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Ideally, analysis of the materials found on a site begins in the field laboratories while excavation is still in progress. Often, however, reconnaissance and excavation are completed in a relatively brief period of time, and the records and preserved remains are taken back to a museum, university, or laboratory for more analysis. This analysis has many aspects, which include describing and classifying objects by form and use, determining the materials from which they were made, dating the objects, and placing them in environmental and cultural contexts. These aspects may be grouped into two broad categories: chronological analysis and contextual analysis.
: Chronological Analysis
Chronological analysis of archaeological materials identifying their time periods and sequence in time is often done first. Archaeologists use two general kinds of dating methods: relative dating, or establishing when the various materials found at a site were made or used in relation to each other, and absolute dating, or assigning a fairly precise, chronometric date to a find.
The oldest method of establishing relative dates is by analyzing stratigraphy the arrangement of strata in a site. This technique is based on the assumption that the oldest archaeological remains occur in the deepest strata of the excavation, the next oldest in the next deepest strata, and so on. By following this assumption, archaeologists can place the materials collected from the various strata into a rough chronological sequence.
If archaeologists digging in an undated site find a distinctive type of pottery for which the date is known, they may conclude that the other materials found in the site along with the pottery bear the same date as the pottery. This is an example of a relative-dating technique called cross dating.
Similarly, archaeologists may assign a date to an artifact based on the geologic region or strata with which the artifact is associated. For example, archaeologists may conclude that hand axes found in the high terrace of the Thames River in England are older than arrow points and pottery found in the lower terrace because they know that the high terrace was formed earlier than the low one.
The association of artifacts with animal or fossil remains can also be used for relative dating. For example, it is known that superbison became extinct in the Great Plains of what is now the United States and were replaced by modern bison. Thus if archaeologists discover one site in which Folsom fluted points (the distinctive tips of a kind of prehistoric man-made weapon) are found imbedded in superbison remains, and they discover a second site in which a different kind of points, called Bajada points, are sticking in the remains of modern bison, they may conclude that Folsom points were made before Bajada points. This kind of relative dating may also be done using plant remains, particularly plant pollen, which is often preserved in archaeological strata.
If archaeologists know how certain types of artifacts styles of pottery or burial objects, for example evolved over time, they may be able to arrange groups of these artifacts in chronological order simply by comparing them. This method is called seriation.
Archaeologists can judge the relative dates of bones by analyzing their fluorine content, since the amount of fluorine in buried bones increases over time. In the 1840 s Dr. Montroville Dickeson proved that a human pelvis found in Natchez, Miss., dated from the same time as mammoth bones found with it because both had accumulated the same proportions of fluorine.
There are many other methods of relative dating. None of them is as accurate as the absolute-dating methods, however, because the assumptions on which many relative-dating techniques are based can be misleading. Nevertheless, sometimes relative dating is the only method available to the archaeologist.
In absolute, or chronometric, dating, a definite age in numbers of years before the present is assigned to an archaeological specimen. When applied correctly, the methods of absolute dating can yield highly accurate dates. The remains found by classical archaeologists coins or written records, for example may have dates already written on them, but this is not always the case. It is never the case for anthropological archaeologists, who study prehistoric materials.
One system of absolute dating, called varve dating, was developed in the early 20th century by Gerard de Geer, a Swedish geologist. He noted that the mud and clay deposited by glaciers into nearby lakes sank to the lake bottom at different rates throughout the year, forming distinct layers, called varves, on the lake bottom. Because each year s layer was different, the researchers were able to establish dates for artifacts or sites associated with a specific varve.
A similar absolute-dating method dendrochronology, or the dating of trees by counting their annual growth rings was first developed for archaeological purposes in the early 1900 s by the American astronomer Andrew Ellicott Douglass. If an ancient structure has wooden parts, archaeologists can compare the number and widths of the growth rings in those parts with sequences from other samples to find out when that structure was built. Other techniques yield absolute dates based on the thickness of the patina, or residue, that forms over time on certain stone artifacts.
