We Can Accurately Determine the Age of a Rock or Fossil by Measuring Its ____.ã¢â‚¬â€¹

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 1 rock layer is older than another. But determining the absolute age of a substance (its age in years) is a much greater claiming. To attain this, scientists use a variety of evidence, from tree rings to the amounts of radioactive materials in a rock.

Lesson Objectives

Define the departure between absolute age and relative age.
Describe iv methods of absolute dating.
Explain what radioactivity is and requite examples of radioactive decay.
Explicate how the decay of radioactive materials helps to establish the age of an object.
Estimate the historic period of an object, given the one-half-life and the amounts of radioactive and girl materials.
Give four examples of radioactive materials that are used to date objects, and explain how each is used.

Tree Rings

In regions outside the tropics, copse grow more quickly during the warm summer months than during the cooler winter. This pattern of growth results in alternating bands of light-colored, low density "early wood" and dark, high density "late wood". Each dark band represents a wintertime; by counting rings information technology is possible to find the historic period of the tree (Figure 11.22). The width of a series of growth rings tin requite clues to past climates and various disruptions such as woods fires. Droughts and other variations in the climate make the tree abound 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 tin can match up the growth rings of living and dead trees. Using logs recovered from onetime buildings and ancient ruins, scientists take been able to compare tree rings to create a continuous tape of tree rings over the past 2,000 years. This tree ring record has proven extremely useful in creating a record of climatic change, and in finding the age of ancient structures.

Effigy 11.22: Cross-section showing growth rings. The thick, light-colored part of each ring represents rapid bound and summertime growth. The thin, dark office of each ring represents slow autumn and winter growth.

Ice Cores and Varves

Several other processes effect in the aggregating of distinct yearly layers that can be used for dating. For case, layers course within glaciers considering at that place tends to be less snowfall in the summertime, allowing a night layer of grit to accumulate on top of the wintertime snow (Figure 11.23). To written report these patterns, scientists drill deep into ice sheets, producing cores hundreds of meters long. Scientists clarify these water ice cores to decide how the climate has changed over time, as well equally 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.

Figure eleven.23: Ice core department 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. The resulting layers, calledvarves, give scientists clues well-nigh past climate weather. For case, an especially warm summer might upshot in a very thick layer of sediment deposited from the melting glacier. Thinner varves can betoken colder summers, because the glacier doesn't melt as much and behave equally much sediment into the lake.

Age of Earth

Figure 11.24: 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 historic period of Earth with indirect techniques. For example, geologists measured how fast streams deposited sediment, in order to try to summate how long the stream had been in beingness. Not surprisingly, these methods resulted in wildly different estimates, from a few one thousand thousand years to "quadrillions of years". Probably the near reliable of these estimates was produced by the British geologist Charles Lyell, who estimated that 240 meg years have passed since the advent of the get-go animals with shells. Today scientists know his estimate was also young; we know that this occurred about 530 million years ago.

In 1892, William Thomson (later known as Lord Kelvin) calculated the age of World in a systematic fashion (Effigy xi.24). He assumed that the Globe began as a ball of molten stone, which has steadily cooled over time. From these assumptions, he calculated that the Globe was 100 million years old. This estimate was a blow to geologists and supporters of Charles Darwin'south theory of development, which required an older World to provide time for evolution to take place.

Thomson'southward calculations, all the same, were soon shown to be flawed when radioactivity was discovered in 1896.Radioactivity is the trend of certain atoms to decay into lighter atoms, emitting energy in the process. Radioactive materials in Globe'south interior provide a steady source of oestrus. Calculations of Earth's age using radioactive decay showed that Earth is actually much older than Thomson calculated.

Radioactive Decay

The discovery of radioactive materials did more than disprove Thomson's gauge of World's age. It provided a fashion to discover the accented age of a rock. To understand how this is done, it is necessary to review some facts well-nigh atoms.

Atoms contain 3 particles: protons, neutrons, and electrons. Protons and neutrons are located in the nucleus, while electrons orbit around the nucleus. The number of protons determines which chemical element y'all're examining. For instance, all atoms of carbon have 6 protons, all atoms of oxygen have eight protons, and all atoms of gold take 79 protons. The number of neutrons, however, is variable. An atom of an element with a dissimilar number of neutrons is anisotope of that element. For example, the isotope carbon-12 contains half-dozen neutrons in its nucleus, while the isotope carbon-xiii has vii neutrons.

