Obsidian

Abstract

Obsidian is silicic volcanic glass that has undergone minimal crystallization, and has been documented in volcanic products spanning rhyolitic to phonolitic compositions. Although most abundant within lava flows, obsidian is also found in volcanic bombs and shallow intrusion margins. Contrary to popular belief, rapid cooling is not required to form obsidian, as sluggish crystal growth in silicic melt allows quenching to amorphous glass over hours to days. Distinctive attributes of obsidian include vitreous luster and conchoidal fracture, which generates exceptionally sharp edges. The color of obsidian ranges widely from common brown-black to orange, green, blue, gray or even near-colorless, and is influenced by the presence of sub-micron-sized crystals. Common textures include flow banding, breccia, devitrification (including spherulites and lithophysae), and vesicular bands. In dry environments obsidian, being a metastable glass, may persist for tens of millions of years. However, obsidian is readily hydrated by external water to generate perlite and pitchstone, with hydration occurring rapidly at hot intrusion margins. Perlite and pitchstone water concentrations (3–10wt%) greatly exceed that of obsidian from lava flows (≤0.5wt%), intrusion margins, and bombs (≤2wt%). Obsidian genesis records the transition from more explosive to gentler silicic eruptions. Proposed mechanisms include the welding of fragmented magma and the collapse of pumiceous magmatic foam within shallow silicic conduits. Obsidian had great importance in antiquity as a raw material for tool manufacture, due to the sharpness of its edges, and may be amongst the first traded commodities. Notable occurrences of obsidian include Mexico, Ethiopia, Kenya, Turkey, Italy, Iceland, Greece, Armenia, Chile, and the western USA. Variants include perlite, pitchstone, pyroclastic obsidian, snowflake obsidian, mahogany obsidian and rainbow obsidian. At high vesicle concentrations (>~50% by volume) obsidian is termed pumice.