Current Questions and New Directions in Archaeological Obsidian Studies
Abstract and Keywords
This article discusses the current status of archaeological obsidian studies, including techniques used in characterization and sourcing studies, obsidian hydration, and regional syntheses. It begins with an overview of obsidian and the unique formation processes that create it before turning to a discussion of the significance of characterization and sourcing techniques for understanding prehistoric obsidian trade and exchange. It then considers the problematic aspects of the term “sourcing,” despite its ubiquitous use in archaeology and archaeometry, along with the use of X-ray fluorescence in the chemical characterization of obsidian. It also explores obsidian hydration dating methods and equations, factors that can affect the date assignments for hydration specimens, and the various uses of obsidian in prehistoric times. Finally, it addresses some important questions relating to obsidian research and suggests new directions in the field.
Approximately fifty years ago, a great leap forward occurred in obsidian studies, marking the beginning of obsidian research as it is conducted today. Two seminal articles were published that changed the way we view this volcanic glass: Cann and Renfrew’s “The Characterization of Obsidian and Its Application to the Mediterranean Region” (1964), which introduced instrumental methods of characterizing and sourcing obsidian, and Friedman and Smith’s “A New Dating Method Using Obsidian” (1960), announcing the use of obsidian hydration—the measurement of a band of absorbed water on the surface—as a dating technique for archaeological specimens. Suddenly, obsidian as a lithic material became both sourceable and dateable, allowing for archaeological research pertaining to trade and exchange, population movements, procurement, and territory; and subsequently innovative new questions on gender, labor, status, and symbol, all with chronological context.
Unfortunately, the sourcing and dating methods used for obsidian remained problematic for at least the first two decades, and we are still refining our instrumentation, analysis methods, and data today. In characterization and sourcing studies, early geologic source libraries were woefully inadequate, and data were hard to manipulate and interpret prior to the development of more robust computing platforms. Obsidian hydration practitioners soon discovered that environmental context, including temperature, humidity, soil acidity, fire history, and even microenvironments, as well as obsidian chemistry, played a significant role in the hydration rate of individual obsidian sources. Archaeologists remained skeptical of the utility of these new techniques at first, relying more consistently on visual sourcing of obsidian and typologies or radiometric dating of associated artifacts for chronological information.
In 1976, the first comprehensive volume on obsidian studies was published (R. E. Taylor, Advances in Obsidian Glass Studies: Archaeological and Geochemical Perspectives), and in 1998 a new volume updated the status of the field (M. S. Shackley, Archaeological Obsidian Studies: Method and Theory,) providing a summary of new directions in obsidian study. Additional summary volumes have been published more recently that provide an expanded overview of the field (Glascock et al. 2007a; Liritzis and Stevenson 2012; Dillian 2014) and outline regional applications of obsidian research (see, for example, Shackley 2005; Kuzmin and Glascock 2010; Hughes 2011; Levine and Carballo 2014).
Today, archaeometric obsidian studies continue to grow, particularly through the refinement of sourcing and dating methodologies, and the addition of new geochemical sources to databases in understudied areas such as East Africa. But obsidian studies are not limited to laboratory analyses. Instead, it is what we do with these obsidian data that is important in advancing knowledge about our past. In the pages that follow, I will provide a summary of the current status of the field, including techniques used in characterization and sourcing studies, obsidian hydration, and regional syntheses, as well as some comments on new directions in obsidian studies moving forward.
What Is Obsidian?
Obsidian is a volcanic glass that forms during rapid cooling of high-silica, rhyolitic lavas (Blatt and Tracy 1996: 29). Rhyolitic lavas are those that contain concentrations of silica (SiO2) as high as 70%–75% and aluminum (Al2O3) concentrations between 10% and 15% (Glascock et al. 1998: 18). Rhyolitic lava is extremely viscous, and this viscosity contributes to its ability to form a glass. To grow crystals, mineral particles must diffuse through the magma to crystal surfaces. In high-viscosity magmas, diffusion rates are low, so crystals will grow more slowly. Rhyolitic melts that erupt and cool before crystals have time to form result in a glass (Carmichael et al. 1974: 156).
The unique formation processes that create obsidian are important to understand because they contribute to its chemical makeup, and subsequently its characterization and provenance assessment. Rhyolitic magma forms as a liquid deep within the earth at high temperature and pressure. During this liquid stage, elements circulate throughout the melt, which contributes to the homogeneity of the resulting obsidian flow. Concurrently, the magma chamber’s wall rocks melt and/or leach minerals into the melt and therefore add new components to the magma’s elemental composition. At the same time, minerals also crystallize and subtract other elements from solution. Liquid magma is at equilibrium such that for a given temperature and pressure, minerals are dissolved within the melt and the liquid is saturated with these minerals. When the magma begins to rise within the crust and slowly starts decreasing in pressure and losing heat to the surrounding matrix, the minerals that were formerly in solution are no longer in equilibrium and begin to crystallize from the liquid. Eventually, the magma will again reach equilibrium for a given temperature and pressure. However, temperature and pressure are continuously changing, and as a result the magma will continue to evolve until the temperature reaches the point at which the magma begins to solidify or the magma erupts, effectively halting the system. Because each magma pool is the product of a unique combination of pressure, temperature, contributing wall rocks, and crystallization processes, the chemical composition of the products formed from that magma pool, such as obsidian, will be unique, and they will even be distinctive for different eruptive events of a single volcano as the magma continues to evolve over time.
