Why do some beaches have black sand?

Answer: Volcanic rock eroded to sand

Decomposed marine organisms — Wrong. Organic matter can darken sand slightly, but true black sand comes from volcanic minerals (basalt, obsidian) eroded to grains.

Burned vegetation washed up — Wrong. Charcoal can create dark spots, but natural black sand beaches are from volcanic rock weathering into tiny grains over time.

Volcanic rock eroded to sand ✓ — Correct! Black sand beaches form where volcanic rock (basalt, lava) weathers and erodes into sand grains. The dark minerals (magnetite, iron) create jet-black beaches. Waves, wind, and time break down volcanic rock. Found near volcanoes (Hawaii, Iceland, Canary Islands). Some black sand is magnetic!