They provide evidence of former surface conditions and the life-forms that existed under those conditions. The sequence of a layered sedimentary series is easily defined because deposition always proceeds from the bottom to the top. This principle would seem self-evident, but its first enunciation more than years ago by Nicolaus Steno represented an enormous advance in understanding.
Known as the principle of superposition , it holds that in a series of sedimentary layers or superposed lava flows the oldest layer is at the bottom, and layers from there upward become progressively younger. On occasion, however, deformation may have caused the rocks of the crust to tilt, perhaps to the point of overturning them. Moreover, if erosion has blurred the record by removing substantial portions of the deformed sedimentary rock, it may not be at all clear which edge of a given layer is the original top and which is the original bottom.
Identifying top and bottom is clearly important in sequence determination, so important in fact that a considerable literature has been devoted to this question alone. Many of the criteria of top—bottom determination are based on asymmetry in depositional features. Oscillation ripple marks, for example, are produced in sediments by water sloshing back and forth.
When such marks are preserved in sedimentary rocks, they define the original top and bottom by their asymmetric pattern. Certain fossils also accumulate in a distinctive pattern or position that serves to define the top side.
Absolute dating - Wikipedia
In wind-blown or water-lain sandstone , a form of erosion during deposition of shifting sand removes the tops of mounds to produce what are called cross-beds. The truncated layers provide an easily determined depositional top direction. The direction of the opening of mud cracks or rain prints can indicate the uppermost surface of mudstones formed in tidal areas. When a section of rock is uplifted and eroded, as during mountain-building episodes, great volumes of rock are removed, exposing a variety of differently folded and deformed rock units.
The new erosion surface must postdate all units, dikes, veins, and deformation features that it crosses. Even the shapes formed on the erosional or depositional surfaces of the ancient seafloor can be used to tell which way was up. A fragment broken from one bed can only be located in a younger unit, and a pebble or animal track can only deform a preexisting unit—i.
In fact, the number of ways in which one can determine the tops of well-preserved sediments is limited only by the imagination, and visual criteria can be deduced by amateurs and professionals alike. One factor that can upset the law of superposition in major sediment packages in mountain belts is the presence of thrust faults.
Such faults , which are common in compression zones along continental edges, may follow bedding planes and then cross the strata at a steep angle, placing older units on top of younger ones. In certain places, the fault planes are only a few centimetres thick and are almost impossible to detect. Relative ages also can be deduced in metamorphic rocks as new minerals form at the expense of older ones in response to changing temperatures and pressures. In deep mountain roots, rocks can even flow like toothpaste in their red-hot state.
Local melting may occur, and certain minerals suitable for precise isotopic dating may form both in the melt and in the host rock. In the latter case, refractory grains in particular may record the original age of the rock in their cores and the time of melting in their newly grown tips. Analytical methods are now available to date both growth stages, even though each part may weigh only a few millionths of a gram see below Correlation. Rocks that flow in a plastic state record their deformation in the alignment of their constituent minerals.
Such rocks then predate the deformation. If other rocks that are clearly not deformed can be found at the same site, the time of deformation can be inferred to lie between the absolute isotopic ages of the two units. Igneous rocks provide perhaps the most striking examples of relative ages.
Magma , formed by melting deep within Earth, cuts across and hence postdates all units as it rises through the crust, perhaps even to emerge at the surface as lava. Black lava, or basalt , the most common volcanic rock on Earth, provides a simple means for determining the depositional tops of rock sequences as well as proof of the antiquity of the oceans.
Pillow shapes are formed as basaltic lava is extruded i. The shapes of pillows in ancient basalts provide both a direct indication of depositional top and proof of underwater eruption. They are widespread in rocks as old as 3. Basaltic lava rocks that are common where ancient continents have been rifted apart are fed from below by near vertical fractures penetrating the crust.
Absolute dating
Material that solidifies in such cracks remains behind as dikes. Here the dikes must be younger than all other units. A more interesting case develops when a cooled older crust is fractured, invaded by a swarm of dikes, and subsequently subjected to a major episode of heating with deformation and intrusion of new magma. In this instance, even though the resulting outcrop pattern is extremely complex, all of the predike units can be distinguished by the relic dikes present.
The dikes also record in their newly formed minerals components that can be analyzed to give both the absolute age and the temperature and pressure of the second event. Because dike swarms are commonly widespread, the conditions determined can often be extrapolated over a broad region.
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Dikes do not always continue upward in a simple fashion. In some cases, they spread between the layers of near-horizontal sedimentary or volcanic units to form bodies called sills. In this situation, fragments of the host rock must be found within the intrusive body to establish its relatively younger age. Once most or all of the relative ages of various strata have been determined in a region, it may be possible to deduce that certain units have been offset by movement along fractures or faults while others have not.
Dikes that cross fault boundaries may even be found. Application of the simple principle of crosscutting relationships can allow the relative ages of all units to be deduced. The principles for relative age dating described above require no special equipment and can be applied by anyone on a local or regional scale. They are based on visual observations and simple logical deductions and rely on a correlation and integration of data that occurs in fragmentary form at many outcrop locations. We welcome suggested improvements to any of our articles. You can make it easier for us to review and, hopefully, publish your contribution by keeping a few points in mind.
