As we learned in the ahead lesson, index fossils and superposition are efficient methods of identify the relative age of objects. In various other words, you deserve to use superposition come tell you that one absent layer is older than another. But determining the absolute age of a problem (its period in years) is a much better challenge. To accomplish this, scientists use a selection of evidence, native tree ring to the amounts of radioactive materials in a rock.

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Lesson Objectives

Define the difference between absolute age and also relative age.Describe four methods of pure dating.Explain what radiation is and also give examples of radioactive decay.Explain just how the degeneration of radioactive products helps to develop the age of an object.Estimate the period of an object, provided the half-life and also the amounts of radioactive and daughter materials.Give four examples of radioactive materials that are provided to date objects, and explain exactly how each is used.

Tree Rings

In regions external the tropics, trees grow much more quickly throughout the warmth summer months than throughout the cooler winter. This sample of expansion results in alternative bands of light-colored, low density “early wood” and also dark, high thickness “late wood”. Each dark tape represents a winter; by counting ring it is possible to discover the period of the tree (Figure 11.22). The broad of a series of development rings deserve to give hints to previous climates and also various disturbances such as forest fires. Droughts and other sports in the climate do the tree flourish slower or quicker than normal, which reflects up in the widths the the tree rings. This tree ring sports will appear in all trees cultivation in a certain region, so scientists can complement up the growth rings of living and also dead trees. Making use of logs recovered from old buildings and ancient ruins, scientists have been maybe to to compare tree rings to produce a constant record the tree rings over the past 2,000 years. This tree ring record has proven extremely valuable in creating a document of climate change, and in recognize the period of ancient structures.


Ice Cores and also Varves

Several various other processes an outcome in the build-up of distinctive yearly class that have the right to be offered for dating. Because that example, layers kind within glaciers because there has tendency to be less snowfall in the summertime, permitting a dark great of dust to accumulate on peak of the winter snow (Figure 11.23). To study these patterns, scientists drill deep into ice sheets, creating cores hundreds of meters long. Scientists analyze these ice cores to determine exactly how the climate has readjusted over time, and also to measure concentrations of atmospheric gases. The longest cores have actually helped to form a document of polar climate stretching numerous thousands of year back.


Another instance of yearly class is the deposition of sediments in lakes, specifically the lakes the are located at the finish of glaciers. Fast melting of the glacier in the summer results in a thick, sandy deposit that sediment. This thick layers alternating with thin, clay-rich great deposited throughout the winter. The result layers, called varves, offer scientists clues about past climate conditions. For example, specifically warm summer might an outcome in a an extremely thick great of sediment deposited from the melt glacier. Diluent varves deserve to indicate colder summers, since the glacier no melt as much and carry as much sediment into the lake.

Age of Earth


While tree rings and other annual layers are useful for dating reasonably recent events, they room not of lot use top top the huge scale the geologic time. Throughout the 18th and also 19th centuries, geologists tried to estimate the period of planet with indirect techniques. Because that example, geologists measured how quick streams deposited sediment, in stimulate to try to calculate how long the stream had been in existence. No surprisingly, this methods caused wildly various estimates, indigenous a few million years to “quadrillions of years”. Most likely the most reliable the these approximates was created by the brother geologist Charles Lyell, who approximated that 240 million years have actually passed due to the fact that the appearance of the first animals with shells. Now scientists know his estimate was as well young; we recognize that this occurred about 530 million year ago.

In 1892, wilhelm Thomson (later recognized as lord Kelvin) calculate the age of earth in a systematic fashion (Figure 11.24). The assumed the the Earth started as a sphere of molten rock, which has actually steadily cooled over time. From these assumptions, that calculated the the planet was 100 million years old. This calculation was a punch to geologists and supporters the Charles Darwin’s theory of evolution, which required an older planet to provide time for advancement to take place.

Thomson’s calculations, however, to be soon displayed to be flawed once radioactivity was found in 1896. Radioactivity is the propensity of particular atoms to degeneration into lighter atoms, emitting power in the process. Radioactive products in Earth’s interior carry out a steady source of heat. Calculations that Earth’s age using radioactive decay showed that earth is actually lot older than Thomson calculated.

Radioactive Decay

The discovery of radioactive materials did more than disprove Thomson’s estimate of Earth’s age. It detailed a means to discover the absolute period of a rock. To understand just how this is done, that is necessary to review some facts around atoms.

