Natural History: Island Formation
In this section we will examine the forces that built and changed Kaua'i. You will hear words like hot spot, shield volcano, formation, caldera, collapse crater, landslides, breccia, columnar basalt, alluvium, depression and erosion. By the time we are through, you should know what they mean, and be able to apply them to the ahupua'a of Nawiliwili Bay.
This section of 'Ainakumuwai draws text and photos from Hawai'i Natural History Association's Geology of Hawaiian Volcanoes web site and from Volcanoes in the Sea - The Geology of Hawai'i by Gordon A. Macdonald and Agatin T. Abbot. Information was also drawn from Kaua'i's Geologic History by Chuck Blay and Robert Siemers; and finally, Peter Reiner's website The Structure and Growth of the Island of Kaua'i.

Although Kaua'i is the oldest of the eight major Hawaiian Islands, it is a younger member of the Hawaiian-Emperor Volcanic Chain. These islands and submerged seamounts were formed when the Pacific Plate of the Earth's crust moved over a hot spot below it. About 40 million years ago, the Pacific Plate changed direction from north to northwest.
We can see this at the Hawaiian- Emperor Bend above.
Mejii Seamount is approximately 80 million years old; Midway Island is 27.7 million years old; Kaua'i is 5.1 million years old, and the Big Island of Hawai'i is less than half a million years old. Island formation has been occurring over this hot spot for at least 80 million years. (Blay)
Each of these seamounts and islands began as a submarine volcano on the ocean floor. Loihi, off the southeast coast of the Big Island is an example of this stage. It should break the surface in a few hundred thousand years. To the left, underwater lava forms pillow lava, the beginning of an island. We can see examples of pillow lava above the Menehune Ditch in Waimea Valley.
Above sea level, island formation continues with thousands of far-spreading thin lava flows that build broadly rounded, dome-shaped mountains known as shield volcanoes.
Above is a cross section of the many thin lava
flows that made the Kaua'i Shield Volcano
5.1 million years ago. These narrow layers are
tilted towards the lava vent that produced them,
and are known as the Napali Formation.
Photo by Agatin T. Abbot Used with permission.
To the right is Hualalai, on the Big Island. A young Kaua'i may have resembled this shield volcano before landslides, shield collapse, erosion and later eruptions altered her innocent face.
Geologist Peter Reiner's latest research indicates that Kaua'i may have actually been formed by two shield volcanoes. Check his website link in Sources and Credits for details.
During shield building, calderas may form when the top of the shield collapses as the magma (underground lava) beneath it moves away.
When collapsed calderas are filled with new lava, it ponds in horizontal layers that cool much slower than shield flows; this makes them thicker and more resistant to erosion. Visible above are horizontal layers that filled the caldera formed at the top of the Kauai shield. These layers are part of the Olokele Formation, exposed in Po'omau Canyon by the river's erosion. Photo by Agatin T Abbot
Used with permission.
The caldera at the top of Kaua'i's shield volcano is 10-12 miles across, the largest in the Hawaiian Islands. It was filled in by the later Olokele Formation to form one of the higher bogs (high rainfall, limited drainage, no porous soil) on Earth, the Alakai Swamp.
This is a picture of the caldera's border. On the left are the thin layers of the Napali Formation. They are separated from the thick and horizontal Olokele Formation on the right by a diagonal layer of breccia (mass of rock fragments).



