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In addition to being tilted horizontally, the layers have been faulted dashed lines on figure. Applying the principle of cross-cutting relationships, this fault that offsets the layers of rock must have occurred after the strata were deposited.

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The principles of original horizontality, superposition, and cross-cutting relationships allow events to be ordered at a single location. However, they do not reveal the relative ages of rocks preserved in two different areas. In this case, fossils can be useful tools for understanding the relative ages of rocks. Each fossil species reflects a unique period of time in Earth's history.

There are two main methods determining a fossils age, relative dating and absolute dating. Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages. Absolute dating is used to determine a precise age of a fossil by using radiometric dating to measure the decay of isotopes, either within the fossil or more often the rocks associated with it. Dating dinosaurs and other fossils Fossils themselves, and the sedimentary rocks they are found in, are very difficult to date directly. Instead, other methods are used to work out a fossil's age. These include radiometric dating of volcanic layers above or below the fossils or by comparisons to similar rocks and fossils of known ages. Relative age dating tells us which fossils are older and which fossils are younger. It does not tell us the age of the fossils. To get an age in years, we use radiometric dating of the rocks. Not every rock can be dated this way, but volcanic ash deposits are among those that can be dated.

The principle of faunal succession states that different fossil species always appear and disappear in the same order, and that once a fossil species goes extinct, it disappears and cannot reappear in younger rocks Figure 4.

Fossils occur for a distinct, limited interval of time.

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In the figure, that distinct age range for each fossil species is indicated by the grey arrows underlying the picture of each fossil. The position of the lower arrowhead indicates the first occurrence of the fossil and the upper arrowhead indicates its last occurrence - when it went extinct. Using the overlapping age ranges of multiple fossils, it is possible to determine the relative age of the fossil species i.

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For example, there is a specific interval of time, indicated by the red box, during which both the blue ammonite and orange ammonite co-existed. If both the blue and orange ammonites are found together, the rock must have been deposited during the time interval indicated by the red box, which represents the time during which both fossil species co-existed. In this figure, the unknown fossil, a red sponge, occurs with five other fossils in fossil assemblage B.

Fossil assemblage B includes the index fossils the orange ammonite and the blue ammonite, meaning that assemblage B must have been deposited during the interval of time indicated by the red box. Because, the unknown fossil, the red sponge, was found with the fossils in fossil assemblage B it also must have existed during the interval of time indicated by the red box. Fossil species that are used to distinguish one layer from another are called index fossils.

Index fossils occur for a limited interval of time. Usually index fossils are fossil organisms that are common, easily identified, and found across a large area.

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Because they are often rare, primate fossils are not usually good index fossils. Organisms like pigs and rodents are more typically used because they are more common, widely distributed, and evolve relatively rapidly. Using the principle of faunal succession, if an unidentified fossil is found in the same rock layer as an index fossil, the two species must have existed during the same period of time Figure 4.

If the same index fossil is found in different areas, the strata in each area were likely deposited at the same time. Thus, the principle of faunal succession makes it possible to determine the relative age of unknown fossils and correlate fossil sites across large discontinuous areas. All elements contain protons and neutronslocated in the atomic nucleusand electrons that orbit around the nucleus Figure 5a.

In each element, the number of protons is constant while the number of neutrons and electrons can vary. Atoms of the same element but with different number of neutrons are called isotopes of that element. Each isotope is identified by its atomic masswhich is the number of protons plus neutrons. For example, the element carbon has six protons, but can have six, seven, or eight neutrons.

Thus, carbon has three isotopes: carbon 12 12 Ccarbon 13 13 Cand carbon 14 14 C Figure 5a. C 12 and C 13 are stable. The atomic nucleus in C 14 is unstable making the isotope radioactive. Because it is unstable, occasionally C 14 undergoes radioactive decay to become stable nitrogen N The amount of time it takes for half of the parent isotopes to decay into daughter isotopes is known as the half-life of the radioactive isotope.

Most isotopes found on Earth are generally stable and do not change. However some isotopes, like 14 C, have an unstable nucleus and are radioactive. This means that occasionally the unstable isotope will change its number of protons, neutrons, or both.

This change is called radioactive decay. For example, unstable 14 C transforms to stable nitrogen 14 N. The atomic nucleus that decays is called the parent isotope.

The product of the decay is called the daughter isotope.

