Geologic Time Scale
The Geologic Time Scale (GTS) is the internationally standardized chronological framework that divides Earth's 4.54-billion-year history into hierarchical time units—eons, eras, periods, epochs, and ages—based on stratigraphic evidence, fossil assemblages, and radiometric dating. The boundaries between units correspond to major geological or biological events such as mass extinctions, sea-level changes, or tectonic reorganizations. The GTS is maintained by the International Commission on Stratigraphy (ICS) and is essential for communicating geological ages, correlating rock formations globally, and understanding Earth's evolutionary and environmental history.
Major Divisions of the Geologic Time Scale
| Eon | Era | Start (Ma) | Duration (Ma) | Key Events |
|---|---|---|---|---|
| Phanerozoic | Cenozoic | 66 | 66 | Rise of mammals, grasses, modern ocean basins |
| Phanerozoic | Mesozoic | 252 | 186 | Dinosaurs, first birds, flowering plants; K-Pg extinction |
| Phanerozoic | Paleozoic | 539 | 287 | Cambrian explosion, fish, amphibians, land plants |
| Proterozoic | (Precambrian) | 2500 | 1961 | Eukaryotes, multicellular life, Snowball Earth events |
| Archean | (Precambrian) | 4000 | 1500 | First life, prokaryotes, earliest continental crust |
| Hadean | (Precambrian) | 4540 | 540 | Earth formation, heavy bombardment, no rock record |
Interactive Tools
International Commission on Stratigraphy — Official GTS Chart
Official, downloadable Geologic Time Scale chart with all eons, eras, periods, and absolute ages in PDF and SVG formats.
Open ToolKhan Academy — Geologic Time
Video introduction to the GTS structure, major boundaries, and how it was constructed.
Open ToolSmithsonian Deep Time — Geologic Time Explorer
Interactive geological timeline with fossil evidence and major events for each period.
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Fossil
A fossil is the preserved remains, trace, or impression of a once-living organism, typically found in sedimentary rock and dating to more than 10,000 years ago. Fossilization requires rapid burial by sediment, which prevents decay and allows mineral replacement of organic tissues (permineralization), leaving a durable record of past life. Fossils are the primary evidence for reconstructing the history of life on Earth, documenting evolutionary relationships, ancient environments, and the timing of major biological events through the fossil record.
Stratigraphic Layer
A stratigraphic layer (stratum, plural strata) is a single bed or layer of sedimentary rock, volcanic ash, or other deposited material that was laid down during a specific interval of geological time, distinguished from adjacent layers by differences in composition, texture, or fossil content. The principle of superposition, established by Nicolas Steno in 1669, states that in an undisturbed sequence, lower layers are older than upper ones, making stratigraphy the primary method for establishing relative geological time. Correlation of strata across regions using index fossils and radiometric dating allows geologists to reconstruct the geological history of continents and oceans.
Radiocarbon Dating
Radiocarbon dating (¹⁴C dating) is a radiometric technique used to determine the age of organic materials up to approximately 50,000 years old by measuring the decay of the radioactive carbon isotope ¹⁴C. Living organisms continuously incorporate ¹⁴C from the atmosphere through photosynthesis and respiration; at death, ¹⁴C incorporation ceases and the remaining ¹⁴C decays at a known rate with a half-life of 5,730 years. Developed by Willard Libby in 1949, for which he received the 1960 Nobel Prize in Chemistry, radiocarbon dating revolutionized archaeology, geology, and environmental science by providing absolute ages for recent geological and archaeological materials.
The word "geologic" derives from Greek "ge" (earth) and "logos" (study). The concept of a unified geological time scale emerged from the work of early 19th-century geologists including William Smith, Charles Lyell, and Adam Sedgwick, and was further refined into a numerical framework after the discovery of radioactivity by Henri Becquerel in 1896.