270 IGNEOUS ROCKS/Obsidian Figure Close up of the Rocche Rosse obsidian flow, Lipari, showing alternations of fresh and devitrified glass layers Reproduced with permission from Guest JE, Cole PD, Duncan AM, and Chester DK (2003) Volcanoes of Southern Italy Bath: Geological Society Publishing House block lava: the surface has arcuate surface ridges (ogives) and talus margins The flow is several tens of metres thick and is typical of high-viscosity silicic lavas There are devitrified layers and lenses alternating with the black obsidian (Figure 2) There are also obsidian flows on the nearby island of Vulcano Armenia The Quaternary Gutansar volcano in Armenia shows a progression from basalts and andesites, through liparites, transitional liparite obsidians, to massive obsidian flows There is a fissure extrusion of black obsidian at the base, changing gradually to brownish and greyish slightly swelled glass, then to perlite and pumice The obsidian extrusion has a fanlike structure (Figure 3) The distribution of the obsidian bodies indicates very shallow magma chambers Such viscous magmas increase in volume and heave up continuously to the surface with regular accelerations, the process being one of tectovolcanic displacements Newberry Caldera, Oregon Newberry Caldera in the Oregon Cascades (USA), km in diameter, lies atop of a volcano initiated in the Pliocene, but its main activity occurred during the Pleistocene and the last eruptions occurred just over 2000 years ago The obsidian flows lie within the caldera and are among the most recent eruptives They present expanses of barren, shiny black obsidian, almost devoid of vegetation, and surfaces sharp as razors can cut the legs of the unwary geologist There are four separate steep-sided flows of glistening black obsidian The Big Obsidian Flow (Figure 4) covers 1.5 km2 and has its own dome of pumiceous obsidian plugging the vent Two smaller flows straddle a north-east-trending fissure from which they were erupted Another small flow was erupted from a vent on the caldera wall This development at Newberry Caldera is undoubtedly the most spectacular development of obsidian flows known, but the terrain is extremely difficult inside the caldera California Obsidian in the central eastern Sierra Nevada mountains of California, USA, at Mono Craters, occurs in a flow of viscous lava, as lenses and bands interleaved with rhyolite (Figure 5) The obsidian is very regularly banded (Figure 6) Two other spectacular obsidian flows occur further north, at Glass Mountain, California (Figure 7), in the Medicine Lake region of the Cascade Range, overlooking the Nevada and Oregon borders These flows are less than 1000 years old and are associated with small obsidian domes They display remarkable flow structures, of curved pressure ridges akin to those on the surface of glaciers