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As we learned in the 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.
But determining the absolute age of a substance its age in years is a much greater challenge. 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 The width of a series of growth rings can give clues to past climates and various disruptions such as forest fires.
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 Are index fossils used for absolute dating 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. 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 in a thick, sandy deposit of sediment. These thick layers alternate with thin, clay-rich layers deposited during the winter. The resulting layers, called varvesgive scientists clues about past climate conditions.
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. 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. Today scientists know his estimate was too young; we know that this occurred about million years ago. He assumed that Are index fossils used for absolute dating 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. Radioactivity is the tendency of certain atoms to decay into lighter atoms, emitting energy in the process. 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 of neutrons, however, is variable. An atom of an element with a different of neutrons is an isotope of that element. For example, the isotope carbon contains 6 neutrons in its nucleus, while the isotope carbon has 7 neutrons.
Some isotopes are radioactivewhich means they are unstable and likely to decay. 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 decay eventually in the formation of stable daughter products. 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.
Scientists measure the rate of radioactive decay with a unit called half-life. 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 of radioactive atoms. After the third year three half-lives After Are index fossils used for absolute dating 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 Radiometric Dating of Rocks In the process of radiometric datingseveral isotopes are used to date rocks and other materials. Using several different isotopes helps scientists to check the accuracy of the ages that they calculate.
Carbon is stable and s for Carbon is also stable and s 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. Carbon is removed from the atmosphere by plants during the process of photosynthesis. Animals consume this carbon when they eat plants or other animals that have eaten plants. Therefore carbon dating can be used to date plant and animal remains.
Examples include timbers from an old building, bones, or ashes from a fire pit. Carbon dating can be effectively used to find the age of materials between and 50, years old. Potassium decays to argon with a half-life of 1. Because argon is a gas, it can escape from molten magma or lava.
Therefore any argon that is found in a crystal probably formed as a result of the decay of potassium Measuring the ratio of potassium to argon will yield a good estimate of the age of the sample. Potassium is a common element found in many minerals such as feldspar, mica, and amphibole.
The technique can be used to date igneous rocks fromyears to over a billion years old. Because it can be used to Are index fossils used for absolute dating geologically young materials, the technique has been useful in estimating the age of Are index fossils used for absolute dating containing the bones of human ancestors. Two isotopes of uranium are used for radiometric dating. Uranium decays to form lead with a half-life of 4. Uranium decays to form lead with a half-life of million years. Uranium-lead dating is usually performed on crystals of the mineral zircon Figure When zircon forms in an igneous rock, the crystals readily accept atoms of uranium but reject atoms of lead.
Therefore, if any lead is found in a zircon crystal, it can be assumed that it was produced from the decay of uranium. Uranium-lead dating can be used to date igneous rocks from 1 million years to around 4. Some of the oldest rocks on Earth have been dated using this method, including zircon crystals from Australia that are 4. Radiometric dating can only be used on materials that contain measurable amounts of radioactive materials and their daughter products.
This includes organic remains which compared to rocks are relatively young, less thanyears old and older rocks. Ideally, several different radiometric techniques will be used to date the same rock. Agreement between these values indicates that the calculated age is accurate. In general, radiometric dating works best for igneous rocks and is not very useful for determining the age of sedimentary rocks.
To estimate the age of a sedimentary rock deposit, geologists search for nearby or interlayered igneous rocks that can be dated. For example, if a sedimentary rock layer is sandwiched between two layers of volcanic ash, its age is between the ages of the two ash layers. Using a combination of radiometric dating, index fossils, and superposition, geologists have constructed a well-defined timeline of Earth history. For example, an overlying lava flow can give a reliable estimate of the age of a sedimentary rock formation in one location.
Index fossils contained in this formation can then be matched to fossils in a different location, providing a good age measurement for that new rock formation as well. As this process has been repeated all over the world, our estimates of rock and fossil ages has become more and more accurate. Techniques such as superposition and index fossils can tell you the relative age of objects, which objects are older and which are younger. Other types of evidence are needed to establish the absolute age of objects in years.
Geologists use a variety of techniques to establish absolute age, including radiometric dating, tree rings, ice cores, and annual sedimentary deposits called varves. Radiometric dating is the most useful of these techniques—it is the only technique that can establish the age of objects older than a few thousand years. The concentrations of several radioactive isotopes carbon, potassium, uranium and and their daughter products are used to determine the age of rocks and organic remains.
Geologic History.Are index fossils used for absolute dating
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