Geologists often need to know the age of material that they find. They use absolute dating methods, sometimes called numerical dating, Most absolute dates for rocks are obtained with radiometric methods. of parent and daughter isotopes in rocks now, you can calculate when the rocks were formed. rocks and fossils in number of years could be determined through radiometric age dating or absolute age dating. This packet on determining age of rocks. But determining the absolute age of a substance (its age in years) is a much . In the process of radiometric dating, several isotopes are used to date rocks and.
Such addition or subtraction may occur if the material mineral or rock has been weathered or metamorphosed. Therefore, material to be dated must be carefully examined to determine whether such processes may have taken place.
Because the dating method depends upon comparing the ratio of parent to daughter element, the assumption must be made that the amount of daughter element initially present be zero or else be determinable.
DETERMINING AGE OF ROCKS AND FOSSILS
Igneous rocks and highly metamorphosed rocks are the best candidates for radiometric dating because for them, for reasons that won't be discussed here, it can relatively easily be determined whether the initial amount of daughter element present was zero or, if it wasn't zero, what was the initial amount.
The 'age' of an igneous rock refers to the time when the magma or lava from which it formed cooled below a certain temperature. A useful material for dating that time is the mineral zircon, a minor but common constituent of igneous rocks. As magma or lava solidifies, the elements zirconium Zrsilicon Si and oxygen O link together to form zircon crystals.
Absolute dating - Wikipedia
If uranium U atoms are in the vicinity, they may be incorporated into the zircon in place of Zr atoms. This substitution is possible because the size and charge of the U is similar to that of Zr.
That is, the U can 'fit' in the sites normally occupied by Zr. Any lead Pb in the vicinity cannot be incorporated in the zircon because it can't 'fit' in any of the sites. Assuming the zircon has not been affected by weathering or metamorphism, any Pb subsequently found in the zircon must have come from decay of the U; it was not there to start with.
It is true that not all minerals that crystallize from a magma or lava form simultaneously, but except for extremely young igneous rocks, the time required for solidification is very short compared compared to the age of the rock.
Accurate radiometric dating of metamorphic rocks is more difficult. During metamorphism, preexisting minerals may be altered and new minerals may be formed. For preexisting minerals, there is the distinct possibility that during metamorphism, parent or daughter elements may have been added or lost.
If this happens, attempts to determine an accurate original premetamorphic age of the material will be frustrated. For example, loss of some of the daughter element will give a deceptively young age; addition of daughter element will give a deceptively old age.
However, if the rock is highly metamorphosed, the situation is more propitious. For example, in the K-Ar system, all of the daughter element Ar may be lost from some preexisting minerals. Or else, completely new mineral grains may develop that contain the parent element K but totally lack the daughter element Ar.
In either case, these minerals constitute new 'closed' systems with zero initial daughter element and, if dated, give the age of the metamorphic event. The age of a sedimentary rock refers to the time when loose sediment is turned into rock becomes 'lithified'.
Sedimentary rocks are varied and complex, but for many of them, the sedimentary particles out of which they are made consist of material eroded from prexisting rocks.
After transportation and deposition, the particles are bound together in some fashion, perhaps by a 'cement'.
Those processes do not reset the clock: Thus, if the particles are dated, the ages obtained refer to the ages of the rock from which they were derived. In consequence, for many sedimentary rocks, the constitutent grains have widely varying ages.
To get the age of the sedimentary rock itself, the material dated has to have formed at the time of consolidation of the rock. For most sedimentary rocks, there is no such material that is datable contains suitable parent-daughter elements.
Sedimentary rocks must, therefore, be dated by 'bracketing'. The method involves determining the absolute ages of slightly younger and slightly older objects to set limits within which the unknown age must lie. You know that Agnes is a 'middle child', younger than her sister Mary, who has just turned 7, and older than her brother John, who is 4 and about to celebrate his 5th birthday.
With this knowledge, you have Agnes' age 'bracketed'. Agnes is more than 4 and less than 7 years old. Sedimentary rocks whose absolute ages can't be determined directly may be established by dating associated lava flows. In the example to the right, the numbered layers are sedimentary rocks. After deposition of layers 12 and 13, lava flow 'X' was erupted. Then, layers were deposited, followed by eruption of lava flow 'Y'. Finally, layers 17 and 18 were deposited. Radiometric dating of the lava flows established their ages as 2 million years for 'Y' and 1 million years for 'X'.
