You found a glassy mineral with no visible crystals in it. What can you tell about the formation of this mineral?

You found a glassy mineral with no visible crystals in it. What can you tell about the formation of this mineral?

• A. It was formed by slowly cooling magma
• B. It was formed by rapidly cooling magma
• C. It formed through metamorphic transformation
• D. It formed by precipitation from a solution

Answer: (B)

A glassy texture means the molten material cooled so quickly that its atoms couldn’t arrange into crystals. When magma erupts or comes into contact with cold air or water, it can be quenched in seconds, yielding a non-crystalline glass like obsidian. If cooling were slow, crystals would grow large and visible; metamorphic rocks form under heat and pressure with recrystallization into minerals, not a glassy, non-crystalline appearance. Crystallization from a solution produces minerals by precipitation, not a glass. So the mineral’s glassy, crystal-free form points to rapid cooling of magma.

A glassy texture means the molten material cooled so quickly that its atoms couldn’t arrange into crystals. When magma erupts or comes into contact with cold air or water, it can be quenched in seconds, yielding a non-crystalline glass like obsidian. If cooling were slow, crystals would grow large and visible; metamorphic rocks form under heat and pressure with recrystallization into minerals, not a glassy, non-crystalline appearance. Crystallization from a solution produces minerals by precipitation, not a glass. So the mineral’s glassy, crystal-free form points to rapid cooling of magma.