Some elements are particularly variable in the liquid melt due to their inability to combine into solid forms. These are called incompatible elements, and they can be uniquely diagnostic in characterization and sourcing of obsidian. Examples of these incompatible elements routinely used in obsidian sourcing studies include rubidium (Rb), strontium (Sr), barium (Ba), yttrium (Y), and zirconium (Zr).
Physically, obsidian is a glass with the properties of a liquid in all respects except for the ability to flow easily (Cann 1983: 227). As a result, obsidian fractures conchoidally, making it an ideal material for stone tool manufacture. Most obsidian, however, is neither uniform nor high quality, and therefore obsidian flows that could have been used for prehistoric tool production are rare. Many flows contain phenocrysts, inclusions, or vesicles, which hinder conchoidal fracture. Further limiting its availability, natural glass decomposes into perlite within a few million years of its formation, so only relatively recent obsidian flows usually contain glassy nodules large enough to have been used for stone tools.
Obsidian forms in a variety of different colors, with black and red as the most frequently observed colors, but gray, green, lavender, and brown are also found. Translucency may vary from clear to opaque, and banded or mossy color arrangements are common. Differences in obsidian appearance can be attributed to diverse factors such as microlites, gas bubbles, chemical variation, oxidation, or incorporation of foreign material into still liquid lava. Color, in some cases, was an important factor in the selection and use of particular obsidian sources in prehistory (for an example, see Hughes 1978), yet color, translucency, and sheen—especially when assessed visually—can vary within a single obsidian source or flow and are therefore unreliable markers of geologic provenance. Reliable source assignments are generally only possible through geochemical techniques.
In 1964, J. R. Cann and Colin Renfrew published a research article in the Proceedings of the Prehistoric Society (New Series) outlining the importance of new characterization and sourcing techniques for understanding prehistoric obsidian trade and exchange. This was a watershed moment in archaeological obsidian studies. Prior to Cann and Renfrew’s seminal work, obsidian was recognized as widely traded, appearing in assemblages far from the closest known sources, but other than the identification of a few visually distinctive sources, pinpointing its provenance remained impossible.
Unfortunately, the term “sourcing” is problematic (Shackley 2008: 196), despite its ubiquitous use in archaeology and archaeometry. In general, obsidian artifacts are “sourced” by analyzing the material to determine the chemical or elemental makeup of a sample (i.e., “characterizing”) and then matching that chemical fingerprint of the artifact to the chemical fingerprint of known geologic obsidian outcrops or flows. This is a statistical exercise in that the archaeometrist is determining the most likely fit between artifact and obsidian flow in order to assess the provenance of an obsidian sample. The limitations of this are twofold: first, in the accuracy of the analytical method; and second, in the comprehensiveness of the geologic database. Studies of characterization methodologies have demonstrated good comparative outcomes when data are generated by experienced archaeometrists using instruments calibrated to published geologic standards (Shackley 1998a). New geologic source databases are continually being generated and published for previously understudied areas through ongoing geologic and archaeological survey (Kuzmin et al. 2002; Glascock et al. 2006; Negash et al. 2006; Negash and Shackley 2007; Kuzmin et al. 2008; Ndiema et al. 2011; Reuther et al. 2011).
The definition of an obsidian “source” can also be problematic (Frahm 2012). Some archaeologists and archaeometrists use the term to designate a single locus on the landscape, in other words, where obsidian was likely procured or where geologic samples for source libraries were taken. Others use “source” to designate a statistical cluster of data indicating a chemical group in which a number of samples have similar geochemical compositions. However, both uses of the term “source” can be misleading. Some geologic loci may contain multiple discrete flows with different chemical signatures (Tykot 1998; Ambroz et al. 2001; Frahm 2012), others may have nodules of different chemical signatures intermingled and indistinguishable from one another on the landscape (Frahm 2012), while some singular chemical groups may be spread over many miles due to secondary geomorphological redeposition of obsidian from its original outcrop (Shackley 2005). This may matter to archaeometrists as we distinguish between statistical clusters of geochemical data, but an obsidian “source” as used by flintknappers in the past may be more than one chemical group at a point on the landscape, or a single chemical group at many points on the landscape.
To determine these geochemical data, chemical characterization techniques such as X-ray fluorescence (XRF) (Shackley 1998a, 2005), instrumental neutron activation analysis (NAA) (Glascock et al. 1998; Glascock et al. 2007b; Speakman and Glascock 2007), inductively coupled plasma mass spectrometry (ICP-MS) (Tykot 1998; Speakman and Neff 2005; Speakman et al. 2007), and proton-induced X-ray emission-proton-induced gamma-ray emission (PIXE-PIGME) (Summerhayes et al. 1998) are methods frequently used to characterize lithic materials such as obsidian. Through chemical characterization, it is possible to generate elemental data that are then matched with the best fit geologic point of origin, or provenance. Provenance is the term used to designate the geologic origin of a sample, while provenience is used to indicate the place of archaeological recovery of an artifact. Chemical characterization techniques used to generate data for assessing provenance are effective because of the unique nature of obsidian’s composition, in that proportions of trace elements—those elements present in concentrations of less than 1%—tend to vary between sources yet remain relatively homogenous within single flow events (Glascock et al. 1998; Shackley 1998a; Tykot 1998). Thorough sampling is necessary to determine the homogeneity of specific geologic sources prior to source assignments of archaeological specimens.
