Essays on the Marshallese Past

The Atolls of the Marshall Islands


The Marshall Islands (Aelon Kein Ad), comprising 29 atolls and 5 islands, are located in the north-west equatorial Pacific, about 3,790km west of Honolulu, about 2,700km north of Fiji and 1,500km east of Ponape.[1] The atolls of the Marshall Islands, comprising well over 1,200 islands and islets, are scattered about in an ocean area of well over 1.8 million km2. The combined ocean area encompassed in the 200 mile EEZ is well over 3.8 million km2. By contrast, the total enclosed lagoonal area of the 28 atolls is only slightly more than 11,500 km2, while the total combined land area of the atolls is as little as 180 km2. With the exception of the two northwestern atolls, Enewetak and Ujelang, the Marshall Islands are arranged in two island chains running roughly NNW to SSE: the western Ralik Chain and the eastern Ratak Chain.

Not counting the five islands, Jemo, Jabwat, Kili, Lib and Mejit, the atolls of the Marshall Islands range from very small, with less than 3.5km2, such as Nadikdik (Knox) Atoll to very large. With 2,173km2 lagoonal area, Kwajalein Atoll has the distinction to be the atoll with the world's largest lagoon.

Map of the Republic of the Marshall Islands

Underwater (bathymetric) maps show that there is also an abundance of underwater seamounts, some of which reach almost to the surface, such as Keats Bank east of Arno Atoll. Most of these guyots are aligned along the same axes as the Ralik and Ratak Chains, so that these bathymetric features as a whole have recently been termed Ralik Ridge and Ratak Ridge.

The Marshall Islands consist of atolls and raised islands. In his initial classification of atoll formation, Charles Darwin distinguished between three stages:

Following Darwin's subsidence theory, the atolls of the Marshall Islands were initially formed as apron reefs and fringing reefs around volcanic islands, similar to Ponape; increased subsidence of the volcanic cone and ongoing reef growth led to the development of almost barrier reefs or barrier reefs, such as Palau. Final subsidence led to the development of almost atolls, such as Chuuk Lagoon. In addition there are table reefs, where the inner lagoon of the atoll has been closed off and even completely infilled.

Darwin's subsidence theory could be proven right during the deep-drilling experiments undertaken on Enewetak Atoll in conjunction with the nuclear testing programme. While the earlier cores drilled on Bikini still encountered a limestone formation at 775 metres, the two cores drilled at Enewetak encountered a volcanic base at 1271 and 1405 metres respectively.

The reefs in the Marshall Islands consist of fully developed atolls, such as Majuro, almost table reefs, such as Wake/Eneen-Kio, and table reefs, such as Mejit.

Water Supply

The water suppy (hydrology) of some of the Marshall Islands' atolls has been studied in great detail. Fresh surface water bodies such as lakes are rare, and the flat landscape, permeability of the underlying rock, and the composition of soils and reef derived sediments prevent the occurrence of streams or creeks.

The atolls of the Marshall Islands.

Atoll (name) Number
of
Islets
Land area (km2) Lagoon area (km2) Ratio land to lagoon area (km2)
English Marshallese area rank area rank ratio rank
Ailinginae Ailiginae 25 2.80 19 105.96 19 2.64 11
Ailinglaplap Aellaplap 52 14.69 3 750.29 6 1.96 16
Ailuk Aelok 35 5.36 16 177.34 17 3.02 10
Arno Arno 83 12.95 4 338.69 12 3.82 8
Aur Aur 42 5.62 15 239.78 14 2.34 14
Bikar Pikaar 6 0.49 30 37.40 26 1.32 19
Bikini Pikinni 36 6.01 12 594.14 9 1.01 23
Ebon Epoon 22 5.75 14 103.83 20 5.54 4
Enewetak newetak 40 5.85 13 1,004.89 3 0.58 29
Erikup dkup 14 1.53 25 230.30 15 0.66 27
Jabwat Jebat 1 0.57 29 - 32 100.00 32
Jaluit Jlwj 84 11.34 5 689.74 7 1.64 17
Jamo Jemo 1 0.16 31 - 33 100.00 33
Kili Kle 1 0.93 28 - 31 100.00 31
Kwajalein Kuwajleen 93 16.39 1 2,173.78 1 0.75 26
Lae Lae 17 1.45 26 17.66 27 8.21 3
Lib Ellep 1 0.93 28 - 31 100.00 31
Likiep Likiep 64 10.26 6 424.01 10 2.42 13
Majuro Mjro 64 9.17 8 295.05 13 3.11 9
Maloelap Maloelap 71 9.82 7 972.72 4 1.01 22
Mejit Mjej 1 1.86 22 - 30 100.00 30
Milli Mile 84 14.94 2 759.85 5 1.97 15
Nadikdik Nadikdik 18 0.98 27 3.42 29 28.79 2
Namo Namo 51 6.27 11 397.64 11 1.58 18
Namorik Namdik 2 2.77 20 8.42 28 32.92 1
Rongelap Rolap 61 7.95 10 1,004.32 2 0.79 25
Rongerik Rodik 17 1.68 24 143.95 18 1.17 21
Taka Tke 5 0.57 29 93.14 22 0.61 28
Taongi Taongi 11 3.24 18 78.04 23 4.15 7
Ujae Ujae 14 1.86 22 185.94 16 1.00 24
Ujelang Ujla~ 32 1.74 23 65.97 24 2.63 12
Utirik Utrk 6 2.43 21 57.73 25 4.22 6
Wotho Wtto 13 4.33 17 94.92 21 4.56 5
Wotje Wjj 72 8.18 9 624.34 8 1.31 20

