Glassy rocks can form either from extremely rapid cooling or high viscosity due to high silica contents.

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Glassy rocks, also known as volcanic glass or obsidian, can form either from extremely rapid cooling of lava or magma, which prevents the formation of crystal structures, or from a high viscosity due to high silica contents, which slows down the formation of crystals and results in a glass-like texture.

Glassy Rocks Can Form Either from Extremely Rapid Cooling or High