The utility of chemical characterization studies in sourcing archaeological obsidian samples has been proven many times over, and submitting obsidian artifacts for X-ray fluorescence analyses or other characterization methods has become standard practice among archaeologists in areas where obsidian is commonly recovered. Yet there is an underlying prerequisite for adequate geologic analysis to occur prior to and concurrently with archaeological sourcing studies. Research investigating the chemical homogeneity of obsidian sources has revealed that individual flows within rhyolite domes sometimes possess trace element chemical differences vast enough to warrant false assignment to distant sources (Hughes and Smith 1993; Hughes 1994; Shackley 1998a; Tykot 1998). Further complications for sourcing results can include secondary sources of obsidian that were often used prehistorically but did not necessitate travel to or quarrying at the original geologic source, such as nodules that have been transported through erosion in river systems and drainages over hundreds of miles (Shackley 1992, 1998a, 2000, 2005).
Because methods for chemical characterization of obsidian have been discussed in detail elsewhere (Taylor 1976; Davis et al. 1998; Glascock et al. 1998; Shackley 1998b, 2005, 2010, 2011a; Summerhayes et al. 1998; Speakman and Neff 2005; Beckhoff et al. 2006; Glascock et al. 2007b; Speakman et al. 2007; Speakman and Glascock 2007; Glascock 2010), I will only elaborate on X-ray fluorescence here, largely because with the advent of portable XRF instruments, obsidian studies have shifted from centralized archaeometric laboratories and practitioners to localized obsidian characterization and sourcing, sometimes even taking in the field. This shift has significant pros and cons, which I will discuss in more detail next.
X-ray fluorescence (XRF) is one of the most commonly employed chemical characterization methods used on lithic artifacts made from volcanic materials (Freund 2013; Hughes and Smith 1993; Glascock et al. 1998; Shackley 1998a, 2005, 2010, 2011a; Tykot 1998; Glascock 2010; Liritzis and Zacharias 2010). It is often the most appropriate method for archaeological specimens because samples do not need to be damaged or destroyed for analysis. New portable X-ray fluorescence (pXRF) technology is further ideal in archaeological contexts because it enables analysis of obsidian specimens in situ in the field or museum to obtain geochemical data (Sr, Y, Zr, Ti, Nb, and Rb) without necessitating transport of artifacts to a separate laboratory facility and allowing for large-scale geologic characterization to take place directly on a quarry site, natural outcrop, or obsidian flow. This also permits researchers to reexamine and retest specimens on site as needed and without additional cost.
Portable X-ray fluorescence instruments have resulted in a transition in who performs and interprets characterization data, specifically shifting from obsidian analyses being conducted by professional archaeometrists at a handful of commercial or academic laboratories, to many obsidian analyses being conducted in the field laboratory or even directly on site by the archaeologists themselves. These instruments are frequently small and potentially handheld, with an appearance similar to that of a radar gun and a price tag that ranges from approximately $20,000 to $50,000, with used instruments selling for considerably less. The biggest advantage of pXRF technology is that they can be used on objects that cannot be moved to a laboratory, either due to size, transport restrictions (i.e., from countries that will not permit collections to be exported), or museum collections. Additionally, there has been a demonstrated advantage to on-site analyses that can help inform the direction and scope of ongoing research (Frahm et al. 2014).
A number of archaeometrists have recognized that there are potential problems with accuracy and interpretation of data when pXRF instruments are used, particularly by inexperienced analysts (Shackley 2010, 2011a; Goodale et al. 2012; Speakman and Shackley 2013; Frahm 2013a, 2013b; Fauman-Fichman 2014). In some cases, the data, in the form of an assessment of the elemental composition of an obsidian object, are not accurately reproduced from instrument to instrument, or they do not conform to the published and accepted elemental composition of geologic obsidian standards. However, this does not mean that all pXRF data are bad, but instead, that best practices, including analysis of well-documented obsidian standards concurrently with each analysis run, are important in maintaining rigor (Speakman and Shackley 2013).
Some demonstrated variability exists in the sensitivity of different characterization methods and between individual instruments (Glascock 1999; Shackley 2010; Goodale et al. 2012; Frahm 2013a, 2013b). Manufacturers and archaeometry practitioners are trying to devise ways to make their data comparable between instruments. One step toward solving this problem is through the calibration of these machines using a suite of source standards, such as those compiled by the Missouri University Research Reactor and used in the factory calibration of all pXRF instruments sold by Bruker AXS, one of the larger pXRF manufacturers that markets to archaeologists. In theory, calibrating instruments to known standards should make all instrument data comparable, and therefore, source samples run on one instrument can produce data that may be used by others in determining provenance. However, a study of Bruker’s pXRF instruments conducted in 2012 concluded that “it is the responsibility of the pXRF user to evaluate and modify any factory calibrations as appropriate (or generate their own) to ensure that data are valid and reliable. Factory calibrations, while useful and informative, should never be accepted by the researcher as the final ‘solution’ without first evaluating performance against known reference materials” (Speakman 2012: 8). This caution applies not just to instruments sold by Bruker AXS, but to all pXRF instruments and all manufacturers.
