Discussion
A glassy or vitreous texture, as in obsidian, forms when the melt is
Answer: A
Correct answer: chilled too rapidly for crystals to grow
Crystallisation requires atoms to migrate into an ordered lattice, which takes time. Lava chilled almost instantly - by contact with water or air at the surface - freezes with its atoms still disordered, giving a natural glass. Obsidian is the result, and being a glass rather than a crystalline aggregate it fractures conchoidally, which is why it was so widely used for blades.
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