Wai‘anae Ecological Characterization

Geological Formation and Features

Formation of the Hawaiian Islands

The distinctive linear shape of the Hawaiian Island chain resulted from the Pacific Plate moving over a deep, stationary hot spot in the Earth’s mantle. The hot spot is located beneath the present-day position of the Big Island of Hawai‘i. Heat from the hot spot produces a persistent source of magma by partly melting the overriding Pacific Plate. The magma rises through the mantle and crust to erupt onto the seafloor, gradually building the shield volcano.

Continuous eruptions build the volcanoes, and the heat of the hot spot causes the plate to expand and rise to great heights (over 10,000 feet) from the ocean floor. Together, these processes form the island volcanoes. The plate continues to move and eventually carries the island beyond the hot spot, removing it from its volcanic source, and the volcano becomes extinct. As one island volcano becomes extinct, another develops over the hot spot, and the cycle is repeated.

As the plate moves away from the hot spot, its temperature drops, and the massive islands slowly subside into the ocean. As the islands subside, coastal areas are inundated by the sea, and strong wave action forms sea cliffs. Erosion from heavy rain over hundreds of thousands of years and massive landslides further degrades the island. The rounded shell, or shield of the volcano, is cut with deep valleys, canyons, and scarps. The islands of Moloka‘i, O‘ahu, Kaua‘i, and Ni‘ihau prominently demonstrate these erosional features. Maui is currently subsiding, and erosion is beginning to sculpt its landscape. For information on the effects of erosion in Wai‘anae, please see the Makai: Ocean Processes, Waves and Erosion section.

When an island is removed from its volcanic source, fringing reefs develop around the edges of the island. As the island continues to subside and water depth increases, the reef dies due to lack of sunlight. New reefs grow in the shallow water, sometimes on top of the older, deeper reef. As the land mass is completely subsided and eroded, the reefs continue to grow and form an atoll, a ring-shaped group of islands with an interior lagoon, such as in the atolls of the Northwestern Hawaiian Islands. When the rate of subsidence overtakes the growth rate of the reefs, the atoll sinks below the surface to become a flat-topped seamount. Seamounts extend from the Kure Atoll in the Northwestern Hawaiian Islands all the way north to the western edge of the Aleutian Islands, where they are being subducted under the North American Plate.

Geologic Formation of the Wai‘anae Moku

The island of O‘ahu was formed by two volcanoes. Formation of the Wai‘anae volcano began approximately four million years ago, followed by the Ko‘olau volcano a million years later. The newly forming island of O‘ahu was the easternmost island of the volcanic chain, located directly above the hot spot (see above). Kaua‘i and Ni‘ihau were located to the northwest, similar to how Maui and Lanai are northwest of the Big Island of Hawai‘i today. The Wai‘anae volcano rose from the ocean floor (20,000 feet below sea level) to more than 10,000 feet above the ocean. Its eruptions were so violent and frequent that the forces of erosion did not have an impact on the round shield of the volcano; instead, rain sank directly into porous lava rather than cutting streams and valleys. The Ko‘olau volcano appeared approximately a million years later as a separate island to the east of Wai‘anae. The two volcanoes continued building and gradually filled the ocean between them to form a single island (see figure).

Geologic Features of the Wai‘anae Moku

Eventually, the violent eruptions of Wai‘anae subsided, and a crater formed at its center near the present Kolekole Pass. Lava flowed down the east side of the crater to create the Leilehua Plateau, or the current Schofield Plain. Lava flows did not break through the west side of the crater, and as a result, new lavas did not build the western flank of the volcano. Erosion has been working on the west side of the crater since its beginnings, resulting in the deep, majestic valleys of Mākua, Mākaha, and Lualualei.

The Wai‘anae mountain range is approximately 22 miles long, with narrow ridges and steep slopes as the predominant features. Most of these features were formed through erosion before the Ko‘olau mountain range rose high enough to intercept the prevailing trade winds and rainfall. The maximum rainfall at Mount Ka‘ala, the highest point in the Wai‘anae range and on O‘ahu at 4,025 feet, is approximately 100 inches per year; however, it would likely exceed 300 inches per year if not for the barrier of the Ko‘olau Mountains. For more information on rainfall in Wai‘anae, please see the Ke a‘o Ulu: Rains and Flooding section.

The Wai‘anae mountain range has been significantly eroded by the rain, sea waves, and landslides, resulting in amphitheater valleys including Lualualei and Mākua to the west. The striking features of the range are the great flat-floored valleys that slope up to the majestic and steep pali (cliffs) that join the back of the valleys.

The summit of Mount Ka‘ala is a plateau, containing a moist and nearly inaccessible montane bog. The plateau was formed by hundreds of feet of dense, resistant ‘a‘ā (Hawaiite) lava flows. It is an only slightly eroded remnant of the original volcano that created the Wai‘anae mountain range, the oldest terrain on O‘ahu.

Reference Cited

Stearns, H.T., and K.N. Vaksvik. 1935. "Geology and ground-water resources of the Island of Oahu, Hawaii." Division of Hydrography Bulletin 1: 479.

Related References

Hawaii Center for Volcanology. 1998. Waianae Volcanic Series. http://www.soest.hawaii.edu/GG/HCV/waianae.html

Jokiel, P.L. 2004. Jokiel's Illustrated Scientific Guide to Kaneohe Bay, Oahu. Hawaii Coral Reef Assessment and Monitoring Program. http://cramp.wcc.hawaii.edu/Study_Sites/Oahu/Kaneohe_Sector/
Jokiels_Illustrated_Scientific_Guide_to_Kaneohe_Bay/

State of Hawaii Department of Land and Natural Resources (DLNR). 1990. Mount Kaala Natural Area Reserve Management Plan. Natural Area Reserves System Program.

U.S. Geological Survey (USGS). 1998. Hawaiian Volcanoes. Hawaii Volcano Observatory. http://hvo.wr.usgs.gov/volcanoes/

U.S. Geological Survey (USGS). 1999. Hotspots: Mantle Thermal Plumes. http://pubs.usgs.gov/publications/text/hotspots.html

University of North Dakota. 2004. Evolution of Hawaiian Volcanoes. http://volcano.und.nodak.edu/vwdocs/vwlessons/evolution/introduction.html

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