Geoarchaeology

Archaeology is rapidly becoming a markedly more scientifically based field as part of a trend that started with the “New Archaeology” of the 1970s. Archaeological geology covers the wide range of geological sciences that are applied to archaeology during excavation and postseason, or postexcavation, sorting, classifying, and analyzing.

Dating Methods

Archaeological geology is the application of geological methods and techniques to archaeology. The two disciplines have become so closely intertwined at times that some have spoken for a new term to describe their partnership: geoarchaeology. A term aimed more specifically at the contributions of the physical and chemical sciences associated with archaeology (such as potassium-argon dating) is archaeometry. Without such scientific methods, archaeology becomes guesswork at best; dating of finds, for example, should be derived from empirical data, or information that can be proved through experiment and observation, rather than deduced from theory without corroboration. Archaeology is, basically, deductions made about an artifact from the context in which it is found. Geology helps define and date that context, thereby providing the empirical information from which speculation about the artifact can be derived.

The principle of superposition was probably one of the first geological methods that archaeology utilized. This law states that a layer superimposed on another layer should be younger, having been laid down after the lower, or older, layer. This study of stratigraphy is a keystone to archaeological dating but constitutes relative rather than absolute dating. One of the better-known methods of absolute dating is carbon-14 dating. Carbon-12 and carbon-14, an isotope of carbon-12, are elements that exist in all living organisms. Once the organism dies, plant or animal, the input of carbon-14 from the environment stops, and the remaining carbon-14 begins to decay. The amount of carbon-14 left relative to the amount of carbon-12 is used to calculate the amount of time the organism has been dead, using the known half-life of carbon-14 (5,730 years).

Archaeomagnetism (the term is the archaeological equivalent of geology's paleomagnetism) is another dating method, but one that uses changes in the Earth's magnetic field as recorded by archaeological artifacts such as kilns. The Earth experiences continual changes in the intensity and polarity of its magnetic field. If a clay artifact has been heated to its Curie point, or temperature, its magnetic particles will align in the direction of the polarity of the Earth's magnetic field in which they cool. Geological identification of the polarity pinpoints the time of firing. Thermoluminescence dating is also a method used on clay. A piece of pottery is heated to 500 degrees Celsius, and the ensuing emission of light is measured as an indication of the length of time since its firing, as the energy of the thermoluminescence increases as it is stored up over time. Another method for dating inorganic objects—in this case, volcanic rock—is potassium-argon dating. When volcanic rock is newly solidified, it contains no argon, but it does contain potassium-40. As the potassium-40 decays, it becomes argon-40. Because the half-life of potassium-40 is known (one billion years), the amount of argon-40 gives the absolute age of the rock.

and Petrology

Geological techniques are also used for locating and recording archaeological sites. Remote sensing is a technique by which data are collected on a site in a “remote” rather than a hands-on way. Photographic images are a major part of remote sensing. Aerial photography, for example, involves photographing the landscape from airplanes or satellites. Images and patterns that record electromagnetic radiation provide another source of remote-sensing data, as does soil resistivity surveying. Soil resistivity surveying is a method used to map buried features by finding electrical conductivity differences between the features and the soil around them. The ease or resistance with which the current penetrates the soil is the basic principle. To test the resistivity, four electrodes are inserted into the ground. Two generate the current, and two measure the drops in voltage; an equation taking into account the distance between the electrodes, the amperage, and the drop in voltage then gives the total resistance to conductivity.

Petrology, or the study of different aspects of rocks, is a standard feature of archaeological geology. Most techniques focus on the study of thin sections or powdered samples of rock—for example, using scanning electron microscopes and X-ray diffraction, respectively. LiDAR (light detection and ranging), ground-penetrating radar (GPR), and magnetometry are also sometimes used in areas with thick vegetation. Additionally, archaeologists utilize geological studies of cryoturbation (freeze-thaw cycles in soil), argilliturbation (shrinking and swelling cycles in clays), aeroturbation (disturbances by gas, wind, and air), aquaturbation (disturbances from the movement of water), and seismiturbation (disturbances by earthquakes). Studying these types of environmental disturbance helps identify the site-disturbing processes at work.

Application of Geoarchaeological Methods

Many aspects of the geological sciences can be applied to archaeological sites. It is the nature of the site that determines the appropriate method to use. For example, a historical, archaeological site, or one that is dated to within the parameters of recorded history, would not probably be a site at which radiocarbon dating would be useful: The artifacts would not be old enough for a dating method that yields figures in thousands of years. Conversely, a prehistoric site such as the possibly Iron Age Caer Cadwgan, a probable hillfort in Wales, would benefit primarily from radiocarbon dating: The site is probably about three thousand years old, and charcoal and bone—excellent types of samples for carbon-14 testing—are the main elements found in excavation at this site. This site has also yielded small glass beads, items that would perhaps be datable by thermoluminescence.

Historical sites, in general, are not the prime candidates for archaeological geology that prehistoric sites are. Prehistoric archaeology, in contrast, depends completely on geological analysis for some conclusions because it predates any written records. There are no fortuitously preserved documents to fall back on for verification. Archaeological geology can accomplish much, but it is most useful for four principal processes during an excavation: locating a site, recording and analyzing the features of a site, and dating a site.

