Beachrock: Hookipa Park, Maui, Hawaii

by Steve Erickson and Marc McGowan

Beachrock is a friable to well-cemented sedimentary rock, formed in the intertidal zone in a tropical or subtropical region. Beachrock can consist of sand or gravel (detrital and/or skeletal particles) which is cemented by calcium carbonate.

Beachrock is currently forming in the intertidal zone on the north shore of Maui, Hawaii, at Hookipa Park. The exposure is 380m long and at its widest point is 5 m wide. The beachrock consists mainly of red algae and molluscan fragments. Intraclasts are also present and they make up approximately 10 per cent of the framework grains.

According to Meyers (1987, Journal of Sedimentary Petrology), the cement of the beachrock is cryptocrystalline high-magnesian calcite (average mole percent magnesian carbonate=15.5).

The beachrock dips approximately 5 degrees seaward and stands about 1.5 meters above water level. It has been suggested that precipitation of the calcite cement is induced by carbon dioxide degassing in the vadose zone. Water splashing the beachrock will find its way into pore spaces and may further affect the cementation process.

The surface of the beachrock has semi-circular burrows that are on average 1 to 3 inches in diameter. They are initially formed by burrowing gastropods and further enlarged by erosive action of the surf.

Marc McGowan has included several of his "off topic" favorite photographs which provide an additional chronicle of the trip. He hopes everyone enjoys them.

Getting ready to tackle the two-hour hike over lava flows to watch the active volcanism on the south shore of the Big Island. Entry point of lava into the sea is barely visible. We left at 3:30 p.m., arrived at the entry point at 5:30, and stayed to see the glow of lava in the steam cloud until well after dark, hiking back to the vehicles from 7:30-9:30, aided only by flashlights.

Standing at the White Rabbit fault zone on the south side of Kilauea Caldera. Mauna Loa is in the background. The summit of Mauna Loa is 13, 677' above sea level, and the mountain stands over 31,000' above the floor of the Pacific, making it one of the largest mountains on earth.

Marc McGowan providing a scale for the silversword plant in the crater of Haleakala. This is only one of three places on Earth where the Silversword is found, along with the summit areas of Mauna Loa and Mauna Kea.

Hanging loose with the natives