CHARACTER AND ORIGIN OF THE SANDY BEACHES OF HAWAII ISLAND’S YOUTHFUL SHIELD VOLCANIC MOUNTAIN COMPLEX

The trade wind-impacted, subtropical, geologically youthful, shield volcanic mountain complex of the Big Island of Hawaii provides a prime location for the investigation of the sedimentological processes that dictate the character and origin of Hawaiian beaches. The merged tops of the island’s five young, < 500,000 yrs bp, volcanic mountains constitute > 62% of the landmass of Hawaii’s eight main high islands, yet the complex contains < 5% of the archipelago’s modern sandy beaches. Only 6.5% of the Big Island’s 500 km perimeter, ~31 km, is rimmed with beach sand; however, the island displays greater variability in composition than that of the beaches of Hawaii’s older islands, ranging to > 5 my. This process sedimentological analysis emphasizes the unprecedented sampling and quantitative petrographic examination of epoxy-impregnated sand samples from 58 of the Big Island’s 66 publically accessible sand beaches.

Beaches along the more than 290 km of the island’s windward, northeast and southeast coasts, from its northernmost point to South Point, are composed predominately of olivine-rich volcanic sand. Northeast coasts receive fluvially contributed grayish-black, crystalline lava rock terrigenous sediment along its wet, trade wind-impacted older Kohala and Mauna Kea mountain portions. Southeastern beaches are composed dominantly of jet-black, hyaloclastic contributed directly from volcanically active Kilauea and Mauna Loa. Olivine-rich Papakolea “Green Sand” Beach near South Point provides an example of a unique geological phenomenon produced by the uncommon combination of normal geological processes. In dramatic contrast, beaches present along 160 km of the dry leeward, Kona coastal zone, South Point to Kawaihae, are composed predominately of yellowish to whitish bioclastic carbonate sand derived from abrasion of shallow fringing marine reefs. A decrease in the coral/coralline algal ratio of beach sand bioclasts from south to north appears indicative of the age and relative subsidence rate, from high to low respectively, of the tremendous volcanic edifices of Mauna Loa, Hualalai and Kohala.

The Big Island’s dynamic geologically diverse setting, with its varied sand beaches, provides an ideal natural laboratory for the demonstration of the process approach to sedimentological analysis.