Geology of the Marshall Islands-Atolls

Geology of the Marshall Islands: Atolls

The Marshall Islands first emerged 70 million years ago when volcanic cores erupted at presently extinct hotspots south of the equator. Around 40 million years ago, while the Pacific plate continued to move in a northwesterly direction, the volcanoes began to subside. The islands were initially high, volcanic islands, but over the course of the ensuing 40 million years, they slowly sank back into the ocean from which they came, propelled by their own weight.

Microscopic organisms called polyps, which thrive in warm waters with high salinity, salvaged the remaining rim of what was once a volcano to the extent that, with botanical seeds transported by birds and ocean currents, small islets had taken form on the reefs capable of sustaining life approximately 3,000 years ago. These islets formed the circular rings encircling sheltered lagoons producing the coral atolls we see today.

Due to east-northeast prevailing wind, waves, and currents, the reef edge is highest and is cut by numerous surge channels on the east-northeast (windward) side of the atolls. Also on the windward side of the atolls, reef debris carried by waves and currents have built up islands that are more numerous and larger, with the largest islands located on seaward projections of the reef, where waves from several directions can attack the reef. Although the reef is continuously eroded on the windward side, the erosion there is more than balanced by rapid organic growth. On the west-southwest (leeward) side of the atolls, growth of the reef is less rapid and gaps are present in the reef creating passes to atoll lagoons.

The atoll lagoons are circular to rectangular, and except for numerous “coral knolls”, their basins have flat floors. The maximum depth of the coral atoll lagoons is proportional to the size of the atoll. The maximum depth of Majuro Atoll lagoon is approximately 180 feet, Rongelap and Enewetak Atoll lagoons are 210 feet; Bikini is 192 feet and Rongerik is 156 feet.

While most water consumed in the Marshall Islands comes from stormwater, fresh water may be naturally stored under larger islands in what are called Ghyben-Herzberg lenses. A Ghyben-Herzberg lens is formed when rain water percolates into the ground depressing the salt water beneath it forming a profile that has the appearance of a lens. Freshwater, being less dense than seawater, tends to “float” on the saltwater. The Ghyben-Herzberg relationship states that the freshwater zone should extend to a depth below the sea level equal to 40 times the height of the water table above sea level.

Source: Geology of Bikini and Nearby Atolls Bikini and Nearby Atolls: Part 1, Geology. Geological Survey Professional Paper 260-A, 1954.

Bikini Atoll
Bikini Atoll

Source: https://www.papahanaumokuakea.gov/monument_features/physical_atoll_formation.html

Bikini Atoll
Bikini Atoll

Bikini Atoll

Bikini Atoll
Bikini Atoll

Ghyben-Herzberg freshwater lens