Advances in the physical sciences during the 20th century greatly improved absolute-dating methods. One of the best-known and most valuable techniques is radiocarbon dating (also called radioactive carbon dating, carbon dating, and carbon-14 dating). All living things contain small amounts of carbon-14, a radioactive form of carbon. After death, this carbon-14 changes, or decays, into a more stable form of carbon. Archaeologists can determine the age of once-living things such as bones, wood, and ash by measuring the amount of carbon-14 remaining in the specimen.
- Carbon-14 Production
Radiocarbon dating cannot be used to make accurate age measurements of very old materials materials more than about 70,000 to 100,000 years old. For such objects, archaeologists can use similar techniques involving other chemical elements. Potassium-argon dating, for example, can be used to date rocks millions of years old. A related dating method called fission-track dating can be used on certain stone samples of almost unlimited age. Another modern dating method, thermoluminescence dating, can be used to find out when ancient pieces of pottery or other fired-clay objects were made.
- Carbon-14 Decay Over Time
: How Archaeologists Estimate the Age of Finds
Prior to about 1950 archaeological sites were dated almost exclusively from inscriptions in the more recent sites or by geological calculations based on soil sequences and the estimated rates of deposit of the soil layers. Tree-ring counts (one ring per year) extended the time scale back a few thousand years in some regions. Analysis of the amounts of fluorine in human and animal bones lying together showed whether or not they were of the same age.
With very ancient specimens, radioactive dating techniques are now used. However, these methods are not absolutely accurate. Age usually must be estimated within wide margins because of variables that cannot be calculated exactly. From time to time, as the techniques become more refined or as sources of error are eliminated, new ages may be assigned to remains.
Two radioactive techniques are frequently used to date the age of fossil humans radiocarbon analysis and potassium-argon analysis. In 1948 physicist Willard F. Libby and his coworkers discovered how to date charcoal and other organic material by carbon-14 testing. Radiocarbon dating traces remains back some 35,000 years. Potassium-argon testing, which utilizes the rate of decay to stable argon-40 of the radioactive potassium-40 found in most rocks, gives dates for sites more than 500,000 years old. Dates can also be obtained by measuring the amount of surface decomposition on certain stone tools or the amount of thermoluminescence seen when ancient pottery is heated.
: Relative & Absolute Dating
Dating is crucial for physical anthropologists, as well as for geologists and archaeologists. It is a method that allows them to determine how old something is whether it be a layer of rocks, a human-like fossil, or a collection of pottery.
There are two kinds of dating: relative and absolute. Relative dating shows the order in which events occurred but does not tell exactly when they occurred. Methods of absolute dating indicate with a fair degree of precision how old something is. Of the two types of dating, the determination of relative age relationships came into use first. Absolute dating depends upon technological advances that have been made in the 20th century.
Geologists and archaeologists have long used relative dating methods to determine the approximate age of the Earth and of fossils and artifacts. Geologists examine the many strata of the Earth s crust to determine the intervals of time from one layer of rock to another. Archaeologists also use the principle of layering to verify the sequence of human cultures.
Another method of determining relative age is fluorine dating. It is based upon the principle that fossil bones absorb the element fluorine from the soil in which they are buried. The longer they are buried, the more fluorine the bones will contain. Determining the amount of fluorine is often not a practical means of relative dating because it requires many samples from an immediate area.
Absolute dating attempts to pinpoint when a given rock, fossil, or other object reached its present condition. The basic method for determining absolute age is called radiometry measuring the rate of radioactive decay of an element. This can be done with a high degree of accuracy, although no method is infallible without a great deal of corroborative testing.
- Radioactive Decay Law
One of the types of absolute dating that has been used by physical anthropologists is potassium-argon dating. It is a method of determining the time of origin of rocks and thereby of the fossils found within them by measuring the amount of decay of potassium-40, a radioactive isotope of the element potassium, into the element argon, one of the rare gases. The half-life of potassium-40, which is the time it will take one half of any quantity of it to decay into potassium, is 1,265,000,000 years. Potassium-argon dating has been used to measure the ages of a wide variety of objects, from meteorites 4,500,000,000 years old to volcanic rocks only 20,000 years old. Such dating techniques applied to the remains and surroundings of ancient human beings have constantly pushed back the estimated age of mankind. By the early 1980 s man was believed to be at least 3 million years old. This is based on the dating of a number of remarkable discoveries of fossil remains made in the Great Rift Valley of Africa, at sites in Ethiopia, Kenya, and Tanzania.
