As we learned in the previous lesson, index fossils and superposition are effective methods of determining the relative age of objects. In other words, you can use superposition to tell you that one rock layer is older than another. To accomplish this, scientists use a variety of evidence, from tree rings to the amounts of radioactive materials in a rock. In regions outside the tropics, trees grow more quickly during the warm summer months than during the cooler winter. Each dark band represents a winter; by counting rings it is possible to find the age of the tree Figure
Droughts and other variations in the climate make the tree grow slower or faster than normal, which shows up in the widths of the tree rings. These tree ring variations will appear in all trees growing in a certain region, so scientists can match up the growth rings of living and dead trees. Using logs recovered from old buildings and ancient ruins, scientists have been able to compare tree rings to create a continuous record of tree rings over the past 2, years.
This tree ring record has proven extremely useful in creating a record of climate change, and in finding the age of ancient structures.
Figure The thick, light-colored part of each ring represents rapid spring and summer growth. The thin, dark part of each ring represents slow autumn and winter growth. Ice Cores and Varves Several other processes result in the accumulation of distinct yearly layers that can be used for dating.
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For example, layers form within glaciers because there tends to be less snowfall in the summertime, allowing a dark layer of dust to accumulate on top of the winter snow Figure To study these patterns, scientists drill deep into ice sheets, producing cores hundreds of meters long.
Scientists analyze these ice cores to determine how the climate has changed over time, as well as to measure concentrations of atmospheric gases. The longest cores have helped to form a record of polar climate stretching hundreds of thousands of years back.
Another example of yearly layers is the deposition of sediments in lakes, especially the lakes that are located at the end of glaciers. Rapid melting of the glacier in the summer results in a thick, sandy deposit of sediment. These thick layers alternate with thin, clay-rich layers deposited during the winter. For example, an especially warm summer might result in a very thick layer of sediment deposited from the melting glacier.
While tree rings and other annual layers are useful for dating relatively recent events, they are not of much use on the vast scale of geologic time.
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During the 18th and 19th centuries, geologists tried to estimate the age of Earth with indirect techniques. For example, geologists measured how fast streams deposited sediment, in order to try to calculate how long the stream had been in existence.
Probably the most reliable of these estimates was produced by the British geologist Charles Lyell, who estimated that million years have passed since the appearance of the first animals with shells.
The most common is U U is found in many igneous rocks, soil and sediment. U decays to Pb with a half-life of million years. Due to its long half-life, U is the best isotope for radioactive dating, particularly of older fossils and rocks. Furthermore, the slow radioactive decay of 40 K shows that there was insufficient time since cooling for measurable amounts of 40 Ar to have accumulated in the rock. Therefore, radiogenic argon (40 Ar) was already present in the rocks as they formed. Radiometric age dating should no longer be sold to the public as providing reliable, absolute ages. Radioactive decay is used in carbon dating, fracking and radiotherapy. Half life graph showing carbon dating Dating rocks. The age of the rock can be calculated if the ratio of uranium to.
Today scientists know his estimate was too young; we know that this occurred about million years ago. He assumed that the Earth began as a ball of molten rock, which has steadily cooled over time.
From these assumptions, he calculated that the Earth was million years old. It provided a way to find the absolute age of a rock. To understand how this is done, it is necessary to review some facts about atoms. Atoms contain three particles: protons, neutrons, and electrons.
Protons and neutrons are located in the nucleus, while electrons orbit around the nucleus. For example, all atoms of carbon have six protons, all atoms of oxygen have eight protons, and all atoms of gold have 79 protons. The number of neutrons, however, is variable. For example, the isotope carbon contains 6 neutrons in its nucleus, while the isotope carbon has 7 neutrons.
This means the atom will spontaneously change from an unstable form to a stable form. There are two forms of nuclear decay that are relevant in how geologists can date rocks Table If an element decays by losing an alpha particle, it will lose 2 protons and 2 neutrons. If an atom decays by losing a beta particle, it loses just one electron. So what does this have to do with the age of Earth? Radioactive materials decay at known rates. As time passes, the proportion of radioactive isotopes will decrease and the proportion of daughter isotopes will increase.
A rock with a relatively high proportion of radioactive isotopes is probably very young, while a rock with a high proportion of daughter products is probably very old. The half-life of a radioactive substance is the amount of time, on average, it takes for half of the atoms to decay.
For example, imagine a radioactive substance with a half-life of one year. When a rock is formed, it contains a certain number of radioactive atoms. After the third year three half-lives After four years four half-lives6.
If you find a rock whose radioactive material has a half life of one year and measure 3. The decay of radioactive materials can be shown with a graph Figure Using several different isotopes helps scientists to check the accuracy of the ages that they calculate.
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Carbon is stable and accounts for Carbon is also stable and accounts for 1. Carbon is radioactive and is found in tiny amounts.
Carbon is produced naturally in the atmosphere when cosmic rays interact with nitrogen atoms. The amount of carbon produced in the atmosphere at any particular time has been relatively stable through time.
Radioactive carbon decays to stable nitrogen by releasing a beta particle. The nitrogen atoms are lost to the atmosphere, but the amount of carbon decay can be estimated by measuring the proportion of radioactive carbon to stable carbon As a substance ages, the relative amount of carbon decreases.