Some isotopes areradioactive, which 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 (eleven.1):

Particle	Composition	Effect on Nucleus
Alpha	2 protons, 2 neutrons	The nucleus contains 2 fewer protons and two fewer neutrons.
Beta	one electron	One neutron decays to grade a proton and an electron, which is emitted.

If an chemical element decays by losing an alpha particle, it will lose ii protons and two neutrons. If an atom decays by losing a beta particle, it loses just one electron.

Then what does this have to practice with the age of Globe? Radioactive decay eventually results in the formation of stabledaughter products. Radioactive materials disuse at known rates. Every bit 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 stone with a high proportion of daughter products is probably very quondam.

Scientists measure the rate of radioactive decay with a unit calledhalf-life. The half-life of a radioactive substance is the amount of time, on average, it takes for one-half of the atoms to decay. For case, imagine a radioactive substance with a half-life of ane year. When a rock is formed, it contains a certain number of radioactive atoms. After one yr (one half-life), half of the radioactive atoms accept rust-covered to grade stable daughter products, and l% of the radioactive atoms remain. After some other year (two one-half-lives), half of the remaining radioactive atoms have decayed, and 25% of the radioactive atoms remain. Subsequently the third year (3 half-lives), 12.5% of the radioactive atoms remain. Subsequently four years (four half-lives), 6.25% of the radioactive atoms remain, and after 5 years (five one-half-lives), only iii.125% of the radioactive atoms remain.

If yous notice a rock whose radioactive cloth has a half life of one yr and measure three.125% radioactive atoms and 96.875% daughter atoms, you can assume that the substance is five years old. The disuse of radioactive materials can be shown with a graph (Figure 11.25). If you observe a rock with 75% of the radioactive atoms remaining, about how old is information technology?

Figure xi.25: Decay of an imaginary radioactive substance with a half-life of one twelvemonth.

Radiometric Dating of Rocks

In the process ofradiometric dating, several isotopes are used to date rocks and other materials. Using several unlike isotopes helps scientists to check the accuracy of the ages that they calculate.

Carbon Dating

Earth'due south atmosphere contains three isotopes of carbon. Carbon-12 is stable and accounts for 98.ix% of atmospheric carbon. Carbon-xiii is besides stable and accounts for one.i% of atmospheric carbon. Carbon-fourteen is radioactive and is institute in tiny amounts. Carbon-14 is produced naturally in the atmosphere when cosmic rays interact with nitrogen atoms. The amount of carbon-fourteen produced in the atmosphere at any item time has been relatively stable through time.

Radioactive carbon-14 decays to stable nitrogen-xiv by releasing a beta particle. The nitrogen atoms are lost to the atmosphere, but the amount of carbon-fourteen decay can be estimated past measuring the proportion of radioactive carbon-fourteen to stable carbon-12. Every bit a substance ages, the relative corporeality of carbon-14 decreases.

Carbon is removed from the atmosphere by plants during the process of photosynthesis. Animals eat this carbon when they swallow plants or other animals that have eaten plants. Therefore carbon-14 dating can be used to date plant and brute remains. Examples include timbers from an former building, bones, or ashes from a fire pit. Carbon dating can be effectively used to find the historic period of materials between 100 and l,000 years quondam.

Potassium-Argon Dating

Potassium-twoscore decays to argon-xl with a half-life of one.26 billion years. 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 consequence of the decay of potassium-twoscore. Measuring the ratio of potassium-twoscore to argon-40 will yield a skilful estimate of the historic period of the sample.

Potassium is a mutual chemical element constitute in many minerals such as feldspar, mica, and amphibole. The technique can exist used to date igneous rocks from 100,000 years to over a billion years old. Because it can exist used to appointment geologically young materials, the technique has been useful in estimating the age of deposits containing the bones of human ancestors.

Uranium-Lead Dating

Two isotopes of uranium are used for radiometric dating. Uranium-238 decays to form lead-206 with a half-life of 4.47 billion years. Uranium-235 decays to grade pb-207 with a half-life of 704 million years.

Uranium-pb dating is usually performed on crystals of the mineral zircon (Effigy 11.26). When zircon forms in an igneous rock, the crystals readily accept atoms of uranium just reject atoms of lead. Therefore, if any lead is found in a zircon crystal, it tin exist assumed that it was produced from the decay of uranium.

