Honolulu Volcanics

The Honolulu Volcanics are a group of volcanoes which form a volcanic field on the island of Oʻahu, Hawaiʻi, more specifically in that island's southeastern sector and in the city of Honolulu from Pearl Harbor to the Mokapu Peninsula. It is part of the rejuvenated stage of Hawaiian volcanic activity, which occurred after the main stage of volcanic activity that on Oʻahu built the Koʻolau volcano. These volcanoes formed through dominantly explosive eruptions and gave rise to cinder cones, lava flows, tuff cones and volcanic islands. Among these are well known landmarks such as Diamond Head and Punchbowl Crater.

Volcanic activity began less than one million years ago and occurred at between 40 and 30 separate volcanic vents, some of which are submarine. Sea level varied during the activity of the volcanic field, and some volcanic eruptions have been dated through correlation with individual sea level fluctuations. The field erupted various kinds of lavas of mostly basaltic type with a high content of xenoliths. During eruptions, ascending magma often underwent interactions with water and thus caused steam explosions and the formation of particular volcanic structures such as tuff cones. The last eruption took place 35,000 or 76,000 years ago and future hazardous eruptions are possible.

The Honolulu Volcanics are a series of volcanoes in the southeastern sector of Oʻahu and includes dikes, lava flows, spatter cones, tephra deposits, tuff cones, and mesas where the surrounding terrain has been eroded away. Vents span the area southeast of a line between Mokapu Peninsula and Pearl Harbor, and extend from the ridges of Koʻolau volcano to below sea level and to the coast plain of southern Oʻahu.

The system takes its name from Honolulu, the capital of Hawaiʻi, as craters are scattered in and around the city. The volcanic system includes well-known landmarks of Honolulu such as Diamond Head, Koko Head, Punchbowl Crater, Rabbit Island, Tantalus, Hanauma Bay (notable as a snorkeling site) and the Mokapu Peninsula, which is the location of Marine Corps Base Hawaiʻi. The United States military has made use of some of the volcanic islands that were formed by the Honolulu Volcanics. The Koko area is designated as the Koko Head Regional Park and Hanauma Bay is also a state park. Parts of this system are among the best known volcanic vents of Hawaiʻi.

About 30–40 vents have been identified. Most cinder cones on Oʻahu are quite large, over 76 metres (250 ft) high and up to 0.80 kilometres (0.5 mi) wide. Some of the lava flows filled deep valleys cut into the older Koʻolau volcano and displaced streams that previously ran through these valleys; for example, water passing over a lava flow in Kamanaiki Valley forms a waterfall. Together with sediments coming down from the mountains and coral reef growth, the deposits of the Honolulu Volcanics have formed the coastal plain on which the city of Honolulu and military installations are built.

The vents of the Honolulu Volcanics follow northeastward-trending alignments that are at right angles to the rift zone of the Koʻolau volcano. From northwest to southeast these are the Haʻikū Rift, the Tantalus Rift, the Kaimukī/Kaʻau Rift and the Koko Head/Koko Rift, but each rift has had eruptions at different times and with different compositions. It is not clear whether these alignments are in any way related to the structure of the previous Koʻolau volcano, instead of being controlled by the crust of the Pacific Ocean, but the trends along the Koko and Tantalus Rifts are parallel to that of the flexural arch of Hawaiʻi Island. There is also a hypothetical "Diamond Head fault" that may be associated with earthquakes on Oahu that occurred in 1948, 1951 and 1961–1981, but it is not parallel to these alignments and its very existence is questionable.

Submarine vents are also known, including a 300-metre (980 ft) high solitary cone with two ridges off northeast Oʻahu, which is covered by pillow lavas and volcaniclastic sediments. At least five cones are found off the southwestern extension of the Koko Rift where they are situated on a southwestward extending ridge. Another set of submarine vents is found south of Diamond Head. It was once proposed that some seamounts (underwater mountains) off northeastern Oʻahu such as Tuscaloosa Seamount are related to the volcanic series; today however they are considered to be fragments of the giant Nuʻuanu Slide off northeastern Oʻahu.

Most of the present-day shoreline of Hawaiʻi Kai was formed by Honolulu Volcanics; the Kuapā Pond is a leftover lake between the new shoreline and the old shoreline of the Koʻolau volcano. The volcanic vents there include Koko Head, the Hanauma Bay craters, Kahauloa crater bay, a wave-eroded cone, Koko Crater and Kalama cinder cone; Koko Head cones are heavily eroded, and the sea has broken into one of the cones, exposing its structure in outcrops. Koko Head is the largest cone of the Honolulu Volcanics and the crater of Koko Crater is about 1 kilometre (0.62 mi) wide. Hanauma Bay close to Kalanianaʻole Highway is located 13 kilometres (8.1 mi) east of Honolulu and is a 0.4 by 0.8 kilometres (0.25 mi × 0.50 mi) large, 18 metres (59 ft) deep compound crater/tuff cone with several associated dykes and lava flows- It was breached by the sea and coral reefs grow within it. Together with Kahauloa and Kalama all these vents form the Koko Rift. Farther northeast lie the Kaupō lava flow and the islands of Kāohikaipu and Mānana; all these except Mānana are also in the Koko Rift. This 15 kilometres (9.3 mi) long rift includes at least 12 separate volcanic vents and its vents appear to have formed in a single eruption.