These new factory calibrations are designed to lessen archaeometrists’ concerns about the comparability of instruments and methods. Specifically, can source data generated from one instrument or analysis be used as a geologic source library for comparison with characterization data obtained from another instrument or analysis? In other words, are the results of an analysis accurate and repeatable on another instrument? Some pXRF users have stated that internal consistency is sufficient and accuracy might not matter, but this has been criticized for veering widely from best practices in archaeometry (Shackley 2010).
Ellery Frahm (2013a, 2013b) has argued that the accuracy of the pXRF instrument, as measured by how well the resulting data compare to published obsidian standards, might not matter as long as it is possible to use these data to answer archaeological questions. He was able to demonstrate that off-the-shelf calibrations and settings for pXRF instruments yielded source assignments that matched those achieved through laboratory-based, benchtop XRF analysis in 94% of cases (Frahm 2013a, 2013b). However, performing analyses in this way, without repeatable analyses of published obsidian standards and without empirical calibrations, has been severely criticized for a lack of accuracy (Shackley 2010; Speakman and Shackley 2013). Specifically, the problem is that the characterization data may be revealing elemental concentrations that are not correct—either higher or lower than the actual concentration—and more dangerously, that they are not skewed directionally. These errors can occur through operator error, such as by analyzing a sample that is too small, or if the sample is not making good contact with the analysis window, the sample has some kind of unrecognized surface contamination, or other factors. However, sometimes errors can also arise unexpectedly.
Most of the time, problems with the accuracy of characterization data can be recognized and corrected, particularly when an experienced archaeometrist examines the data (for an unusual example of this, see Shackley and Dillian 2002). However, this lack of accuracy is most concerning when the instrument and data are in the hands of an inexperienced archaeometrist who might not realize the errors appearing in the instrument output. When an off-the-shelf pXRF instrument is used by an experienced operator, as in the case of Frahm’s study, the archaeometrist will presumably recognize anomalous data and reanalyze a sample or conduct an analysis using an alternate method. However, echoing Shackley (2010), it is important for pXRF practitioners to make sure they understand the instrumentation, geology, and data output in order to make valid source assignments based on characterization data (see Fauman-Fichman 2014 for a cautionary tale about erroneous source assignments).
Yet despite potential problems with the use of pXRF in obsidian analyses, the introduction of this new instrumentation has resulted in a massive democratization of the field. Now pXRF analyses are conducted by archaeologists instead of in specialized archaeometry laboratories, and even sometimes on site, where the data can be used to help drive ongoing research methodologies in real time. Rapid pXRF analysis, in which source assessments can be revealed in mere seconds, can further revolutionize the way in which obsidian is integrated in research design through the analysis of much larger numbers of samples (Frahm et al. 2014). Particularly in regions where the export of artifacts to an archaeological laboratory is limited, rapid pXRF analysis can yield significantly more information, including the identification of rare sources in an archaeological assemblage (Frahm et al. 2014). Finally, the ability to analyze artifacts in restricted regions or collections has resulted in both the identification of additional geologic sources and the clarification of previously unknown patterns of lithic resource use that are changing the way in which we understand the prehistoric past.
The advent of obsidian hydration dating in the 1960s also marked a watershed in archaeological obsidian studies, yet problems quickly became apparent when dates obtained through obsidian hydration were compared with those achieved through other methods. Hydration dates are affected by factors such as environmental conditions and chemical compositions that were not anticipated when the technique was first described and published. Obsidian hydration dating is performed by cutting a small sample from an artifact and then measuring the thin hydration rind that has formed on the surface of archaeological obsidian. Calendar dates are based on rind thickness calibrated to additional factors such as obsidian chemistry, relative humidity, and effective hydration temperature (EHT). Despite the use of this technique for over 50 years in archaeology (Friedman and Smith 1960; Friedman and Long 1967; Meighan 1976), there are still questions and problems with its implementation that hydration specialists seek to overcome (Meighan 1976, 1981, 1983; Michels and Tsong 1980; Friedman and Trembour 1983; Friedman et al. 1994; Ridings 1996; Anovitz et al. 1999; Hull 2001). However, in situations where other methods of dating are unavailable, it offers one potential source of chronological information. Unfortunately, obsidian hydration does require the removal of a small piece of the artifact and is therefore destructive, so it is not ideal for some specimens or collections.
Obsidian hydration works because of the way in which a fresh surface of obsidian, such as one exposed when a flake is removed from a core, will react with water in the atmosphere. Obsidian will hydrate in the presence of both liquid water and water vapor, though most moisture occurs as the latter (Stevenson et al. 1998: 183). Water vapor creates a thin molecular layer of water on the surface of the obsidian, which is continually replenished through interaction with the surrounding matrix. Two processes occur when a fresh surface of obsidian is exposed to water: first, water slowly diffuses into the obsidian, creating a hydration rind; and second, the glass actively dissolves until silica saturation of the water layer is achieved. Obsidian hydration is possible because the rate at which water is absorbed by the obsidian occurs at a faster rate than the dissolution of the obsidian. However, obsidian hydration is both a physical and a chemical process. As the obsidian matrix absorbs molecular water, hydronium ions replace mobile cations, such as sodium, in the obsidian (Stevenson et al. 1998: 183). These processes create a birefringence layer visible under a polarizing microscope, and it is this layer that is measured in obsidian hydration dating.
