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.
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.
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.
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
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.
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.