34 



COLIN 



25-foot-high cap of poured cement plates covers the crypt. 

 The seaward crater, Lacrosse crater, was not altered dur- 

 ing construction of the Cactus crypt. Nolan et al. (1975) 

 described the distribution of substrate types in these 

 craters and fish assemblages occurring in them (Chapter 7 

 of this volume). 



The third atomic crater is on the west side of Boken 

 Island in the north of Encwetak (Fig. 6). It is similar in size 

 to the Runit craters, roughly 200 m in diameter, 10 m 

 deep, and is connected to the sea via the reef flat. 



Two of the thermonuclear craters are located between 

 Boken and Bocinwotme Islands (Fig. 7). The MIKE crater 

 is the western one, roughly 1.8 km in diameter and 56 m 



The last crater is some 7 km southwest of Bokoluo, 

 the westernmost of the northern islands. The device was 

 exploded from a boat anchored over the shallow lagoon 

 margin. This produced a crater which excavated 

 northwestward in the shallow reef and reef flat but is very 

 open to the lagoon to the southwest. It is roughly 1.7 km 

 in diameter. 



Other Physiographic Effects from 

 Nuclear Tests 



A large area of reef flat and seaward reef face cleaved 

 away in the area north of the MIKE crater sometime 



Fig. 7 Mike (right) and KOA thermonuclear craters on the northern reef at Enewetak Atoll, 

 photographed from 10,000 feet. The larger Mike crater Is about 1.6 km across. The island of 

 Boken with the small Seminole crater is seen on the left side of the photograph. The section 

 of outer reef face which cleaved off after the KOA test can he seen seaward of the Mike 

 crater at the bottom of the photograph. [Photo by P. L. Colin.] 



deep. The KOA crater to the east is slightly smaller, about 

 1.5 km in diameter. Both blasts were detonated on islands 

 which disappeared with formation of the craters. A third 

 island, Bogairikk (not shown in Fig. 1), was largely elim- 

 inated with formation of the craters and is now 

 represented solely by remnants on the sandbar west of 

 Boken. The MIKE crater breaches the shallow reef into the 

 lagoon at the 10 to 15 m depth contour. The KOA crater 

 is still separated from the actual lagoon by a shallow txjt- 

 tom of less than 6 to 10 m depth but is confluent with the 

 MIKE crater on its west side. A minimum of about 400 m 

 of reef flat separated the MIKE crater from the op>en 

 ocean; a slightly greater margin exists between the ocean 

 and KOA crater. 



between 1952 and 1958 (Fig. 7). The section of reef did 

 not break away as a result of the MIKE test but was split 

 off sometime later. About 300 m of the reef face, running 

 as much as 60 m inward on the reef flat, fell away, and 

 there is no bottom visible in aerial photos over what was 

 once reef flat. This represents an exposure of underlying 

 reef structure which is of unprecedented magnitude (see 

 Chapter 4 of this volume for details). Direct examination of 

 this scarp reveals that it is vertical to slightly overhanging 

 with relatively sparse benthic organisms on its upper sur- 

 face. 



Other nuclear-produced phenomena still visible at 

 Enewetak include ejecta trails on the reef flat produced by 

 thermonuclear tests, particularly in the area of the craters, 



