absolute dating

Absolute dating.

Learn about this topic in these articles:

major reference.

Although relative ages can generally be established on a local scale, the events recorded in rocks from different locations can be integrated into a picture of regional or global scale only if their sequence in time is firmly established. The time that has…

European culture.

New methods of absolute dating, including radiocarbon dating, revolutionized the understanding of this phase in prehistoric Europe. They showed that many supposedly interdependent developments had in fact developed independently and been separated by centuries. The Metal Ages of Europe thus must be understood as indigenous local inventions and…

relative age.

Precise isotopic ages are called absolute ages, since they date the timing of events not relative to each other but as the time elapsed between a rock-forming event and the present. Absolute dating by means of uranium and lead isotopes has been improved to the point that for rocks 3…

Absolute dating.

Absolute dating is the process of determining a specific date for an archaeological or palaeontological site or artifact. Some archaeologists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies a certainty and precision that is rarely possible in archaeology. Absolute dating is usually based on the physical or chemical properties of the materials of artifacts, buildings, or other items that have been modified by humans. Absolute dates do not necessarily tell us when a particular cultural event happened, but when taken as part of the overall archaeological record they are invaluable in constructing a more specific sequence of events.

Absolute dating contrasts with the relative dating techniques employed, such as stratigraphy. Absolute dating provides a numerical age for the material tested, while relative dating can only provide a sequence of age.

Radiometric Techniques Edit.

Radiocarbon dating Edit.

One of the most widely used and well-known absolute dating techniques is carbon-14 (or radiocarbon) dating, which is used to date organic remains. This is a radiometric technique since it is based on radioactive decay. Carbon-14 is an unstable isotope of normal carbon, carbon-12. Cosmic radiation entering the earth’s atmosphere produces carbon-14, and plants take in carbon-14 as they fix carbon dioxide. Carbon-14 moves up the food chain as animals eat plants and as predators eat other animals. With death, the uptake of carbon-14 stops. Then this unstable isotope starts to decay into nitrogen-14. It takes 5,730 years for half the carbon-14 to change to nitrogen; this is the half-life of carbon-14. After another 5,730 years only one-quarter of the original carbon-14 will remain. After yet another 5,730 years only one-eighth will be left. By measuring the proportion of carbon-14 in organic material, scientists can determine the date of death of the organic matter in an artifact or ecofact.

Limitations Edit.

Because the half-life of carbon-14 is 5730 years carbon dating is only reliable about up to 60,000 years, radiocarbon is less useful to date some recent sites. See radiocarbon dating. This technique usually cannot pinpoint the date of a site better than historic records.

A further issue is known as the "old wood" problem. It is possible, particularly in dry, desert climates, for organic materials such as from dead trees to remain in their natural state for hundreds of years before people use them as firewood or building materials, after which they become part of the archaeological record. Thus dating that particular tree does not necessarily indicate when the fire burned or the structure was built. For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating. The development of accelerator mass spectrometry (AMS) dating, which allows a date to be obtained from a very small sample, has been very useful in this regard.

Potassium-argon dating Edit.

Other radiometric dating techniques are available for earlier periods. One of the most widely used is potassium-argon dating (K-Ar dating). Potassium-40 is a radioactive isotope of potassium that breaks down into argon-40, a gas. The half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14. With this method, the older the specimen, the more reliable the dating. Furthermore, whereas carbon-14 dating can be done only on organic remains, K-Ar dating can be used only for inorganic substances: rocks and minerals. As potassium-40 in rocks gradually breaks down into argon-40, the gas is trapped in the rock until the rock is heated intensely (as with volcanic activity), at which point it may escape. When the rock cools, the buildup of argon resumes. Dating is done by reheating the rock and measuring the escaping gas. The date received from this test is for the last time that the object was heated. Common dates tested are the firing of ceramics (archaeology), and the setting of rocks (geology).

Relative Vs. Absolute Dating: The Ultimate Face-off.

Our planet inherits a large number of artifacts and monuments bestowed upon us by older historic civilizations. These remains are subjected to dating techniques in order to predict their ages and trace their history. This ScienceStruck post enlists the differences between the absolute and relative dating methods.

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Our planet inherits a large number of artifacts and monuments bestowed upon us by older historic civilizations. These remains are subjected to dating techniques in order to predict their ages and trace their history. This ScienceStruck post enlists the differences between the absolute and relative dating methods.

