Hanauma Bay ~ Oahu ~ Hawaii
As you can see, a crater where once lay an active volcano...
About 32,000 years ago Hanauma Bay was created in a violent series of explosions. Hydromagmatic blasts happened when a volcanic vent opened underwater. The erupting magma blew up in a bursts of steam, hot gases, rock, coral, and fine ash that blanketed the area. The ash chemically cemented together over time into the firm rock called tuff .
The Bay is now a flooded crater surrounded by several cratered cones. It is likely that its oceanside rim was low to begin with because the ash was carried away by tidal currents and trade winds. The present opening to the sea is probably the result of wave erosion.