Using the Law of Superposition: Layers 12 and 13 must be older than 2 million years. Layers 14 to 16 must be younger than 2 million years but older than 1 million years.
Layers 17 and 18 must be younger than one million years. In the example to the right, after sedimentary layers 21 to 25 were deposited, lava flow 'Q' was erupted. Then, sedimentary layer 26 to 28 were deposited. At some time after the deposition of the first half of layer 27, igneous intrusive 'P' was emplaced. Radiometric dating of the lava flow and the intrusive established their ages as 5 million years for lava flow 'Q' and 2 million years for intrusive 'P'.
Because they are under the lava flow, layers are older than 5 million years. Because they are above the lava flow, layers 26 to 28 are younger than 5 million years.
Using the Law of Cross-Cutting Relationships: Because they are cut by the intrusive, layer 26 and 27 are older than 2 million years. Because it is not cut by the intrusive, it is not known whether layer 28 was deposited before or after the intrusive was emplaced.
Its age can only be designated as less than 5 million years. In the example shown below, two of the layers at location 'X', 24 and 28, are fossiliferous, containing fossil assemblages B and A respectively. Their ages have been determined by bracketing: July Thermoluminescence[ edit ] 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 process frees electrons within minerals that remain caught within the item.
Heating an item to degrees Celsius or higher releases the trapped electronsproducing light. This light can be measured to determine the last time the item was heated. Radiation levels do not remain constant over time. Fluctuating levels can skew results — for example, if an item went through several high radiation eras, thermoluminescence will return an older date for the item. Many factors can spoil the sample before testing as well, exposing the sample to heat or direct light may cause some of the electrons to dissipate, causing the item to date younger.
It cannot be used to accurately date a site on its own. However, it can be used to confirm the antiquity of an item. Optically stimulated luminescence OSL [ edit ] Optically stimulated luminescence OSL dating constrains the time at which sediment was last exposed to light.
Then, count the number of pieces of candy left with the M facing down. These are the parent isotope that did not change during the first half life. The teacher should have each team report how many pieces of parent isotope remain, and the first row of the decay table Figure 2 should be filled in and the average number calculated. The same procedure of shaking, counting the "survivors", and filling in the next row on the decay table should be done seven or eight more times.
Each time represents a half life.
Each team should plot on a graph Figure 3 the number of pieces of candy remaining after each of their "shakes" and connect each successive point on the graph with a light line. AND, on the same graph, each group should plot points where, after each "shake" the starting number is divided by exactly two and connect these points by a differently colored line.
After the graphs are plotted, the teacher should guide the class into thinking about: Is it the single group's results, or is it the line based on the class average? U is found in most igneous rocks. Unless the rock is heated to a very high temperature, both the U and its daughter Pb remain in the rock. A geologist can compare the proportion of U atoms to Pb produced from it and determine the age of the rock.
The next part of this exercise shows how this is done. Each team is given a piece of paper marked TIME, on which is written either 2, 4, 6, 8, or 10 minutes.
The team should place each marked piece so that "U" is showing. This represents Uranium, which emits a series of particles from the nucleus as it decays to Lead Pb- When each team is ready with the pieces all showing "U", a timed two-minute interval should start. During that time each team turns over half of the U pieces so that they now show Pb This represents one "half-life" of U, which is the time for half the nuclei to change from the parent U to the daughter Pb A new two-minute interval begins.
Continue through a total of 4 to 5 timed intervals. That is, each team should stop according to their TIME paper at the end of the first timed interval 2 minutesor at the end of the second timed interval 4 minutesand so on. After all the timed intervals have occurred, teams should exchange places with one another as instructed by the teacher. The task now for each team is to determine how many timed intervals that is, how many half-lives the set of pieces they are looking at has experienced.
The half life of U is million years.
Both the team that turned over a set of pieces and the second team that examined the set should determine how many million years are represented by the proportion of U and Pb present, compare notes, and haggle about any differences that they got.