- High School Earth Science/Absolute Ages of Rocks
- Absolute Age: Definition & Dating
- Dating | geochronology | hebujelysofu.tk
- General considerations
There are many different types of absolute age dating methods because many different types of materials exist. Each material and situation has an optimal method that should be used in determining its age. To unlock this lesson you must be a Study. Login here for access. Did you know… We have over college courses that prepare you to earn credit by exam that is accepted by over 1, colleges and universities.
High School Earth Science/Absolute Ages of Rocks
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Find a degree that fits your goals. Try it risk-free for 30 days. Add to Add to Add to. Want to watch this again later? Imagine braving the desert heat for days or even weeks as you dig for dinosaur bones. You find something extraordinary and want to know as much about it as you can. In this lesson, you'll learn how scientists determine the absolute age of materials.
What is Absolute Age? Try it risk-free No obligation, cancel anytime. Want to learn more? Select a subject to preview related courses: Lesson Summary In absolute age dating, scientists determine the age of Earth materials as precisely as possible. Register to view this lesson Are you a student or a teacher? I am a student I am a teacher. Unlock Your Education See for yourself why 30 million people use Study. Become a Member Already a member?
Absolute Age: Definition & Dating
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Help and Review 28 chapters lessons. Mineral Types, Properties, and Earth and Celestial Rocks: The diagram on the right shows varves that accumulated simultaneously during the same 8-year time interval at locations A, B, and C. Note that although the thicknesses vary from location to location, the ratios of thicknesses remain constant. At location A, from oldest to youngest, the thicknesses for the layers are 0.
The ratio of the thicknesses is 5: That is, the pattern is the same at at the three locations. It is important to note that the pattern is random. The position within the series of any layer, therefore, is unique.
Dating | geochronology | hebujelysofu.tk
Layers formed at the same time, such as layer 'X', may be recognized. That is, equivalent layers may be correlated. Correlation using pattern matching makes it possible to determine, in a location where deposition has ceased, the absolute ages of the layers. This is accomplished by comparison with a location where annual deposition continues. Consider the varve sequences at locations A, B and C. At A, yearly deposition continues, so the absolute ages of layers can be determined by counting down from the top. At locations B and C, deposition stopped at some time in the past.
Indeed, some of the top layers may have been removed by erosion. However, by pattern matching, five layers within the series at A can be correlated with five layers at the top of B. Similarly, by pattern matching, five layers in the series at B can be correlated with five layers at the top of C. Since the ages of the layers at A are known by counting down from the top, layers at B that correlate with them can also be assigned ages.
Then, the ages of the rest of the layers at B may be determined by counting down. In similar fashion, layers at C that correlate with layers at B may be assigned ages, and the rest of the layers at C may be assigned by counting down. Using this method, ages of varves that formed tens of thousands of years ago may be determined.
For example, varves close to forty thousand years old have been dated in Japan. Pattern matching is also used to date trees by examining growth rings dendrochronology. Ages up to 14, years have been determined in this fashion. An archeologist finds a dried out, abandoned flood plain at location 'A'. He drills a hole and extracts a drill core that shows a series of layers of sediment one of which contains pottery fragment 'X'. The archeologist then contacts his colleague who is working in a nearby area location 'B' where there is a modern floodplain to which a layer of sediment is added every year.
He asks his colleague to extract and send him a drill core from location 'B', making sure to include and label the most recent layer, deposited in She does so, and also includes another drill core from a third location 'C', where she has recently worked. She tells him that location 'C', like location 'A', is also a dried out, abandoned floodplain.
The first archeologist wants to know in what year the layer containing pottery fragment 'X' was deposited. In what year was the layer formed that contains pottery fragment 'X'? My answer to Question 1: The layer containing 'X' was deposited in: Indeed, dating of lake sediments using varves was undertaken as early as Their disadvantage is that they are restricted to sites where annual deposition has occurred and the absolute age of at least one layer can be determined with confidence by some other means for example, by counting or by pattern matching with places where annual deposition continues through to today.
Places satisfying these requirements are relatively few. Another disadvantage is that over geologic time, preservation of such layers is limited. Absolute age determination by varve counting is only suitable for materials less than several tens of thousands of years old.
These limitations are overcome in radiometric dating. Radioactive elements, such as certain isotopes of uranium, thorium, rubidium, potassium, carbon and others, have the property that over set periods of time, known as their 'half lives' which are different for each radioactive element , half of their atoms decay to form atoms of different elements.
For example, over the course of million years, half the atoms of the 'parent' element uranium U decay to form atoms of the 'daughter' element lead Pb Over the next million years, half of the remaining U atoms change to Pb, and so on. By comparing the ratios of U to Pb that are found in the material today, the time when the process started may be ascertained see table below. Examples of radioactive parent-daughter pairs and their half lives include: U - Pb 4. An error of that magnitude may be quite acceptable for such old rocks. After careful analysis, a geochronologist determines that an unweathered, unmetamorphosed mineral sample contains 7 trillion atoms of the radioactive element K and trillion atoms of its decay product A How many years ago was the sample formed?
The number of years ago that the sample formed is: It is important to choose a radioactive parent-daughter pair whose half life is appropriate for the age of the material being dated. On the one hand, the half life should be short enough so that a measurable amount of the daughter element has formed. On the other hand, if the half life is too short, the amount of parent element left may not be measurable. Thus, K-Ar dating would not be appropriate for a material that is 50, years old, as hardly any daughter element would have formed.
Similarly, C dating is not be appropriate for materials older than about 70, years as the amount of the parent element left becomes too small to be measured accurately. Radiometric dating depends on certain assumptions. The most fundamental assumption is that the half life of a parent-daughter pair does not change through time. Experimentally and theoretically, that assumption seems justified. Also, successful cross-checking of ages using different dating techniques on the same sample supports the constancy of half lives.
For example, C dates may be checked against ages determined through varve counting. A second assumption is that the system is closed. Although with clever detective work many complex time sequences or relative ages can be deduced, the ability to show that objects at two separated sites were formed at the same time requires additional information. A coin, vessel, or other common artifact could link two archaeological sites, but the possibility of recycling would have to be considered. It should be emphasized that linking sites together is essential if the nature of an ancient society is to be understood, as the information at a single location may be relatively insignificant by itself.
Similarly, in geologic studies, vast quantities of information from widely spaced outcrops have to be integrated. Some method of correlating rock units must be found. In the ideal case, the geologist will discover a single rock unit with a unique collection of easily observed attributes called a marker horizon that can be found at widely spaced localities.
Any feature, including colour variations, textures, fossil content, mineralogy , or any unusual combinations of these can be used. It is only by correlations that the conditions on different parts of Earth at any particular stage in its history can be deduced. In addition, because sediment deposition is not continuous and much rock material has been removed by erosion , the fossil record from many localities has to be integrated before a complete picture of the evolution of life on Earth can be assembled.
Using this established record, geologists have been able to piece together events over the past million years, or about one-eighth of Earth history, during which time useful fossils have been abundant. The need to correlate over the rest of geologic time, to correlate nonfossiliferous units, and to calibrate the fossil time scale has led to the development of a specialized field that makes use of natural radioactive isotopes in order to calculate absolute ages.
The precise measure of geologic time has proven to be the essential tool for correlating the global tectonic processes that have taken place in the past. Precise isotopic ages are called absolute ages, since they date the timing of events not relative to each other but as the time elapsed between a rock-forming event and the present.
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The same margin of error applies for younger fossiliferous rocks, making absolute dating comparable in precision to that attained using fossils. 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. 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. 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.
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.
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.