Erosion Always Wins: Kauai


Waimea Canyon from the lower lookout, Kauai, Hawai'i

Waimea Canyon from the lower lookout, Kauai, Hawai’i

In WC’s Geomorphology class, on the first day, the professor (whose name WC has forgotten) told us, “If you only take one thing away for this course, it should be this: Erosion always wins.”

If you need proof of that claim, you should visit Kauai, the oldest of the principle islands of the Hawai’ian Islands. And the most eroded.

It’s easy to describe in general terms the geologic reason for the existence of the Hawai’ian Islands, a chain of northwesterly-trending volcanic islands in the middle of the vast Pacific Ocean. A plume – a “hotspot” – in the mantle punches up through the Pacific Plate, pumping immense amounts of lava out onto the ocean floor.1

Eventually the pile of lava rises more than 14,000 feet above the sea floor and breaks the surface of the Pacific. And an island is born. As the plume pushes still more lava up, a shield volcano is formed, a dome-shaped mountain of basalt, ash and cinders. That’s what’s happening now at Mauna Loa on the Big Island, Mauna Loa is more than 30,000 feet tall, measured from the seabed, and contains an estimated 18,000 cubic miles of rock. All created in about 600,000 years, a geologic eyeblink.

As tectonic forces shift the Pacific Plate northwesterly, the “hot spot” punches through the ocean floor again, and, eventually, a new island is born. The result is the Hawai’ian chain of islands, extending from the Big Island in the southeast to Kauai in the northwest, and a series of atolls beyond that.

Erosion starts at once, of course. Ocean waves, the big rollers that make Hawai’i a surfing paradise, strike the new rock like jack hammers. So long as the level of volcanic activity is high enough, the islands grow in size. The Big Island is still growing, although its future is being written. The underwater eruptions of Loihi, off the southeast corner of the Big Island, are already underway. In a few millennia, give or take, it will reach the surface and Loihi will be the newest island.

Once an island shifts far enough northwest, its volcanic action ceases. The island building stops. Erosion becomes the dominant force. The island starts to erode away. Four kinds of erosion: rain, waves, landslip and subsidence.2 The trade winds, a bit stronger in the slightly northerly latitudes of Kauai, push the moisture-laden air against the mountainous islands. As the air is forced up the air cools, and that moisture condenses out as rain. Lots of rain. Twenty-four feet of rain a year on top of Kauai. The water is a major erosive force. Among other things, it has carved the Waimea Canyon, 3,000 feet deep into the old volcano in places.

Ocean waves do their part. The Napali Coast – better ascribed as the Napali Cliffs – are a consequence of the waves battering the flank of the volcano. It’s also the location of some massive landslips, where huge chunks of the mountain have careened down the step, submerged slopes of the island. Subsidence is more subtle.

The ocean bottom is lifted up by the volcanic plume, a kind of immense, gentle hill that extends a variable distance around the plume. The further the portion of the Pacific Plate carrying Kauai migrates away from the active plume, the greater the subsidence. On Kauai, you can find old beaches five to seven feet below the current waterline. Further northwest, erosion has reduced the volcanic islands to atolls, rings of coral supported by the old basalt. Still further northwest, there are only seamounts, immense mountains of basalt that no longer approach sea level.

But while the land lasts, it’s the forces of erosion that give us the terrific scenery. Which certainly includes that Waimea Canyon and the Napali Cliffs on Kauai. Enjoy them while they last.3 Because erosion always wins.

 

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  1. Why a plume? Why right there? It’s “not well understood.” There are lots of theories WC may go into another time. 
  2. There are also isostatic forces. All that rock weighs a lot. It presses down on the oceanic crust it sits on. But as the island erodes away, isostatic forces matter somewhat less. Less weight, less compression. The forces involved and their role are not well understood. See Note 1. 
  3. Kauai may be the oldest of the major Hawai’ian Islands, but it isn’t old in geological terms: about 2.7 million years old, is all. An arriviste, in fact. Idaho doesn’t have any mountains that young. Nor does Alaska. Heck, Alaska has glaciers older than that. 

2 thoughts on “Erosion Always Wins: Kauai

  1. No idea how long you’re on Kauai, but with your penchant for birding I can envision most of the Usual Locations and won’t mention them. But…..

    A singular place we discovered on our own last May was something off the Top 10 (etc) most visited locations, but we found it hit all the buttons (it always helps when one finds it on one’s own). So, it gets the nod for
    * geologically really interesting – a sinkhole accessible only via a crawl-through tunnel
    * archeologically truly superb – the sinkhole being perhaps the richest archeological site in Oceania
    * birding terrific – easy access in the surrounding wetlands to the largest number of endemics I think you’ll see in one place (mostly waterfowl & waders)

    Just be sure you’re there when the cave’s gate is unlocked. Which it usually is during mid-day hours. And there’s one of the island’s better beaches around the corner.

    Makauhawi Cave

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