"This "Curved" bay was declared a protected marine life conservation area and underwater park in 1967. This is THE BEST protected cove if you are new to scuba diving or snorkeling with a diverse population of marine life and a large, rich coral reef. Advanced snorkelers and divers can also enjoy the beauty of this bay as the reef extends throughout the cove into deeper water. The bay floor is actually the crater of an ancient volcano that flooded when the exterior wall collapsed and the ocean rushed in.
There is a large sandy beach perfect for sunbathing, relaxing and picnicking. Several hiking trails extend along coastline and along the ridge overlooking the bay providing breathtaking lookouts.
The curvature of the bay usually provides protection from large ocean waves and allows swimmers a terrific opportunity to view the marine and reef life in a safer, protected environment." SOURCE
"The Hawaiian Islands archipelago is the most isolated island chain in the world. It extends from the Island of Hawai'i to Kure Atoll in the northwest, a distance of over 1,500 miles. The island Hawai'i is the youngest of the islands.
The Islands are a group of volcanos that have risen up over a "hot spot" of molten rock that wells up from deep in the earth's interior. As the ocean floor slowly moves over this spot, islands are born. They are then carried towards the northwest as they erode and slowly sink below the waves.
O'ahu first rose above the sea nearly 4 million years ago as the Wai'anae volcano. The younger Ko'olau volcano, on the east side of O'ahu, is about 2.7 million years old. It is larger than the Wai'anae, and makes up most of the island. Nearly 2 million years ago, the eastern flank of the Ko'olau volcano collapsed and slid into the sea, leaving the steep mountain range that we see today on the windward side.
A quiet period followed the eruptions that formed the Ko'olaus until energetic rejuvenation-stage volcanism started to occur about a million years ago. This "Honolulu Volcanic Series" created many well-known landmarks including Diamond Head, Punchbowl, and Hanauma Bay.
The present site of Hanauma Bay was underwater 100,000 years ago. The ocean lapped at the base of the Ko'olaus and a coral reef grew in the area.
The youngest craters and cones of the Honolulu Series are part of Koko Head Regional Park, which lies alongKoko Head Crater the Koko rift zone. This region has had the most recent volcanic activity on O'ahu, and it may not be over yet.
The lava flows that created the Hanauma Bay region started about 40,000 years ago. Koko Crater, the tallest and best preserved cinder and ash cone, is possibly one of the latest volcanic vents. Radiocarbon dates of volcanic rock and reef indicate that the last volcanic event in this area occurred just 7,000 years ago.
The youngest craters and cones of the Honolulu Series are part of Koko Head Regional Park, which lies along the Koko rift zone. This region has had the most recent volcanic activity on O'ahu, and it may not be over yet.
The lava flows that created the Hanauma Bay region started about 40,000 years ago. Koko Crater, the tallest and best preserved cinder and ash cone, is possibly one of the latest volcanic vents. Radiocarbon dates of volcanic rock and reef indicate that the last volcanic event in this area occurred just 7,000 years ago.
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.
The violent explosions that created Hanauma Bay and this southeastern part of the island blasted up through a previouslyWhite Coral Limestone And Dark Basalt Rock Imbedded In The Walls Of Tuff established coral reef. As you walk down to or around the Bay, you will see fragments of white coral limestone and dark basalt rock imbedded in the walls of tuff.
In some areas you may find large pieces of rock and coral that were ejected by eruptions. These large fragments fell into the unconsolidated ash and compressed the layers of ash below them, creating bomb sags in the tuff. If you look carefully, you can sometimes see the shock pattern that the rock left in the tuff.
Small crystals of green olivine can also be seen in the crater walls and the sands of the bay, particularly around the "Toilet Shock Pattern That The Rock Left In The TuffBowl" section of the Bay. Olivine is a mineral that is found in areas of recent volcanic activity, like that which happened here.
Sea cliffs formed as the crater walls of Hanauma Bay eroded. Wave action, weathering and daily heating and drying of the walls undercut the tuff. This caused large areas to fall into the sea or on the bench (ledge) that developed below the cliffs.
Although the bench was exposed to wave action, water prevented the bench from drying out and weakening. The ledge formed due to the erosion of the cliffs by wind and salt, rather than the bench being degraded by waves." SOURCE