Here is a diagram of the same picture, showing the different layers
Photo and diagram by G. A. Macdonald (Geology and Ground-Water Resources of the Island of Kaua'i)
On the east side of Kaua'i, the Lihu'e Depression was thought to be a collapse in the Napali Formation. Three and a half million years ago, the lava of the Koloa Volcanics began to cover the floor of the Lihu'e Depression. The last eruption of the Koloa Volcanics Series was half a million years ago in Wailua.
Sleeping Giant (Nonou) and Kalepa Ridge are
remnants of the Napali Formation
Geologist Peter Reiners says," These lavas [the Koloa Volcanics] have very strange and interesting compositions, and commonly bring up pieces of mantle rock from very deep within them when they erupt. In the Lihu'e basin they extend down some 1200 feet below sea level, suggesting that something pretty strange might have happened in this region quite awhile ago." We can see the thick horizontal ponded layers of the Koloa Volcanics at the top of Opaeka'a Falls .
At the southern border of the Lihu'e Depression is the Ha'upu Caldera, and its associated ridge. The ridge is part of the Napali Formation of the original Kaua'i Shield, while the Ha'upu Caldera is really the remains of a ponded lava plug that filled the original caldera about the same time as the Alakai Swamp was formed. The erosion resistant horizontal layers of this plug are clearly visible, and are known as the Ha'upu Formation.
That the Ha'upu Formation did not mark the vent of an independent volcano is shown by the fact that the lava beds around it do not slope away from it, but continue to slope away from the center of the island. (G.A. Macdonald) The golf course in the foreground is on the lava of the Koloa Volcanics.
Photo by Randy Wichman


Kilohana, a small shield volcano, was also created by the Koloa Volcanics. Visible at its top is a collapse crater, too small to be called a caldera.
Another factor involved in changing the face of Kaua'i is landslides. In the 1980s, the US Geological Survey used sonar to map the sea floor surrounding the Hawaiian Islands. Geologists discovered about 70 major giant landslides that cover half of the flanks of the Hawaiian Ridge (Moore and others, 1989; 1994)
Kauai has two or three major landslide areas. On the north shore, the landslides helped to shape the steep Napali cliffs.. On the south shore, the slide was a part of the slump known as the Makaweli Depression. By redirecting the flow of streams, the Makaweli Depression was responsible for the erosion of the Waimea Canyon. On the east side, a slide may have been part of the collapse that created the Lihu'e Depression.
Other destructive forces constantly changing Kaua'i are erosion and island subsidence (sinking). In about two to three million years, these forces will reduce Kaua'i to the size(one mile long, 900 feet high) of Nihoa, the next Hawaiian island to the northwest.

Why don't the ahupua'a of Nawiliwili Bay have deeply eroded valleys like those of the Napali cliffs? There are two main reasons.
Photo by Peter Reiner

First, any deep valleys on the east side that formed in the original shield were lost in the collapse that created the Lihu'e Depression. Second - after the collapse, streams had to cut and recut their valleys as a series of Koloa Volcanics lava flows filled in their earlier erosive effort.
This relief model displays clearly the valleys cut by Hule'ia, Pu'ali, and Nawiliwili streams. Note the radial stream erosion surrounding Kilohana. In addition to carving their valleys, these streams also carried the alluvium (eroded material) and deposited it along the base of the Ha'upu Ridge, as well as the lower reaches of each stream, enriching the valley soil for agriculture.





Here's what radial stream erosion on Kilohana really looks like.
Photo by David Boynton, pilot Casey Riemer of Jack Harter Helicopters.

Lava at Nawiliwili has some very special qualities. A form of lava that cooled very slowly in dikes (rock formed when magma is forced into a crack and cools there) is called columnar basalt.
As it slowly cools, columnar basalt separates into columns of four, five, or six sides. We can see this lava behind the dock at Nawiliwili, or while surfing puka moi at Kalihiwai. Above, the small columnar basalt at Nawiliwili Bay.
A really unique columnar basalt was quarried (excavated) in Kipu, and can be seen in a bridge built in the early 1900's, over the Hule'ia River. There is no other bridge like this in the islands A corral was built of the same material on the ranch. (Hobey)




It is the size of the columns that is so remarkable. This dike rock was important for the fabrication (making) of tools

This bridge was built so that the children of the workers at Kipu could go to school in Hule'ia. Before the stone bridge, a Hawaiian trail went through here to the fishing grounds at Kipu Kai.
Photo courtesy of the Baker Collection, Kaua'i Historical Society. The Rice Bridge
The next time you travel around the ahupua'a of Nawiliwili Bay - try look. You will see the evidence of island formation all around you. Once Kaua'i was born, native plants arrived and began a slow change into species found nowhere else on the planet.
Created June 2001