4.4. How obsidian is formed

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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).