O‘ahu used to be nearly twice as big as it is now. (Thompson) The Island consists of two major shield volcanoes: Waiʻanae and Koʻolau; the eroded remnants of which are the Waiʻanae Range and the Koʻolau Range.
Waiʻanae is the older of the two (breaking the ocean surface ~3.9 to ~2.8 million-years ago) and makes up the western part of O‘ahu. Koʻolau volcano started as a seamount above the Hawaiian hotspot around 4-million years ago. It broke sea level some time prior to 2.9-million years ago.
Koʻolaupoko, one of O‘ahu’s six ancient moku (districts,) is bounded by Kalaeoka‘ō‘io, which is a point near the center of the northeast coast at Kualoa; the crest of the Koʻolau Range to the west; and Makapuʻu Point on the southeastern tip of the island.
This expanse also generally delineates the extent of Koʻolau Volcano – effectively from and including Kāne‘ohe to Kailua to Waimānalo. A significant landslide and ongoing erosion reshaped the volcano.
About 2-million years ago, much of the northeast flank of Koʻolau volcano was sheared off and material was swept onto the ocean floor (named the Nuʻuanu Avalanche) – one of the largest landslides on Earth.
The Pali is the remaining edge of the giant basin, or caldera, formed by the volcano. At its base are the towns of Kāne’ohe, Kailua and Waimānalo – beyond that, open ocean. The other half of the caldera, an area the size of Brooklyn, tore away and tumbled into the ocean. (Sullivan)
Material swept more than 140-miles north of O‘ahu and Molokai. For the last 85-miles of its journey, the avalanche traveled uphill by about 1000 feet, leaving jumbled blocks – once part of O‘ahu – scattered over more than 9,000-square miles of seafloor. (Sinton)
Residual ridges, remnants of the old Koʻolau volcano, extend northeast from the Pali. These include the Mokulua islands, Olomana, Kapaʻa (where the quarry is,) Mahinui (Oneawa, Kalaheo) and Keolu Hills.
Mōkapu Peninsula (where Marine Corps Base Hawai‘i is situated) is evidence of subsequent secondary volcanic eruptions that formed Ulupaʻu Crater (the large hill on the Kailua side of the peninsula,) Pu‘u Hawaiiloa (the central hill that originally had the base control tower, now has radar (‘the hill’,)) Pyramid Rock and the nearby Moku Manu (Bird Island.)
Following a period of dormancy, Koʻolau erupted about 1-million years ago (known as the Honolulu Volcanic Series) and created landmarks such as Lēʻahi (Diamond Head,) Hanauma Bay, Pūowaina (Punchbowl Crater) and Āliapaʻakai (Salt Lake.) Another tuff cone is Mānana Island (Rabbit Island.)
Pounded by the tradewind and rains, the windward side of O‘ahu is more weathered than the leeward areas of the island, and now this vast caldera wall is reduced to a line of sheer cliffs stretching from Makapuʻu to Kualoa and beyond.
The flat valley floors are extensively eroded, and are now mostly joined, studded here and there with isolated remnant peaks and ridges connected to the central range. (Klieger)
Coral reefs and marine terraces were formed at different elevations due to the changing sea levels over time. There are some broad lowland areas in the lower reaches of deeply alluvial valleys. (Moberly)
It was the broadness of this coastal plain (which included swampy areas near the shore) that distinguished Koʻolaupoko from other areas of O‘ahu, and most likely helped account for the intensity and productiveness of agriculture.
The abundant rainfall produces constantly flowing streams that supported the vast expanse of wetland kalo (taro) lo‘i (pondfields) that once extended throughout Koʻolaupoko. (Klieger)
Because Kāne’ohe Bay has a deep lagoon between an outer reef and the shore, the reef is considered by some geologists to be a barrier reef, the only example in Hawaii. Several fishponds lined the bay.
Mokoliʻi (Chinaman’s Hat) and Mokuoloe (Coconut Island) are erosional remnants of the bedrock Koʻolau basalt; Kapapa and Kekepa (Turtleback) Islands are of limestone; and Ahu O Laka Island is a sand bar that is uncovered at low tide. (Moberly)