What remains of Koʻolau is the western half of the original volcano that was destroyed in prehistoric times when the entire eastern half—including much of the summit caldera—slid cataclysmically into the Pacific Ocean.

Remains of this ancient volcano lie as massive fragments strewn nearly 100 miles (160 km) over the ocean floor to the northeast of Oʻahu.

The volcano remained dormant for hundreds of thousands of years, during which time erosion ate away at the initially smooth slopes of the shield-shaped mountain; and the entire mass subsided considerably.

[5] The Ko‘olau Range is the erosion of remnants of a massive shield volcano, but estimates show evidence that it erupted approximately 2.5 million years ago.

In 1795, the newly-formed Hawaiian Kingdom conducted a battle resulting in the triumphant conquest of O'ahu on the range within part of the Nu‘uanu Pali Lookout, under the command of Kamehameha the Great, as his troops forced all of the warriors up the valley to fall to their deaths below the cliffs.