Geology
The Hawaiian Islands are a group of volcanoes that have risen up over a hot spot, which is a section of the Earth's surface that has exhibited volcanism for an extended period of time. Volcanic chains such as the Hawaiian Islands form as a result of the movement of a tectonic plate across fixed hot spot beneath the surface. In the case of the Hawaiian Islands, the Pacific plate has moved slowly northwestward over such a hotspot.
Approximately 3.9 million years ago, the Waiʻanae volcano created the island of Oʻahu. About 2.5 million years ago, the Koʻolau volcano erupted on the ocean floor, and continued to grow in elevation until about 1.7 million years ago, when it went dormant. Most of the eastern or windward portion of Oʻahu are remnants of this volcano. Most of the familiar geographic landmarks of eastern Oʻahu were created by eruptions from Koʻolau from about 500,000 to 10,000 years ago. The eastern flank of the Koʻolau volcano including the caldera slid into the sea, leaving the Koʻolau mountain range that can be seen today on the windward side of the island.
The Hanauma Crater was created about 32,000 years ago during the Honolulu volcanic series, the latest (and perhaps final) burst of volcanic activity to occur on Oʻahu. Tens of thousands of years ago, a series of volcanic vents opened along the southeast shoreline of Oʻahu. Unlike the gentle lava flows currently building the island of Hawaiʻi, the late-stage eruptions on Oʻahu were violent explosions. The volcanic vents that formed Hanauma Crater opened on the sea floor. Upwelling magma vaporized the ocean water and steam explosions atomized the magma into fine ash. The explosions built cones of ash, which solidified into tuff. The eruptions shattered the sea floor—coral reef and basalt—and scattered pieces that are now embedded in the tuff. Wave erosion eventually cut through the low, southeast wall of the crater, forming the current bay.
Read more about this topic: Hanauma Bay