Did You Know?

Although both relative and absolute dating methods are used to estimate the age of historical remains, the results produced by both these techniques for the same sample may be ambiguous.

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Geological specimens that are unearthed need to be assigned an appropriate age. To find their age, two major geological dating methods are used. These are called relative and absolute dating techniques. Absolute dating, also called numerical dating, arranges the historical remains in order of their ages. Whereas, relative dating arranges them in the geological order of their formation.

The relative dating techniques are very effective when it comes to radioactive isotope or radiocarbon dating. However, not all fossils or remains contain such elements. Relative techniques are of great help in such types of sediments.

Relative Dating Vs. Absolute Dating.

Relative Dating.

➤ It determines if an object/event is younger or older than another object/event from history. ➤ Relative dating is qualitative. ➤ This technique helps determine the relative age of the remains. ➤ It is less specific than absolute dating. ➤ Relative dating is comparatively less expensive and time-efficient. ➤ It works best for sedimentary rocks having layered arrangement of sediments.

The following are the major methods of relative dating.

Stratigraphy: The oldest dating method which studies the successive placement of layers. It is based on the concept that the lowest layer is the oldest and the topmost layer is the youngest.

Biostratigraphy: An extended version of stratigraphy where the faunal deposits are used to establish dating. Faunal deposits include remains and fossils of dead animals.

Cross dating: This method compares the age of remains or fossils found in a layer with the ones found in other layers. The comparison helps establish the relative age of these remains.

Fluorine dating: Bones from fossils absorb fluorine from the groundwater. The amount of fluorine absorbed indicates how long the fossil has been buried in the sediments.

Absolute Dating.

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➤ It determines the age of a rock/object using radiometric techniques. ➤ Absolute dating is quantitative. ➤ This technique helps determine the exact age of the remains. ➤ It is more specific than relative dating. ➤ Absolute dating is expensive and time-consuming. ➤ It works best for igneous and metamorphic rocks.

The following are the major methods of relative dating.

Radiometric dating: This technique solely depends on the traces of radioactive isotopes found in fossils. The rate of decay of these elements helps determine their age, and in turn the age of the rocks.

Amino acid dating: Physical structure of living beings depends on the protein content in their bodies. The changes in this content help determine the relative age of these fossils.

Dendrochronology: Each tree has growth rings in its trunk. This technique dates the time period during which these rings were formed.

Thermoluminescence: It determines the period during which certain object was last subjected to heat. It is based on the concept that heated objects absorb light, and emit electrons. The emissions are measured to compute the age.

Differentiation Using a Venn Diagram.

A Venn diagram depicts both dating methods as two individual sets. The area of intersection of both sets depicts the functions common to both. Take a look at the diagram to understand their common functions.

When we observe the intersection in this diagram depicting these two dating techniques, we can conclude that they both have two things in common:

1.Provide an idea of the sequence in which events have occurred. 2.Determine the age of fossils, rocks, or ancient monuments.

Although absolute dating methods determine the accurate age compared to the relative methods, both are good in their own ways.

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Absolute dating.

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 .

Radiometric dating.

Most absolute dates for rocks are obtained with radiometric methods. These use radioactive minerals in rocks as geological clocks.

The atoms of some chemical elements have different forms, called isotopes. These break down over time in a process scientists call radioactive decay. Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter. Each isotope is identified with what is called a ‘mass number’. When ‘parent’ uranium-238 decays, for example, it produces subatomic particles, energy and ‘daughter’ lead-206.

Isotopes are important to geologists because each radioactive element decays at a constant rate, which is unique to that element. These rates of decay are known, so if you can measure the proportion of parent and daughter isotopes in rocks now, you can calculate when the rocks were formed.

Because of their unique decay rates, different elements are used for dating different age ranges. For example, the decay of potassium-40 to argon-40 is used to date rocks older than 20,000 years, and the decay of uranium-238 to lead-206 is used for rocks older than 1 million years.

Radiocarbon dating measures radioactive isotopes in once-living organic material instead of rock, using the decay of carbon-14 to nitrogen-14. Because of the fairly fast decay rate of carbon-14, it can only be used on material up to about 60,000 years old. Geologists use radiocarbon to date such materials as wood and pollen trapped in sediment, which indicates the date of the sediment itself.