This radioactivity can be used for dating, since a radioactive 'parent' element decays into a stable 'daughter' element at a constant rate. For geological purposes, this is taken as one year.
Another way of expressing this is the half-life period given the symbol T. The half-life is the time it takes for half of the parent atoms to decay.
Many different radioactive isotopes and techniques are used for dating. All rely on the fact that certain elements particularly uranium and potassium contain a number of different isotopes whose half-life is exactly known and therefore the relative concentrations of these isotopes within a rock or mineral can measure the age. For an element to be useful for geochronology measuring geological timethe isotope must be reasonably abundant and produce daughter isotopes at a good rate.
Either a whole rock or a single mineral grain can be dated. Some techniques place the sample in a nuclear reactor first to excite the isotopes present, then measure these isotopes using a mass spectrometer such as in the argon-argon scheme. Others place mineral grains under a special microscope, firing a laser beam at the grains which ionises the mineral and releases the isotopes.
The isotopes are then measured within the same machine by an attached mass spectrometer an example of this is SIMS analysis. This is a common dating method mainly used by archaeologists, as it can only date geologically recent organic materials, usually charcoal, but also bone and antlers.
All living organisms take up carbon from their environment including a small proportion of the radioactive isotope 14C formed from nitrogen as a result of cosmic ray bombardment. The amount of carbon isotopes within living organisms reaches an equilibrium value, on death no more is taken up, and the 14C present starts to decay at a known rate.
The amount of 14C present and the known rate of decay of 14C and the equilibrium value gives the length of time elapsed since the death of the organism.
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This method faces problems because the cosmic ray flux has changed over time, but a calibration factor is applied to take this into account. Radiocarbon dating is normally suitable for organic materials less than 50 years old because beyond that time the amount of 14C becomes too small to be accurately measured.
This scheme was developed in but became more useful when mass spectrometers were improved in the late s and early s.
For biological objects older than 50, years, scientists use radioactive dating to determine the age of rocks surrounding where the material was found. By dating rocks, scientists can approximate ages of very old fossils, bones and teeth. Radiocarbon dating was invented in the s by Willard F. Libby. 1. How do scientists find the age of planets (date samples) or planetary time (relative age and absolute age)?We have rocks from the Moon (brought back), meteorites, and rocks that we know came from Mars. We can then use radioactive age dating in order to date the ages of the surfaces (when the rocks first formed, i.e. when the lava cooled and crystallized).
However, both Rb and Sr easily follow fluids that move through rocks or escape during some types of metamorphism. This technique is less used now. The dual decay of potassium K to 40Ar argon and 40Ca calcium was worked out between and This technique has become more widely used since the late s.
Its great advantage is that most rocks contain potassium, usually locked up in feldspars, clays and amphiboles. However, potassium is very mobile during metamorphism and alteration, and so this technique is not used much for old rocks, but is useful for rocks of the Mesozoic and Cenozoic Eras, particularly unaltered igneous rocks. Argon-Argon dating 39ArAr. This technique developed in the late s but came into vogue in the early s, through step-wise release of the isotopes.
This technique uses the same minerals and rocks as for K-Ar dating but restricts measurements to the argon isotopic system which is not so affected by metamorphic and alteration events.
It is used for very old to very young rocks. The decay of Sm to Nd for dating rocks began in the mids and was widespread by the early s. It is useful for dating very old igneous and metamorphic rocks and also meteorites and other cosmic fragments.
Geologic History 5 Radioactive Dating
However, there is a limited range in Sm-Nd isotopes in many igneous rocks, although metamorphic rocks that contain the mineral garnet are useful as this mineral has a large range in Sm-Nd isotopes. This technique also helps in determining the composition and evolution of the Earth's mantle and bodies in the universe.
The Re-Os isotopic system was first developed in the early s, but recently has been improved for accurate age determinations. The main limitation is that it only works on certain igneous rocks as most rocks have insufficient Re and Os or lack evolution of the isotopes.
The use of radiometric dating was first published in by Bertram Boltwood and is now the principal source of information about the absolute age of rocks and other geological features, including the age of the Earth itself, and can be used to date a wide range of natural and man-made materials. Radioactive dating is a method of dating rocks and minerals using radioactive isotopes. This method is useful for igneous and metamorphic rocks, which cannot be dated by the stratigraphic correlation method used for sedimentary rocks. Over naturally-occurring isotopes are known. radiometric dating: Determination of the absolute age of rocks and minerals using certain radioactive isotopes. relative dating: Rocks and structures are placed into chronological order.
This technique is good for iron meteorites and the mineral molybdenite. This system is highly favoured for accurate dating of igneous and metamorphic rocks, through many different techniques. It was used by the beginning of the s, but took until the early s to produce accurate ages of rocks. The great advantage is that almost all igneous and metamorphic rocks contain sufficient U and Pb for this dating. It can be used on powdered whole rocks, mineral concentrates isotope dilution technique or single grains SHRIMP technique.
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It has revolutionised age dating using the U-Pb isotopic system. Using the SHRIMP, selected areas of growth on single grains of zircon, baddeleyite, sphene, rutile and monazite can be accurately dated to less than years in some cases. It can even date nonradioactive minerals when they contain inclusions of zircons and monazite, as in sapphire grains.
It can help fix the maximum age of sedimentary rocks when they contain enough accessory zircon grains usually need about grains.