When first published, obsidian hydration dating generated a great deal of enthusiasm, but subsequent studies have demonstrated that the hydration process and its measurement are not as straightforward as initially thought. Specifically, date calculations based on hydration rim measurements may be incorrect due to an incomplete understanding of the hydration process; environmental factors such as fire or excessive heat; or measurement errors resulting from inaccurate temperature data, imprecise equipment, or operator error.
Hydration dating methods and equations are being refined (Stevenson et al. 1998; Stevenson et al. 2000; Rogers 2011, 2013, 2015a, 2015b; Duke and Rogers 2013; Rogers and Duke 2014), but it has been suggested that the equations currently in use are merely crude tools for estimating approximate dates and should by no means be viewed as accurate assessments of chronological data (Anovitz et al. 1999: 735). However, accelerated hydration experiments have been used as one means for refining and reevaluating hydration equations (Origer et al. 1997). Accelerated hydration, subjecting obsidian specimens to pressure, heat, and steam in a laboratory setting for controlled periods of time, is used to calculate hydration rate differences between and among obsidian sources. These data are used to refine the equations used to compute calendar dates from hydration measurements.
Environmental dynamics such as fire and excessive heat are known to affect the hydration rim thickness of obsidian specimens (Anderson and Origer 1997; Origer et al. 1997; Loyd 2002; Steffen 2002). Experimental heating has shown that hydration rims disappear or become indistinct following burning, even at relatively low temperatures (Anderson and Origer 1997; Soloman 2002). However, accelerated hydration of burned samples has resulted in hydration rims similar to those which formed on previously unhydrated specimens during the same period (Origer et al. 1997). This could obviously pose potential problems in environments that burn periodically or in those situations where the burning history of the area is unknown (Skinner 2002). Experiments dealing with the effects of fire on hydration rim measurements have questioned the lowest temperature thresholds at which hydration rims are altered and found that temperatures as low as 150°C can result in diffuse hydration rinds, particularly in situations of extended durations of heat exposure (Deal 2002). Imprecise temperature data, including differences in surface and subsurface temperatures, microclimates, or shaded areas, can also affect hydration rates and calculations. Small-scale, regionally specific climatic data, including surface and subsurface temperatures, may be necessary for calculating age estimates from hydration rim measurements (Hull 2001).
The chemical composition of obsidian is also a variable factor in hydration rate. Above all, intrinsic water strongly affects hydration rate. Compositional variability between sources is accounted for to some degree in the hydration formula, which is specific to a particular source. However, chemical variability within a single source is rarely addressed in hydration analyses, though it is now recognized that structural water content varies within sources (Stevenson et al. 2000). Geochemical characterization studies have also revealed that compositional variability is a major issue in the identification and sourcing of archaeological specimens (Shackley 2000) and, by extension, may be a factor in hydration studies as well. If obsidian flows are highly variable in chemistry and water content, compositionally specific hydration rate formulas may be necessary for accurate assessment of chronological data.
Finally, operator or equipment error can have drastic effects on the date assignments for hydration specimens. Comparative studies of the hydration rim measurements obtained by independent operators on the same specimens have shown a wide range of variation (Jackson 1984; Stevenson et al. 1989). New methodologies for detecting and measuring hydration thicknesses are increasingly gaining popularity, though they may add higher costs to a traditionally low-cost and low-tech dating technique. Experimental use of secondary ion mass spectrometry (Riciputi et al. 2000; Stevenson et al. 2001; Stevenson et al. 2004) and infrared photoacoustic spectroscopy (Stevenson et al. 2001) for detecting and measuring hydration rims has shown some success, but it has yet to gain widespread use in archaeology.
In areas where tool-quality obsidian was present in prehistory, it was a notable component of the lithic tool kit. Obsidian forms an exceptionally sharp edge when flaked, but that cutting edge also dulls quickly, meaning that it is ideal for fine cutting or piercing of soft materials, but not durable enough for tasks such as chopping. Yet its value was important enough that it was used throughout human prehistory, and people traveled long distances or engaged in extensive trading in order to obtain it.
Obsidian exploitation in Africa predates the evolution of modern Homo sapiens, suggesting that even our earliest ancestors recognized the superior cutting edge that could be achieved with this material. One of the earliest uses of obsidian in prehistory has been documented at the Early Stone Age site of Melka Konture in central Ethiopia. This site, dating to as early as 1.6 million years ago, revealed hominin procurement and use of obsidian (Morgan et al. 2012). However, obsidian data also indicated that these early hominins had a relatively restricted territory, demonstrated through obsidian sourcing, which showed a heavy reliance on Balchit obsidian, located only about 10 km distant. Artifacts from this site are primarily associated with Developed Oldowan (H. habilis) and Acheulean (H. erectus) technologies. Hominins during the Early Stone Age used lithic sources close to home (Negash et al. 2006). This pattern has been demonstrated at hominin sites at Koobi Fora in Kenya as well, where hominins may have been transporting lithic materials such as basalt as much as 20 km, but transport distance appears to increase through time (Braun et al. 2009), perhaps with the arrival of H. erectus and later H. sapiens.