The table below shows characteristics of some common radiometric dating methods. Geologists choose a dating method that suits the materials available in their rocks. There are over 30 radiometric methods available.

Dating method.

Material dated.

Age range dated.

Carbon-14 to nitrogen-14 (radiocarbon)

Organic remains, archaeological artefacts.

Up to 60,000 years ago.

Tephra, loess, lake sediments.

Up to 100,000 years ago.

10,000 to 400 million years ago.

Potassium-40 to argon-40.

20,000 to 4.5 billion years ago.

Uranium-238 to lead-206.

1 million to 4.5 billion years ago.

All radiometric dating methods measure isotopes in some way. Most directly measure the amount of isotopes in rocks, using a mass spectrometer. Others measure the subatomic particles that are emitted as an isotope decays. Some measure the decay of isotopes more indirectly. For example, fission track dating measures the microscopic marks left in crystals by subatomic particles from decaying isotopes. Another example is luminescence dating, which measures the energy from radioactive decay that is trapped inside nearby crystals.

Difference Between Absolute and Relative Dating.

The main difference between absolute and relative dating is that the absolute dating is a technique to determine the numerical age of a rock or a fossil whereas the relative dating is a technique that determines the relative age. Furthermore, absolute dating can be done with the use of radiometric dating while relative age is determined with respect to other layers.

Absolute dating and relative dating are two techniques used in geology to evaluate the age and the period of a fossil or rock.

Key Areas Covered.

Key Terms.

Absolute Dating, Amino Acid Dating, Dendrochronology, Methods of Dating, Numerical Dating, Radiometric Dating, Relative Dating, Thermoluminescence.

What is Absolute Dating.

In geology, absolute dating is a technique that determines the exact numerical age of a historical remaining. Since it evaluates the exact age of the sample, absolute ageing is also called numerical dating . The four techniques used in absolute dating are radiometric dating, amino acid dating, dendrochronology, and thermoluminescence.

Radiometric dating : It determines the age of the sample by measuring the amount of a particular radioactive isotope present in the sample. The age can be determined by the rate of decay of that particular isotope. The type of radioactive isotope used depends on the type of sample. One of the most popular and widely used types of radioactive isotope in this type of techniques is the carbon-14.

Figure 1: Radiocarbon Date Calibration Curve.

What is Relative Dating.

Relative dating is the technique used to determine the age by comparing the historical remaining to the nearby layers. It is a less advanced technique when compared to absolute dating. Some methods used in relative dating are stratigraphy, biostratigraphy, and cross dating.

Stratigraphy : This technique assumes that the lowest layer is the oldest while the topmost layer is the youngest layer. It is one of the oldest methods of relative dating.

Figure 2: Igneous Rock Layers.

Similarities Between Absolute and Relative Dating.

Absolute and relative dating are the two types of techniques used to determine the age of a historical remaining. Both techniques help to understand the order of formation of the historical remaining.

Difference Between Absolute and Relative Dating.

Definition.

The absolute dating refers to a technique used to determine the exact age of the artefact or a site using methods such as carbon dating while relative dating refers to a technique used to determine which object or item is older in comparison to the other one.

Significance.

Absolute dating determines the numerical age while relative dating arranges the fossils in an order.

Methods.

The four methods involved in absolute dating are radiometric dating, amino acid dating, dendrochronology, and thermoluminescence while biostratigraphy, stratigraphy, and cross dating are involved in the relative dating.

Precision.

The precision in absolute ageing is high while the precision of the relative ageing is low.

Quantitative/Qualitative.

Absolute age is a quantitative measurement while relative age is a qualitative measurement.

Work Better for.

Absolute dating works better for igneous and metamorphic rocks while relative dating works better for sedimentary rocks having layered arrangement of sediments.

Cost and Time.

Absolute dating is expensive and takes time while relative dating is less-expensive and efficient.

Conclusion.

Absolute dating is the technique that determines the exact age of a historical remaining while relative dating gives the order of age of several samples. Therefore, absolute dating is a quantitative measurement while relative dating is a qualitative measurement. The main difference between absolute and relative dating is the precision of the measurement.

Reference:

1. “Absolute Dating.” Science Learning Hub, Available Here 2. “Relative Dating.” Science Learning Hub, Available Here.

