www.ck12.org Chapter 11. HS Evidence About Earth’s Past

11.3 Absolute Ages of Rocks

Lesson Objectives

• Define the differences between absolute age and relative age.• Describe four methods of absolute dating.• Explain what radioactivity is and give examples of radioactive decay.• Explain how the decay of radioactive materials helps to establish the age of an object.• Estimate the age of an object, given the half-life and the amounts of radioactive and daughter materials.• Give four examples of radioactive materials that are used to date objects, and explain how each is used.• Describe how scientists know earth is billions of years old.

Vocabulary

• absolute age• daughter product• half-life• ice core• parent isotope• radioactive isotope• radioactivity• radiometric dating• tree ring

Introduction

What was missing from the early geologic time scale? While the order of events was given, the dates at which theevents happened were not. With the discovery of radioactivity in the late 1800s, scientists were able to measure theabsolute age, or the exact age of some rocks in years. Absolute dating allows scientists to assign numbers to thebreaks in the geologic time scale. Radiometric dating and other forms of absolute age dating allowed scientists toget an absolute age from a rock or fossil.

Tree Ring Dating

In locations where summers are warm and winters are cool, trees have a distinctive growth pattern. Tree trunksdisplay alternating bands of light-colored, low density summer growth and dark, high density winter growth. Eachlight-dark band represents one year. By counting tree rings it is possible to find the number of years the tree lived(Figure 11.29).

The width of these growth rings varies with the conditions present that year. A summer drought may make the treegrow more slowly than normal and so its light band will be relatively small. These tree-ring variations appear in alltrees in a region. The same distinctive pattern can be found in all the trees in an area for the same time period.

Scientists have created continuous records of tree rings going back over the past 2,000 years. Wood fragments fromold buildings and ancient ruins can be age dated by matching up the pattern of tree rings in the wood fragment in

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FIGURE 11.29Cross-section showing growth rings.

question and the scale created by scientists. The outermost ring indicates when the tree stopped growing; that is,when it died. The tree-ring record is extremely useful for finding the age of ancient structures.

An example of how tree-ring dating is used to date houses in the United Kingdom is found in this article: http://www.periodproperty.co.uk/ppuk_discovering_article_013.shtml.

Ice Cores and Varves

Other processes create distinct yearly layers that can be used for dating. On a glacier, snow falls in winter but insummer dust accumulates. This leads to a snow-dust annual pattern that goes down into the ice (Figure 11.30).Scientists drill deep into ice sheets, producing ice cores hundreds of meters long. The information scientists gatherallows them to determine how the environment has changed as the glacier has stayed in its position. Analyses of theice tell how concentrations of atmospheric gases changed, which can yield clues about climate. The longest coresallow scientists to create a record of polar climate stretching back hundreds of thousands of years.

FIGURE 11.30Ice core section showing annual layers.

Lake sediments, especially in lakes that are located at the end of glaciers, also have an annual pattern. In the summer,the glacier melts rapidly, producing a thick deposit of sediment. These alternate with thin, clay-rich layers depositedin the winter. The resulting layers, called varves, give scientists clues about past climate conditions (Figure 11.31).A warm summer might result in a very thick sediment layer while a cooler summer might yield a thinner layer.

Age of Earth

During the 18th and 19th centuries, geologists tried to estimate the age of Earth with indirect techniques. Whatmethods can you think of for doing this? One example is that by measuring how much sediment a stream deposited

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FIGURE 11.31Ancient varve sediments in a rock outcrop.

in a year, a geologist might try to determine how long it took for a stream to deposit an ancient sediment layer.Not surprisingly, these methods resulted in wildly different estimates. A relatively good estimate was produced bythe British geologist Charles Lyell, who thought that 240 million years had passed since the appearance of the firstanimals with shells. Today scientists know that this event occurred about 530 million years ago.

In 1892, William Thomson (later known as Lord Kelvin) calculated that the Earth was 100 million years old (Figure11.32). He did this systematically assuming that the planet started off as a molten ball and calculating the time itwould take for it to cool to its current temperature. This estimate was a blow to geologists and supporters of CharlesDarwin#8217;s theory of evolution, which required an older Earth to provide time for geological and evolutionaryprocesses to take place.

