Water levels recede primarily due to the Pleistocene ice age. Pearl Harbor is being created by this process as it erodes above sea level. On the windward side of the Ko'olau, in the center of the range at the site of an old caldera, Kawainui marsh starts first as an ocean bay. Later it closes off to become a marsh, the largest fresh-water marsh on the islands. Koko Head, Koko Crater, Sea Life Park area, last set to erupt.
Water level rises again due to melting of ice caps.
Pearl Harbor is formed by water filling the valleys created between the mountain ranges. Waikiki is a marshy lagoon. Theoretically there still could be an eruption on O'ahu though it is not highly likely. About one half of the original land mass of O'ahu has slipped into the sea through erosion and landslides. In a few million years, O'ahu will vanish, much like the islands of the North-West chain above Kaua'i.