Image Courtesy:

1. “Radiocarbon Date Calibration Curve” By HowardMorland – Own work based on information from: Reimer, P.J., et al. C.E. (2004). “IntCal04 Terrestrial radiocarbon age calibration”. Radiocarbon 46: 1029-58. (CC BY-SA 3.0) via Commons Wikimedia 2. “Relative dating of fossils” By Jillcurie – Own work (CC BY-SA 3.0) via Commons Wikimedia.

About the Author: Lakna.

Lakna, a graduate in Molecular Biology & Biochemistry, is a Molecular Biologist and has a broad and keen interest in the discovery of nature related things.

Absolute dating.

Absolute dating is the process of determining an age on a specified time scale in archaeology and geology. Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty of accuracy. [1] [2] Absolute dating provides a numerical age or range in contrast with relative dating which places events in order without any measure of the age between events.

In archeology, absolute dating is usually based on the physical, chemical, and life properties of the materials of artifacts, buildings, or other items that have been modified by humans and by historical associations with materials with known dates (coins and written history). Techniques include tree rings in timbers, radiocarbon dating of wood or bones, and trapped charge dating methods such as thermoluminescence dating of glazed ceramics. [3] Coins found in excavations may have their production date written on them, or there may be written records describing the coin and when it was used, allowing the site to be associated with a particular calendar year.

In historical geology, the primary methods of absolute dating involve using the radioactive decay of elements trapped in rocks or minerals, including isotope systems from very young (radiocarbon dating with 14 C ) to systems such as uranium-lead dating that allow acquisition of absolute ages for some of the oldest rocks on earth.

Contents.

1 Radiometric techniques 1.1 Radiocarbon dating 1.1.1 Limitations 1.2 Potassium-argon dating 2 Luminescence dating 2.1 Thermoluminescence 2.2 Limitations 2.3 Optically stimulated luminescence (OSL) 3 Dendrochronology 4 Amino acid dating 5 See also 6 References 7 Further reading.

Radiometric techniques.

Radiometric dating is based on the known and constant rate of decay of radioactive isotopes into their radiogenic daughter isotopes. Particular isotopes are suitable for different applications due to the type of atoms present in the mineral or other material and its approximate age. For example, techniques based on isotopes with half lives in the thousands of years, such as Carbon-14, cannot be used to date materials that have ages on the order of billions of years, as the detectable amounts of the radioactive atoms and their decayed daughter isotopes will be too small to measure within the uncertainty of the instruments.

Radiocarbon dating.

One of the most widely used and well-known absolute dating techniques is carbon-14 (or radiocarbon) dating, which is used to date organic remains. This is a radiometric technique since it is based on radioactive decay. Cosmic radiation entering the earth’s atmosphere produces carbon-14, and plants take in carbon-14 as they fix carbon dioxide. Carbon-14 moves up the food chain as animals eat plants and as predators eat other animals. With death, the uptake of carbon-14 stops. It takes 5,730 years for half the carbon-14 to change to nitrogen; this is the half-life of carbon-14. After another 5,730 years only one-quarter of the original carbon-14 will remain. After yet another 5,730 years only one-eighth will be left. By measuring the carbon-14 in organic material, scientists can determine the date of death of the organic matter in an artifact or ecofact.

Limitations.

The relatively short half-life of carbon-14, 5730 years, makes the reliable only up to about 75,000 years. The technique often cannot pinpoint the date of an archeological site better than historic records, but is highly effective for precise dates when calibrated with other dating techniques such as tree-ring dating.

An additional problem with carbon-14 dates from archeological sites is known as the "old wood" problem. It is possible, particularly in dry, desert climates, for organic materials such as from dead trees to remain in their natural state for hundreds of years before people use them as firewood or building materials, after which they become part of the archaeological record. Thus dating that particular tree does not necessarily indicate when the fire burned or the structure was built. For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating. The development of accelerator mass spectrometry (AMS) dating, which allows a date to be obtained from a very small sample, has been very useful in this regard.

Potassium-argon dating.

