Chapter 3 



Ph\;siograph\; of Eneivetak Atoll 



PATRICK L. COLIN 



Motupore Island Research Department 

 (Jniuersity of Papua New Guinea 

 Port Moresby/. Papua New Guinea 



LOCATION AND SIZE 



Coral atolls have been variously defined and, without 

 considering unusual cases, can be described as more or 

 less continuous reef (largely corals and other calcium car- 

 bonate producing organisms), which surrounds a deeper 

 lagoon and drops steeply to oceanic depths on the seaward 

 margin. All islands are typically low, derived from reef rub- 

 ble and sand. Enewetak Atoll conforms to all aspects of 

 this description and in many respects is a "textbook" atoll. 

 It has a large elliptical lagoon, approximately 41 islands on 

 its rim, a few passages between the lagoon and ocean, and 

 narrow shelves dropping steeply into deep water on all 

 sides. The subsurface geology of Enewetak and Bikini have 

 been extensively examined, and these results are reported 

 in the U. S. Geological Survey Professional Papers 260 

 series. 



Enewetak Atoll is located in the northwestern Marshall 

 Islands with its center at approximately 11°30'N; 

 162n5'E (Fig. 1). It is 220 km from the nearest land, 

 Ujelang Atoll to the southwest; 310 km from Bikini Atoll 

 to the east; and about 410 to 460 km from other atolls 

 (Ujae, Wotho, Ailinginae, Rongelap) to the southeast to 

 east. To the north occur Wake Island, about 1000 km 

 northeast, and Marcus (Tora Shima) Island, about 1600 

 km northwest. To the west are the Marianas, the nearest 

 being about 1700 km. All islands of the Marshall Islands 

 are low, most being coral atolls. The high islands nearest 

 to Enewetak are Ponape, to the southwest, and Kusaie, to 

 the south, both about 580 km distant. 



Enewetak is a relatively large atoll, somewhat elliptical 

 in shape, about 33 by 41 km in size, with the islands, reef 

 flat, and lagoon covering about 1000 km . It is the third 

 largest atoll in the Marshall Islands, exceeded by Kwajalein 

 (the largest atoll in the world) and Rongelap. By world- 

 wide standards, it is not exceptionally large. The majority 

 of the area of Enewetak is the lagoon, with the reef flat 

 and the islands covering progressively less area. Table 1 



provides information on the area covered by various 

 environments at Enewetak. 



WEATHER AND CLIMATE 



Weather at Enewetak is dominated by the surrounding 

 marine conditions. Since all islands are low and of small 

 area, they do not alter weather conditions by their pres- 

 ence. The atoll is semiarid, with rainfall averaging only 

 about 1700 mm per year, and has a distinct wet-dry 

 annual cycle. Air temperatures are relatively high and very 

 stable, with a mean annual temperature of about 28°C. 

 Solar radiation is intense, and humidity is consistently 

 high. 



At almost 12°N, Enewetak is within the trade wind 

 belt with nearly consistent easterly winds. The atoll is sub- 

 ject to tropical storms and typhoons at irregular intervals 

 which greatly affect the marine and terrestrial environ- 

 ments. The meteorology of Enewetak is discussed in 

 Chapter 6 of this volume. 



ENVIRONMENTS OF ENEWETAK 

 The Lagoon 



The lagoon is the largest component of the atoll. It is 

 relatively deep by atoll standards, averaging about 54 m, 

 with a reported maximum of 71 m. The lagoon bottom 

 generally slopes from the lagoon rim toward the center. At 

 a distance of 2 to 4 km from the rim, the lagoon bottom is 

 essentially flat at a depth of about 45 m. Even the outer- 

 most 2 to 4 km of the lagoon has generally low slope gra- 

 dients on its bottom because of the horizontal distance 

 required to reach 45 m depth. The only areas with signifi- 

 cant slopes, except along the flanks of patch reefs and 

 coral pinnacles, on the soft bottom of the lagoon occur 

 shallower than 25 m. Below that depth, except for small- 

 scale undulations, there is little variation in the soft bottom 

 from the flat and horizontal. The area above 25 m depth 

 is also affected by wave action and currents which can 

 affect sediment distribution. 



Most of the lagoon bottom is relatively inaccessible to 

 human observers. The depths are below those practical for 

 sustained diving operations and, generally, must be 

 observed or sampled remotely. The area of the lagoon bo' 



27 