By 0.7–1.0 million years ago, hominins were making Acheulean bifaces out of obsidian, as evidenced by finds of more than 100 artifacts at the Upper Sites at Kariandusi in Kenya. This site, first excavated by Louis Leakey, contained Acheulean bifaces proposed to have come from the Eburru area to the south (Gowlett and Crompton 1994). At Gadeb, Ethiopia, an obsidian artifact may have come from a source approximately 100 km away (Clark and Kurashina 1979; Noll 2000). Some of the assemblages from Olorgesaile dating between about 400,000 and 340,000 years ago contain as much as 55% obsidian in the lithic assemblage (Haradon 2010: 163). At the Halibee site in Ethiopia, dating between 106,000 and 54,000 years ago, obsidian artifacts have been sourced to Kone about 200 km away (Morgan 2009). By the time modern Homo sapiens appeared on the landscape in Africa, obsidian procurement, exchange, and use were relatively well established in areas where obsidian was available. Obsidian was a significant part of the prehistoric toolkit in East Africa during the Holocene (Ndiema et al. 2011). Even today, obsidian continues to be used for hide production in southwestern Ethiopia (Brandt 2015).
The procurement of obsidian was not necessarily a complex task. For example, at some sources, obsidian may simply be picked up from river alluvium. The Cow Canyon obsidian source located in eastern Arizona has completely weathered away, so that marekenites of Cow Canyon obsidian were potentially collected from the area of the original outcrop and down into the Gila River system. The Mule Creek obsidian source of western New Mexico also eroded into the Gila River. As a result, both types of obsidian were available across a wide, and overlapping, geographic region. Mule Creek and Cow Canyon obsidian could be obtained from any location where it occurred in the alluvium (Shackley 1998a).
At the Melos, Greece, quarries of Sta Nychia and Demenegaki, prehistoric people obtained obsidian by simply digging into a soft matrix to remove large, unfractured cobbles of tool-quality obsidian. There, initial knapping took place at the quarry before cores and blades were removed for use or exchange (Torrence 1986).
In contrast, the obsidian mines of Pico de Orizaba, Veracruz, Mexico, were highly organized, specialized operations (Stocker and Cobean 1984). Extensive tunnel systems were recorded at Valle del Ixtetal, one of several obsidian sources at Pico de Orizaba, some measuring as long as 70 meters. At the mine, wooden levers were used to follow natural fractures in the obsidian and remove large blocks, which were then manufactured into uniform blade cores (Stocker and Cobean 1984: 92–93).
Some of the earliest studies of obsidian procurement focused on questions of exchange, and obsidian was used to develop the classic “fall-off curves” of lithic exchange in the Neolithic Near East (Dixon et al. 1968; see Frahm 2012 for a detailed summary). From an economic viewpoint, exchange served to move obsidian through space from producers at quarries and workshops to consumers. In parts of the United States, where sourcing of lithic materials is standard practice, the procurement and exchange of lithics has been used to reconstruct extensive seasonal rounds and territorial boundaries, such as large-scale studies in California and the Great Basin that have been used to infer cycles of mobility that incorporated obsidian procurement into seasonal or annual rounds (Hughes 1978: 53; Bettinger 1982: 103–127; Hughes and Bettinger 1984; Lyneis 1984; Bennyhoff and Hughes 1987: 161; Luhnow 1997; Jones et al. 2003; Jones et al. 2012). Studies of territory and mobility traditionally focused on subsistence needs as a central focus of seasonal mobility, as suggested by Binford (1980), but the archaeological investigations of these behaviors often require traceable, durable archaeological artifacts or an ethnohistoric record. In one innovative example, XRF analysis of obsidian debitage was used to delineate the boundary between the Gumbatwas and Kokiwas bands of the Modoc Nation, and differences of obsidian use between the two bands were clearly visible in archaeological assemblages. The Kokiwas Modoc were using Blue Mountain obsidian for most, and in some instances all, of their stone tool requirements, whereas the Gumbatwas Modoc utilized Medicine Lake sources, including Glass Mountain, Grasshopper Flat, Lost Iron Wells, Cougar Butte, and East Medicine Lake. This demonstrated the differences in obsidian procurement and use patterns that represented territorial boundaries (Luhnow 1997).
In other areas, where obsidian was not readily accessible, it was exchanged over very long distances. For example, in the United States, the presence of western US obsidian in Mississippian and Hopewellian contexts in the middle United States has been clearly documented, indicating transport or exchange distances over 2,500 km, possibly through one or more bursts of exchange between the Rocky Mountains region, specifically Obsidian Cliff in Yellowstone, and the Ohio and Mississippi River valleys (Griffin 1965; Griffin et al. 1969; Anderson et al. 1986; Hatch et al. 1990; Hughes 1992, 1995; Hughes and Fortier 1997; Lepper et al. 1998; DeBoer 2004). Other obsidian artifacts have been found on Early Woodland sites in Wisconsin (Stoltman and Hughes 2004: 751), and a scraper was found at Spiro Mounds in Oklahoma that was made from obsidian originating in Hidalgo, Mexico (Barker et al. 2002: 103). In the mid-Atlantic United States, exceptionally rare obsidian artifacts have been determined to have come from sources in California, Utah, Nevada, and Idaho, suggesting transport and exchange over 3,500 km, probably through casual, down-the-line exchange networks (Dillian et al. 2010).