Other radiometric dating techniques are available for earlier periods. One of the most widely used is potassium-argon dating (K-Ar dating). Potassium-40 is a radioactive isotope of potassium that decays into argon-40. The half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14, allowing much older samples to be dated. Potassium is common in rocks and minerals, allowing many samples of geochronological or archeological interest to be dated. Argon, a noble gas, is not commonly incorporated into such samples except when produced in situ through radioactive decay. The date measured reveals the last time that the object was heated past the closure temperature at which the trapped argon can escape the lattice. K-Ar dating was used to calibrate the geomagnetic polarity time scale.

Luminescence dating.

Thermoluminescence.

Thermoluminescence testing also dates items to the last time they were heated. This technique is based on the principle that all objects absorb radiation from the environment. This process frees electrons within minerals that remain caught within the item. Heating an item to 500 degrees Celsius or higher releases the trapped electrons, producing light. This light can be measured to determine the last time the item was heated.

Limitations.

Radiation levels do not remain constant over time. Fluctuating levels can skew results – for example, if an item went through several high radiation eras, thermoluminescence will return an older date for the item. Many factors can spoil the sample before testing as well, exposing the sample to heat or direct light may cause some of the electrons to dissipate, causing the item to date younger. Because of these and other factors, Thermoluminescence is at the most about 15% accurate. It cannot be used to accurately date a site on its own. However, it can be used to confirm the antiquity of an item.

Optically stimulated luminescence (OSL)

Optically stimulated luminescence (OSL) dating, a type of optical dating, constrains the time at which sediment was last exposed to light. During sediment transport, exposure to sunlight 'zeros' the luminescence signal. Upon burial, the sediment accumulates a luminescence signal as natural ambient radiation gradually ionises the mineral grains. Careful sampling under dark conditions allows the sediment to be exposed to artificial light in the laboratory which releases the OSL signal. The amount of luminescence released is used to calculate the equivalent dose (De) that the sediment has acquired since deposition, which can be used in combination with the dose rate (Dr) to calculate the age.

Dendrochronology.

Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree rings , also known as growth rings . Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year. This has three main areas of application: paleoecology, where it is used to determine certain aspects of past ecologies (most prominently climate); archaeology, where it is used to date old buildings, etc.; and radiocarbon dating, where it is used to calibrate radiocarbon ages (see below).

In some areas of the world, it is possible to date wood back a few thousand years, or even many thousands. Currently, the maximum for fully anchored chronologies is a little over 11,000 years from present. [4]

Amino acid dating.

Amino acid dating is a dating technique [5] [6] [7] [8] [9] used to estimate the age of a specimen in paleobiology, archaeology, forensic science, taphonomy, sedimentary geology and other fields. This technique relates changes in amino acid molecules to the time elapsed since they were formed. All biological tissues contain amino acids. All amino acids except glycine (the simplest one) are optically active, having an asymmetric carbon atom. This means that the amino acid can have two different configurations, "D" or "L" which are mirror images of each other. With a few important exceptions, living organisms keep all their amino acids in the "L" configuration. When an organism dies, control over the configuration of the amino acids ceases, and the ratio of D to L moves from a value near 0 towards an equilibrium value near 1, a process called racemization. Thus, measuring the ratio of D to L in a sample enables one to estimate how long ago the specimen died. [10]

relative vs absolute dating.

Our planet inherits a large number of artifacts and monuments bestowed upon us by older historic civilizations. These remains are subjected to dating techniques in order to predict their ages and trace their history. This Buzzle article enlists the differences between the absolute and relative dating methods.

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2. Absolute age dating.

Chapter contents:

Absolute age dating deals with assigning actual dates (in years before the present) to geological events. Contrast this with relative age dating, which instead is concerned with determining the orders of events in Earth's past. The science of absolute age dating is known as geochronology and the fundamental method of geochronology is called radiometric dating .

Scholars and naturalists, understandably, have long been interested in knowing the absolute age of the Earth, as well as other important geological events. In 1650, Archbishop James Ussher famously used the genealogy of the Old Testament of the Bible (e.g., Genesis, Chapter 5)--and the human lifespans recorded in it--to estimate the age of the Earth; he concluded that the Earth was young in age, having formed in 4004 B.C., or about 6,000 years ago.

Archbishop James Ussher ( 1581-1656 ) (public domain; WikiMedia Commons).

In the 1800's, practitioners of the young science of geology applied the uniformitarian views of Hutton and Lyell (see the introduction to this chapter) to try to determine the age of the Earth. For example, some geologists observed how long it took for a given amount of sediment (say, a centimeter of sand) to accumulate in a modern habitat, then applied this rate to the total known thickness of sedimentary rocks. When they did this, they estimated that the Earth is many millions of years old.

We now know that this estimate is far, far too young*. But, unlike Ussher's calculation, this estimate was on the order of millions of years, rather than 6,000. Geologists were beginning to accept the views of Hutton that the Earth is unimaginably ancient. [*In part, this estimate is so low because these early geologists did not recognize that unconformities--which represent missing units of time, often caused by erosion--are rampant in the rock record, as well as the fact that some metamorphic rocks were once sedimentary, and thus left out of their calculations.]

What key discovery, then, allowed geologists to begin assigning absolute age dates to rocks and ultimately discover the age of the Earth? The answer is radioactivity.

Radiometric dating.

Hypotheses of absolute ages of rocks (as well as the events that they represent) are determined from rates of radioactive decay of some isotopes of elements that occur naturally in rocks.

Elements and isotopes.

In chemistry, an element is a particular kind of atom that is defined by the number of protons that it has in its nucleus. The number of protons equals the element's atomic number. Have a look at the periodic table of the elements below. Carbon's (C) atomic number is 6 because it has six protons in its nucleus; gold's (Au) atomic number is 79 because it has 79 atoms in its nucleus.

Periodic table of the elements. Image by Sandbh - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=55162370.

Even though individual elements always have the same number of protons, the number of neutrons in their nuclei sometimes varies. These variations are called isotopes . Isotopes of individual elements are defined by their mass number , which is simply the number of protons + the number of neutrons.

Consider, for example, the three different isotopes of Carbon:

Carbon-12: 6 protons, 6 neutrons Carbon-13: 6 protons, 7 neutrons Carbon-14: 6 protons, 8 neutrons.

Most isotopes are stable, meaning that they do not change. Some isotopes are unstable, however, and undergo radioactive decay.

Radioactive decay.

Radioactive decay involves unstable isotopes shedding energy in the form of radiation, causing their numbers of protons and neutrons to change, in turn resulting in one element changing into another.

As a matter of convention, we call the atomic nucleus that undergoes radioactive decay the parent and the resulting product the daughter product (or, decay product).

The rate at which a particular parent isotope decays into its daughter product is constant. This rate is determined in a laboratory setting and is typically represented by its half-life . A half-life is the amount of time needed for half of the parent atoms in a sample to be changed into daughter products. This is illustrated in the chart below.

Relationship between the amount of radioactive parent atoms in a sample relative to the number of daughter atoms over the passage of time, measured in half-lives. Image by Jonathan R. Hendricks. This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

At the start time (zero half-lives passed), the sample consists of 100% parent atoms (blue diamonds); there are no daughter products (red squares) because no time has passed. After the passage of one half-life, 50% of the parent atoms have become daughter products. After two half-lives, 75% of the original parent atoms have been transformed into daughter products (thus, only 25% of the original parent atoms remain). After three half-lives, only 12.5% of the original parent atoms remain. As more half-lives pass, the number of parent atoms remaining approaches zero.

Based on this principle, geologists can count the number of parent atoms relative to daughter products in a sample to determine how many half-lives have passed since a mineral grain first formed. Consider the example shown below.

An example of how the initial number of radioactive parent atoms (blue diamonds) in two mineral grains (gray hexagons) changes over time (measured in half-lives) relative to the number of daughter products (red squares). Image by Jonathan R. Hendricks. This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

The left-most box in the figure above represents an initial state, with parent atoms distributed throughout molten rock (magma). As the magma cools, grains of different minerals begin to crystalize. Some of these minerals (represented above as gray hexagons) incorporate the radioactive parent atoms (blue diamonds) into their crystalline structures; this marks the initiation of the "half-life clock" (i.e., the start time, or time zero). After one half-life has passed, half (50%, or four) of the parent atoms in each mineral grain have been transformed into their daughter products (red squares). After two half-lives have passed, 75% (six) of the original parent atoms in each grain have been transformed into daughter products. How many parent atoms would remain if three half-lives passed?

Calculating radiometric dates.

