How is Obsidian Formed? (GeoShort)

Have you ever heard of obsidian?  Mostly likely you have; it is the rock that was commonly used by early humans to make arrowheads and other weapons.  Obsidian is a very beautiful rock types, but it's formation is often misunderstood.  In this GeoShort, we briefly highlight how obsidian is formed and discuss how unique it is.  Obsidian is actually a glass, meaning it has no crystals in it!  In fact, obsidian is naturally very unstable at Earth's surface so it breaks down quickly.  This means that obsidian is usually a very young rock; no ancient obsidian occurs on Earth! 

Obsidian has been used for thousands of years by humans.  However, obsidian formation is usually misunderstood by people!  In this GeoShort we discuss how obsidian forms, which has more to do with the composition of the magma than the cooling rate.  The felsic (silica-rich) magma type that obsidian forms from is very thick and sticky.  This means that it cannot flow very quickly (a term called viscosity, or the resistance to flow) which also inhibits elements from diffusing within the molten rock.  The lack of diffusion means that crystals cannot form quickly, and the magma cools before mineral grains can form.  This creates a glass! Obsidian is often used in lapidary work as well as it polishes well and often occurs in beautiful colors! 

 
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