SAND IN HAWAII
Jodi N. Harney
Beaches in Hawaii may be made of 'black sand' derived from the erosion of volcanic rocks, of 'white sand' made by marine organisms, or a mixture of both. On the windward side of the Big Island, for example, black sand beaches are very common. The beach at South Point is almost entirely green sand composed of olivine, a common mineral found in the volcanic rocks here in Hawaii. On the Kona coast of the Big Island and on the other islands, the beaches have a range of mixed compositions, some with a high volcanic (detrital) component, some dominated by calcareous (reef-derived) sediment. Beach and submarine sands in Kailua Bay on windward Oahu (my research area) are almost entirely composed of calcarous, reef-derived material. On average, only about 5% of the sand grains are volcanic minerals or rock fragments. Every beach is unique and has its own source and type of sediments.
Because Hawaii does not have a continental source of quartz sand like mainland beaches, the 'white' beaches and marine sediments here in the islands are primarily composed of the carbonate shells and skeletons of marine organisms, such as corals, algae, molluscs, foraminifera, echinoderms, and bryozoans.
Some animals (such as corals and molluscs) and plants (or 'algae' such as Halimeda and 'coralline' algae) living on reefs and in shallow marine waters make hard skeletons of calcium carbonate (CaCO3). These organisms are quite successful at building reefs and making sand, as long as the environment they live in is a healthy one.
The production of CaCO3 occurs at different rates depending on the organism, location, setting, and physical factors such as water depth, amount of light, and wave energy. The table below lists some "production rates" for corals and algae in Hawaii. Click on the table to view a larger version.
As the reef is built, it is also broken down into sand by other animals ("bioerosion") and by wave action ("mechanical erosion"). These processes indirectly make sand for our beaches.
A sediment budget is a quantitative estimate of particulate sources, sinks, and losses within a geographically well-defined natural system. It includes the production of calcareous sediments by reef-dwelling organisms and also the addition of detrital sediments derived from the erosion of volcanic terrain. Check out my sediment budget for Kailua Bay.