Monday, November 6, 2017

Hole in Kalalau ridge visible in unique light

Hole in Kalalau ridge at Keanapuka. Author photo,
enlarged and color corrected by John Wehrheim

The Hawaiian Islands are constantly eroding, and an example of that erosion is a huge hole through one of the prominent ridges of Kalalau Valley on Kaua`i—a window through a rock wall.

For most people viewing it, it looks like something brand new, because they haven’t seen it in that location before. But perhaps it’s not so new, and only being noticed now because it is only readily visible in absolutely perfect lighting.

The nearly circular hole on the northeast wall of the valley is a distinct feature visible near sunset this week from along the Pihea Trail, which runs along the cliffs at the back Kalalau Valley.

It is only barely visible from the Pu`u O Kila Lookout, from which the pierced ridge is viewed nearly end-on. Most of the popular photos of the valley are taken from the lookout. 

The hole is best viewed when the ridge can be viewed from the side--a half mile or so from the lookout along Pihea Trail.

Was it always there, with the light just perfect in late afternoon on November 5, 2017, so that it was suddenly visible?

Famed Isle photographer John Wehrheim said he has viewed the same ridge from the same vantage point and from closer, and he never noticed the big hole or puka. But the name of that part of the valley wall, Keanapuka, (cave open at both ends) suggests there was at least some feature like it at that location.

“I’ve not seen it before and I’ve experienced this view many times and from much closer vantages.  But that light is uniquely perfect for separating the puka from the ridge. My guess is that the puka can only be seen clearly and obviously at this time of year with its long low angle of light,” Wehrheim said.

His enlarged and color corrected version of my image is shown ar the top of this story

So, a sheet of rock fell out of a narrow ridge, creating an opening—at some point. But did it happen recently, or decades ago, or was it an existing feature that got bigger?

Geologist Chuck Blay reviewed the images and said it may be an unnoticed older feature in a particularly thin ridge on the Hawaiian landscape.

“I can see why the hole in the ridge may have been there without notice for some time.  The lighting seems to be just right for it to be noticeable at the time you saw it.  From its shape and location it doesn't seem probable that it just all of a sudden developed,” Blay said.

The key to the visibility this week is that the near face of the ridge is in late afternoon shadow, but the cliff face behind it is in full sun, so the window-like brightness emphasizes the feature. Otherwise, the greenery of the near side of the cliff would be indistinguishable from the greenery immediately beyond it.

Also, the sunlight is at the perfect angle to illuminate the rock that forms the inner wall of the puka. That helps put a pale gray border around the hole and enhances its visibility.

These kinds of features—holes right through a rocky ridge—are not uncommon in the islands. But they are also transient, as the islands’ lava cliffs, piers and pillars erode from wind and water, and even from feral animal traffic.

There is a feature on Kaua`i that was commonly called the Hole in the Mountain, although it has mostly closed because of a collapse of its roof. It was famous in Hawaiian legend as a hole pierced in the mountain by a prodigious spear-throw from a Hawaiian hero.

Along the Na Pali Coast, there is a hole through a ridge that separates the two beaches of Honopu Valley.

On the Big Island, there is the Holei Sea Arch in Hawai`i Volcanoes National Park, and there was the Onomea Arch north of Hilo, which has since collapsed.

On Molokai’s north shore, there is a walk-through cave called Keanapuka just along Hakaaano. There are arches on Lana`i, Lehua, South and East Kauai, and elsewhere in the Islands.

And there are lots more in these dynamic islands. Why does the rock of the Islands erode so quickly? Geologist Blay has an answer.

“The shield volcanics in Hawaii are rather chemically metastable owing that they were derived from the melting of upper mantle material which is stable at high temperature and pressure but unstable at earth surface temperature and pressure,” he said.

And in that is a caution. Hawaiian cliffs are notoriously crumbly and risky to walk under or climb on.

“The lava rocks of Kauai are mostly pretty rotten,” Blay said.

“There is a good reason that sand size lava rock fragments are rather uncommon in most of the beaches of the island.  They fragment and dissolve before they get to the coastal zone.

“We all know that rock climbing is not a good idea in areas like Na Pali and Waimea Canyon,” he said.

© Jan TenBruggencate 2017

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