Effigy 11.26: Zircon crystal.

Uranium-lead dating can be used to date igneous rocks from 1 million years to around four.v billion years old. Some of the oldest rocks on Earth have been dated using this method, including zircon crystals from Australia that are iv.4 billion years old.

Limitations of Radiometric Dating

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 100,000 years one-time) 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 all-time for igneous rocks and is not very useful for determining the age of sedimentary rocks. To approximate the age of a sedimentary stone deposit, geologists search for nearby or interlayered igneous rocks that can be dated. For example, if a sedimentary rock layer is sandwiched betwixt two layers of volcanic ash, its historic period is betwixt the ages of the two ash layers.

Using a combination of radiometric dating, alphabetize fossils, and superposition, geologists accept constructed a well-divers timeline of Globe history. For example, an overlying lava flow tin can give a reliable estimate of the age of a sedimentary rock formation in i location. Index fossils contained in this germination tin so be matched to fossils in a unlike location, providing a good age measurement for that new rock formation also. As this procedure has been repeated all over the world, our estimates of stone and fossil ages has become more than and more authentic.

Lesson Summary

Techniques such as superposition and index fossils can tell y'all the relative historic period of objects, which objects are older and which are younger. Other types of evidence are needed to establish the accented age of objects in years. Geologists utilise a variety of techniques to found absolute age, including radiometric dating, tree rings, ice cores, and annual sedimentary deposits chosen varves.

Radiometric dating is the most useful of these techniques—it is the but technique that can plant the age of objects older than a few thousand years. The concentrations of several radioactive isotopes (carbon-14, potassium-40, uranium-235 and -238) and their daughter products are used to decide the historic period of rocks and organic remains.

Review Questions

What four techniques are used to determine the accented age of an object or effect?
A radioactive substance has a half-life of 5 million years. What is the age of a stone in which 25% of the original radioactive atoms remain?
A scientist is studying a slice of cloth from an ancient burial site. She determines that forty% of the original carbon-14 atoms remain in the cloth. Based on the carbon decay graph (Figure 11.27), what is the approximate age of the cloth?

Effigy 11.27: Radioactive disuse of Carbon-14
Which radioactive isotope or isotopes would you utilize to date each of the following objects? Explain each of your choices.
- A 4 billion twelvemonth one-time slice of granite.
- A one million twelvemonth old bed of volcanic ash that contains the footprints of hominids (human ancestors).
- The fur of a woolly mammoth that was recently recovered frozen in a glacier.
- A fossilized trilobite recovered from a bed of sandstone that is nearly 500 one thousand thousand years quondam.
The principle of uniformitarionism states that the nowadays is the cardinal to the by. In other words, the processes that we come across happening today probably worked in a like way in the past. Why is it important to assume that the rate of radioactivity has remained abiding over time?

Vocabulary

absolute age: The age of an object in years.
alpha particle: Particle consisting of two protons and two neutrons that is ejected from the nucleus during radioactivity.
beta particle: Particle consisting of a single electron that is ejected from the nucleus during radioactive disuse. A beta particle is created when a neutron decays to form a proton and the emitted electron.
daughter production: Stable substance that is produced by the decay of a radioactive substance. For case, uranium-238 decays to produce pb-207.
half-life: Corporeality of time required for half of the atoms of a radioactive substance to decay and form daughter products.
water ice core: Cylinder of ice extracted from a glacier or ice canvas.
radioactive: Substance that is unstable and probable to emit energetic particles and radiation.
radioactivity: Emission of high-energy particles and/or radiation by certain unstable atoms.
radiometric dating: Procedure of using the concentrations of radioactive substances and girl products to estimate the age of a material. Every bit substances age, the amounts of radioactive atoms decrease while the amounts of daughter materials increase.
tree ring: Layer of forest in a tree that forms in one year. You can determine the age of a tree by counting its rings.
varve: Thin layer of sediment deposited on a lakebed over the grade of one yr usually found at the bottom of glacial lakes.

Points to Consider

Why are techniques like tree rings, ice cores, and varves only useful for events that occurred in the last few yard years?
Why was information technology so important for Darwin and his followers to prove that the World was very onetime?
Why is it important to utilize more one method to find the age of a rock or other object?

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Source: https://courses.lumenlearning.com/earthscience/chapter/absolute-ages-of-rocks/