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Archaeologists use many different techniques to determine the age of a particular artifact, site, or part of a site. Two broad categories of dating or chronometric techniques that archaeologists use are called relative and absolute dating. Stratigraphy is the oldest of the relative dating methods that archaeologists use to date things. Stratigraphy is based on the law of superposition-like a layer cake, the lowest layers must have been formed first. In other words, artifacts found in the upper layers of a site will have been deposited more recently than those found in the lower layers. Cross-dating of sites, comparing geologic strata at one site with another location and extrapolating the relative ages in that manner, is still an important dating strategy used today, primarily when sites are far too old for absolute dates to have much meaning.

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Mar 17,   Dating refers to the archaeological tool to date artefacts and sites, and to properly construct history. All methods can be classified into two basic categories: a) Relative dating methods: Based on a discipline of geology called stratigraphy, rock layers are used to decipher the sequence of historical geological events. Relative techniques can determine the sequence of events but not the Author: Johnblack. The study of strata is called stratigraphy, and using a few basic principles, (K-Ar) method, that allows dating of materials that are beyond the limit of radiocarbon dating (Table 1). Archaeological stratigraphy, radiocarbon dates, and the study of associated materials indicates that the beginnings of Megaliths in south Asia is in the very late second millennium BC, and comes with the mass production of iron.

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Stratigraphic Dating. Stratigraphy refers to layers of sediment, debris, rock, and other materials that form or accumulate as the result of natural processes, human activity, or both. An individual layer is called a stratum; multiple layers are called strata. The potassium-argon dating method, like radiocarbon dating, relies on measuring radioactive emissions. The Potassium-Argon methoddates volcanic materials and is useful for sites dated between 50, and 2 billion years ago. It was first used at Olduvai Gorge. A recent modification is Argon-Argon dating, used recently at Pompeii. In their study of stratigraphy students will use an activity sheet to: 1. Interpret archaeological strata using the law of superposition. 2. Apply cross-dating to determine the age of other artifacts. Materials: Five books of any size, "Site Near Richfield" and "Cross-dating" activity sheets for each student.

Fortunately, the Ancient Image Galleries. The principle of superposition states that in an undeformed sequence of sedimentary rocks, each layer of rock is older than the one above it and younger than the one below it Figures 1 and 2.

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Accordingly, the oldest rocks in a sequence are at the bottom and the youngest rocks are at the top. Sometimes sedimentary rocks are disturbed by events, such as fault movements, that cut across layers after the rocks were deposited. This is the principle of cross-cutting relationships.

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The principle states that any geologic features that cut across strata must have formed after the rocks they cut through Figures 2 and 3. According to the principle of original horizontality, these strata must have been deposited horizontally and then titled vertically after they were deposited.

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. 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.

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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. 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. In the figure, that distinct age range for each fossil species is indicated by the grey arrows underlying the picture of each fossil.

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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.

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.

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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.

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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. 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.

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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.

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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.

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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.

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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. 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.

Rock \u0026 Fossil Correlation

This method is known as radiometric dating. Some commonly used dating methods are summarized in Table 1. 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.

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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. 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.

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Thus, radiocarbon dating is only useful for measuring things that were formed in the relatively recent geologic past. At an archaeological site, strata exposed during excavation can be used to relatively date sequences of events.

At the heart of this dating technique is the simple principle of superposition: Upper strata were formed or deposited later than lower strata.

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Without additional information, however, we cannot assign specific dates or date ranges to the different episodes of deposition. In this example, archaeologists might radiocarbon date the basket fragment or bone awl in Stratum E, and they could use artifact seriation to obtain fairly precise date ranges for Strata A, B, C, and E.

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