Waiʻanae 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 Oʻahu.

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.

One theory to explain this erosion pattern is that a large landslide cut away the western portion of the volcano.

The faults from this huge landslide weakened the rock, making the western part of the mountain much more susceptible to erosion than the eastern side.