Atoms contain 3 particles: protons, neutrons, and electrons. Protons and neutrons are located in the nucleus, while electron orbit roughly the nucleus. The variety of protons identify which element you’re examining. Because that example, all atoms of carbon have six protons, all atoms the oxygen have eight protons, and all atoms of gold have 79 protons. The number of neutrons, however, is variable. One atom that an facet with a different variety of neutrons is an isotope of that element. For example, the isotope carbon-12 includes 6 neutrons in that is nucleus, if the isotope carbon-13 has actually 7 neutrons.

Some isotope are radioactive, which means they are unstable and likely to decay. This way the atom will certainly spontaneously readjust from an unstable form to a secure form. There are two develops of nuclear degeneration that are appropriate in just how geologists can date rocks (Table (11.1):

ParticleCompositionEffect top top Nucleus
Alpha2 protons, 2 neutronsThe nucleus has two under protons and also two under neutrons.
Beta1 electronOne neutron decays to type a proton and also an electron, i m sorry is emitted.

If an element decays by losing an alpha particle, it will shed 2 protons and 2 neutrons. If one atom decays by shedding a beta particle, it loses simply one electron.

So what go this have to do v the period of Earth? Radioactive decay eventually outcomes in the development of stable daughter products. Radiation materials degeneration at known rates. As time passes, the ratio of radioactive isotopes will certainly decrease and also the proportion of daughter isotopes will certainly increase. A rock v a reasonably high relationship of radioactive isotope is probably an extremely young, while a rock with a high proportion of daughter products is probably an extremely old.

Scientists measure up the price of radioactive degeneration with a unit called half-life. The half-life that a radioactive substance is the quantity of time, top top average, the takes for fifty percent of the atoms to decay. Because that example, imagine a radioactive substance through a half-life that one year. When a absent is formed, it consists of a certain variety of radioactive atoms. After ~ one year (one half-life), fifty percent of the radiation atoms have decayed to form stable daughter products, and also 50% of the radioactive atoms remain. After another year (two half-lives), fifty percent of the remaining radioactive atoms have decayed, and 25% that the radioactive atoms remain. After ~ the 3rd year (three half-lives), 12.5% of the radioactive atom remain. After 4 years (four half-lives), 6.25% the the radioactive atoms remain, and after 5 year (five half-lives), just 3.125% of the radioactive atom remain.

If you discover a absent whose radiation material has a fifty percent life of one year and measure 3.125% radioactive atoms and also 96.875% daughter atoms, you deserve to assume the the problem is 5 years old. The degeneration of radioactive materials have the right to be shown with a graph (Figure 11.25). If you find a rock with 75% the the radioactive atom remaining, about how old is it?


Radiometric date of Rocks

In the procedure of radiometric dating, numerous isotopes are supplied to day rocks and other materials. Utilizing several various isotopes helps scientists to inspect the accuracy the the periods that lock calculate.

Carbon Dating

Earth’s atmosphere contains three isotope of carbon. Carbon-12 is stable and accounts for 98.9% the atmospheric carbon. Carbon-13 is also stable and accounts for 1.1% that atmospheric carbon. Carbon-14 is radioactive and is found in tiny amounts. Carbon-14 is developed naturally in the atmosphere when cosmic rays connect with nitrogen atoms. The amount of carbon-14 produced in the atmosphere at any certain time has actually been relatively stable v time.

Radioactive carbon-14 decays to secure nitrogen-14 by publication a beta particle. The nitrogen atom are lost to the atmosphere, but the lot of carbon-14 degeneration can be estimated by measure the proportion of radiation carbon-14 to secure carbon-12. Together a problem ages, the family member amount that carbon-14 decreases.

Carbon is removed from the environment by plants throughout the procedure of photosynthesis. Pets consume this carbon once they eat plants or other pets that have consumed plants. Because of this carbon-14 dating have the right to be provided to day plant and also animal remains. Examples include timbers from an old building, bones, or ashes indigenous a fire pit. Carbon dating deserve to be successfully used to discover the age of materials between 100 and also 50,000 years old.

Potassium-Argon Dating

Potassium-40 decays come argon-40 with a half-life that 1.26 billion years. Due to the fact that argon is a gas, it deserve to escape native molten magma or lava. Therefore any type of argon the is discovered in a decision probably created as a an outcome of the degeneration of potassium-40. Measure up the proportion of potassium-40 come argon-40 will yield a an excellent estimate the the age of the sample.

Potassium is a common facet found in countless minerals such together feldspar, mica, and amphibole. The method can be used to day igneous rocks from 100,000 year to over a billion year old. Because it have the right to be supplied to date geologically young materials, the method has been advantageous in estimating the age of deposits containing the skeleton of person ancestors.

