Pages in category "Geochronological dating methods". The following 17 pages are in this category, out of 17 total. This list may not reflect recent changes (learn .
Table of contents
- Early views and discoveries
- Geochronology - New World Encyclopedia
- Category:Geochronological dating methods
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- General considerations
It is, however, important not to confuse geochronologic and chronostratigraphic units. As noted above, various dating methods are used in geochronology.
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- Geochronology - Wikipedia.
Each method has a certain degree of uncertainty, but the reliability of the results can be enhanced by using several techniques. By measuring the amount of Radioactive decay of a radioactive isotope with a known half-life , geologists can establish the absolute age of the parent material. A number of radioactive isotopes are used for this purpose, and depending on the rate of decay, are used for dating different geological periods.
With the exception of the radiocarbon method, most of these techniques are actually based on measuring an increase in the abundance of a radiogenic isotope, which is the decay-product of the radioactive parent isotope. Luminescence dating techniques observe 'light' emitted from materials such as quartz, diamond, feldspar, and calcite. Many types of luminescence techniques are utilized in geology, including optically stimulated luminescence OSL , cathodoluminescence CL , and thermoluminescence TL. Thermoluminescence and optically stimulated luminescence are used in archaeology to date "fired" objects such as pottery or cooking stones, and can be used to observe sand migration.
Incremental dating techniques allow the construction of year-by-year annual chronologies, which can be fixed that is, linked to the present day and thus calendar or sidereal time or floating. Marker horizons are geological units in different geographic locations but of the same age. They allow age-equivalence to be established between different sites.
The same margin of error applies for younger fossiliferous rocks, making absolute dating comparable in precision to that attained using fossils.
Early views and discoveries
To achieve this precision, geochronologists have had to develop the ability to isolate certain high-quality minerals that can be shown to have remained closed to migration of the radioactive parent atoms they contain and the daughter atoms formed by radioactive decay over billions of years of geologic time. In addition, they have had to develop special techniques with which to dissolve these highly refractory minerals without contaminating the small amount about one-billionth of a gram of contained lead and uranium on which the age must be calculated.
Since parent uranium atoms change into daughter atoms with time at a known rate, their relative abundance leads directly to the absolute age of the host mineral. In fact, even in younger rocks, absolute dating is the only way that the fossil record can be calibrated. Without absolute ages, investigators could only determine which fossil organisms lived at the same time and the relative order of their appearance in the correlated sedimentary rock record.
Geochronology - New World Encyclopedia
Unlike ages derived from fossils, which occur only in sedimentary rocks, absolute ages are obtained from minerals that grow as liquid rock bodies cool at or below the surface. When rocks are subjected to high temperatures and pressures in mountain roots formed where continents collide, certain datable minerals grow and even regrow to record the timing of such geologic events. When these regions are later exposed in uptilted portions of ancient continents, a history of terrestrial rock-forming events can be deduced.
Episodes of global volcanic activity , rifting of continents, folding, and metamorphism are defined by absolute ages. The results suggest that the present-day global tectonic scheme was operative in the distant past as well. Continents move, carried on huge slabs, or plates, of dense rock about km 62 miles thick over a low-friction, partially melted zone the asthenosphere below.
In the oceans , new seafloor, created at the globe-circling oceanic ridges , moves away, cools, and sinks back into the mantle in what are known as subduction zones i.
Where this occurs at the edge of a continent, as along the west coast of North and South America, large mountain chains develop with abundant volcanoes and their subvolcanic equivalents. These units, called igneous rock , or magma in their molten form, constitute major crustal additions. By contrast, crustal destruction occurs at the margins of two colliding continents, as, for example, where the subcontinent of India is moving north over Asia.
Great uplift, accompanied by rapid erosion, is taking place and large sediment fans are being deposited in the Indian Ocean to the south. Rocks of this kind in the ancient record may very well have resulted from rapid uplift and continent collision. When continental plates collide, the edge of one plate is thrust onto that of the other.
The rocks in the lower slab undergo changes in their mineral content in response to heat and pressure and will probably become exposed at the surface again some time later. Rocks converted to new mineral assemblages because of changing temperatures and pressures are called metamorphic. Virtually any rock now seen forming at the surface can be found in exposed deep crustal sections in a form that reveals through its mineral content the temperature and pressure of burial. Such regions of the crust may even undergo melting and subsequent extrusion of melt magma, which may appear at the surface as volcanic rocks or may solidify as it rises to form granites at high crustal levels.
