Koolau Range is a name given to the fragmented remnant of the eastern or windward shield volcano of the Hawaiian island of Oahu. It is not a mountain range in the normal sense, because it was formed as a single mountain called Koolau Volcano (koolau means "windward" in Hawaiian, cognate of the toponym Tokelau). What remains of Koolau 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 over the ocean floor to the northeast of Oahu. The modern Koolau mountain forms Oahu's windward coast and rises behind the leeward coast city of Honolulu — on its leeward slopes and valleys are located most of Honolulu's residential neighborhoods.
The volcano is thought to have first erupted on the ocean floor more than 2.5 million years ago. It eventually reached sea level and continued to grow in elevation until about 1.7 million years ago, when the volcano became dormant. 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. The highest elevation perhaps exceeded 3000 m (10,000 ft), although today it is only 960 m (3149 ft; Puu Konahuanui).
After hundreds of thousands of years of dormancy, Koolau volcano began to erupt again. Some thirty eruptions over the past 500,000 years or so have created many of the landmarks around eastern Oahu, such as Diamond Head, Hanauma Bay, Koko Head, Punchbowl Crater, Tantalus, and Āliapaakai. Geologists do not always agree on the dates of these more recent eruptions, some dating them to around 32,000 years ago, others to as recently as 10,000 years ago. Geologists believe that there is at least a remote possibility that Koolau volcano will erupt again.