Obsidian is produced from felsic lava, rich in the lighter elements such as silicon, oxygen, aluminum, sodium and potassium. It is commonly found within the margins of rhyolitic lava flows known as obsidian flows. These fluxes have a high silica content, which gives them a high viscosity. The high viscosity inhibits the diffusion of atoms through the lava, which inhibits the first step (nucleation) in the formation of mineral crystals. Along with rapid cooling, this results in the formation of a natural glass from the lava.
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