: Radioactivity – Absolute Dating
Late in the 19th century, scientists discovered an amazing activity in certain kinds of matter. Through the ages, atoms of these substances have been shooting off particles and emitting radiations (together called rays) without anyone suspecting that this was happening. Scientists also found that nothing could be done to change the emissions. The application of heat, electricity, or any other force made no difference whatsoever. Emission seemed to be an unchangeable property of the substances.
Many vital uses have come from this discovery. A special use of the element uranium led to the development of nuclear weapons and nuclear energy. Doctors also have found that the rays can penetrate living tissues for short distances and affect the tissue cells. Like x-rays, they can disrupt chemical bonds in the molecules of important chemicals within cells, and so they help in treating cancers and other diseases.
In time, scientists learned how to make all other elements give off these rays. These include the major elements that make up our bodies. If such radioactive elements are placed in the body through food or by other methods, the rays can be traced through the body. This use of tracer elements is extremely helpful in expanding knowledge of our life processes. (See Radioactive Decay Law page )
Geologists have learned how to use radioactivity to determine the age of rocks. From this they obtain new checks on the ages of mountain ranges and even the age of the Earth itself. The study of radioactivity continues to contribute to the understanding of the nature of atoms, and from this, scientists are learning how energy and matter interact to bring about everything that happens in the physical universe.
: Units for Measuring Radioactivity
The unit of measurement of the radioactivity of a substance is the curie. One curie equals about 37 billion emissions per second . Matter emitting half of this amount per second would represent 1/2 curie of radiation .
Another unit of measurement used in radioactivity is the radius (rho) of the nucleus. The value of rho for a particular element is approximated by multiplying 1.2 x 10 15 meter by the cube root of the element s atomic number, the number of protons in the nucleus or . (1.2 x 10 15 meter would have to be multiplied by about 20 trillion to equal 1 inch or .) Using this formula and the fact that a nucleus is roughly spherical, it follows that the density of particles is about the same in all nuclei. This density is incredibly high. If a cubic centimeter of matter were as dense as this, it would weigh about 250 million tons. Neutron stars, which are thought to be the result of supernovas, have densities of this magnitude, with an amount of mass roughly equal to that of the sun packed into a star about 12 miles (19 kilometers) in diameter.
: Carbon Dating
Without the element carbon, life as we know it would not exist. Carbon provides the framework for all tissues of plants and animals. These tissues are built of elements grouped around chains or rings made of carbon atoms. Carbon also provides common fuels coal, coke, oil, gasoline, and natural gas. Sugar, starch, and paper are compounds of carbon with hydrogen and oxygen. Proteins such as hair, meat, and silk contain carbon and other elements such as nitrogen, phosphorus, and sulfur.
More than six and a half million compounds of the element carbon, many times more than those of any other element, are known, and more are discovered and synthesized regularly. Hundreds of carbon compounds are commercially important but the element itself in the forms of diamond, graphite, charcoal, carbon black, and fullerene is also indispensable.
Carbon occurs in nature as the sixth most abundant element in the universe and the 19th element in order of mass in the Earth s crust. As the element in the forms of graphite, diamond, and fullerene it is a minor part of the Earth s crust, but compounds of carbon with other elements are very common. The chemical symbol for an atom of carbon is C. Some common natural substances rich in carbon are coal, petroleum, natural gas, oil shale, limestone, coral, oyster shells, marble, dolomite, and magnesite. Limestone, coral, and oyster shells are largely calcium carbonate, CaCO3. Marble, dolomite, and magnesite also contain calcium, magnesium, and carbon. (See Properties of Carbon table page )
- Properties of Carbon
Coal, petroleum, natural gas, and oil shale are mainly compounds of carbon and hydrogen derived from plant and animal sources deposited in the Earth millions of years ago and subjected to high pressure. These deposits were once a part of what is called the carbon cycle, a dynamic system of change still occurring. Through photosynthesis, plants use sunlight to convert carbon dioxide from the air and convert water from the soil into plant tissues such as cellulose and into an energy source such as sugar. Plants release oxygen into the air as the carbohydrates sugar and cellulose are synthesized. Animals eat the plants, breathe in oxygen from the air and oxidize the carbohydrates, or use them as fuel, which releases energy to the animal. Eventually the products of animal metabolism carbon dioxide, water, and other waste products are returned to the atmosphere and the Earth. The cycle repeats itself endlessly.
