Diamond Head is part of the system of cones, vents, and their associated eruption flows that are collectively known to geologists as the Honolulu Volcanic Series, eruptions from the Koʻolau Volcano that took place long after the volcano formed and had gone dormant. The Honolulu Volcanic Series is a series of volcanic eruption events that created many of Oʻahu’s well-known landmarks, including Punchbowl Crater, Hanauma Bay, Koko Head, and Mānana Island in addition to Diamond Head.
Diamond Head, like the rest of the Honolulu Volcanoes, is much younger than the main mass of the Koʻolau Mountain Range. While the Koʻolau Range is about 2.6 million years old, Diamond Head is estimated to be about 200,000 years old and inactive for 150,000 years.
The eruption that built up Diamond Head was probably very brief, lasting no more than a few days. It was probably explosive, since when the cinder cone was originally formed, the sea level is thought to have been higher and the vent burst erupted over a coral reef. Another factor probably contributing to the eruption’s explosive nature was that rising magma would have come into contact with the water table. The eruption’s relatively brief length is thought to explain why the cone today is so symmetrical.
A nearby eruption that took place at about the same time as the Diamond Head eruption was the eruption that built the Black Point lava shield. Since the type of eruptions that built Diamond Head tend to be monogenetic, geologists don’t believe Diamond Head will erupt again.
Some say that Diamond Head isn’t a volcano. It is rather like a vent of the Ko’olau Range that was formed. After the eruption that formed Diamond Head, the Ko’olau Range became extinct. Punchbowl Crater and Koko Head is also considered as an extinct volcanic vent because it was formed around the time of the Honolulu Volcanic Series. None of these volcanic vents were formed by the Mount Ka’ala volcano though.