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In the example, 14 C is the parent and 14 N is the daughter. Some minerals in rocks and organic matter e. The abundances of parent and daughter isotopes in a sample can be measured and used to determine their age. This method is known as radiometric dating. Some commonly used dating methods are summarized in Table 1.

Relative Dating

The rate of decay for many radioactive isotopes has been measured and does not change over time. Thus, each radioactive isotope has been decaying at the same rate since it was formed, ticking along regularly like a clock. For example, when potassium is incorporated into a mineral that forms when lava cools, there is no argon from previous decay argon, a gas, escapes into the atmosphere while the lava is still molten.

When that mineral forms and the rock cools enough that argon can no longer escape, the "radiometric clock" starts. Over time, the radioactive isotope of potassium decays slowly into stable argon, which accumulates in the mineral. The amount of time that it takes for half of the parent isotope to decay into daughter isotopes is called the half-life of an isotope Figure 5b. When the quantities of the parent and daughter isotopes are equal, one half-life has occurred.

If the half life of an isotope is known, the abundance of the parent and daughter isotopes can be measured and the amount of time that has elapsed since the "radiometric clock" started can be calculated. For example, if the measured abundance of 14 C and 14 N in a bone are equal, one half-life has passed and the bone is 5, years old an amount equal to the half-life of 14 C.

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If there is three times less 14 C than 14 N in the bone, two half lives have passed and the sample is 11, years old. However, if the bone is 70, years or older the amount of 14 C left in the bone will be too small to measure accurately. Thus, radiocarbon dating is only useful for measuring things that were formed in the relatively recent geologic past. Luckily, there are methods, such as the commonly used potassium-argon K-Ar metho that allows dating of materials that are beyond the limit of radiocarbon dating Table 1.

Comparison of commonly used dating methods.

Jun 12,   Scientists called geochronologists are experts in dating rocks and fossils, and can often date fossils younger than around 50, years old using radiocarbon dating. This . Relative dating to determine the age of rocks and fossils Geologists have established a set of principles that can be applied to sedimentary and volcanic rocks that are exposed at the Earth's. Fossils above a specific layer are inferred to be younger than that layer, and those below are older, in line with the law of superposition, a key scientific principle of stratigraphy. Dating of the fossils contributes to a clearer timeline of evolutionary history.

Radiation, which is a byproduct of radioactive decay, causes electrons to dislodge from their normal position in atoms and become trapped in imperfections in the crystal structure of the material.

Dating methods like thermoluminescenceoptical stimulating luminescence and electron spin resonancemeasure the accumulation of electrons in these imperfections, or "traps," in the crystal structure of the material. If the amount of radiation to which an object is exposed remains constant, the amount of electrons trapped in the imperfections in the crystal structure of the material will be proportional to the age of the material.

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These methods are applicable to materials that are up to aboutyears old. However, once rocks or fossils become much older than that, all of the "traps" in the crystal structures become full and no more electrons can accumulate, even if they are dislodged. The Earth is like a gigantic magnet. It has a magnetic north and south pole and its magnetic field is everywhere Figure 6a.

Just as the magnetic needle in a compass will point toward magnetic north, small magnetic minerals that occur naturally in rocks point toward magnetic north, approximately parallel to the Earth's magnetic field. Because of this, magnetic minerals in rocks are excellent recorders of the orientation, or polarityof the Earth's magnetic field.

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Small magnetic grains in rocks will orient themselves to be parallel to the direction of the magnetic field pointing towards the north pole. Black bands indicate times of normal polarity and white bands indicate times of reversed polarity. Through geologic time, the polarity of the Earth's magnetic field has switched, causing reversals in polarity.

The Earth's magnetic field is generated by electrical currents that are produced by convection in the Earth's core. During magnetic reversals, there are probably changes in convection in the Earth's core leading to changes in the magnetic field.

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The Earth's magnetic field has reversed many times during its history. When the magnetic north pole is close to the geographic north pole as it is todayit is called normal polarity.

Reversed polarity is when the magnetic "north" is near the geographic south pole. Using radiometric dates and measurements of the ancient magnetic polarity in volcanic and sedimentary rocks termed paleomagnetismgeologists have been able to determine precisely when magnetic reversals occurred in the past.

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Combined observations of this type have led to the development of the geomagnetic polarity time scale GPTS Figure 6b. The GPTS is divided into periods of normal polarity and reversed polarity. Geologists can measure the paleomagnetism of rocks at a site to reveal its record of ancient magnetic reversals. Every reversal looks the same in the rock record, so other lines of evidence are needed to correlate the site to the GPTS.