Uranium-Lead Dating

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

Uranium-lead date is usually performed on crystals the the mineral zircon (Figure 11.26). As soon as zircon forms in one igneous rock, the crystals readily accept atoms of uranium however reject atoms of lead. Therefore, if any type of lead is uncovered in a zircon crystal, it can be assumed the it was developed from the degeneration of uranium.


Uranium-lead dating deserve to be provided to date igneous rocks from 1 million years to roughly 4.5 billion year old. Some of the earliest rocks on earth have been dated utilizing this method, including zircon crystals native Australia that space 4.4 billion years old.

Limitations of Radiometric Dating

Radiometric dating can only be supplied on products that contain measurable quantities of radiation materials and their daughter products. This contains organic stays (which compared to rocks are reasonably young, less than 100,000 years old) and also older rocks. Ideally, several different radiometric methods will be used to day the exact same rock. Agreement in between these values shows that the calculated age is accurate.

In general, radiometric dating works finest for igneous rocks and also is not an extremely useful because that determining the age of sedimentary rocks. To estimate the age of a sedimentary absent deposit, geologists search for surrounding 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 periods of the 2 ash layers.

Using a combination of radiometric dating, index fossils, and superposition, geologists have constructed a well-defined timeline of planet history. For example, one overlying lava circulation can offer a dependable estimate of the age of a sedimentary rock formation in one location. Table of contents fossils consisted of in this development can then be suitable to fossils in a various location, offering a great age measurement because that that new rock formation as well. As this procedure has been repeated all over the world, our estimates of rock and fossil periods has become much more and an ext accurate.

Lesson Summary

Techniques such together superposition and index fossils have the right to tell you the relative period of objects, i m sorry objects space older and which are younger. Other types of evidence are necessary to establish the absolute period of objects in years. Geologists usage a variety of approaches to develop absolute age, including radiometric dating, tree rings, ice cream cores, and annual sedimentary deposits dubbed varves.

Radiometric date is the most valuable of this techniques—it is the only technique that can create the period of objects older 보다 a few thousand years. The concentrations of number of radioactive isotope (carbon-14, potassium-40, uranium-235 and also -238) and their daughter commodities are supplied to determine the age of rocks and also organic remains.

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Review Questions

What four techniques are offered to identify the absolute age of an item or event?A radiation substance has a half-life of 5 million years. What is the period of a rock in which 25% that the original radioactive atom remain?Which radioactive isotope or isotopes would you usage to date each that the following objects? define each of your choices.A 4 billion year old item of granite.A one million year old bed of volcanic ash that has the footprints that hominids (human ancestors).The fur of a woolly mammoth the was newly recovered frozen in a glacier.A fossilized trilobite recovered indigenous a bed the sandstone the is about 500 million years old.The rule of uniformitarionism says that the present is the crucial to the past. In other words, the procedures that we check out happening now probably functioned in a similar means in the past. Why is it necessary to assume the the rate of radioactive degeneration has remained consistent over time?


absolute ageThe period of an object in years.alpha particleParticle consists of 2 protons and also two neutrons that is ejected from the nucleus throughout radioactive decay.beta particleParticle consisting of a single electron the is ejected from the nucleus throughout radioactive decay. A beta fragment is created when a ghost decays to kind a proton and the emitted electron.daughter productStable substance that is developed by the decay of a radiation substance. Because that example, uranium-238 decays to produce lead-207.half-lifeAmount the time required for half of the atoms of a radioactive problem to decay and form daughter coreCylinder of ice cream extracted native a glacier or ice cream sheet.radioactiveSubstance that is unstable and likely come emit energetic particles and also radiation.radioactivityEmission the high-energy particles and/or radiation by certain unstable atoms.radiometric datingProcess of utilizing the concentration of radiation substances and also daughter assets to calculation the period of a material. As substances age, the amounts of radioactive atoms decrease while the quantities of daughter materials increase.tree ringLayer of timber in a tree that forms in one year. You have the right to determine the period of a tree by counting its rings.varveThin class of sediment deposited on a lakebed over the course of one year usually found at the bottom of glacial lakes.

Points come Consider

Why are approaches like tree rings, ice cream cores, and varves only useful for occasions that occurred in the last couple of thousand years?Why was it so crucial for Darwin and also his followers to prove that the planet was really old?Why is it vital to use more than one technique to uncover the period of a rock or various other object?