How are stalactites formed?

• A. Stalactites form from mineral deposits as dripping water evaporates and leaves behind minerals that accumulate over time.
• B. Stalactites form from volcanic ash cementation.
• C. Stalactites form from wind-blown sand accumulation.
• D. Stalactites form from the collapse of caves ceiling.

Answer: (A)

Stalactites form when mineral-rich water drips from a cave ceiling and deposits minerals as it hangs there. The water carries dissolved calcium carbonate; as it drips, carbon dioxide escapes and the water can evaporate a bit, causing calcite to precipitate and stay behind on the ceiling. Over many drips and a long time, these mineral deposits grow downward into icicle-like formations. This is a slow speleothem-building process, and it explains why stalactites hang from the ceiling rather than forming on the floor or from other processes. The other options don’t describe mineral deposition from dripping water: volcanic ash cementation would bind ash layers, wind-blown sand would build up in deserts, and ceiling collapse isn’t how stalactites form.