It is not a mountain range in the normal sense, because it was formed as a single mountain called Koolau Volcano. 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 over the ocean floor to the northeast of Oahu. Kāneʻohe Bay is what remains of the ancient volcano's summit caldera after the slide. The modern Koolau mountain forms Oahu's windward coast and rises behind the leeward coast city ofHonolulu — on its leeward slopes and valleys are located most of Honolulu's residential neighborhoods. is visible on the right side. 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 ; today, the summit of the tallest peak, Puu Konahuanui is only.
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, Koko Head, Koko Crater, Punchbowl Crater, Tantalus, and Āliapaakai, and are collectively known as the Honolulu Volcanic Series, or simply Honolulu Volcanics. According to the US Geological Survey, the most recent eruptions in this series of activity occurred between about 70,000 to 100,000 years ago. There is a possibility that Koolau volcano could erupt again; however, the chance of such an eruption occurring in "our lifetimes, or even those of many future generations" is remote.
Transportation
There are three roads that tunnel through the southern part of the Koolau Range, connecting Honolulu to the Windward Coast. From leeward to windward:
