Koko Head is a prominent geological landmark on the southeastern coast of Oʻahu, Hawaii, known for its steep hiking trail. Koko Head is indeed a volcano, though it is not a massive shield volcano like those that form the main Hawaiian Islands. Its structure is the result of a highly explosive type of eruption that occurred relatively late in Oʻahu’s volcanic history.
Koko Head’s Geological Identity
The precise geological classification for Koko Head is a tuff cone. A tuff cone is a volcanic vent built up from layers of volcanic ash and rock fragments, unlike lava-flowing shield volcanoes. The formation involves magma rising through the Earth’s crust and violently encountering groundwater or surface water.
This interaction triggers a powerful phreatomagmatic explosion, where the rapid heating of water flashes it into steam. The steam shatters the magma into fine particles of ash and lapilli, which then rain down to create the cone shape. The resulting rock is called tuff, a solidified mix of volcanic ash and other debris. Koko Head’s appearance as a steep, bowl-shaped hill is a direct result of this explosive formation process.
The Formation of the Koko Head Complex
Koko Head is a feature of the Honolulu Volcanic Series, a sequence of much younger, secondary eruptions that occurred on Oʻahu. This series represents a resurgence of volcanic activity long after the shield-building phase had ended. This rejuvenated stage created many of the dramatic landmarks seen today, including Diamond Head.
The eruptions that formed the Koko Head complex began roughly between 30,000 and 70,000 years ago, a relatively recent period in geological time. Koko Head and its surrounding features are concentrated along the Koko Rift Zone, marking the latest episode of volcanism on Oʻahu. Scientists classify Koko Head as an extinct volcano, meaning it is not expected to erupt again.
Related Volcanic Features
The Koko Head complex includes several other prominent geographical features, all sharing the same explosive origin. The most visually dominant feature is Koko Crater, a separate, much larger tuff cone formed during the same late Pleistocene volcanic event.
Another related landmark is Hanauma Bay, one of Oʻahu’s most famous natural areas. Hanauma Bay was originally one of Koko Head’s volcanic vents, specifically a tuff ring with a large depression. The ocean later breached one side of the ring, flooding the crater and creating the sheltered, crescent-shaped bay known worldwide for its marine life.