ORIGIN OF THE FLUTED CLIFFS OF THE NA PALI COAST, KAUAI: OBSERVATIONS, ISSUES, AND SPECULATIONS

The deeply grooved (“fluted”) escarpment along the Na Pali coast on the NW side of Kauai is one of the most dramatic geomorphic features on planet Earth. Its spectacular character arises from a combination of height (~600 m) and steepness with the drainage network superimposed on it, consisting of nearly parallel, very steep (45-80°), wide (30-100m), and deep gullies, eroded by runoff, and separated by sharp divides. The original escarpment is thought to be a consequence of wave action on the NW coast.

Although there are small rockslide scars on the cliffs, and wave-induced rockfall right at the coastline, the fluted cliffs are not significantly sculpted by mass movement. Rocks strong enough to stand as steep 600 m cliffs without failing generally are not significantly erodible by runoff, affording uniqueness to Hawaiian escarpments. Erodibility is a consequence of chemical weathering of the thin-bedded Na Pali Formation basalts. Proxy evidence suggests that rotten basalts and clay weathering products are a few tens of meters deep. Decomposition of basalt must proceed downward at the about the same rate as removal of mass by erosion. The cliffs incorporate aspects of both “transport-limited” and “weathering-limited” slopes.

The best-developed fluted cliffs occur in a middle zone of the Na Pali coast where annual rainfall is 75-150cm/year, hypothetically because the climate is too arid farther SW, and too wet farther NE, resulting in thick cliff vegetation.

The fluted cliffs retreat parallel to themselves because as much material is eroded at the bases of gullies as higher up, and because no debris collects at the bases of gullies. Erosion proceeds by episodic removal of thin sheets of weathering products.

Below the very steep fluted cliffs are slopes with lesser gradients (30-45˚). Conceivably these lower slopes are developed on a lower unit of Na Pali basalts that behaves mechanically differently than an upper unit. But no such lithologic difference has been described or is obvious. A tentative hypothesis is that the lower slopes are a footslope left behind as the fluted cliffs retreat parallel to themselves.

Parallel retreat of slopes and production of basal footslopes are traditionally regarded as processes associated with desert environments, but Hawaiian examples indicate they can occur even in tropical environments.