Diamond Head, known to Native Hawaiians as Lēʻahi, is an iconic landmark along the Honolulu skyline. This distinctive volcanic formation has a unique profile and rich geological history, shaping the landscape of Oahu.
The Final Eruption
The last eruptive event that formed Diamond Head occurred approximately 300,000 years ago. This was a singular, explosive event, not a prolonged period of continuous volcanic activity. The eruption was phreatomagmatic, where rising magma interacted violently with groundwater or seawater.
This interaction generated steam explosions, propelling ash, cinders, and volcanic rock fragments high into the atmosphere. As these materials settled and solidified, they formed layers of tuff, a type of rock composed of compacted volcanic ash. The eruption was brief, likely lasting only a few days to a month, which contributed to the cone’s symmetrical shape. Diamond Head is considered a monogenetic volcano, meaning it resulted from a single eruptive phase and is not expected to erupt again.
Geological Origins
Diamond Head is classified as a tuff cone, a geological structure distinct from the broad shield volcanoes that characterize much of Hawaii. Tuff cones are created by the explosive fragmentation of magma when it encounters water, leading to the rapid accumulation of ash and debris. This process contrasts with the effusive lava flows typical of shield volcanoes like Mauna Loa.
The formation of Diamond Head is part of the Honolulu Volcanic Series, a later stage of volcanism on Oahu. This series occurred long after the initial shield-building phase that created the island’s main mountain ranges, such as the Koʻolau Volcano, which formed millions of years earlier. During Diamond Head’s eruption, prevailing winds played a role in shaping its structure, causing the southwestern rim to be higher due to the direction of ash deposition.
Diamond Head as a Landmark
Today, Diamond Head is a dormant volcano, its volcanic past evident in its pronounced crater and solidified slopes. It is a natural landmark with a distinct profile. The site is a popular destination for visitors and residents, offering a hiking trail that leads to its summit.
From the summit, panoramic views of Waikiki and the Oahu coastline are visible. Diamond Head has been designated a Hawaii State Monument and a National Natural Landmark since 1968. The name “Diamond Head” was given by 19th-century British sailors who mistakenly thought the calcite crystals found there were diamonds.