Besides the wide occurrence of carbon in compounds, two allotropes, or forms, of the element diamond and graphite are deposited in widely scattered locations around the Earth. The third form, fullerene, is unstable in comparison to these two forms and is thus not found widely.
A diamond, no matter what the size, may be considered to be a single molecule of carbon atoms, each joined to four other carbons in regular tetrahedrons, or triangular prisms. The crystal structure is called a face-centered cubic lattice. Diamond is extremely hard but brittle and has a high specific gravity of 3.51. Its high refractive index of 2.42 is a measure of how far diamond can refract, or bend, light. This property gives the diamond brilliance and fire. A diamond can be cleaved, or split, along its crystal faces into smaller pieces with the sides of the cleavage remaining smooth. This property is very important to the diamond cutter and the jeweler.
Graphite, the second allotrope of carbon, was known in antiquity. Natural deposits of graphite have been called black lead, silver lead, and plumbago, which is another name for the lead ore galena. The largest deposits of graphite are in Sri Lanka but the highest quality graphite comes from Madagascar. Other sources are North Korea, Mexico, Canada, Siberia, and New York. In contrast to that of diamond, the structure of graphite consists of layers of carbon atoms joined in regular hexagons by strong bonds. The layers are held together by long-range, relatively weak attractive forces called Van der Waals forces. The layers can slide over each other easily, which accounts in part for the lubricating property of graphite.
Amorphous carbon is not generally called a third allotrope because it is a form of graphite consisting of microscopic crystals. Amorphous carbon is obtained by heating any of a variety of carbon-rich materials to 1,200. to 1,800. F (650. to 980. C) in a limited amount of air so that complete combustion does not occur. Coal, for example, is heated to give coke; natural gas or petroleum to give carbon black (also called lampblack and channel black); wood to give charcoal; bone to give bone char; petroleum coke or coal to give baked carbon, carbon arcs, or carbon electrodes.
In 1785 it was discovered that activated carbon from the carbonization of wood and charcoal removes color from solutions for example, the brown color from raw sugar solutions. Activated carbon is still used in the beet sugar industry and bone char is favored for the same purpose in the cane sugar industry. Other foodstuffs commonly decolorized by activated carbon include vinegar, soup stock, whiskey, gelatin, and oils and fats. Activated carbon is also used to adsorb the toxic gases used in chemical warfare, to adsorb organic vapors, and to reclaim solvents. All these uses depend on the adherence of impurities to the enormous surface area of the finely divided carbon.
Most carbon black is used in the manufacture of tires; it improves the strength of rubber and resists scraping. The rest is used in making printing inks for newspapers and magazines, and in paints, lacquers, enamels, and carbon paper.
Fullerene, a hollow cluster of carbon atoms that resemble the geodesic domes made by architect R. Buckminster Fuller, was first postulated to exist in 1985. In 1990 its existence was confirmed, and methods of synthesizing it in mass quantities were invented. The most studied form, known as buckminsterfullerene or the buckyball, has 60 carbon atoms arranged into a five-sided and six-sided geometry to resemble a soccerball. It is suited for use as a lubricant, superconductor, radioactive shield, hard coating, battery, and ball bearing. Through experimentation, scientists concluded that fullerene exists in interstellar space and in soot from the burning of certain gases on Earth. In 1992 it was found for the first time in rock sediments formed more than 600 million years ago. Because fullerene is unstable when exposed to air it is not found in large quantities naturally.
