Exploring Hanauma Geology
Hanauma Bay is located on the island of Oahu, one
of the eight main islands in the 
Hawaiian
archipelago that make up the State of Hawaii. The islands are made up
of volcanoes that formed as the Pacific plate passed over the Hawaiian
hotspot. The Pacific plate travels in a northwestern direction several
centimeters a year! Active volcanoes on Hawaii and Loihi islands are currently
over the hotspot.
Oahu was formed by two shield volcanoes, Waianae in the west and Koolau in the east. Waianae volcano erupted between 3.9 and 2.8 million years ago. Koolau volcano erupted between 2.7 and 1.8 million years ago. Towards the end of Koolau's shield building stage 1.8 million years ago, the northeastern flank of the volcano collapsed in a giant submarine landslide. The steep cliffs, or pali, seen on the windward side of the island is the eroded fault scarp of this massive landslide.
About 100,000 years ago, volcanism on Ko'olau was
renewed. Several famous landmarks including Diamond Head, Punchbowl, and
Hanauma Bay were formed in this period known as the rejuvenation stage
of volcanism.
The vents that formed Koko Head, Hanauma Bay, Kahauloa (Rifle Range), Koko Crater, and several smaller cones erupted along what is known as the Koko Fissure. It is believed that these vents erupted 30-35 thousand years ago but new ages suggest that they may have erupted earlier around 80 thousand years ago.
Most of the vents along Koko Fissure are tuff cones or tuff rings, volcanoes of hardened ash. These volcanoes were explosive unlike eruptions seen at Puu Oo on Kilauea. Magma that produced these volcanoes came into contact with seawater causing it to explode through a shallow marine environment that ran along the Koolau's former coastline. The craters of Hanauma Bay are thus made up of ash, pieces of Koolau basalt, and fragments of the old reef, and not layers of lava.
The outer crater wall was perhaps breached
by the ocean 10 thousand years ago. The oldest coral in the bay is dated
at 7,000 years old. Today, the youngest corals are growing out towards
the ocean near the mouth of the bay.
© 2003-2004 Hanauma Bay Education Program,
University of Hawaii Sea Grant Extension Service
100 Hanauma Bay Road ~ Honolulu, HI 96825 ~ Ph: (808) 397-5840 ~ E-mail:
hanauma@hawaii.edu
Last updated
June 3, 2004
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