For a guy born and raised in Hawaii, I was pretty ignorant of the actual processes that gave rise to the islands we lived on, and it wasn't until I wanted to understand how the terrain from Pololu to Waipio Valleys came to be that I realized the extent of my lack of knowledge. For example, I didn't know that the massive landslide that helped to create the current Kohala coastline was not such an unusual geological event for the Hawaiian Islands, and in fact, landslides of this magnitude come with the territory, so to speak.
Every major island in Hawaii has at least one coast with towering cliffs facing the ocean as a result of such landslides. Massive pieces broke off the eastern part of Kauai, and a large section of the north flank of Wai'ale'ale left a towering sea cliff that later eroded into the spectacular Na Pali Coast. On Oahu, most of Wai'anae Volcano's western flank slid into the ocean, and the Nu'uanu Slide of Ko'olau Volcano's eastern flank was one of the largest known slides on earth. It scattered debris from Ko'olau in a swath 32 km (20 mi) wide as far as 193 km (120 mi) to the northeast. One block of debris was measured at 28 km (18 mi) long and 1.6 km (1 mi) thick, as large as a sea mount.
Molokai, Maui, Lanai, and Kaho'olawe existed as one land mass during the last ice age. The Wailau Slide of the northern flank of East Molokai was the third largest slide in Hawaii shattering into fragments that scattered 161 km (100 mi) to the north and creating a sea cliff that stood 1,829 m (6,000 ft.) high. Today even after the sinking of the island and erosion, it is still the highest sea cliff in the world at 1,128 m (3,700 ft). Lanai is a mere shadow of a volcano that lost most of its mass in the Clark Slide. For the Big Island of Hawaii, besides the Kohala Volcano landslide, there was also the Alika Slide 100,000 years ago that spread part of the western flank of Mauna Loa 100 km (62 mi) into the Hawaiian Deep.
Why didn't I know any of this? I'd like to say that it was probably because I took at face value the beautiful beaches and mountains I enjoyed in my youth. Like most people in Hawaii, I didn't really care how it had all come to be, although I was well aware that we should protect what we had. But the truth is that besides the most basic description of how our islands came to be from volcanoes rising from the sea, it wasn't covered in the public schools I attended. (Of course, this was decades ago. Hopefully, education in Hawaii provides a better appreciation of Hawaii now.)
At this point, having realized the importance of massive landslides in creating the Hawaii I thought I knew, I thought I should similarly check what I'd been taught while growing up:
Hot spot: Everyone learned that the Hawaiian Islands are the result of island building through volcanoes growing over a hot spot in the Pacific Plate. A volcano will appear over the hot spot, and as it grows, it breaks the surface creating an island. The island continues to increase in size until the volcano moves away from the hot spot, because although the plate is moving up to 9 cm (3.5 in) a year, the hot spot is stationary. The whole mechanism is like an island creation conveyor belt. The Hawaiian Island chain actually extends over 1,770 km (1,100 mi) to Kure (past Midway Island) in the Northwest Hawaiian Islands, but the submerged islands and seamounts continue all the way to the Aleutian and Kurile Trenches on the ocean floor. The oldest seamount in the northwest is about 70 million years old.
What I didn't know: In spite of geologists' certainty about Hawaii's fate as it moves away from the hot spot, they are not sure what causes the hot spot or the plumes that create them or even from what depth they come from. There are various theories ranging from asteroid impacts to chemical changes from pressure, but all await further research and confirmation. There are about 42 hot spots on earth.
A sinking feeling: We learned that the Hawaiian Islands we lived on would eventually sink below the waves due to erosion. Waimea Canyon on Kauai was touted as a spectacular example of erosion, and the small islands and atolls to the northwest of Hawaii as our inevitable fate.
What I didn't know: A Hawaiian island starts sinking even before the volcano appears above sea level to create an island. All volcanoes in Hawaii are shield volcanoes. Due to the hot spot, Hawaii magmas are the hottest in the world, and as such, are more fluid, so the slopes of shield volcanoes are less steep and it takes more volume to attain height. The amount of magma pumped out by a Hawaiian volcano is so great that the oceanic crust begins to sag under the weight of the volcano before it breaches the ocean surface, and as long as the volcano continues to build itself higher, the more the island will also sink. Sinking only stops when no more magma is added to the mass of the island. Today the newest island, the Big Island of Hawaii, is sinking even as it grows, and a gauge in Hilo has recorded this at 2.5 to 5 cm (1 to 2 in) a year.
Here today, gone tomorrow: Growing up near Diamond Head as a child, I often went to Waikiki Beach and knew the shoreline from Ala Moana Beach to Black Point intimately. Maybe the emotion of childhood memories got in the way, but I never imagined that that shoreline was significantly different in the past or could radically change in the future.
What I didn't know: This is less about not knowing and more about not connecting the dots, but when you combine sinking islands with changes in sea level due to the coming and going of ice ages, it's obvious that coastlines will change. There are ancient shorelines now submerged on the undersea slopes of Kohala and Mauna Kea as deep as 1.2 km (4,000 ft) from previous ice ages. There are also such shorelines above sea level, an example being the ancient sea cliffs standing above the coastal plain west of Waimea on Kauai. On a coastal plain formed when the sea level was much higher, an ancient shoreline still exists ringing the area around Pearl Harbor, and Pearl Harbor itself is actually made up of drowned valleys. When the sea level dropped during the last ice age, streams cut valleys which in turn were flooded as the sea level rose again at the end of the last ice age. I didn't realize how dynamic a past the beach I played at had. Hawaii really is the geological equivalent of an action movie.
