GG 103 Spring 2004
Class 26
April 15, 2004

Regional Geology of Kau`i, Ni`ihau and NW Hawaiian chain

Kaua`i, Ni`ihau and the rest of the islands in the Hawaiian chain are all on a fairly continuous ridge that is separated from the eastern Hawaiian Islands by the Kaua`i channel.

This geologic map shows that Kaua`i is a single large shield volcano. The large circular feature in vertical rules is the Olokele caldera, the largest caldera known in the Hawaiian Islands. It was 15-20 km across when the shield volcano was active.

The large feature just south of the caldera is the Makaweli Depression, a fault-bounded graben that represents the southern flank of the caldera that collapsed. Kaua`i's volcano also has two other calderas that formed on its flanks -- Haupu and Lihue calderas. These are unusual -- they are the only two flank calderas in the Hawaiian Islands.

The volcanic rocks of the Kaua`i shield volcano are called the Waimea Canyon volcanic series and are typical Hawaiian tholeiitic basalt. Post-caldera volcanics eventually filed the caldera. These are called the Olokele Formation and are mostly tholeiitic, but the latest lavas are alkalic.

The youngest of the Waimea Canyon lavas are about 3 my old. The book says that from about 3 mybp to 1.5 mybp, the Kaua`i volcano was dormant and erosion took over. However, recent work has shown that a period of post-erosional volcanism began at least 2.65 my ago, so there was not much of a hiatus of volcanism in northwest Kaua`i. The 1.5 my old volcanics were all erupted in east and SE Kaua`i. The youngest dated rocks are 500,000 yr old. The ///-ruled areas on the geologic map are exposures of post-erosional Koloa volcanics. These are mostly very alkalic rocks. At least 40 vents have been recognized in eastern Kaua`i. They range from small spatter cones to large cinder cones. Some of the lavas are very thick -- there are 650 m exposed in the east wall of Hanalei Valley.

Why is Kaua`i generally circular? Did it not suffer slumping? No, there were major slides off both the north and south flanks of Kaua`i. Because of post-slumping subsidence and erosion, the island has assumed a nearly circular shape.

The most obvious geologic feature of Kaua`i is its advanced stage of erosion. Very deep canyons cut down from the center of the island to the ocean. Another feature of Kaua`i is the large amount of white sand beach. Because volcanism stopped over a million years ago, corals and other shell-producing animals have been able to establish themselves around the island. The coral and shells are broken down by wave action and white sand beaches accumulate.

?Just west of Kaua`i is the island of Ni`ihau. It is also the remnant of a single shield volcano made of tholeiitic basalt. The island is elongated in a west-SW direction. There are many dikes oriented NE-SW at the east-central part of the island. All of the lava flows slope SW - what does this mean? Only the SW flank of the shield volcano remains; the summit and NE flank have been removed.

Following a period of erosion, a post-erosion period of volcanism poured new lavas out onto the surface of the eroded island. Some of these eruptions were moderately explosive and several tuff cones were formed, including a large one that is exposed as Lehua Island to the north.
Northwest of the main Hawaiian Islands are a series of small islands and submerged remnants.
Most of the islands are just small residual pinnacles of volcanic rock surrounded by coral reefs, because the island chain has been subsiding and it has been many my since these volcanoes were active.

This diagram shows the relative topography along the Hawaiian chain. Note that with increasing age to the NE, the volcanoes are eroded and sink below sea level.

Web Section

For this class there is one last set of Virtual Field Trips with a quiz. Click here to get to the Kauai Virtual Field trips and here to get to the Kauai VFT Quiz.

 Page last modified on April 10, 2004