FIGURE 11.32Lord Kelvin.

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Thomson#8217;s calculations were soon shown to be flawed when radioactivity was discovered in 1896. Radioac-tivity is the tendency of certain atoms to decay into lighter atoms, a process that emits energy. Radioactive decayof elements inside Earth#8217;s interior provides a steady source of heat, which meant that Thomson had grosslyunderestimated Earth#8217;s age.

Radioactive Decay

Radioactivity also provides a way to find the absolute age of a rock. To begin, go back to the Earth’s Mineralschapter and review the material about atoms.

Some isotopes are radioactive; radioactive isotopes are unstable and spontaneously change by gaining or losingparticles. Two types of radioactive decay are relevant to dating Earth materials (Table 11.1):

TABLE 11.1: Types of Radioactive Decay

Particle Composition Effect on NucleusAlpha 2 protons, 2 neutrons The nucleus contains two fewer pro-

tons and two fewer neutrons.Beta 1 electron One neutron decays to form a pro-

ton and an electron. The electron isemitted.

The radioactive decay of a parent isotope (the original element) leads to the formation of stable daughter product,also known as daughter isotope. As time passes, the number of parent isotopes decreases and the number of daughterisotopes increases (Figure 11.33).

FIGURE 11.33A parent emits an alpha particle to createa daughter.

An animation of radioactive decay: http://lectureonline.cl.msu.edu/ mmp/applist/decay/decay.htm.

Radioactive materials decay at known rates, measured as a unit called half-life. The half-life of a radioactivesubstance is the amount of time it takes for half of the parent atoms to decay. This is how the material decaysover time (see Table 11.2).

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TABLE 11.2: Radioactive Decay

No. of half lives passed Percent parent remaining Percent daughter produced0 100 01 50 502 25 753 12.5 87.54 6.25 93.755 3.125 96.8756 1.563 98.4377 0.781 99.2198 0.391 99.609

Pretend you find a rock with 3.125% parent atoms and 96.875% daughter atoms. How many half lives have passed?If the half-life of the parent isotope is 1 year, then how old is the rock? The decay of radioactive materials can beshown with a graph (Figure 11.34).

FIGURE 11.34Decay of an imaginary radioactive sub-stance with a half-life of one year.

An animation of half-life: http://einstein.byu.edu/ masong/htmstuff/Radioactive2.html

Notice how it doesn#8217;t take too many half lives before there is very little parent remaining and most of theisotopes are daughter isotopes. This limits how many half lives can pass before a radioactive element is no longeruseful for dating materials. Fortunately, different isotopes have very different half lives.

Radiometric decay is exponential. Learn how exponential growth and decay can be described mathematically in thisvideo (I&E 1e): http://www.youtube.com/watch?v=UbwMW7Q6F3E (4:46).

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MEDIAClick image to the left for more content.

RThe Scientific Method Made Easy explains scientific method succinctly and well (I&E 1a, 1b, 1c, 1d, 1f,1g, 1j,1k): http://www.youtube.com/watch?v=zcavPAFiG14&feature=related (9:55).

Radiometric Dating of Rocks

Different isotopes are used to date materials of different ages. Using more than one isotope helps scientists to checkthe accuracy of the ages that they calculate.

Radiocarbon Dating

Radiocarbon dating is used to find the age of once-living materials between 100 and 50,000 years old. This range isespecially useful for determining ages of human fossils and habitation sites (Figure 11.35).

FIGURE 11.35Carbon isotopes from the black materialin these cave paintings places their cre-ating at about 26,000 to 27,000 years BP(before present).

The atmosphere contains three isotopes of carbon: carbon-12, carbon-13 and carbon-14. Only carbon-14 is radioac-tive; it has a half-life of 5,730 years. The amount of carbon-14 in the atmosphere is tiny and has been relativelystable through time.

Plants remove all three isotopes of carbon from the atmosphere during photosynthesis. Animals consume this carbonwhen they eat plants or other animals that have eaten plants. After the organism#8217;s death, the carbon-14 decays

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to stable nitrogen-14 by releasing a beta particle. The nitrogen atoms are lost to the atmosphere, but the amountof carbon-14 that has decayed can be estimated by measuring the proportion of radioactive carbon-14 to stablecarbon-12. As time passes, the amount of carbon-14 decreases relative to the amount of carbon-12.