By counting the numbers of parent atoms remaining in a sample relative to the number originally present, it is possible to determine the number of half-lives that have passed since the initial formation of a mineral grain (that is, when it became a "closed system" that prevented parent and daughter atoms from escaping). You might be wondering how it is possible to know the number of parent atoms that were originally in a sample. This number is attained by simply adding the number of parent and daughter atoms currently in the sample (because each daughter atom was once a parent atom).

The next step in radiometric dating involves converting the number of half-lives that have passed into an absolute (i.e., actual) age. This is done by multiplying the number of half-lives that have passed by the half-life decay constant of the parent atom (again, this value is determined in a laboratory).

To summarize, the key piece of information that needs to be determined from a mineral specimen in order to determine its absolute age is its age in number of half lives.

This can be mathematically determined by solving for y in this equation:

where N p = the number of parent atoms currently in the sample, N 0 = number of parent atoms present in the sample when the system became closed (so, N 0 = N p + N d , where N d = the number of daughter atoms currently in the sample), λ = the decay constant, which for half-life is 0.5, and y = the number of half-lives that have passed.

Let's work through a hypothetical example problem. Suppose you analyzed a mineral sample and found that it contained 33,278 parent atoms and 14,382 daughter atoms. Further, suppose that the half-life of the parent atom is 2.7 million years. How old is the mineral sample?

First, we know that: N p = 33,278 ; N 0 = N p + N d = 33,278 + 14,382 = 47,660 ; and that λ = 0.5 . So,

33,278 / 47,660 = (1 - 0.5 ) y.

log 0.698 = y * log 0.5.

log 0.698 / log 0.5 = y.

So, we conclude that 0.518 half-lives have passed since the formation of this mineral sample. To determine the absolute age of this mineral sample, we simply multiply y (= 0.518 ) times the half life of the parent atom (=2.7 million years).

Thus, the absolute age of sample = y * half-life = 0.518 * 2.7 million years = 1.40 million years.

As noted above, a radiometric date tells us when a system became closed, for example when a mineral containing radioactive parent elements first crystalized. An individual mineral grain may have a long history after it first forms. For example, it may erode out of an igneous rock and then be transported long distances and over long periods of time before it is finally deposited, becoming one grain among billions in a layer of sedimentary rock (e.g., sandstone). If a radiometric date were to be attained from this mineral grain, it would tell us when the mineral first formed, but not when the sedimentary rock formed (it would, however, tell us the maximum possible age of the sedimentary rock layer).

Further, heating mineral grains to great temperatures can cause them to leak parent and daughter material, resetting their radiometric clocks. This can be a concern when calculating radiometric dates from samples of metamorphic rocks, which are sedimentary or igneous rocks that have been altered by great amounts of heat and/or pressure. The melting involved with metamorphic change can reset the radiometric clock. For example, suppose an igneous rock formed 2.0 billion years ago. If it were subjected to metamorphism 1.2 billion years ago, radiometric dating would tell us that a sample from the rock is 1.2 billion years old, not 2.0 billion years old.

Variation in half-lives among different isotopes.

As noted above, the rate at which a given radioactive isotope decays into its daughter product is constant. This rate, however, varies considerably among different radioactive isotopes. Further, many radioactive isotopes undergo a series of transformations--some of which have half-lives that persist for only very short amounts of time--before they are converted into their final daughter products.

Below are some of the decay series that are commonly used in radiometric dating of geological samples. Note the great variations in their half-lives.

Parent isotope Final decay product Half-life Uranium-238 Lead-206 4.47 billion years Uranium-235 Lead-207 710 million years Potassium-40 Argon-40 1.25 billion years Rubidium-87 Strontium-87 50 billion years Carbon-14 Nitrogen-14 5,730 years.

Note that the half-life for the rubidium-87 to strontium-87 series is 50 billion years! Since the entire universe is 13.8 billion years old, we know that not enough time has passed for even half (i.e., one half-life) of the universe's supply of rubidium-87 to decay into strontium-87.

At the other end of the spectrum, note the very short half-life of carbon-14: 5,730 years. The is the isotope that is used in "carbon dating." Carbon-14 forms in Earth's upper atmosphere. Both it and carbon-12 (which is stable, meaning that it does not undergo radioactive decay) are incorporated into the tissues of plants as they grow. After a plant dies, the carbon-12 in its tissues remains stable, but the carbon-14 decays into nitrogen-14. The ratio of carbon-14 relative to carbon-12 in a sample, therefore, may be used to determine the age of organic matter derived from plant tissues. Because of its short half-life, carbon-14 can only be used to date materials that are up to about 70,000 years old (beyond this point, the amount of carbon-14 remaining becomes so small that it is difficult to measure). Because of its precision, it is nevertheless very useful for dating organic matter from the near recent geological past, especially archeological materials from the Holocene epoch.

