Erosion of Fluted Cliffs on the Na Pali Coast, Kauai, Hawaiian Islands

Abstract:

Precipitous sea cliffs on the northwest side of Kauai, sculpted into deep bedrock gullies and sharp divides, are known as “Na Pali”, Hawaiian for “the cliffs”, and are commonly referred to as fluted cliffs. Rising 500 m above the coast, they constitute some of the most dramatic scenery on planet Earth and are a world famous tourist attraction. After the discovery of flank collapses on some Hawaiian volcanoes the great Na Pali escarpment was hypothesized to have been generated by a megalandslide, but bathymetric data show the shallow submarine shelf off the Na Pali coast bears no landslide deposits. It is likely that wave erosion at the base of the local thin-bedded lavas, particularly in winter, is responsible for the escarpment.

 Superimposed on the escarpment are the deep gullies, or grooves, and divides, which constitute drainage networks that happen to be developed on very steep slopes. Networks are poorly developed on relatively young parts of the escarpment and well developed on older parts, which feature coalescing tributaries and very deep gullies. In most cases gullies shallow going upslope, and many fade out completely just below drainage divides, just like in drainage networks developed on more gently sloping landscapes. The drainage networks show that the basalts along this coast are relatively easily eroded by runoff.

 Rocks easily eroded by runoff generally do not stand in 500-m-high cliffs because they are not mechanically strong enough to resist failure and mass movement. The key to development of high fluted cliffs is a mix of mechanical strength and erodibiity, a combination whose rarity explains why analogs to the Na Pali cliffs are not found outside the Hawaiian Islands. The combination here is provided by basalt which is strong where unweathered but rotten and in some case completely decomposed close to the surface, courtesy of the Hawaiian climate. Weathering must proceed downward at about the same rate as removal of mass by runoff erosion, otherwise surface erosion would cease or mass movement would dismantle the cliffs. Steepness of the escarpment is in part maintained by wave erosion, but in valleys that have eroded headward back from the coast valley walls are just as steep as cliffs right on the coast. This suggests a type of pedimentation is at play, possibly aided by groundwater sapping.