On landslides, what else I didn't know: I didn't know that massive landslides were actually due to the structural weakness of a Hawaiian island. When a Hawaiian volcano appears deep below the surface on the ocean floor, the immense pressure at those depths keep the magma from exploding so the lava flows appear like pillows and build up upon each other. At more shallow depths, water pressure is insufficient to prevent explosions, so lava shatters into small pieces of debris. Much of this weak debris hundreds of meters thick form the submarine foundations of the islands, and subsequent lava flows cover this pile of sediment. Eventually, the landmass grows so large that the sediment layer gives way triggering landslides. Submarine slopes also tend to be more steep than subaerial portions of the shield volcanoes, so they give way even though the landmass above sea level may not seem unstable. These massive slide scarps take place once every 100,000 to 200,00 years especially when a volcano has reached its maximum size.
Sea level is not level: Lunar tides aside, I was so sure that sea level meant sea level; that is, sea level in Hawaii is the same as sea level in Stockholm and everywhere else.
What I didn't know: Apparently, the upwelling of the hot spot brings an excess of dense mantle material to create a bulge in the Pacific Plate 400 km (248.5 mi) wide called the Hawaiian Swell. This means the force of gravity is higher over the bulge than elsewhere, and therefore, more water is attracted and flows into the area. Sea level in Hawaii is 22 m (72 ft) higher than "true sea level."
Having gone through this remedial education, I've learned some things about how the northeastern coast of Kohala and the sea cliffs there were formed. Kohala Volcano was built up in two phases of eruptions, the Pololu formation which have the oldest rocks on the Big Island from 780,000 years ago and the Hawi alkalic basalt flows from 250,000 years ago. Erosion had already cut gullies and valleys into the Pololu formation when Hawi lavas covered much of Kohala Volcano.
As a result of Hawi volcanism, one of the most unusual ecosystems exists at the summit of Kohala. Huge volumes of ash from the Hawi phase eruptions spread over a vast area, which weathered into soil that contained impermeable clay called hardpan just below the surface. The hardpan trapped water on the nearly level ground at the summit creating a swamp now supporting a unique cloud rainforest environment with many rare and endemic species. Where the hardpan fractured, water seeped down and appeared in nearby valley walls as springs that eroded the ground under the water creating sinks and collapsing valley walls.
At its maximum size, Kohala Volcano was about 1.6 km (1 mi) higher and probably twice as large as it is now. At that point about 300,000 to 250,000 years ago, a massive landslide took place leaving a slide scarp. From this point, the diagram on page 40 in "Roadside Geology of Hawaii" tells the story of how Pololu Valley and valleys like it formed better than I can.
Streams continued their carving turning gullies into valleys while Kohala Volcano sank. Seawater flooded the valley floors while streams, landslides, and lava flows continue to fill the valleys with sediment creating broad flatlands between the high and steep walls of the valleys. The original depth of the valley can be found by projecting the valleys downward to where they meet below the surface of the valley floor, and in most cases, sediment fill can be hundreds of meters deep.
One day Kohala Volcano will sink so much below sea level that there won't be enough sediment to fill the valley floors. Many undersea canyons that have completely drowned show up in the undersea topography of the Hawaiian islands. Pololu Valley seems to be on its way as it continues for a short distance under the ocean up to the apparent headwall of the great landslide.
One important feature of Kohala Volcano is the horst and graben structure near the summit. This was formed when vertical magma dikes pushed the rift zone apart and created faults along the rift. Blocks between the faults are pushed upward (horsts) or drop down (grabens).
Two grabens run like culverts from northwest to southeast, so surface water near the summit flows to the backs of Waipio and Waimanu Valleys on one end and Honokane and Pololu Valleys on the other. Having cooled underground into dense rock, the magma dikes also act as impermeable walls that groundwater cannot flow through, so groundwater also ends up in the same areas as surface water. The accumulation of water at both ends of the grabens and dikes have been largely responsible for the deep canyons, and towards the backs of the valleys, some of the highest waterfalls in Hawaii such as Waihilau Falls in Waimanu Valley which is 790 m (2,600 ft) high.
The forces of water create not only the valleys but the profile of the coast. Seen from above, erosion of the shoreline is most evident between Pololu Valley and Waipio Valley. While this is partly due to the wave action undercutting the sea cliffs creating eventual collapse of the cliff face and eroding the beaches at the valley openings, the constant stream of water out of the valleys is also a factor.
That made me wonder what the original shape of Kohala Volcano must have been, but when I made an inquiry to one of the University of Hawaii's geology experts who teaches a course on the formation of the Hawaiian Islands, he provided recent studies showing that it would be very difficult for an accurate description given the extent to which Kohala Volcano is covered by volcanics from Mauna Loa and Mauna Kea. I took this as a sign to bring this learning journey to an end for now.
Lipman, Peter, et al., "The Giant Submarine Alika Debris Slide, Mauna Loa, Hawaii," Journal of Geophysical Research, Vol. 93, No. B5, pp. 4279-4299, May 10, 1988.
Volcano Watch: February 16, 2006 post, USGS Hawaiian Volcano Observatory web site, accessed November 29, 2013.
Hawaiian Volcanism, Volcano World (web site), Oregon State University, (accessed November 2013).
MacDonald, Gordon, "Volcanoes of Hawaii," G MacDonald
Hazlett, Richard and Hyndman, Donald, "Roadside Geology of Hawaii," Mountain Press Publishing Company, 1996, p. 55-56,