A video of carbon-14 decay is seen here: http://www.youtube.com/watch?v=81dWTeregEA; a longer explanationis here: http://www.youtube.com/watch?v=udkQwW6aLik&feature=related.

Potassium-Argon Dating

Potassium-40 decays to argon-40 with a half-life of 1.26 billion years. Argon is a gas so it can escape from moltenmagma, meaning that any argon that is found in an igneous crystal probably formed as a result of the decay ofpotassium-40. Measuring the ratio of potassium-40 to argon-40 yields a good estimate of the age of that crystal.

Potassium is common in many minerals, such as feldspar, mica, and amphibole. With its half-life, the technique isused to date rocks from 100,000 years to over a billion years old. The technique has been useful for dating fairlyyoung geological materials and deposits containing the bones of human ancestors.

Uranium-Lead Dating

Two uranium isotopes are used for radiometric dating.

• Uranium-238 decays to lead-206 with a half-life of 4.47 billion years.• Uranium-235 decays to form lead-207 with a half-life of 704 million years.

Uranium-lead dating is usually performed on zircon crystals (Figure 11.36). When zircon forms in an igneous rock,the crystals readily accept atoms of uranium but reject atoms of lead. If any lead is found in a zircon crystal, it canbe assumed that it was produced from the decay of uranium.

FIGURE 11.36Zircon crystal.

Uranium-lead dating is useful for dating igneous rocks from 1 million years to around 4.6 billion years old. Zirconcrystals from Australia are 4.4 billion years old, among the oldest rocks on the planet.

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Limitations of Radiometric Dating

Radiometric dating, or the process of using the concentrations of radioactive substances and daughter products toestimate the age of a material, is a very useful tool for dating geological materials but it does have limits:

1. The material being dated must have measurable amounts of the parent and/or the daughter isotopes. Ideally,different radiometric techniques are used to date the same sample; if the calculated ages agree, they are thought tobe accurate.

2. Radiometric dating is not very useful for determining the age of sedimentary rocks. To estimate the age of asedimentary rock, geologists find nearby igneous rocks that can be dated and use relative dating to constrain the ageof the sedimentary rock.

Using a combination of radiometric dating, index fossils, and superposition, geologists have constructed a well-defined timeline of Earth history. With information gathered from all over the world, estimates of rock and fossilages have become increasingly accurate.

All of this evidence comes together to pinpoint the age of Earth at 4.6 billion years. A video discussing the evidencefor this is found here: http://www.youtube.com/watch?v=w5369-OobM4

The age of Earth is also discussed in this video: http://www.youtube.com/watch?v=lplcRdNDcps&feature=channel

Lesson Summary

• Earth is very old, and the study of Earth#8217;s past requires us to think about times that were millions oreven billions of years ago.

• Techniques such as superposition and index fossils can tell you the relative age of objects, which objects areolder and which are younger.

• Geologists use a variety of techniques to establish absolute age, including radiometric dating, tree rings, icecores, and annual sedimentary deposits called varves.

• The concentrations of several radioactive isotopes (e.g. carbon-14, potassium-40, uranium-235 and -238) andtheir daughter products are used to accurately determine the age of rocks and organic remains.

Review Questions

1. Name four techniques that are used to determine the absolute age of an object or event.

2. A radioactive substance has a half-life of 5 million years. What is the age of a rock in which 25% of the originalradioactive atoms remain?

3. A scientist is studying a piece of cloth from an ancient burial site. She determines that 40% of the originalcarbon-14 atoms remain in the cloth. Based on the carbon-decay graph (Figure 11.37), what is the approximate ageof the cloth?

4. Which radioactive isotope or isotopes would you use to date each of the following objects? Explain each of yourchoices.

4.) A fossilized trilobite from a bed of sandstone that is about 500 million years old. 3.) The fur of a woollymammoth that was recently recovered, frozen in a glacier. 2.) A 1-million-year-old bed of volcanic ash thatcontains the footprints of human ancestors. 1.) A 4-billion-year-old piece of granite.

