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Radioactive decay can be used to date igneous rocks by assuming making some assumptions, finding the percentages of parent to daughter products and using experimentally determined half-lives. Start with the assumptions that the rock 1. That there have been no factors that have influenced the rate of radioactive decay during the "life" of the rock. Second determine the percentages of the parent to the daughter elements in the rock. The percentages can be be used to calculate the number or fractions of the half lives that have occurred to create the percentages. Third multiple the experimentally determined half live the parent to daughter transformation by the number of half lives determined by the percentages.

## How can radioactive decay be used to date rocks?

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 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 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 Cross-section showing growth rings. 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. Ice core section showing annual layers. 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. Lord Kelvin. 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 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 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-lives , 6.

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 Decay of an imaginary radioactive substance with a half-life of one year. Using several different isotopes helps scientists to check the accuracy of the ages that they calculate. 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.

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 from , years to over a billion years old. Because it can be used to date geologically young materials, the technique has been useful in estimating the age of deposits 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. Zircon crystal. 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 than , years 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.

## Answer: Because the elements used for dating need to be re-set by volcanism. Radioactive elements decay at a certain constant rate and this is the basis of However, sedimentary rocks can be age dated if a volcanic ash. Radioactive decay can be used to date igneous rocks by assuming no usually no radioactive material found in sedimentary layers and the.

Radiometric dating , radioactive dating or radioisotope dating is a technique used to date materials such as rocks or carbon , in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. Together with stratigraphic principles , radiometric dating methods are used in geochronology to establish the geologic time scale.

The most widely known form of radiometric dating is carbon dating.

Originally fossils only provided us with relative ages because, although early paleontologists understood biological succession, they did not know the absolute ages of the different organisms. It was only in the early part of the 20th century, when isotopic dating methods were first applied, that it became possible to discover the absolute ages of the rocks containing fossils.

## How Do Scientists Determine the Age of Dinosaur Bones?

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 The width of a series of growth rings can give clues to past climates and various disruptions such as forest fires.

## The Cambrian Explosion at Qingjiang

Radioactive elements decay at a certain constant rate and this is the basis of radiometric dating. But, the decay elements need to be set, much like you would re-set a stop watch for a runner, to ensure an accurate measurement. When minerals get subducted into the Earth and come back as volcanic magmas or ash, this essential re-sets the radiometric clock back to zero and therefore a reliable age date is possible. Sedimentary rocks may have radioactive elements in them, but they have been re-worked from other rocks, so essentially, there radiometric clock has not been re-set back to zero. However, sedimentary rocks can be age dated if a volcanic ash horizon or a diabase sill or dyke can be found within the sequence. For example, if you find a dinosaur bone in a sedimentary sequence and you find an ash layer 10 meter above the bone and another ash layer 20 meters below it, you can determine the age of the two ash layers. You can then infer that the dino must have lived some time between these two age dates. Why is it difficult to date sedimentary rocks using radiometric dating techniques?

This diagram shows a selection of rock layers, or stratigraphic columns, from the Koobi Fora geologic formation on the eastern shore of Lake Turkana in Kenya. This area is a ridge of sedimentary rock where researchers have found more than 10, fossils, both human and other hominins, since

Geologists use radiometric dating to estimate how long ago rocks formed, and to infer the ages of fossils contained within those rocks. Radioactive elements decay The universe is full of naturally occurring radioactive elements. Radioactive atoms are inherently unstable; over time, radioactive "parent atoms" decay into stable "daughter atoms. When molten rock cools, forming what are called igneous rocks, radioactive atoms are trapped inside.

## How can radiometric dating be used to date sedimentary rocks?

Despite seeming like a relatively stable place, the Earth's surface has changed dramatically over the past 4. Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free. These changes typically occur so slowly that they are barely detectable over the span of a human life, yet even at this instant, the Earth's surface is moving and changing. As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils. A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved. However, by itself a fossil has little meaning unless it is placed within some context. The age of the fossil must be determined so it can be compared to other fossil species from the same time period. Understanding the ages of related fossil species helps scientists piece together the evolutionary history of a group of organisms. For example, based on the primate fossil record, scientists know that living primates evolved from fossil primates and that this evolutionary history took tens of millions of years. By comparing fossils of different primate species, scientists can examine how features changed and how primates evolved through time.

## Dating Rocks and Fossils Using Geologic Methods

Can we date sedimentary rocks using radiometric dating techniques? Sedimentary rocks cannot be dated directly using radiometric dating, which is based on the idea that when rocks are in liquid form, their radiometric clock resets. This technique is generally used to date igneous and metamorphic rock, which are rocks that were once melted due to extreme heat and pressure. Radiometric dating determines how long ago the liquid rock solidified into solid rock. Sedimentary rock on the other hand consists of sedimentary particles which were removed and deposited somewhere else by some sort of fluid generally wind and water. The sedimentary particles predate the rock which they form.

Geologists often need to know the age of material that they find. They use absolute dating methods, sometimes called numerical dating, to give rocks an actual date, or date range, in number of years. This is different to relative dating, which only puts geological events in time order. Most absolute dates for rocks are obtained with radiometric methods. These use radioactive minerals in rocks as geological clocks.

## Why is it difficult to date sedimentary rocks using radiometric dating techniques?

This riddle exemplifies how time normally results in decay and destruction. So, why are people so interested in the topic of time? Perhaps the biggest reason is because of evil: Humans have a limited lifespan to get things done, so we try to synchronize schedules in order not to waste time. We have a limited endurance for boredom or pain, so before the use of anesthesia, the best surgeons were the quickest. Although some complain that the Lord delays His coming Mt

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## Dating Fossils – How Are Fossils Dated?

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How Old is that Rock?
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