In instances of long-distance exchange, obsidian’s importance may have been more than merely pragmatic. Qualities unique to obsidian, such as color, luster, sheen, and translucency, may have made its value more important than as mere toolstone (Dillian 2002; Torrence 2005; Dillian et al. 2010; Torrence et al. 2013). This continues even today, when a simple search on the craft site Etsy yields more than 10,000 items made of obsidian, mostly in the form of jewelry that highlights obsidian’s glossy black color and shiny luster (Oct. 16, 2015, https://www.etsy.com/search?q=Obsidian). Throughout prehistory, people traded over long distances to obtain nodules of obsidian that could have been used for a wide variety of tools or personal items (Torrence 1986; Hatch et al. 1990; Hughes 1992, 1995; Shackley 1992; Hughes and Fortier 1997; Lepper et al. 1998; Glascock 2002; Yacobaccio et al. 2002; DeBoer 2004; Rosen et al. 2005; Boulanger et al. 2007; Dillian et al. 2007, 2010; Tykot 2011). Torrence suggests that this is because unmodified fragments of unusual lithic material “are more easily linked to distant, unknown, unpeopled and mysterious places than are products that exhibit identifiable skills and/or forms which come from populated and possibly known places or individuals” (2005: 366).
The field of obsidian studies is changing rapidly, and much of this has been driven by new technology. The biggest change over the last few years has been the advent of portable instrumentation that brings obsidian sourcing directly into the field. As portable X-ray fluorescence gains in popularity, obsidian sourcing has evolved from a laboratory-based, expensive, and time-consuming analysis, costing approximately $20 to $35 per sample with at best a few days’ turnaround time to mail and receive samples from archaeometry laboratories. Instead, it has now become a more economical and immediate component of research design and project implementation, with obsidian analyses taking place in the field or field laboratory concurrent with excavations. This will enable two important outcomes: first, archaeologists will be able to analyze a much larger number of obsidian samples from any site, potentially revealing unusual or unexpected sources represented in small numbers in archaeological assemblages and allowing for regional syntheses of obsidian exchange and procurement; and second, archaeologists will be able to alter research design based on source data revealed in real time, rather than waiting for results after the field season has ended, permitting changes in research design based on these data and enabling more reflexive methodologies.
Large-scale studies of obsidian circulation across broad geographic regions may offer new insight into population movements and interactions that no longer focus on a single site or small sample, but instead, can include thousands, even hundreds of thousands, of samples across regions or even continents. In the United States, regional syntheses have provided valuable information on a large scale, but they have been performed mainly by archaeologists who operate sourcing labs or are partnering with researchers at archaeometry laboratories, such as studies in California, the greater US Southwest, and the Great Basin (Jones et al. 2003; Silliman 2003; Shackley 2005; Beck and Jones 2011; Gilreath and Hildebrand 2011; Janetski et al. 2011; Kelly 2011; King et al. 2011; Jones et al. 2012), but new large-scale syntheses exist or are in progress for other areas, such as Mesoamerica, the Mediterranean, and the Pacific Rim (Tykot 1998; Kuzmin and Glascock 2010; Levine and Carballo 2014). As portable X-ray fluorescence technology is obtained by more individuals and institutions, which judging from the numbers of presentations at regional, national, and international conferences, is happening now, these kinds of regional syntheses will become more accessible and can give us a great deal more information about how prehistoric people were using landscapes, interacting with others, and selecting resources.
However, portable X-ray fluorescence studies will only be taken seriously if practitioners utilize best practices to ensure accuracy and validity in their data. As discussed by Frahm, Shackley, Speakman, and others, pXRF practitioners must familiarize themselves with not only the artifacts and their context but also with the regional geology and pXRF instrumentation (Shackley 2010, 2011a, 2011b; Goodale et al. 2012; Speakman 2012; Frahm 2013a, 2013b; Speakman and Shackley 2013). Published geologic standards should be analyzed concurrently with archaeological and geologic specimens in order to ensure accuracy and comparability of data.
Another direction in obsidian studies that has been gaining new traction in the field is the analysis of obsidian as more than mere toolstone. The emphasis on the tangible and intangible value of obsidian to past populations has become a new research question that is explored through differential patterns of use or nonutilitarian uses of this material (Dillian 2002; Torrence et al. 2013; Healey 2014; Healey and Campbell 2015; Lopiparo 2015; Moutsiou 2015; Rath 2015). A recent session at the Society for American Archaeology annual conference in San Francisco, California, highlighted the aesthetic value, social importance, and status indicators that can be essential elements of obsidian use and exchange (Torrence and Dillian [organizers] 2015). Despite obsidian’s exotic beauty and lustrous sheen, it may not always mark high status, but it can instead be an indicator of low status, as in the case of modern-day hideworkers in Ethiopia (Brandt 2015).
