A glassy mineral with no visible crystals indicates formation by rapid cooling of magma.

A glassy mineral with no visible crystals indicates formation by rapid cooling of magma.

• A. Slow cooling of magma
• B. Metamorphic transformation
• C. Erosion by wind
• D. Rapid cooling of magma

Answer: (D)

The key idea is how texture reveals how fast a melt cooled. When molten material cools, atoms must arrange into a crystal lattice. If cooling happens slowly, atoms have time to organize and you get visible crystals. If cooling is extremely rapid, they don’t have time to form crystals at all, and the melt becomes a glassy, amorphous solid with no visible crystals. So a glassy material indicates the melt was quenched quickly, near or at the surface, producing a glassy texture. This is why rapid cooling of magma (lava) is the process that best fits a glassy mineral with no visible crystals. The other options don’t fit: slow cooling of magma would produce crystals large enough to see; metamorphic transformation changes mineral structure under heat and pressure rather than forming glass; erosion by wind is a surface process that wears away rocks and doesn’t create a glassy texture.

The key idea is how texture reveals how fast a melt cooled. When molten material cools, atoms must arrange into a crystal lattice. If cooling happens slowly, atoms have time to organize and you get visible crystals. If cooling is extremely rapid, they don’t have time to form crystals at all, and the melt becomes a glassy, amorphous solid with no visible crystals.

So a glassy material indicates the melt was quenched quickly, near or at the surface, producing a glassy texture. This is why rapid cooling of magma (lava) is the process that best fits a glassy mineral with no visible crystals.

The other options don’t fit: slow cooling of magma would produce crystals large enough to see; metamorphic transformation changes mineral structure under heat and pressure rather than forming glass; erosion by wind is a surface process that wears away rocks and doesn’t create a glassy texture.