Even in volcanically active regions of the world, the geological formation of high quality obsidian is a relatively rare event in nature because a number of features must co-occur for volcanic magma to become tool-quality natural glass. The following description was developed largely from Shackley (2005: 10-15, 189) and Fink and Manley (1987). Obsidian can form when rhyolitic magmas are extruded and quenched in the course of a volcanic eruption. Rhyolitic magmas are silica-rich, acidic melts that are capable of flowing as viscous lavas. As rhyolitic magmas approach the earth's surface, the high water content (up to 10% H2O) begins to escape as vapor, changing the viscosity and the cooling rate of the flow, and resulting in a very low presence of water in obsidian. When water remains trapped in obsidian it sometimes forms bubbles of water vapor, reducing the homogeneity and fracture quality of obsidian for tool production. Fink (1983) found that obsidian emplacement tends to proceed along the following sequence associated with the eruption: (1) tephra fall-out from the initial explosive eruption, (2) basal lava breccia, (3) coarsely vesicular pumice, (4) the principal obsidian flow, (5) finely vesicular pumice, and (6) surface breccia. The best quality obsidian for tool-production often occurs not on the ground surface but slightly underground in subsurface emplacements around a volcanic vent where degassed magma squeezes into rock fractures free of dirt and ash particles. Obsidian over 20 million years old is rarely useable for tool production because, as a geologically unstable material, obsidian gradually devitrifies from a glass into a rock (Francis and Oppenheimer 2004: 163).
|
Characteristic |
Value |
Compare with |
|
Composition |
Rhyolite (Felsic) |
As an intrusive rock it is granite |
|
Silica content |
Rhyolite is usually >70% wt SiO2 |
Basalt: <52%, Andesite: 53-63% wt SiO2 |
|
Water content |
Obsidian: 0.1 - 0.5 H20 |
Perlite: 3-4%, Pitchstone: 4-10% H20 |
|
Age |
Quality obsidian is usually <20 Ma |
Obsidian >66.4 Ma (KT boundary) is devitrified. |
|
Hardness |
Obsidian: 5.0 - 5.5 |
Quartz: 7.0 |
|
Specific gravity |
Obsidian: 2.6 (2600 kg/m3) |
Pumice: 0.64, Water: 1.0, Basalt (solid): 3.0 |
|
Compressive strength |
Obsidian: 0.15 |
Chert: 100 - 300 |
Table 4-4. Characteristics of Obsidian.
During the extrusion of rhyolitic lavas it is the supercooling (instantaneous quenching) of the lava that creates obsidian, an atomically disordered natural glass with the structural properties of non-flowing liquid. This lack of crystalline structure in aphyric obsidian results in an isotropic lithic material with excellent flaking properties and the potential for extremely sharp edges because it has no prevailing fracture direction and it fractures at the molecular level. Obsidian has a low specific gravity as it is acidic, it also lacks crystalline structure, and it has relatively low hardness. Obsidian has high tensile strength but it has extremely low compressive strength and, combined with the non-crystalline structure, the result is implements with relatively brittle characteristics and fragile working edges (Hughes 1998: 367;Luedtke 1994: 93;Obsidian 2006;Speth 1972: 52). The cortex ofobsidian from primary deposits can visually vary widely depending on the context ofemplacement and weathering processes. Obsidian flows that cool where tephra is present can melt a thin layer of the adjacent ash and the fused material appears as a thicker cortex (Figure 4-21).
An important attribute of obsidian for archaeological investigation is that obsidian flows are chemically distinctive allowing artifacts to be chemically linked to their geological source areas. These chemical differences in obsidian are the result of certain elements crystallizing to solids and being removed from the magma as per the Bowen reaction series, resulting in a distinctive geochemistry for lava from most magma chambers and sometimes for each extruded lava flow. Prior to, and during, a volcanic eruption, magma evolves as changes in temperature and pressure causes chemical differentiation and leads certain minerals to crystallize and settle out of the melt.
As magma evolves, further melting and crystallization change the nature of the solids, and crystals that accept the incompatible elements may form in the liquid. Some feldspars, for instance, are good hosts for strontium, as is mica for rubidium. "Evolved" obsidian magmas may contain these crystal "hosts," and the ratio of a given element between the liquid and solid phases will change dramatically. Changes of this kind issue a particular chemical character to a given obsidian…The result of these processes is that the incompatible-element mix of a given obsidian source varies from any other and becomes a sensitive indicator of origin (Shackley 2005: 10-11).
This process creates detectable chemical differences in obsidian that permits methods such as X-ray florescence (XRF), Instrumental Neutron Activation analysis (INAA), and various types of inductively coupled plasma mass spectrometry (ICP-MS), and Proton-induced X-ray emission-proton-induced gamma ray emission (PIXE-PIGME) to chemically characterize the material (Neff and Glascock 1995;Shackley 1998).
The color of obsidian is most often black but it also occurs in red, brown, bronze, purple, blue, green, gray, silver, clear, as well as with banding that includes some of the colors listed above. Obsidian coloration results from the oxidation state of tiny crystals that occur in the melt (Volcano Hazards Program 2000). The black color that is common in obsidian is the result of tiny (< .005 mm) magnetite (iron oxide) crystals, red is usually from hematite present in highly oxidized obsidian, and green results from variations in iron oxidation. Microscopic crystals of various types of feldspars may yield the unique blue, purple, green or bronze colors associated with "rainbow obsidian". Banding results from the folding-in of an oxidized flow surface as the lava continues to move, with each colored streak perhaps reflecting the individual pulses in the obsidian eruption. Gold and silver sheen obsidian is argued to be caused by bubbles of water vapor trapped in the glass that are stretched nearly flat along flow layers (Obsidian 2006). Given the unusual visual qualities of obsidian, the color and banding in a particular nodule are characteristics likely to have influenced human use of the material.