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Igneous rocks are dated according to whether they caused metamorphism in the surrounding rock proof that they intruded into the preexisting rockwhether they cross cut preexisting rocks, or whether sediments were deposited on them after they were formed.
The profile from one location is then compared with profiles from surrounding sites to determine the geologic history of a larger area. If fossils are present in the rocks, they may also be used to correlate rock layers across large distances and, now that absolute time has been established, to determine the age of the rocks.
In this process, you will study the rocks and events in a geologic cross section and put them in the correct order from oldest to youngest. In order to do your best on this activity, you must understand a few of the basic principles that are applicable to relative age relationships between rocks: Principle of original horizontality: Strata that are not horizontal have been deformed by movements of the arth s crust. Principle of faunal succession: Principle of crosscutting relations: The easiest way to do relative age dating is to work from oldest to youngest.
Try to find the oldest rock usually located near the bottom in the diagram below and work your way up. Your first example is the diagram below. Review the principle of original horizontality and the principle of superposition and you will see that the only possible answer to this puzzle is that layer is the oldest and layer is the youngest. Here are some additional hints that will help you with your diagrams: If rocks are folded, the folding is younger that the youngest rock affected.
If they are folded into a syncline a U-shaped fold the youngest rocks are in the core of the fold see figure. The opposite is true for an anticline a big dome-shaped fold. Sedimentary rocks that contain fragments of another rock are younger than the rocks that the fragments came from. Layers and F were then deposited at a later time and are the youngest. Igneous rocks are formed by the solidification of a liquid magma; the therefore can intrude into preexisting rocks or be poured out onto the surface of the earth: If a body of granite contains unmelted inclusions of another rock, the granite is the younger rock.
Granites can intrude into other rocks, even though they may be on the bottom of your geologic diagram. Look carefully for the granitic pattern see below and for irregular contacts between the granite and the country preexisting rock see ctivity figure 2.
The granite may also metamorphose the country rocks: Lava flows may cause contact-metamorphism with the older rocks they lie upon. Metamorphic rocks are preexisting rocks that have been metamorphosed changed into different rocks by large amounts of heat and pressure in a region. These rocks have usually been deformed by large, mountain forming events, and therefore if they are in contact with layered or unmetamorphosed rocks, they are usually the oldest rocks in the sequence considering that if those rocks had been in place when the metamorphism occurred, they also would be metamorphosed!
Metamorphic rocks are older than sedimentary rocks deposited above them or with igneous rocks that may intrude them. Now, familiarize yourself with the rock patterns: Which of the principles apply to sedimentary rocks? Look at the diagrams in Part 2. In figure 1 below, could it be possible to determine an absolute age of these rocks? If yes, explain in detail how you may be able to do this: In figure 3 below, could it be possible to determine an absolute age of these rocks?
How do you determine the relative ages of igneous rocks? List the ways Part 2: These are generally analytical methods, and are carried out in a laboratory. Absolute dates are also relative dates, in that they tell which specimens are older or younger than others.
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Absolute dates must agree with dates from other relative methods in order to be valid. This dating technique of amino acid racimization was first conducted by Hare and Mitterer inand was popular in the s. It requires a much smaller sample than radiocarbon dating, and has a longer range, extending up to a few hundred thousand years. It has been used to date coprolites fossilized feces as well as fossil bones and shells.
These types of specimens contain proteins embedded in a network of minerals such as calcium. Amino acid racimization is based on the principle that amino acids except glycine, a very simple amino acid exist in two mirror image forms called stereoisomers.
Living organisms with the exception of some microbes synthesize and incorporate only the L-form into proteins. When these organisms die, the L-amino acids are slowly converted into D-amino acids in a process called racimization.
The protons are quickly replaced, but will return to either side of the amino acid, not necessarily to the side from which they came. This may form a D-amino acid instead of an L—amino acid. The rate at which the reaction occurs is different for each amino acid; in addition, it depends upon the moisture, temperatureand pH of the postmortem conditions. The higher the temperature, the faster the reaction occurs, so the cooler the burial environment, the greater the dating range.
The burial conditions are not always known, however, and can be difficult to estimate. For this reason, and because some of the amino acid racimization dates have disagreed with dates achieved by other methods, the technique is no longer widely used. Cation-ratio dating is used to date rock surfaces such as stone artifacts and cliff and ground drawings. It can be used to obtain dates that would be unobtainable by more conventional methods such as radiocarbon dating.
Scientists use cation-ratio dating to determine how long rock surfaces have been exposed. They do this by chemically analyzing the varnish that forms on these surfaces.