5. Why is it important to assume that the rate of radioactive decay has remained constant over time?

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FIGURE 11.37Carbon-decay graph.

Further Reading / Supplemental Links

Using tree rings and ice cores to track El Nino events: http://www.pbs.org/wgbh/nova/elnino/reach/living.html

Points to Consider

• Why are techniques for dating, such as using tree rings, ice cores, and varves only useful for events thatoccurred in the last few thousand years?

• Why is it important for geological and biological processes that the earth is very old?• Why is it important to use more than one method to find the age of a rock or other object?

• Image copyright markrhiggins, 2010. http://www.shutterstock.com/. Used under license from Shutterstock.com.• Image copyright p.schwarz, 2010. http://www.shutterstock.com/. Used under license from Shutterstock.com.• Kurt Rosenkrantz/CK-12 Foundation. . CC-BY-SA 3.0.• (a) Adrian Pingstone (Apingstone); (b) Daniel Schwen; (c) Courtesy of US Geological Survey; (d) GD Berlin;

(e) Woudloper. [(a) http://commons.wikimedia.org/wiki/File:Amber.insect.800pix.050203.jpg; (b) http://commons.wikimedia.org/wiki/File:Petrified_wood_closeup_2.jpg; (c) http://commons.wikimedia.org/wiki/File:Cast_and_mold_of_a_clam_shell.jpg; (d) http://commons.wikimedia.org/wiki/File:Ammonit_aus_Pyrit.jpg; (e)http://commons.wikimedia.org/wiki/File:Pecopteris_arborescens.jpg ]. (a) Public Domain; (b) GNU-FDL1.2; (c) Public Domain; (d) CC-BY-SA 2.0 Germany; (e) Public Domain.

• Tobias Alt, modified by CK-12 Foundation. http://commons.wikimedia.org/wiki/File:Grand_Canyon_NP-Arizona-USA.jpg. CC-BY-SA 3.0.

• DanielCD. http://en.wikipedia.org/wiki/File:MucrospiriferSp.jpg. GNU-FDL 1.2.• (a) Tribal; (b) Eli Hodapp. [(a) http://commons.wikimedia.org/wiki/File:Shark_teeth_in_stone.jpg; (b) http

://www.flickr.com/photos/io_burn/1805341269/ ]. (a) GNU-FDL 1.2; (b) CC-BY 2.0.• Eurico Zimbres/Tom Epaminondas. http://commons.wikimedia.org/wiki/Image:Zirc%C3%A3o.jpeg. CC-

BY-SA 2.0 Brazil.• Kurt Rosenkrantz/CK-12 Foundation. . CC-BY-NC 3.0.

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• Image copyright Roger De Marfa, 2010. Used under license from Shutterstock.com. http://www.shutterstock.com/.

• Courtesy of US Geological Survey. http://commons.wikimedia.org/wiki/File:Coprolite.jpg. Public Domain.• Kurt Rosenkrantz/CK-12 Foundation. . CC-BY-SA 3.0.• Kurt Rosenkrantz/CK-12 Foundation. . CC-BY-SA 3.0.• Kurt Rosenkrantz/CK-12 Foundation. . CC-BY-SA 3.0.• Unknown. http://commons.wikimedia.org/wiki/File:Lord_Kelvin_photograph.jpg. Public Domain.• Courtesy of the US Geological Survey. http://esp.cr.usgs.gov/info/kt/stop2b.html. Public Domain.• Image copyright Syringa, 2010. http://www.shutterstock.com. Used under license from Shutterstock.com.• Anne Burgess. http://commons.wikimedia.org/wiki/File:Siccar_Point.jpg. CC-BY-SA 2.0.• John Clerk. http://en.wikipedia.org/wiki/File:Hutton_Unconformity_,_Jedburgh.jpg. Public Domain.• (a) #1044;#1080;#1041;#1075;#1076;; (b) James Emery; (c) Peter Halasz; (d) Vassil. [(a) http://en.wikipe

dia.org/wiki/File:Quetzalcoatlus07.jpg; (b) http://commons.wikimedia.org/wiki/File:Argentinosaurus.jpg; (c)http://commons.wikimedia.org/wiki/File:Kolihapeltis_01_Pengo.jpg; (d) http://commons.wikimedia.org/wiki/File:Ammonite_2582.jpg ]. (a) Public Domain; (b) CC-BY 2.0; (c) CC-BY-SA 2.5; (d) Public Domain.

