Geologic time scale
Standardized system dividing Earth's history into defined time intervals.
The geologic time scale is a system for dividing Earth's 4.54 ± 0.05 billion-year history into standardized intervals based on the rock record and principles of chronostratigraphy and geochronology. It is used primarily by Earth scientists to describe the timing and relationships of events in geologic history. The scale is constructed by arranging rock sequences in chronological order, relying on fundamental changes in stratigraphy that correspond to major geological or paleontological events. Chronostratigraphy organizes all rocks of the Earth's crust into groups based on their relative ages, using principles such as the law of superposition (older strata lie at the bottom), original horizontality (sediments deposit horizontally), lateral continuity (layers extend until thinning or being cut off), cross-cutting relationships (a rock cutting across another is younger), and the law of included fragments (embedded fragments are older than the surrounding rock). Unconformities, representing gaps in the record from erosion or non-deposition, help determine relative timing. The principle of faunal succession allows correlation of strata through distinctive fossil sequences. Geochronology provides precise ages through quantitative measurements like radiometric dating, as well as relative methods such as paleomagnetism and stable isotope ratios. A geochronologic unit is the time interval during which a corresponding chronostratigraphic unit formed; improved dating can refine the age of a geochronologic unit without altering the rock-defined boundary. The International Commission on Stratigraphy (ICS) standardizes global chronostratigraphic units, using Global Boundary Stratotype Sections and Points (GSSPs) to define the lower boundaries of these units, reconciling historical regional scales.
- field
- Earth science (geology, paleontology, geophysics, geochemistry, paleoclimatology)
- governing_body
- International Commission on Stratigraphy (ICS), a constituent body of the International Union of Geological Sciences (IUGS)
- time_span_covered
- about 4.54 ± 0.05 billion years
- largest_unit
- Eon (four formally defined: Hadean, Archean, Proterozoic, Phanerozoic)
- smallest_hierarchical_unit
- Age (96 formal and five informal ages; current age is the Meghalayan)
- current_epoch
- Holocene
- current_period
- Quaternary
Lore & Background
The geologic time scale is a system for dividing Earth’s deep history, spanning approximately 4.54 billion years, into standardized intervals based on major geological and paleontological events. Its appearance is often visualized as a log-spiral diagram that proportionally represents these intervals, with notable events in Earth’s history and the evolution of life marked along the spiral. The scale is built from the rock record using two complementary approaches: chronostratigraphy, which organizes rock sequences into chronostratigraphic units based on their relative ages, and geochronology, which assigns precise numerical ages to those units using techniques like radiometric dating. Defining characteristics include several key principles: the law of superposition (older strata lie below younger in undeformed sequences), original horizontality (sediments initially deposit horizontally), lateral continuity (sediment layers extend until they thin or are cut off), cross-cutting relationships (a rock cutting across another is younger), the law of included fragments (embedded rock fragments are older than the surrounding rock), unconformities (gaps in the record from erosion or non-deposition), and faunal succession (fossil assemblages succeed each other in a predictable order). The scale’s global standardization is overseen by the International Commission on Stratigraphy, which defines Global Boundary Stratotype Sections and Points (GSSPs) to mark the lower boundaries of chronostratigraphic units. These boundaries remain fixed even as absolute ages are refined through improved dating methods.
Reader's Guide
The geologic time scale is the foundational framework for understanding Earth's history, enabling scientists to correlate rock sequences globally and date events such as mass extinctions and mountain building. Its development is overseen by the International Commission on Stratigraphy, which defines global chronostratigraphic units on the International Chronostratigraphic Chart. Boundaries between units are often defined by Global Boundary Stratotype Section and Points (GSSPs), known informally as 'golden spikes,' which mark precise points in specific rock successions. For example, the iridium anomaly from the Chicxulub impact defines the boundary between the Cretaceous and Paleogene. The scale uses units such as eons, eras, periods, epochs, ages, and chrons, with numeric ages expressed in Ma (megaannum), Ga (gigaannum), or ka (kiloannum). While the scale is standardized internationally, regional terms persist in some areas, and absolute ages of boundaries can be refined by improved dating techniques without altering the rock-defined boundaries.
Did You Know?
- The geologic time scale covers a time span of about 4.54 ± 0.05 billion years.
- The International Commission on Stratigraphy (ICS) is responsible for defining standardized international units of geological time.
- A Global Boundary Stratotype Section and Point (GSSP) defines the lower boundary of a stage at a precise point in a specific rock succession.
- The current epoch is the Holocene, and the current age is the Meghalayan.
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