Magmas produced in this way are regarded as recycled crust, whereas others extracted by partial melting of the mantle below are considered primary. Even the oceans and atmosphere are involved in this great cycle because minerals formed at high temperatures are unstable at surface conditions and eventually break down or weather, in many cases taking up water and carbon dioxide to make new minerals.
Category:Geochronological dating methods
If such minerals were deposited on a downgoing i. These components would then rise and be fixed in the upper crust or perhaps reemerge at the surface. Such hot circulating fluids can dissolve metals and eventually deposit them as economic mineral deposits on their way to the surface.
Geochronological studies have provided documentary evidence that these rock-forming and rock-re-forming processes were active in the past. Seafloor spreading has been traced, by dating minerals found in a unique grouping of rock units thought to have been formed at the oceanic ridges, to million years ago, with rare occurrences as early as 2 billion years ago.
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Other ancient volcanic units document various cycles of mountain building. The source of ancient sediment packages like those presently forming off India can be identified by dating single detrital grains of zircon found in sandstone. Magmas produced by the melting of older crust can be identified because their zircons commonly contain inherited older cores. Episodes of continental collision can be dated by isolating new zircons formed as the buried rocks underwent local melting.
Periods of deformation associated with major collisions cannot be directly dated if no new minerals have formed.
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The time of deformation can be bracketed, however, if datable units, which both predate and postdate it, can be identified. The timing of cycles involving the expulsion of fluids from deep within the crust can be ascertained by dating new minerals formed at high pressures in exposed deep crustal sections. In some cases, it is possible to prove that gold deposits may have come from specific fluids if the deposition time of the deposits can be determined and the time of fluid expulsion is known.
Where the crust is under tension, as in Iceland, great fissures develop. These fissures serve as conduits that allow black lava , called basalt , to reach the surface. The portion that remains in a fissure below the surface usually forms a vertical black tubular body known as a dike or dyke. Precise dating of such dikes can reveal times of crustal rifting in the past. Dikes and lava, now exposed on either side of Baffin Bay , have been dated to determine the time when Greenland separated from North America—namely, about 60 million years ago.
Combining knowledge of Earth processes observed today with absolute ages of ancient geologic analogues seems to indicate that the oceans and atmosphere were present by at least 4 billion years ago and that they were probably released by early heating of the planet. The continents were produced over time; the oldest preserved portions were formed approximately 4 billion years ago, but this process had begun about by 4. Absolute dating allows rock units formed at the same time to be identified and reassembled into ancient mountain belts, which in many cases have been disassociated by subsequent tectonic processes.
The most obvious of these is the Appalachian chain that occupies the east coast of North America and extends to parts of Newfoundland as well as parts of Ireland, England, and Norway. Relic oceanic crust , formed between million and million years ago, was identified on both sides of the Atlantic in this chain, as were numerous correlative volcanic and sedimentary units.
General considerations
Evidence based on geologic description, fossil content, and absolute and relative ages leave no doubt that these rocks were all part of a single mountain belt before the Atlantic Ocean opened in stages from about million years ago. Relative geologic ages can be deduced in rock sequences consisting of sedimentary, metamorphic, or igneous rock units. In fact, they constitute an essential part in any precise isotopic, or absolute, dating program. Such is the case because most rocks simply cannot be isotopically dated.
Therefore, a geologist must first determine relative ages and then locate the most favourable units for absolute dating. It is also important to note that relative ages are inherently more precise, since two or more units deposited minutes or years apart would have identical absolute ages but precisely defined relative ages. While absolute ages require expensive, complex analytical equipment, relative ages can be deduced from simple visual observations.
Most methods for determining relative geologic ages are well illustrated in sedimentary rocks. These rocks cover roughly 75 percent of the surface area of the continents, and unconsolidated sediments blanket most of the ocean floor. The seminal work of Smith at clarifying various relationships in the interpretation of rock successions and their correlations elsewhere resulted in an intensive look at what the rock record and, in particular, what the fossil record had to say about past events in the long history of the Earth.
The application of the ideas of Lyell, Smith, Hutton, and others led to the recognition of lithologic and paleontologic successions of similar character from widely scattered areas. It also gave rise to the realization that many of these similar sequences could be correlated. Not convinced that catastrophes caused massive and widespread disruption of the biota, Lamarck preferred to think of organisms and their distribution in time and space as responding to the distribution of favourable habitats.