• (left) Photo by Jastrow; (right) Ovulator. [(left) http://commons.wikimedia.org/wiki/File:Satyr_griffin_Arimaspus_Louvre_CA491.jpg; (right) http://commons.wikimedia.org/wiki/File:Protoceratops-skeleton.jpg ]. (left)Public Domain; (right) GNU-FDL 1.2.

• Courtesy of US Geological Survey, modified by CK-12 Foundation. http://commons.wikimedia.org/wiki/File:Skagit-gneiss-Cascades.jpg. Public Domain.

• (a) Stephen J. Reynolds; (b) Woudloper; (c) Mark A Wilson (Wilson44691). [(a) http://reynolds.asu.edu/geologic_scenery/geologic_scenery_images.htm; (b) http://en.wikipedia.org/wiki/File:Principle_of_horizontal_-continuity.svg; (c) http://en.wikipedia.org/wiki/File:IsfjordenSuperposition.jpg ]. (a) Noncommercial usesOK as long as source is acknowledged; (b) Public Domain; (c) Public domain.

• Lancevortex. http://en.wikipedia.org/wiki/File:Pompeii_Garden_of_the_Fugitives_02.jpg. GNU-FDL 1.2.• PalaeoMal. http://commons.wikimedia.org/wiki/File:Cliona_in_Ostrea_edulis_-_Eemian.JPG. CC-BY-SA 2.5.• Courtesy of US Geological Survey. http://pubs.usgs.gov/of/2004/1216/tz/tz.html. Public Domain.• Courtesy of US Geological Survey. http://commons.wikimedia.org/wiki/File:Geologic_time_scale.jpg. Public

Domain.• Kurt Rosenkrantz/CK-12 Foundation. . CC-BY-SA 3.0.• Charles R. Knight. http://en.wikipedia.org/wiki/File:La_Brea_Tar_Pits.jpg. Public Domain.• Image copyright Galyna Andrushko, 2010. http://www.shutterstock.com. Used under license from Shutter-

stock.com.• (left) Dlloyd; (right) ryan junell. [(left) http://commons.wikimedia.org/wiki/File:Ammonite_Asteroceras.jpg;

(right) http://en.wikipedia.org/wiki/File:Elephant_skull_at_Serengeti_National_Park.jpg ]. (left) GNU-FDL1.2; (right) CC-BY-SA 2.0.

• Kurt Rosenkrantz/CK-12 Foundation. . CC-BY-SA 3.0.• Courtesy of US Geological Survey, provided by Eric Cravens, Assistant Curator, National Ice Core Laboratory.

http://en.wikipedia.org/wiki/File:GISP2D1837_crop.jpg. Public Domain.• Lawrence Murray (lawmurray). http://www.flickr.com/photos/22699083@N04/2284340556/. CC-BY 2.0.• Hermann A. Wiese. http://commons.wikimedia.org/wiki/File:Geol_Garten_7.jpg. CC-BY-SA 2.5.• (a) Mark A Wilson (Wilson44691); (b) Mark A Wilson (Wilson44691); (c) Duncan Wright; (d) H Raab

(Vesta). [(a) http://en.wikipedia.org/wiki/File:WalcottQuarry080509.jpg; (b) http://commons.wikimedia.org/wiki/File:Anomalocaris_Mt._Stephen.jpg; (c) http://commons.wikimedia.org/wiki/File:Brittle_star-fossil.jpg;(d) http://commons.wikimedia.org/wiki/File:Archaeopteryx_lithographica_%28Solenhofener_Specimen%29.jpg ]. (a) Public Domain; (b) Public Domain; (c) GNU-FDL 1.2; (d) CC-BY-SA 3.0.

• HTO. http://en.wikipedia.org/wiki/File:Chauvet_cave,_paintings.JPG. Public Domain.

Opening image courtesy of Barbara Summey/NASA, http://earthobservatory.nasa.gov/IOTD/view.php?id=602, andis in the public domain.

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