Your contribution may be further edited by our staff, and its publication is subject to our final approval. Unfortunately, our editorial approach may not be able to accommodate all contributions. Our editors will review what you've submitted, and if it meets our criteria, we'll add it to the article.
Please note that our editors may make some formatting changes or correct spelling or grammatical errors, and may also contact you if any clarifications are needed. In historical geology , the primary methods of absolute dating involve using the radioactive decay of elements trapped in rocks or minerals, including isotope systems from very young radiocarbon dating with 14 C to systems such as uranium—lead dating that allow acquisition of absolute ages for some of the oldest rocks on earth.
Radiometric dating is based on the known and constant rate of decay of radioactive isotopes into their radiogenic daughter isotopes.
Particular isotopes are suitable for different applications due to the types of atoms present in the mineral or other material and its approximate age. For example, techniques based on isotopes with half lives in the thousands of years, such as carbon, cannot be used to date materials that have ages on the order of billions of years, as the detectable amounts of the radioactive atoms and their decayed daughter isotopes will be too small to measure within the uncertainty of the instruments. One of the most widely used and well-known absolute dating techniques is carbon or radiocarbon dating, which is used to date organic remains.
This is a radiometric technique since it is based on radioactive decay. Carbon moves up the food chain as animals eat plants and as predators eat other animals. With death, the uptake of carbon stops.
It takes 5, years for half the carbon to change to nitrogen; this is the half-life of carbon After another 5, years only one-quarter of the original carbon will remain. After yet another 5, years only one-eighth will be left. By measuring the carbon in organic material , scientists can determine the date of death of the organic matter in an artifact or ecofact. The relatively short half-life of carbon, 5, years, makes dating reliable only up to about 50, years. The technique often cannot pinpoint the date of an archeological site better than historic records, but is highly effective for precise dates when calibrated with other dating techniques such as tree-ring dating.
General considerations
An additional problem with carbon dates from archeological sites is known as the "old wood" problem. It is possible, particularly in dry, desert climates, for organic materials such as from dead trees to remain in their natural state for hundreds of years before people use them as firewood or building materials, after which they become part of the archaeological record. Thus dating that particular tree does not necessarily indicate when the fire burned or the structure was built.
For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating. The development of accelerator mass spectrometry AMS dating, which allows a date to be obtained from a very small sample, has been very useful in this regard.
Other radiometric dating techniques are available for earlier periods. One of the most widely used is potassium—argon dating K—Ar dating. Potassium is a radioactive isotope of potassium that decays into argon The half-life of potassium is 1. Potassium is common in rocks and minerals, allowing many samples of geochronological or archeological interest to be dated. Argon , a noble gas, is not commonly incorporated into such samples except when produced in situ through radioactive decay.
The global tectonic rock cycle
The date measured reveals the last time that the object was heated past the closure temperature at which the trapped argon can escape the lattice. K—Ar dating was used to calibrate the geomagnetic polarity time scale. Thermoluminescence testing also dates items to the last time they were heated.
This technique is based on the principle that all objects absorb radiation from the environment. This is, as I say, contrary to our present understanding of physics, and so is mere unfounded speculation. What is more, the reader should recollect that " radioactive decay " is not the name of one process; it is the name of any process that rearranges the nucleus. So to leave dates produced by different radiometric methods still concordant, nature would somehow have to conspire to fool us by changing the rates of alpha decay , of beta decay , and of electron capture , in such a way that the different dating methods based on these different modes of decay come up with the same dates.
Another point to bear in mind is that a change in the rate of radioactive decay, even if it was carefully coordinated in this way, would still not change every radiometric date in the same direction: It is possible to doubt any particular date obtained by absolute dating methods. But it would be bizarre to doubt the general picture they paint. For what we see is a massive agreement between the different radiometric methods , varves , dendrochronology , sclerochronology , rhythmites , paleomagnetic data, deposition rates, sea floor spreading , and relative dating methods.
For the dates obtained by absolute dating to be wrong in general and yet wrong in such a way as to be in agreement with one another and with other observations, we would have to suppose either that we are looking at an inconceivably massive coincidence, or that the whole Earth is a fraud designed to deceive us. Ideas to the latter effect have actually been proposed from time to time; most notably by the nineteenth century religious zealot Philip Gosse, whose eccentric work Omphalos proposed that the Earth was a mere few thousand years old, but that God had created it to look much older.
To this the Reverend Charles Kingsley memorably answered: That of course would be a theological rather than a geological question, and so is outside the scope of this textbook. What can be said is that geology is a science, and that in science it is necessary to proceed on the basis that the universe is not a lie; because if we believed that, we could believe that anything at all was the case and disregard all evidence to the contrary. The scientific method compels us, then, to disregard the possibility of divine malice; and mere natural processes, being mindless, cannot be actually malevolent.
What, then, of coincidence? Well, there are limits to the degree of coincidence we can believe in, otherwise again we could believe nearly anything. The scientific method requires us to discard such remote possibilities unless there is at least a hint of a shred of evidence for them. We are left with the conclusion that the great majority of the dates produced by absolute dating methods must be reasonably accurate.
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