The varnish contains cations, which are positively charged atoms or molecules. Different cations move throughout the environment at different rates, so the ratio of different cations to each other changes over time. By calibrating these ratios with dates obtained from rocks from a similar microenvironment, a minimum age for the varnish can be determined.
This technique can only be applied to rocks from desert areas, where the varnish is most stable. Although cation-ratio dating has been widely used, recent studies suggest it has potential errors.
Many of the dates obtained with this method are inaccurate due to improper chemical analyses. In addition, the varnish may not actually be stable over long periods of time. Thermoluminescence dating is very useful for determining the age of pottery. Electrons from quartz and other minerals in the pottery clay are bumped out of their normal positions ground state when the clay is exposed to radiation.
This radiation may come from radioactive substances such as uranium, present in the clay or burial medium, or from cosmic radiation.
The longer the radiation exposure, the more electrons get bumped into an excited state. With more electrons in an excited state, more light is emitted upon heating. The process of displacing electrons begins again after the object cools. Scientists can determine how many years have passed since a ceramic was fired by heating it in the laboratory and measuring how much light is given off. Thermoluminescence dating has the advantage of covering the time interval between radiocarbon and potassium-argon datingor 40,—, years.
In addition, it can be used to date materials that cannot be dated with these other two methods. Optically stimulated luminescence OSL has only been used since It is very similar to thermoluminescence dating, both of which are considered "clock setting" techniques.
Minerals found in sediments are sensitive to light. Electrons found in the sediment grains leave the ground state when exposed to light, called recombination. To determine the age of sediment, scientists expose grains to a known amount of light and compare these grains with the unknown sediment. This technique can be used to determine the age of unheated sediments less thanyears old. A disadvantage to this technique is that in order to get accurate results, the sediment to be tested cannot be exposed to light which would reset the "clock"making sampling difficult.
The absolute dating method utilizing tree ring growth is known as dendrochronology. It is based on the fact that trees produce one growth ring each year. The rings form a distinctive pattern, which is the same for all members in a given species and geographical area. The patterns from trees of different ages including ancient wood are overlapped, forming a master pattern that can be used to date timbers thousands of years old with a resolution of one year.
Timbers can be used to date buildings and archaeological sites. In addition, tree rings are used to date changes in the climate such as sudden cool or dry periods.
Dendrochronology has a range of one to 10, years or more.Relative Age Dating I
As previously mentioned, radioactive decay refers to the process in which a radioactive form of an element is converted into a decay product at a regular rate. Radioactive decay dating is not a single method of absolute dating but instead a group of related methods for absolute dating of samples.
Potassium-argon dating relies on the fact that when volcanic rocks are heated to extremely high temperatures, they release any argon gas trapped in them. As the rocks cool, argon 40Ar begins to accumulate. Argon is formed in the rocks by the radioactive decay of potassium 40K. The amount of 40Ar formed is proportional to the decay rate half-life of 40K, which is 1.
In other words, it takes 1. This method is generally only applicable to rocks greater than three million years old, although with sensitive instruments, rocks several hundred thousand years old may be dated.
The reason such old material is required is that it takes a very long time to accumulate enough 40Ar to be measured accurately. Potassium-argon dating has been used to date volcanic layers above and below fossils and artifacts in east Africa. Radiocarbon dating is used to date charcoal, wood, and other biological materials. The range of conventional radiocarbon dating is 30,—40, years, but with sensitive instrumentation, this range can be extended to 70, years.
Radiocarbon 14C is a radioactive form of the element carbon. It decays spontaneously into nitrogen 14N. Plants get most of their carbon from the air in the form of carbon dioxideand animals get most of their carbon from plants or from animals that eat plants. Relative to their atmospheric proportions, atoms of 14C and of a non-radioactive form of carbon, 12C, are equally likely to be incorporated into living organisms.
When the organism dies, however, its body stops incorporating new carbon. The ratio will then begin to change as the 14C in the dead organism decays into 14N.
The rate at which this process occurs is called the half-life. This is the time required for half of the 14C to decay into 14N. The half-life of 14C is 5, years. This allows them to determine how much 14C has formed since the death of the organism. One of the most familiar applications of radioactive dating is determining the age of fossilized remains, such as dinosaur bones.
Radioactive dating is also used to authenticate the age of rare archaeological artifacts.
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Because items such as paper documents and cotton garments are produced from plants, they can be dated using radiocarbon dating. Without radioactive datinga clever forgery might be indistinguishable from a real artifact. There are some limitations, however, to the use of this technique. Samples that were heated or irradiated at some time may yield by radioactive dating an age less than the true age of the object.
Because of this limitation, other dating techniques are often used along with radioactive dating to ensure accuracy.