The archaeological use of soil resistivity for locating sites or individual features was first applied to locate prehistoric stone monuments just after World War II. The differences in ground conductivity are used to locate anomalies, which can range from buried ditches to stone walls. This method was first developed in geology to locate ore deposits, faults, and sinks (sunken land where water can collect). Remote sensing can be a useful method for several different goals. It can be used to locate sites, monitor changes in the archaeological record, reveal the distribution of archaeological sites, or map sites. Aerial photography can be used to locate sites, and then used to map a site and record its features for planning an excavation. On some sites in North Africa, the camera is sent up in a balloon to take the photographs, as the site may be too small or the budget of the excavation too limited for airplane-carried camera work.

Archaeology is a destructive process, and once a site is excavated or even surface collected, it cannot be restored. Remote sensing can, sometimes, take the place of excavation in what is termed nondestructive archaeology. It can also be a useful substitute for excavation when there is not time for a full-scale dig to be mounted, as during times when a formation may be temporarily visible (for example, winter snows revealing significant gradations in the land) but conditions may not be right for excavation. Remote sensing can also preserve at least an image of a site that must be destroyed, as during construction of a road. With this use, it is an invaluable tool in rescue archaeology.

Application of Dating Methods

Petrology is a useful tool in general for archaeology. Petrology can be used on many different types of sites, from prehistoric to historic. It can be used to give a provenance, or origin, for building and sculptural stones as old as Stonehenge or as young as the classical Greek marbles. Archaeomagnetism can be used not only to date artifacts but also to distinguish sources for substances. Obsidian sources, for example, have been found to each have different magnetization strengths. It is possible, then, that the source of ore for coins, which may contain trace amounts of iron, may also be able to be discovered. Archaeomagnetic dating of lava has been used to date the end of the Minoan civilization (about 1500 BCE), which was destroyed by the volcanic eruptions at Santorini (or Thera) in Greece. It is used mainly for early human sites, or for sites up to 10,000 years old.

Thermoluminescence can also be used to date lava as well as burnt flint (implements heated by accident or on purpose to improve certain qualities), burnt stones (heated on a fire and then placed in a food container as a “pot-boiler”), glass (volcanic glass especially), sediments (buried soils), and ceramics. This method is popular because of its absolute dates for a wide range of ages: 50 to 500,000 years old. Potassium-argon dating is also used for inorganic samples, but only for ones that predate humankind. Samples can only be as young as 100,000 years old. Another limitation is that because it is used to date lava, to be useful, the site must be connected to a particular volcanic eruption, and sites such as Santorini and Pompeii are not common (and too young). The usefulness of potassium-argon dating to archaeology is its ability to fix dates for reversals of the Earth's magnetic field, which, in turn, are used to date archaeological sites through archaeomagnetism.

Using petrology to establish the provenance of a rock artifact is of great importance in prehistoric archaeology. The evaluation of rock types found can help to identify the mining or quarrying skills of a culture, its determination of the usefulness of various kinds of rocks, and even some of the places to which people may have traveled. If a certain type of rock is not native to the area in which it is found yet occurs in large quantities, it may be inferred that significant trade or travel occurred. The value that a culture may have placed on a particular rock may also be determined by its use (ceremonial or practical, for example). Use-wear, as opposed to Earth-moving processes that have changed the shape of the rock, is another object of study. Petrological methods such as X-ray diffraction can be used to identify the origin of rocks on a site. Studying the rocks from several sites can be a way of tracing the trade routes of a culture.

Principal Terms

absolute date: a date that gives an actual age, though it may be approximate, of an artifact

Curie point, or Curie temperature: the temperature at which materials containing iron oxides lose their magnetic pattern and align with the Earth's magnetic field

relative date: a date that places an artifact as older or younger than another object, without specifically giving an age for it

remote sensing: any of a wide variety of techniques, such as aerial photography, used for collecting data about the Earth's surface from a distance

stratigraphy: the deposition of artifacts in layers or strata

Bibliography

Butzer, Karl W. Archaeology as Human Ecology: Method and Theory for a Contextual Approach. Cambridge UP, 2006.

Garrison, Erv. Techniques in Archaeological Geology. 2nd ed., Springer, 2018.

Goldberg, Paul, and Richard I. MacPhail. Practical and Theoretical Geoarchaeology. 2nd ed., Wiley-Blackwell, 2022.

Hill, Christopher L. "Geoarchaeology, History." Encyclopedia of Geoarchaeology. Springer International Publishing, 2024, pp. 1-12. doi.org/10.1007/978-3-030-44600-0‗10-1.

Rapp, George, Jr., and Christopher L. Hill. Geoarchaeology. 2nd ed., Yale UP, 2006.

Ryan, William, and Walter Pitman. Noah's Flood: The New Scientific Discoveries About the Event That Changed History. Simon & Schuster, 1998.

Sanyal, Santonu Kumar, and Jeremiah Shuster. "Gold Particle Geomicrobiology: Using Viable Bacteria as a Model for Understanding Microbe–mineral Interactions." Mineralogical Magazine, vol. 85, no. 1, 2021, pp. 117-24. doi.org/10.1180/mgm.2021.19.

Schiffer, Michael, editor. Formation Processes of the Archaeological Record. U of Utah P, 2002.

West, Frederick H., editor. American Beginnings. U of Chicago P, 1996.