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September 29, 1995

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

O'ahu, Ni'ihau, and Kaua'i

In the last few weeks, we have reviewed the eruptive stages of Hawaiian volcanoes and have given brief overviews of the geologic history of Hawai'i Island and of Maui, Moloka'i, Lana'i, and Kaho'olawe, the islands that make up the Maui Complex. This week, we will complete the geologic history of the principal Hawaiian Islands with a brief look at O'ahu, Ni'ihau, and Kaua'i.

Each Hawaiian volcano progresses through a series of eruptive stages that starts with the preshield stage, followed by the shield stage, the postshield stage, and, following a period of erosion, the rejuvenated stage. Not all volcanoes go through the postshield or rejuvenated stages, and preshield stage lavas are presumably buried beneath later lavas on all the volcanoes except Lo'ihi Seamount, which has not yet progressed past the preshield stage. The lavas erupted during each stage are chemically distinct, thereby allowing identification of the different stages. With this brief review of topics previously covered, let's take a geologic look at the three oldest of the principal Hawaiian Islands.

O'ahu consists of two extinct volcanoes, Ko'olau to the east and Wai'anae to the west. Ko'olau Volcano consists of the eruptive products of the shield (2.5-1.7 million years old) and rejuvenated stages; no postshield stage lavas are known. A caldera complex, filled with thick, ponded lavas that have been altered by hot water, occurs in the Kailua region on the northeast shore of the island. The caldera was bisected by the catastrophic collapse of the Nu'uanu landslide, which deposited numerous blocks on the sea floor as far as 100 miles northeast of the island. The largest of these blocks is about the same size as Manhattan Island.

The rejuvenated stage lavas erupted mainly in the Honolulu area, hence their name, the Honolulu Volcanics. These vents and flows appear to be older than 100,000 years; the best-dated vent, at Black Point, is 410,000 years old. The flows and ashes of the Honolulu Volcanics have high contents of sodium and potassium and low contents of silica. Many of the vents erupted through a coral reef that surrounded the island on the south side. These eruptions tended to be explosive, and most vents along the coast are ash, or tuff, cones, such as Diamond Head, Hanauma Bay, and Salt Lake Crater. Flows erupted inland were funneled down valleys, such as Manoa and Nu'uanu Valleys, thereby creating flat valley floors.

Wai'anae Volcano consists of the eruptive products of the shield (3.9-3.5 million years old) and postshield (3.2-2.5 million years old) stages. The shield lavas are overlain by a thick sequence of postshield stage lavas. A post-erosional sequence of lava, once thought to be of the rejuvenated stage, is 2.5 million years old and has been reinterpreted as postshield. The erosional break that separates these lavas from the earlier part of the postshield stage has been attributed to a catastrophic landslide to the southwest named the Wai'anae slump.

Ni'ihau, an extinct volcano with shield- (4.9 million years old), postshield- (4.9 million years old), and rejuvented- (2.5 million to 400,000 years old) stage lavas, has no exposed summit caldera complex; it was apparently removed during a large landslide towards the east. The postshield stage consists of the erosional remnants of a dike and of a single cone. Rejuvenated stage lava, with a narrow range of compositions, covers about a third of the island and forms a small ash, or tuff, cone, Lehua Island, off the north shore. Numerous cones and flows of rejuvenated stage lava also dot the deep sea floor west of Ni'ihau.

Kaua'i consists of at least one extinct volcano, including shield- (5.6-5.0 million years old), rare postshield- (4.94 million years old) and abundant rejuvenated- (3.65 million to 500,000 years old) stage lava. Kaua'i is unique among Hawaiian volcanoes with its enormous caldera complex, no obvious rift zones, and a graben, or downdropped block, on the south side of the caldera complex. Rejuvenated-stage lava has covered much of the eastern half of the island with a broad compositional range of basaltic lavas. Some flows have filled canyons and diverted rivers, only to be eroded through once again. Rejuvenated stage lavas also erupted on the east and southeast submarine flanks of Kaua'i.

Shield-stage lavas filled the caldera and the graben with thick, ponded flows. The Makaweli graben probably formed in response to a large landslide off the southern coast of the island, that can be clearly identified by the levees it left behind and the blocky debris on the sea floor. Other landslides also affected the north and northeast flanks of the island and, perhaps, the east flank, as well. The caldera complex of a second smaller volcano may be represented by thick, ponded lavas in the southeastern part of the island. Deep erosion, weathering of the flows, and voluminous subsequent rejuvenated-stage lava, make unraveling Kaua'i's early history difficult.

The abundance of giant landslides around the principal Hawaiian Islands indicates that such landslides are an integral part of their life history. Fortunately, such catastrophic events occur infrequently - - only once every several hundred thousand years. Aspects of the geology of individual volcanoes, such as the apparent absence of summit calderas, are readily understood within the context of such giant landslides. New undersea data collected within the last decade, combined with previous observations and mapping on the islands, have led to a revolution in our understanding of the geologic processes that built and then modified the Hawaiian Islands.

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