One of the reasons for obsidian’s importance as a meaningful material may have been the spectacular phenomena of obsidian-forming eruptions, some of which could have been witnessed by prehistoric people (Dillian 2002). Rhyolitic eruptions, those that form obsidian, are exceptionally rare. In fact, one of the few rhyolitic eruptions to occur in modern times began on May 1, 2008, at the Chaiten volcano in northern Patagonia, Chile. This was the first rhyolitic eruption to be monitored scientifically and has led to an extensive rewriting of our understanding of how rhyolitic eruptions occur.
In the Chaiten example, earthquakes began only a mere 24 hours before the eruption, which is vastly different from the seismic events preceding basaltic eruptions than can occur for weeks or months (or even years) before an eruption. Instead, magma ascended at a rate of almost a meter per second, leading to a devastating eruption with little warning of the impending hazard (Castro and Dingwell 2009; Pallister et al. 2013). Merely one day later, a large Plinian explosion blew out the mountain’s earlier obsidian dome and released large quantities of pumice and ash, and potentially obsidian (Castro and Dingwell 2009; Pallister et al. 2013). Visually, rhyolitic Plinian eruptions are impressive. Ash spews into the sky as high as 25 to 40 kilometers, pumice fragments from a few centimeters to more than a meter in size may be hurled from the volcano, and the night would be lit with glowing clouds of lightning (Few 2012).
Though the Chaiten volcano has not been documented as a prehistoric source of obsidian artifacts, the kinds of eruptions observed there would have been similar to those producing obsidian elsewhere. When a rare, but visually arresting and potentially devastating natural event such as a rhyolitic eruption occurred, prehistoric people would have certainly taken notice of it and may have incorporated the phenomenon into local practice. At Glass Mountain, for example, obsidian was formed by a massive eruption that was witnessed by local populations. Glass Mountain is a large, tool-quality obsidian source located in the Medicine Lake Highlands of northeastern California. The obsidian eruption is dated to 885 B.P., placing it within a time of late-prehistoric habitation of the region. Archaeological sites within the adjacent Modoc Plateau bracket the eruptive event (Sampson 1985; Hughes 1986; McAlister 1988; Baker et al. 1990; Busby et al. 1990; Gates 1991; Delacorte et al. 1995; Moratto 1995; Bevill and Nilsson 1996; Mikkelsen and Bryson 1997; Gates et al. 2000).
Fieldwork conducted at Glass Mountain revealed a singular pattern of raw material procurement and production at the quarry. Survey, surface sampling, and subsurface testing indicated that large biface production was the primary activity performed at Glass Mountain, evidenced by high percentages of identifiable biface thinning flakes and biface fragments in all stages of production. Retooling activities were conspicuously absent in the debitage and tool assemblages. Though a handful of utilized flakes were found, no projectile points, projectile point fragments, knives, formed scrapers, drills, or other formed tools were observed. The only formalized objects recorded were large bifaces and biface fragments that were produced at the quarry and then transported to the coast of northern California and used as wealth, family heirlooms, and important components of the White Deerskin Dance, a world renewal ceremony practiced by coastal nations. Furthermore, Glass Mountain obsidian is exceptionally rare in other sites, making up only approximately 5% of local assemblages away from the quarry that date within the last 900 years. The restriction of this obsidian to uses that can be categorized as ceremonial derives from the observation of obsidian’s formation at this source, and it marks a connection between the phenomenon of rhyolitic eruption and traditional practice (Dillian 2002).
However, studies such as these require refinements to our chronological control and therefore to improvements in obsidian hydration dating. Hydration dating methods and equations are being improved, which may allow us to gain better and more accurate dates for obsidian specimens (Stevenson et al. 1998; Stevenson et al. 2000; Rogers 2011, 2013, 2015a, 2015b; Duke and Rogers 2013; Rogers and Duke 2014). However, we may also find that optical measurements of obsidian hydration rinds are ultimately flawed or that we may need a better understanding of the way in which obsidian hydrates in order to obtain useable dates for obsidian artifacts (Michels and Tsong 1980; Meighan 1981, 1983; Friedman and Trembour 1983; Friedman et al. 1994; Ridings 1996; Anovitz et al. 1999; Hull 2001). Perhaps new applications of techniques such as secondary ion mass spectrometry (Riciputi et al. 2000; Stevenson et al. 2001; Stevenson et al. 2004) and infrared photoacoustic spectroscopy (Stevenson et al. 2001) for detecting and measuring hydration rims will be the new direction for obsidian dating. Unfortunately, many archaeologists remain skeptical of the utility for obsidian hydration dating today, but hopefully this will change with new technological advances.
When Cann and Renfrew’s “The Characterization of Obsidian and Its Application to the Mediterranean Region” (1964), and Friedman and Smith’s “A New Dating Method Using Obsidian” (1960) were first published, it is unlikely the authors realized how important these contributions would be to obsidian studies and the discipline of archaeology as a whole. An entire field of research was spawned by the ability to source and date obsidian artifacts, rendering a material that was already deemed unique and noteworthy into one that could yield valuable provenance and chronological data. As we move forward in the field, we can ask new questions and gain new insights into prehistoric patterns, networks, belief systems, status, and symbols. We must not forget that the people who used this material in the past may have had similar reactions to ours upon seeing, touching, and using obsidian. It may have been meaningful in ways we have yet to understand. Through new research, new methods, and new instrumentation, our knowledge and interpretation of the past may be greatly enhanced by the study of this remarkable stone.
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