The synthesis of carbon-containing compounds starts from carbon compounds available in nature. The sources of the starting compounds are petroleum for aliphatic hydrocarbons (straight-chain molecules of carbon and hydrogen) and coal or petroleum for aromatic hydrocarbons (rings of carbon and hydrogen). Limestone, from which carbon-containing calcium carbide and acetylene can be made, is extremely important to the chemical industries in countries that have no native petroleum.
Carbon compounds containing boron and silicon are among the hardest substances known. On a standardized scale of hardness called the Mohs scale, where diamond is 10, SiC, silicon carbide (or Carborundum), is 9.15 and B4C, boron carbide, is 9.32. These carbides are used as abrasives on emery wheels. They are chemically inert and nearly indestructible. Carbides formed by the more metallic elements such as iron, cobalt, and nickel, in contrast, are easily decomposed by acids to give hydrocarbons, chiefly methane and hydrogen.
An ordinary carbon atom has six protons and six neutrons in its nucleus; so the atom is called C12. Another isotope, or type, of carbon atom has six protons and seven neutrons in its nucleus and is called carbon-13, or C13. The relative abundance of C12 and C13 in natural sources is 98.89 percent and 1.11 percent respectively. In the air, however, the fast-flying neutrons from cosmic rays keep hitting nitrogen atoms (N14, with seven protons and seven neutrons). Each time a neutron hits, it drives a proton from the nitrogen atom s nucleus and takes its place. Since the atom now has six protons, it is an atom of carbon. It has 14 particles (six protons and eight neutrons), however, in the nucleus; so it is called C14.
This form of carbon decays radioactively. The production and decay are balanced so that C12 and C14 remain always at the same ratio to each other in carbon dioxide. Since the two forms are the same chemically, plants use them for photosynthesis in this same ratio. Because animals eat plants the ratio is found in all living organisms.
Fossils, mummies, and wooden relics, however, no longer exchange carbon with the air. The carbon (C12) that was present at death remains, but the C14 decays radioactively and becomes less in ratio to C12. The changing ratio can be detected easily with a Geiger counter or a scintillation counter; and the amount of change tells the age of the specimen. For example, suppose that the percentage of C14 in a specimen is only half that in the air. Since C14 undergoes a half-life of decay in about 5,730 years, the specimen must be this old.
Radioactive carbon-13 is used as a tracer for many chemical reactions. Chemists can introduce it into food, for example, and then trace the course of the food through the body with a special type of Geiger counter or scintillation counter. This method has been used to trace the steps in photosynthesis.
Leallyn B. Clapp
see Pict 5 & 6
: Potassium Dating
The chemical element potassium is essential to life. In higher animals potassium ions together with sodium ions act at cell membranes in transmitting electrochemical impulses in nerve and muscle fibers and in balancing the activity of food intake and waste removal from cells.
Discovered in 1807 by the English chemist Humphry Davy, who obtained it from molten potassium hydroxide (KOH), potassium, a soft, silver-white metal, was the first metal to be isolated by electrolysis. It belongs to the family of elements known as the alkali metals. It oxidizes rapidly in air and also reacts violently with water, yielding potassium hydroxide and hydrogen gas (which ignites). Because of this, potassium is stored submerged in mineral oil. It is never found alone and is difficult to isolate from its compounds. (See Properties of Potassium table page )
The seventh most abundant element in the Earth s crust, potassium occurs in many silicate rocks and minerals. The major commercial source is salt deposits, but a small fraction is obtained from plant and animal sources. Water-soluble potassium compounds are economically recovered. They are frequently found as dry mineral deposits and as brines. Most potassium is present in insoluble minerals, making it difficult to obtain, but it can be prepared commercially by electrolysis from some refinable minerals.
- Properties of Potassium
Potassium compounds have many commercial uses. Potassium chloride (KCl) is used in preparing other potassium compounds and in fertilizers. Electrolysis of potassium chloride yields potassium hydroxide, also called caustic potash, a water-absorbing substance used in making soaps and detergents. Caustic potash is also used for preparing many potassium salts, such as potassium carbonate (K2CO3), a water-absorbing substance used in making glass and textile dyes and for cleaning and electroplating metals.
