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Under which condition below would you expect a glassy extrusive rock like obsidian to form

User Nikson
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Quick cooling on earths surface
User James Cameron
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Obsidian is an extrusive rock that is formed when magma from underneath the Earth's crust breaches and becomes lava. Once the lava starts cooling, it solidifies and elements are frozen, giving the rock its glassy appearance.
User BenoitParis
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