Age of the Earth.

At the beginning of this chapter, you learned that the Earth is 4.54 billion years old. As it turns out, the oldest dated mineral--a grain of zircon from the Jack Hills of Western Australia--is 4.4 billion years old and the oldest known rock unit--the Acasta Gneiss from the Northwest Territories of Canada--is 4.0 billion years old.

A single grain of zircon, imaged using a scanning electron microscope. Image by Gunnar Ries, Creative Commons BY-SA 2.5.

A sample of 4.0 billion year old Acasta Gneiss from the Northwest Territories of Canada. Image by Mike Beauregard, Wikimedia Commons, Creative Commons Attribution 2.0 Generic license.

If the oldest mineral grain is 4.4 Ga and the oldest rock 4.0 Ga, how then do we know that the Earth is 4.54 Ga? The answer is radiometric dating of meteorite specimens, which we presume to have formed around the same time as the Earth, Sun, and other planetary bodies in our solar system. One such dated meteorite comes from Meteor Crater in Arizona.

The Holsinger Meteorite, which is a piece of the meteor that crashed in ancient Arizona, forming Meteor Crater. Samples from this meteor were used by Clair Patterson to determine the age of the Earth. Image by Marcin Wichary - originally posted to Flickr as The biggest discovered fragment, CC BY 2.0, Link.

Difference Between Relative and Absolute Dating.

June 1, 2011 Posted by Olivia.

Relative vs Absolute Dating.

Dating is a technique used in archeology to ascertain the age of artifacts, fossils and other items considered to be valuable by archeologists. There are many methods employed by these scientists, interested in the old, to get to know the age of items. It is possible to tell the number of years ago a particular rock or archeological site had been formed. Two broad categories of classification methods are relative dating and absolute dating. Though using similar methods, these two techniques differ in certain ways that will be discussed in this article.

As the name implies, relative dating can tell which of the two artifacts is older. This is a method that does not find the age in years but is an effective technique to compare the ages of two or more artifacts, rocks or even sites. It implies that relative dating cannot say conclusively about the true age of an artifact. Absolute dating, on the other hand is capable of telling the exact age of an item using carbon dating and many other techniques that were not there in earlier times.

Relative dating makes use of the common sense principle that in a deposition of layers. A layer that is higher is of later age than a layer that is lower in order. This means that the oldest are the strata that are lying at the bottom. However, age of deposition does not mean the age of artifacts found in that layer. Artifacts found in a layer can be compared with other items found in layers of similar age and placed in order. However, archeologists still require further information to find out the items that are oldest and those that are youngest in the order.

It is left for absolute dating to come up with the precise age of an artifact. This type of dating employs many dating techniques like atomic clocks, carbon dating, annual cycle methods, and trapped electron method. Dendrochronology is another of the popular method of finding the exact age through growth and patterns of thick and thin ring formation in fossil trees. It is clear then that absolute dating is based upon physical and chemical properties of artifacts that provide a clue regarding the true age. This is possible because properties of rock formations are closely associated with the age of the artifacts found trapped within them.

The most popular method of radio dating is radio carbon dating which is possible because of the presence of C-14, an unstable isotope of carbon. C-14 has a half life of 5730 years which means that only half of the original amount is left in the fossil after 5730 years while half of the remaining amount is left after another 5730 years. This gives away the true age of the fossil that contains C-14 that starts decaying after the death of the human being or animal.

In brief:

Relative Dating vs. Absolute Dating.

• Dating techniques are used in archeology to ascertain the age of old artifacts and a broad classification of these methods bifurcates them in relative dating and absolute dating.

• Relative dating comes to a conclusion based upon the study of layer formation of rocks. Upper most layers are considered the youngest while the lowermost deposition is considered as oldest.

• Relative dating does not tell the exact age, it can only compare items as younger and older.

• Absolute dating techniques can tell the exact age of an artifact by employing various techniques, the most popular being C-14 dating.