Neogene and Quaternary.
• Ages are the smallest hierarchical geochronological units. Ninety-six ages are defined, with the Meghalayan being the current one.
• Chronozones represent non-hierarchical intervals often based on magnetic, lithological, or biological stratigraphic markers.

Terminology distinguishes between chronostratigraphic units, which are bodies of rock (eonothems, erathems, systems, series, stages), and their geochronological counterparts (eons, eras, periods, epochs, ages). The hierarchical structure allows geologists to describe time in both relative and absolute terms, linking rock bodies with their age.
The subdivisions ‘early’, ‘middle’, and ‘late’ serve as geochronological descriptors corresponding to the chronostratigraphic terms ‘lower’, ‘middle’, and ‘upper’, enabling consistent communication about temporal position within a larger unit. For example, “Early Triassic” describes the time interval, whereas “Lower Triassic” describes the rocks formed during it.

Construction of the Geologic Record

The time scale reflects key events and transitions recorded in Earth’s geological archive. These include:

• Formation of Earth and early crustal development during the Hadean.
• Appearance of early life in the Archean.
• Atmospheric oxygenation, supercontinent cycles, and multicellular evolution in the Proterozoic.
• Diversification of complex life in the Phanerozoic, including major radiations and mass extinctions.

By linking geological formations with dated horizons, the geologic time scale places such events in chronological context. Physical traces such as volcanic ash layers, magnetic reversals, and fossil assemblages provide markers correlating sequences from different regions. Stratigraphic gaps revealed by unconformities indicate intervals of erosion or non-deposition, highlighting periods where the geological record is incomplete.
Deep-time chronology is further refined through radiometric techniques, stable isotope analysis, palaeomagnetism, and astrochronology. Each technique improves numerical precision and enhances the resolution of events across Earth’s history.

Significance in Earth Sciences

The geologic time scale is essential for interpreting the evolution of the Earth system. It:

• Provides a framework for correlating rock strata across continents.
• Enables reconstruction of past climates and environments.
• Supports studies of biological evolution, extinction patterns, and palaeoecology.
• Helps identify geodynamic processes such as mountain building, sea-level fluctuations, and plate movements.
• Serves as the temporal foundation for disciplines such as geochemistry, sedimentology, and palaeoclimatology.

Because it integrates stratigraphic observations with absolute time measurements, the geologic time scale is a cornerstone of Earth science. It allows researchers to describe Earth’s past in a coherent and internationally standardised format, supporting both scientific understanding and practical applications such as resource exploration and environmental reconstruction.