Why do waterfalls exist?
Show answer & explanation
Answer: Hard rock resists erosion
Rain collects at edges — Wrong. Rain doesn't create waterfalls. Falls form when rivers flow over hard rock ledges that resist erosion while softer rock downstream wears away.
Underground rivers surface — Wrong. Waterfalls aren't underground rivers surfacing. They form when rivers encounter differential erosion—hard rock resists while soft rock erodes, creating cliffs.
Hard rock resists erosion ✓ — Correct! Waterfalls form where rivers cross hard, erosion-resistant rock layers. Downstream soft rock erodes faster, creating a cliff. The waterfall gradually retreats upstream as water undercuts the hard rock ledge. Niagara Falls has retreated 11km in 12,000 years! Glacial erosion can also create waterfalls by carving hanging valleys.