Information such as index fossils or radiometric dates can be used to correlate a particular paleomagnetic reversal to a known reversal in the GPTS. Once one reversal has been related to the GPTS, the numerical age of the entire sequence can be determined.

Relative dating of rocks and fossils

Using a variety of methods, geologists are able to determine the age of geological materials to answer the question: "how old is this fossil? These methods use the principles of stratigraphy to place events recorded in rocks from oldest to youngest.

Absolute dating methods determine how much time has passed since rocks formed by measuring the radioactive decay of isotopes or the effects of radiation on the crystal structure of minerals.

Paleomagnetism measures the ancient orientation of the Earth's magnetic field to help determine the age of rocks. Deino, A. Evolutionary Anthropology 6 : Faure, G. Isotopes: Principles and Applications. Third Edition. New York: John Wiley and Sons Gradstein, F. The Geologic Time Scale2-volume set. Where possible, several different methods are used and each method is repeated to confirm the results obtained and improve accuracy.

Different methods have their own limitations, especially with regard to the age range they can measure and the substances they can date. A common problem with any dating method is that a sample may be contaminated with older or younger material and give a false age. This problem is now reduced by the careful collection of samples, rigorous crosschecking and the use of newer techniques that can date minute samples.

Sedimentary rocks are rarely useful for dating because they are made up of bits of older rocks. Uranium is present in many different rocks and minerals, usually in the form of uranium This form of uranium usually decays into a stable lead isotope but the uranium atoms can also split - a process known as fission. During this process the pieces of the atom move apart at high speed, causing damage to the rock or mineral. This damage is in the form of tiny marks called fission tracks.

When volcanic rocks and minerals are formed, they do not contain fission tracks.

absolute ages of most sedimentary rocks, and their contained fossils, are established indirectly geologists rely on radiometric dating of igneous dikes, lava flows and ash beds, as well as metamorphic dates associated with the sedimentary strata to the age of the rocks. The youngest fossils range in age from 10, years to the oldest fossils: cyanobacteria from Archaean rocks of Western Australia, dated billion years old! The observation that certain fossils were systematically associated with certain rock strata led early geologists to recognize a geological timescale in the 19th century. Day 1, and fossils from oldest dating is underlain by nicolas steno, age of another layer 2 and fossils to determine a rock strata. Index fossils frank k. Absolute dating, feature, a time scale, but it can establish whether one on the principles are descriptions of million years old is provided for comparison.

The number of tracks increases over time at a rate that depends on the uranium content. It is possible to calculate the age of a sample by measuring the uranium content and the density of the fission tracks. The age of volcanic rocks and ash can be determined by measuring the proportions of argon in the form of argon and radioactive potassium within them. Each volcanic eruption produces a new deposit of ash and rock.

Fossils and other objects that accumulate between these eruptions lie between two different layers of volcanic ash and rock.

An object can be given an approximate date by dating the volcanic layers occurring above and below the object. Argon is gas that gradually builds up within rocks from the decay of radioactive potassium.

The heat from a volcanic eruption releases all the argon from the molten rock and disperses it into the atmosphere. Argon then starts to re-accumulate at a constant rate in the newly formed rock that is created after the eruption. This relatively new technique was developed in order to achieve more accurate dates than those obtained from the potassium-argon method. The older method required two samples for dating and could produce imprecise dates if the argon was not fully extracted.

This newer method converts a stable form of potassium potassium into argon Measuring the proportions of argon and argon within a sample allows the age of the sample to be determined. Only one sample is required for this method as both the argon and argon can be extracted from the same sample.

In special cases, bones can be compared by measuring chemicals within them.

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Buried bones absorb chemicals, such as uranium and fluorine, from the surrounding ground and absorb more of these chemicals the longer they remain buried. The rates of absorption depend on a number of factors which are too variable to provide absolute dates. This technique is, however, useful for providing relative dates for objects found at the same site.

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Another useful chemical analysis technique involves calculating the amount of nitrogen within a bone. The level of nitrogen gradually reduces as the bone decays.

Absolute dating is not possible with this method because the rate at which the nitrogen content declines depends on the surrounding temperature, moisture, soil chemicals and bacteria. The technique can, however, provide the relative ages of bones from the same site. Most fossils are found in sedimentary rocks deposited in layers.

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