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 Waianae volcano created the island of Oahu. About 2.5 million years ago, the Koolau volcano breached the ocean's surface 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 Oahu are remnants of this volcano. Most of the familiar geographic landmarks of eastern Oahu were created by eruptions from Koolau from about 500,000 to 10,000 years ago. The eastern flank of the Koolau volcano including the caldera slid into the sea, leaving the Koolau 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) round of volcanic activity to occur on Oahu. Tens of thousands of years ago, a series of volcanic vents opened along the southeast shoreline of Oahu. Unlike the gentle lava flows currently building the island of Hawaii, the late-stage eruptions on Oahu were most often short-lived violent explosions. The volcanic vents that formed Hanauma Crater opened on the sea floor. Upwelling magma vaporized the ocean water and steam explosions blew the magma into fine ash. The explosions built cones or rings 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. Human history Due to the lack of fresh water in the vicinity, the area was not inhabited, although archeologists have found fishing tools and other indications of human presence there. The bay was used as a recreational area by aliʻi (Hawaiian nobility), including King Kamehameha and Queen Ka‘ahumanu, who fished, entertained visitors, and sponsored games there. It was also used as a layover and as a navigational lookout point, since the waters between Oʻahu and Molokaʻi are at times difficult. Hanauma Bay was purchased from the Bernice Pauahi Bishop estate by the City and County of Honolulu, and subsequently opened