Enewtak and nearby atolls

Republic of Marshall Islands

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People of the Marshall Islands

Another Atoll

Geology of Enewetak and Bikini Atolls

What Is An Atoll?
Atolls are coral islands that form around the flanks of volcanic islands. As the volcanic island is subducted and sea levels chagne a ring of islands made, consisting mostly of fossilized coral and reef debris. An area of shallow water is left, where the volcanic island once stood, called a lagoon. Atolls form on basaltic foundations close to sea level, however as they sink the reef keeps building. Over millions of years, through weathering and pressure, coral islands form. Many atolls can be found around the equator in the Western Pacific Ocean.

Enewetak Atoll (and Bikini Atoll) Basics

Enewetak is the most northwestern atoll of the Marshall Islands group located in Micronesia- just North of the equator. It is made up of 40 or so low lying carbonate islands overlying a basaltic volcanoe base. Enewetak is elliptical in shape, measuring 40 kilometers by 32 kilometers. Its lagoon is relatively deep compared to other atolls like Bikini, and is one of the larger atolls of the Marshall Islands group. Its lagoon contains a significant amount of reef and coral activity. Enewetak is situated above two adjoining, submerged basaltic seamounts, and consists of carbonate sand and gravel islands 2-3 meters above sealevel. Enewetak and Bikini have been studied extensively due to US interests in it - done both for geological purposes and military purposes as a nuclear test site.

Geological History of Enwetak Atoll

This atoll sits atop a basaltic volcanoe base that rises tow miles above the ocean floor. The basaltic seamounts at the base of the atoll about 65 to 70 million years ago, during the late Meseozoic times. They eroded and were subductedby early Eocene times, resulting in the formation of the atoll we see today. Over time Anewetak has been characterized by rising and falling sealevels, subsidence, and by litle or no defomation and tilting of the sedimentary beds after deposition. Over vast stretches of time, with the rise and fall of ocean levels, varying limestone and gravel deposits formed. These go from Eocene to Oligocent, to Miocene, and to Pleicene and Pleistocene. Extensive studies have been conducted on Enewetak since the early 1950s when it was simultaneously being used as a nuclear test site.

Geologic Composition and Formation of the Atoll

Enewetak Atoll, along with Bikini, consists of 1200 meters of Tertiary and Quatenary carbonate sidements overlying an olivine basaltic foundation. The atoll itselfis quite porous in nature, with various marine waters and a little freshwater running thoruhg it. At the surface are broken, uncemented, poorly sorted bits of gravel, forminifera, corals and mollusks. As you go down in depth limestone zones rich in Aragonite/high magnesium alternate with calcium rich zones. Farther down are layers of fossilized, calcized limestone, with coral and mollusk fossils. The deeper you go, the more cemented the sediments and rocks become- utill you reach the solid basaltic bedrock. Calcite makes up the primary cementing mineral. There are basicall three zones above the basaltic base: An aragonite rich with corals and fossils, a calcite zone, and a dolomitic zone consisting mainly of dolomitized limestone (calium and magnesium removed in dolomite through dissolution).

Enewetak's formation took place over millions of years, with relative periods of volcanic upheavel and subsidence. As the volcanic base was subducted, and ocean levels fluctuated, the corals of the atoll grew. A limestone cap has repeatedly formed as sea-level frose above the atoll and when it fell layers of recrystalized limestone formed. Over geologic time, millions of years, one sees layers of unaltered limetstone instpersed by layers of re-crystallized limestone. When enough land is above sea-level a thin layer of fresh water can form, as has been the case on Anewetak. The freshwater on Anewetak atoll islandes is rather brackish though, and actually falls in-between mrine and fresh waters.

Effects of US use of Enewetak: A nuclear test site.

Before the US came to Enewetak around 1950 or so, it had been inhabited by natives. The people of Anewetak originally arrived on the atoll's islands centuries ago, coming from the Malaysian and Indonesian lands. As part of the Marshall Islands they had been under the rule of teh Spanish and then Germany. In 1951 the US Military decided to use Anewetak and Bikini as sites for nuclear testing of various hydrogen and atomic bombs. The US had to remove the residents from their islands and move them away to other atolls of the Marshall Islands. They were not allowed back untill the late 70s and early 80s. The effects of the testing done on the atoll are obvious, leaving huge craters and the vaporization of parts of the islands (See pictures below). One island was completly vaporized. In recent decades, with the UN's help, the Republic of the Marshall Islands was formed, however the damages done during the 1950s and 60s remains evident.

Literature Cited

IMAGES MAY BE INSERTED THROUGHOUT THIS SECTION, TOO

This website is part of a Geology 211 class project on Processes in Physical Geology.

Earlham · Geosciences Department · Geociences 211: Physical Geology

Copyright ©-2001 Earlham College. Revised 25 February 2002. Send corrections or comments to danglha@earlham.edu