Dating time scale
- 8.2 Relative Dating Methods
- How is radiometric dating used in geochronology?
- What is the geologic time scale?
- What determines the precision of a dating method?
- What is the half-life of interest in radiometric dating?
- What is the purpose of radiometric dating?
- What is meant by the term radioactive dating?
- How is the age of a rock determined by radiometric dating?
- What type of mass spectrometer is used in radiometric dating?
8.2 Relative Dating Methods
James Hutton see realized geologic processes are slow and his ideas on uniformitarianism i. This section discusses principles of relative time used in all of geology, but are especially useful in stratigraphy. Principle of Superposition: In an otherwise undisturbed sequence of sedimentary strata, or rock layers, the layers on the bottom are the oldest and layers above them are younger.
Principle of Original Horizontality: Layers of rocks deposited from above, such as sediments and lava flows, are originally laid down horizontally. The exception to this principle is at the margins of basins, where the strata can slope slightly downward into the basin.
Of course, all strata eventually end, either by hitting a geographic barrier, such as a ridge, or when the depositional process extends too far from its source, either a sediment source or a volcano. Strata that are cut by a canyon later remain continuous on either side of the canyon. Principle of Cross-Cutting Relationships: Deformation events like folds, faults and igneous intrusions that cut across rocks are younger than the rocks they cut across.
Principle of I nclusions: When one rock formation contains pieces or inclusions of another rock, the included rock is older than the host rock. Principle of Fossil Succession: Evolution has produced a succession of unique fossils that correlate to the units of the geologic time scale. Dating time scale of fossils contained in strata dating time scale unique to dating time scale time they lived and can be used to correlate rocks of the same age across a wide geographic distribution.
Assemblages of fossils refer to groups of several unique fossils occurring together. The photo shows layers of rock on top of one another in order, from the oldest at the bottom to the youngest at the top, based on the principle of superposition. The predominant white layer just below the canyon rim is the Coconino Sandstone. This layer is laterally continuous, even though the intervening canyon separates its outcrops.
The rock layers exhibit the principle of lateral continuity, as they are found on dating time scale sides of the Grand Canyon which has been carved by the Colorado River. In the lowest parts of the Grand Canyon are the oldest sedimentary formations, with igneous and metamorphic rocks at the bottom. The principle of cross-cutting relationships shows the sequence of these events. The metamorphic schist 16 is the oldest rock formation and the cross-cutting granite intrusion 17 is younger.
This illustrates the principle of superposition. The Grand Canyon region lies in Colorado Plateau, which is characterized by horizontal or nearly horizontal strata, which follows the principle of original horizontality.
These rock strata have been barely disturbed from their original deposition, except by a broad regional uplift. Because the formation of the basement rocks and the deposition of the overlying strata is not continuous but broken by events of metamorphism, intrusion, and erosion, the contact between the strata and the older basement is termed an unconformity.
An unconformity represents dating time scale period during which deposition did not occur or erosion removed rock that had been deposited, so there are no rocks that represent events of Earth history during that span of time at that dating time scale.
Unconformities appear in cross-sections and stratigraphic columns as wavy lines between formations. Unconformities are discussed in the next section. The pinching Temple Butte is the easiest to see the erosion, but even between the Muav and Redwall, there is an unconformity. There are three types of unconformities, nonconformity, disconformity, and angular unconformity.
Dating time scale nonconformity occurs when sedimentary rock is deposited on top of igneous and metamorphic rocks as is the case with the contact between the strata and basement rocks at the bottom of the Grand Canyon. The strata in the Grand Canyon represent alternating marine transgressions and regressions where sea level rose and fell over millions of years.
When the sea level was high marine strata formed. When sea-level fell, the land was exposed to erosion creating an unconformity. In the Grand Canyon cross-section, dating time scale erosion is shown as heavy wavy lines between the various numbered strata. This is a type of unconformity called a disconformity, where either non-deposition or erosion took place.
In other words, layers of rock that could have been present, are absent. The time that could have been represented by such layers is instead represented by the disconformity.
Disconformities are unconformities that occur between parallel layers of strata indicating either a period of no deposition or erosion. Notice the flat-lying strata over dipping strata Source: Doug Dolde.
The Phanerozoic strata in most of the Grand Canyon are horizontal. However, near the bottom horizontal strata overlie tilted strata. This is known as the Great Unconformity and is an example of an angular unconformity. The lower strata were tilted by tectonic processes that disturbed their original horizontality and caused the strata to be eroded. Later, horizontal strata were deposited on top of the tilted strata creating an angular unconformity.
Here are three graphical illustrations of the three types of unconformity. The wavy rock is an old metamorphic gneiss, A and F are faults, B is an igneous granite, D dating time scale a basaltic dike, and C and E are sedimentary strata. In the block diagram, the sequence of geological events can be determined by using the relative-dating principles and known properties of igneous, sedimentary, metamorphic rock see, and.
The sequence begins with the folded metamorphic gneiss on the bottom. Next, the gneiss is cut and displaced by the fault labeled A. Both the gneiss and fault A are cut by the igneous granitic intrusion called batholith B; its irregular outline suggests it is an igneous granitic intrusion emplaced as magma into the gneiss. Since batholith B cuts both the gneiss and fault A, batholith B is younger than the other two dating time scale formations.
Next, the gneiss, fault Dating time scale, and batholith B were eroded forming a nonconformity as shown with the wavy line.
This unconformity was actually an ancient landscape surface on which sedimentary rock C was subsequently deposited perhaps by a marine transgression. Next, igneous basaltic dike D cut through all rocks except sedimentary rock E. This shows that there is a disconformity between sedimentary rocks C and E. The top of dike D is level with the top of layer C, which establishes that erosion flattened the landscape prior to the deposition of layer E, creating a disconformity between rocks D and E.
Fault F cuts across all of the older rocks B, C and E, producing a fault scarp, which is the low ridge on the upper-left side of the diagram. The final events affecting this area are current erosion processes working on the land surface, rounding off the edge of the fault scarp, and producing the modern landscape at the top of the diagram. Dating time scale LibreTexts libraries are and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot.
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How is radiometric dating used in geochronology?
Together with stratigraphic principles, radiometric dating methods are used in geochronology to establish the geologic time scale. Among the best-known techniques are radiocarbon dating, potassium–argon dating and uranium–lead dating.
What is the geologic time scale?
The geologic time scale ( GTS) is a system of chronological dating that classifies geological strata ( stratigraphy) in time. It is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events in geologic history.
What determines the precision of a dating method?
The precision of a dating method depends in part on the half-life of the radioactive isotope involved. For instance, carbon-14 has a half-life of 5,730 years.
What is the half-life of interest in radiometric dating?
In these cases, usually the half-life of interest in radiometric dating is the longest one in the chain, which is the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter.
What is the purpose of radiometric dating?
Radiometric dating (or radioactive dating) is any technique used to date organic and also inorganic materials from a process involving radioactive decay. Radiometric dating methods are used in geochronology to establish the geologic time scale. Radiation Dosimetry
What is meant by the term radioactive dating?
Radiometric dating (or radioactive dating) is any technique used to date organic and also inorganic materials from a process involving radioactive decay. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay.
How is the age of a rock determined by radiometric dating?
The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature. This temperature varies for every mineral and isotopic system, so a system can be closed for one mineral but open for another.
What type of mass spectrometer is used in radiometric dating?
Thermal ionization mass spectrometer used in radiometric dating. Radiometric dating calculates an age in years for geologic materials by measuring the presence of a short-life radioactive element, e.g., carbon-14, or a long-life radioactive element plus its decay product, e.g., potassium-14/argon-40.