Potassium nitrate (KNO3), also known as niter or saltpeter, has wide use as a fertilizer and in fireworks and explosives. It also serves as a food preservative. Potassium chlorate (KClO3), as a source of oxygen, is used in fireworks, matches, and explosives. The iodide of potassium (KI) is added to table salt and animal feed to protect against iodine deficiency. It is also used to treat goiter and certain fungal infections. Applications for potassium sulfate (K2SO4) include use as a laxative and in the production of fertilizer, rubber, and potassium carbonate. Potassium cyanide (KCN) is a poison used in some insecticides and is a source for the fumigant hydrogen cyanide. It is also used to extract gold and silver from their ores.
: Contextual Analysis:
Determining the chronology of an artifact is only half of the archaeologist s task; the other half is reconstructing the ancient culture from which the artifact came. This process is called contextual analysis.
The lowest, or most basic, level of contextual analysis consists of analyzing a culture s systems of subsistence and technology that is, the ways in which ancient people adapted to their environment. The next level involves reconstructing their social structures and settlement patterns. Finally, archaeologists try to reconstruct a culture s ethos, or guiding beliefs.
Each of these levels requires different analytical methods. Archaeologists may start reconstructing an ancient subsistence system by determining what the people ate. They may do this through coprology, the examination of fossilized feces, or by analyzing human bones for the presence of certain forms of carbon and nitrogen. The study of the plant remains found in a dig can also provide clues to a people s diet.
By studying ancient tools such as arrow tips, butcher knives, and grinding stones archaeologists can find out how people obtained and prepared their foods. Archaeologists may also be able to determine how ancient people made and used their tools. Studying the work of a modern flint knapper, for instance, may show an archaeologist how ancient people made flint tools. (In archaeology, this type of reasoning or interpretation is called ethnographic analogy.)
When archaeologists attempt to reconstruct ancient social structures, they often use data gathered by ethnographers, social anthropologists, and historians. The excavated materials themselves may also provide hints of ancient social organization. Specialized artifacts that are found concentrated in certain areas may indicate that the ancient culture had full-time craft specialists, and different types of burial arrangements may indicate that social classes existed.
Reconstructing the highest level of a culture, including its values, ethos, or religion, is the most difficult type of contextual analysis. Such items as statues or paintings of figures that appear to be supernatural, buildings that may have been temples, and evidence of religious ceremonies can all be used to help reconstruct ancient systems of beliefs.
The goal of chronological and contextual analysis is to write and publish records of ancient history. Excavated materials have value only if the information gained from them is disseminated through books, magazines, and other publications. Such publications not only keep track of how techniques have changed but also record great archaeological discoveries.
: Historical Particularism
By the beginning of the 20th century, anthropologists in Great Britain, Germany, and the United States were questioning the belief that all societies developed in much the same way. They suggested that each culture was unique because each had its separate geography, history, creativity, and degree of contact with its neighbors.
One of the first to reject evolutionism was a German-born American anthropologist, Franz Boas. Boas emphasized the importance of fieldwork and observation. Fieldwork involves seeking information about a particular group s behavior by gathering data and recording observable behaviors in that group s natural environment.
Boas believed that every aspect of a culture should be recorded and that the anthropologist studying a native culture should not only learn its language but should attempt to think like its people. Boas emphasized the importance of collecting information that described the individuals and their interrelationships in a particular culture. Such information was gathered through the recording of life histories and folklore, and then connecting these details with archaeological and historical data. Boas also believed that similarities among different cultures were the result of similar outside influences rather than to the similarity in thought processes or to any universal laws of development. He stressed the importance of analyzing a culture within its historical context.
Boas is known as the founder of the culture history school of anthropology, which dominated American cultural anthropology for much of the 20th century. Anthropologists who followed Boas theories included Ruth Benedict, Alfred L. Kroeber, Margaret Mead, and Edward Sapir.
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