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Obsidian flows usually consist of fold surfaces, explosion craters, flow banding, and cavities underling the craters.
The explosion craters are commonly located on the surface of the flow and can have diameters of 10-25m (36-91ft), at 5-15m (15-55ft) deep; the accompanying cavities can be 5-15m in length. The cavities form near the surface by a buckling mechanism. Buckling occurs by contrast of stiff upper layer relative to the near liquid flow interior, which forces the upper layer to make an almost triangular shape at the surface, separating it from the lower layer. These cavities can collect magmatic/meteoric water*, and as vapor pressure builds up and exceeds the strength of the surface crust of cavity an explosion results producing the explosion caters.
*Meteoric water: rain water/ground water, not from the magma.
Image depics the event of cavities forming by buckling and exploding due to gas pressure exceeding the strength of the rock. This illustration comes from Castro's Structural origin of large gas cavities in the Big Obsidian Flow, Newberry Volcano.
Flow banding in obsidian is a distinctive characteristic and the abundance of the flow banding. Even with the abundance of these flow bands within obsidian little is known about the origins of these features, but what is known is that these freatures derive from both crystallization and deformation processes. According to a paper by J.M. Castro the flow bands develope from the deformation of microlites (very small phenocryst) by shear strains during the flow of the obsidian. The picture below shows the flow banding as pointed out by the arrow.