Shape Development On Tahiti-Nui

Basalt pebble morphology was studied in nine rivers and on fourteen high- and low-wave-energy beaches around Tahiti-Nui. The study included about 15,000 measurements on 961 pebbles in the 16-256 mm size range. Tahiti was chosen because it consists predominantly of isotropically-wearing basalt, has many rivers, and numerous beaches with different sand-pebble ratios and different wave energy characteristics. The maximum projection sphericity averaged .68 for rivers, .64 for low-wave-energy beaches, and .58 for high-wave-energy beaches. Roundness averaged .38 for rivers, .47 for low-wave-energy beaches, and .55 for high-wave-energy beaches. The oblate-prolate index averaged +0.18 (prolate, rod-like) for rivers, -0.81 for low-wave-energy beaches, and -2.13 (oblate, disc-like) for high-wave-energy beaches. It appears that a sphericity value of under .66 and oblate-prolate index more negative than -1.5 distinguish beach from river pebbles. On sandy, low-wave-energy beaches the smallest pebbles are flattest, while on gravelling, high-wave-energy beaches the largest pebbles are flattest. Thus the optimum sliding size of pebble is a measure of surf vigor. A selective trapping effect causes discs to be more abundant on sandy than on gravelly beaches. Abrasion is the chief cause for the abundance of discs on the beaches; evidence is that as roundness increases from rivers to beaches, sphericity decreases and oblate-prolate index becomes more negative (disc-like). The production of discs on the beaches is predominantly a result of abrasion caused by pebbles sliding back and forth over sand or smaller pebbles in the surf zone. This change in shape is accomplished within distances of a few feet to a few hundred feet of the river mouth. Selective shape sorting is also important on some beaches with waves 1-3 feet high, and sandy beaches tend to selectively trap discs.