Geologic Time Scale: A Comprehensive Guide to Earth’s History
1. Principle of Superposition: When any given stratum was being formed, all the matter resting on it was fluid. Therefore, when the lowest stratum was formed, none of the upper strata existed.
2. Principle of Original Horizontality: Strata which are either perpendicular to the horizon or inclined to it were at one time parallel to the horizon.
3. Principle of Lateral Continuity: When any given stratum was being formed, it was either encompassed at its edges by another solid substance or it covered the whole globe of the earth. Hence, it follows that wherever bared edges of strata are seen, either a continuation of the same strata must be looked for or another solid substance must be found that kept the material of the strata from being dispersed.
4. Principle of Cross-Cutting Relationships: If a body or discontinuity cuts across a stratum, it must have formed after that stratum.
5. Law of Included Fragments: Small fragments of one type of rock embedded in a second type of rock must have formed first and were included when the second rock was forming.
6. Unconformities: Gaps in rock layers show periods when no sediment was deposited or when erosion occurred, breaking the sequence. These geologic features represent a gap in the geologic record.
7. Principle of Faunal Succession: Fossils in rock layers appear in a specific order, helping to match layers even when there are gaps. This allows for a correlation of strata even when the horizon between them is not continuous.
Geochronology and Chronostratigraphy are two related but distinct concepts in geology. Geochronology (AGE OF ROCK) refers to intervals of time, defining when events occurred in Earth’s history, making it an abstract concept. Chronostratigraphy, on the other hand, deals with the physical units of rock (strata) formed during specific time intervals. This connects layers of rock (strata) to specific points in time, helping us create a timeline. For example, in Western Europe, chalk rock layers are part of the Cretaceous System (a unit of rock), which were deposited during the Cretaceous Period (a time interval) when shallow seas covered the region. Both geochronology and chronostratigraphy follow a parallel hierarchical structure, with terms like the Silurian Period (time) corresponding to the Silurian System (rocks). Maintaining both systems is essential as it distinguishes the physical reality of rock layers from the abstract concept of the time during which they formed, ensuring clarity and precision in studying Earth’s history.
The geologic time scale represents deep time based on events throughout Earth history—a time span of about 4.54 ± 0.05 Ga (4.54 billion years). It chronologically organizes strata, and subsequently time, by observing fundamental changes in stratigraphy that correspond to major geological or paleontological events. Historically, regional geologic time scales were used due to litho- and biostratigraphic differences around the world in time-equivalent rocks. The geological time scale and the geologic column are closely related. The geologic column is a physical record of rock layers that show the sequence of Earth’s history, while the geological time scale organizes this history into time intervals like eons, eras, and periods. The time scale is based on data from the geologic column, where scientists study the rock layers and their fossils to assign specific time periods, linking the physical record to a chronological framework.
Geochronological Units (Time-Based):
Eon: The largest division of time (e.g., Phanerozoic Eon).
Era: A subdivision of an eon (e.g., Mesozoic Era).
Period: A subdivision of an era (e.g., Cretaceous Period).
Epoch: A subdivision of a period (e.g., Late Cretaceous Epoch).
Age: The smallest time unit, often used to refer to specific events within an epoch (e.g., Campanian Age).
Chronostratigraphic Units (Rock-Based):
System: Corresponds to a period and refers to the rock layers deposited during that time (e.g., Cretaceous System).
Series: Corresponds to an epoch and refers to the rock layers formed during that time (e.g., Late Cretaceous Series).
Stage: Corresponds to an age and refers to rock layers deposited during that time (e.g., Campanian Stage).
Eons: The longest periods in Earth’s history. Divided into four eons:
1. Hadean Eon (not explicitly mentioned but precedes Archean).
2. Archean Eon: Up to 2.5 Ga (billion years ago).
3. Proterozoic Eon: From 2.5 Ga to 542 Ma (million years ago).
4. Phanerozoic Eon: From 542 Ma to the present.
The Precambrian consists of the Archean and Proterozoic eons.
Eras are subdivisions of eons, especially for the Phanerozoic:
Paleozoic Era: Up to 251 Ma.
Mesozoic Era: From 251 Ma to 65.5 Ma.
Cenozoic Era: From 65.5 Ma to the present.
The Precambrian can also be subdivided into eras (e.g., Paleoproterozoic, Mesoproterozoic, and Neoproterozoic).
Ages/Stages: Ages are the smallest recognized units of geologic time, subdivisions of epochs, used for very detailed studies of Earth’s history. Example: Within the Pleistocene Epoch, there are several ages such as the Gelasian Age (2.58–1.8 Ma). The corresponding rock layers formed during an age are called stages.
Chrons are short intervals of geological time, often lasting thousands to tens of thousands of years, identified primarily through paleomagnetic studies, which record Earth’s magnetic field reversals in rock layers. While they are a key unit in magnetostratigraphy, their usage is limited outside this field. In the Quaternary Period, chrons are further refined using techniques like marine isotope stages, which rely on changes in global climate and ocean composition, providing a detailed framework for dating recent geological events.
Phanerozoic Periods named after areas where rocks were first described
Cambrian (Wales—Roman for Wales); Devonian (Devon from England); Permian (area in Russia); Jurassic (Jura Mountains, France); Ordovician & Silurian (names of ancient Welsh tribes); Carboniferous (character of the rock); Cretaceous (Latin for chalk)
Type area principle—a region where rocks of that age occur that could act as a reference for other occurrences of similar rocks—mainly the fossil content was the means of correlation—strata containing the same fossils were considered of the same period.
The Precambrian (4.6 Ga–542 Ma) is divided into three eons: Hadean (4.6–4.0 Ga): Earth’s formation, crust stabilization, and early oceans.
Archean (4.0–2.5 Ga): First life (prokaryotes, stromatolites), continental crust formation. Neoarchean; Mesoarchean; Paleoarchean; Eoarchean
Proterozoic (2.5 Ga–542 Ma): Oxygenation of the atmosphere, emergence of eukaryotes, and multicellular life.
The Proterozoic Eon has three eras:
Paleoproterozoic (2.5–1.6 Ga): Oxygen buildup and supercontinent formation.
Mesoproterozoic (1.6–1.0 Ga): First sexually reproducing organism
Neoproterozoic (1.0 Ga–542 Ma): Glaciations (“Snowball Earth”) and early animals (Ediacaran biota).