High Quality Content by WIKIPEDIA articles! Wai anae Range (sometimes referred to as the Waianae Mountains) is the eroded remains of an ancient shield volcano that comprises the western half of the Hawaiian Island of O ahu. ITs crest, at Mount Ka ala, is the highest peak on O ahu.LIke the neighboring Ko olau, the Wai anae Range is not a mountain range in the sense most people are familiar with, as the mountain range as a whole was created from a single volcano.THe oldest lavas dated from the volcano are about 3.9 million years old. ABout 3.2 million years ago, the volcano's activity changed, the rate of eruption started to decrease and the composition of the lava erupted from the volcano changed. THe volcano is thought to have last erupted about 2.5 million years ago.WHen active, the Wai anae volcano's center of activity was at present-day Lualualei Valley. INtense erosion on the western flank of the mountain has destroyed much of those flanks. THus, the mountain today is much smaller than it was when the volcano was active.