Bokak Atoll - Physical Features

Physical Features

The atoll is roughly crescent-shaped, measuring about 11 miles (18 km) by 5.6 miles (9.0 km), and oriented in a north-south direction. The atoll reef is unbroken except for a 22 yards (20 m) wide channel in the west. Ten islets lie on the eastern and southeastern reef. The more important named islets, from north to south, are North Island, Kamwome, Bwdije, Sibylla, Bokak, and Bwokwla. Sibylla is the largest, measuring approximately 4.5 miles (7.2 km) in length and up to 333 yards (304 m) in width. Kamwome Islet to the north-east of Sibylla is the second largest, while Bokak (Taongi), after which the atoll is named, lies to the south of Sibylla. Based on the results of drilling operations on Enewetak (Eniwetok) Atoll, in the nearby Ralik Chain of the Marshall Islands, Bokak may include as much as 4,600 feet (1,400 m) of reef material atop a basalt rock base. As most local coral growth stops at about 150 feet (46 m) below the ocean surface, such a massive stony coral base suggests a gradual isostatic subsidence of the underlying extinct volcano, which itself rises 10,000 feet (3,000 m) from the surrounding ocean floor. Shallow water fossils taken from just above Enewetak's basalt base are dated to about 55mya.

High boulder and sand ridges indicate a history of severe storms and are a feature of the islets. Inland on the wider islets are sand and rubble flats, while back from the lagoon sides are low sand and gravel ridges. Soils are mostly very immature, a mixture of coarser coral sand and gravel of various textures with very little humus accumulation. The lagoon is shallow, probably not exceeding 30 metres (98 ft) depth, and has many coral heads and patch reefs, some reaching the surface.

The lagoon water level is up to 0.9 metres (3.0 ft) higher than the surrounding ocean due to an influx of wind-driven waters over the windward ocean reef and the presence of only one narrow reef passage on the leeward side. Water cascades over the coral-covered rim and flats of the sloping leeward reef. A massive algal ridge lines the outer edge of the windward reef, while the south and west reefs are coral-covered narrow flats where landings can be made in quiet weather. A very small algal rim, 4-6" (100–150 mm) high, on lagoon shores of the westernmost islets, on east-facing lagoon reef-fronts and on the windward edges of coral patches in the lagoon, may be a feature unique to Taongi. This rim is maintained by the constant flow of water over the reef flat.