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Diamond Head, Hawaii

Diamond Head is the name of a volcanic tuff cone on the Hawaiian island of Oʻahu and known to Hawaiians as Lēʻahi, most likely from lae 'browridge, promontory' plus ʻahi 'tuna' because the shape of the ridgeline resembles the shape of a tuna's dorsal fin. Its English name was given by British sailors in the 19th century, who mistook calcite crystals embedded in the rock for diamonds.




Geology

Diamond Head is part of the complex 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 Manana Island in addition to Diamond Head.
Diamond Head, like the rest of the Honolulu Volcanics, 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 extinct 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 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.
Source: Wikipedia