Fresh groundwater on the atolls consist of a lens of (lighter) freshwater floating on top of (heavier) marine waters in the subsurface island rock strata (Ghyben-Herzberg lens). Rainwater percolates down through the islands' surface to collect in the lens. The consistency and permeability of the rock strata maintains the integrity of the lens, slowing the mixing of the freshwater lens with surrounding marine water. The thickness and size of the lens for a particular island depends on many factors but tends to be larger for larger and rounder islands subjected to higher rainfall.

The distribution of impermeable rock (i.e. beachrock, conglomerates, breccia), tidal fluctuations, and other gravitational forces influence the mixing rates of the fresh groundwater with surrounding marine waters, and therefore, influence the size and salinity of the lens. Salt spray from wind and breaking waves, and mineral dissolution also increase the salinity of groundwater lenses. The small size and narrow shape of many islands, especially on the dryer northern atolls, prevent formation of groundwater lenses with potable (fresh) water, discouraging permanent human habitation.

Artificially excavated areas, such as taro pits, and natural depressions formed during typhoon events may contain standing ground-water and have a swamp-like appearance. The groundwater lens is tapped by small wells for washing, bathing, laundry and also toilet flushing water. These wells are commonly restricted to the build-on area.

Sea-level history

The islands of the Marshall Islands were not stable in geological time, but exposed to a number of influences, one of which is the change in the sea-level of the world's oceans. The sea-level as we see it today is known to have undergone fluctuations in the past. In simplified terms, depending on the extent and length of world-wide cooling connected with the ice ages, varying amounts of water were bound in form of ice on the ice sheets of the North and South Poles, as well as the northern European, northern American and Himalayan glaciers. These water masses, were thereby taken out of the circulation cycle, thus lowering the sea-level of the world's oceans.

The sea-level minimum during the last ice age was about 120m below present. During the times of sea-level lowered to such an extent, islands were in fact areas of high elevation, covered with a different vegetation and exposed to erosion. Global warming at the end of the individual ice ages freed the water again and led to sea-level rises. The additional weightload of water on the ocean bottom and the dynamics of the, flexible, earthmantle caused the sea-level to rise to a hight greater than before, and then gradually fell back to a stable equilibrium, rather than to rise directly to a new uniform and stable level.

Nothing is known about the pre-Holocene (pre-8000BC) sealevel history of Majuro Atoll due to the absence of geological deep-drilling programs. Thus evidence from elsewhere in the Marshall Islands needs to be drawn to create a picture of previous conditions. A great amount of pre-Holocene sea-level research resulted from the analysis of the deep-drilling cores on Bikini and Enewetak. Here it could be shown that the limestone cores showed zones where calcitic limestones are overlain and underlain by aragonitic sediments, suggesting intermittent times of exposure to air, i.e. substantially lower sea-level than present. This is borne out by Thorium/ Uranium dates on corals from the drill cores, which showed a break between 6000 and 100,000 before present, indicating a substantial period of time of interrupted reef growth because of exposure of the island.

During this time, Enewetak most likley looked similar to Ponape or Kosrae in general appearance, with the marked exception that the underlying rock substrate was limestone, rather than volcanic rock, and that the top of the island was level, in fact a table reef. The island of Nauru before the onset of phosphate mining would be the best direct comparison. A fringing reef would wind around the shore which was bordered by a mangrove swamp. A dense coastal vegetation zone would gradually make way to an inland forest which would climb the slopes of the table reef and which would cover its top. We can expect the plateau of the island to be sculpted with numerous fissures and pinnacles, many of which may today form the bases for inner lagoonal patchreefs.

Little is known about the Holocene sea-level history of Marshall Islands' atolls, as studies of raised coral reefs and microatolls, as well as dating assessments of coral cores to determine of reef growth have not been undertaken. It has been argued that the modern relative sea-level was achieved prior to ~5000/6000 BP in the equatorial Pacific. The relative sea-level at its maximum was at least locally 1.0 to 1.5m higher than today and has subsequently fallen smoothly until the present day.

A sea-level higher than present implies that any islands existing by then, if there were any, were higher, which then were levelled by the erosional forces of wind and water to their existing height. The general elevation of the sand cays of the Marshall Islands is usually less than 6 to 8 feet above MHWL (mean high water level). Some elevations are higher, but all cases observed so far by the author indicate human activities as the underlying cause.


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Bibliographic citation for this document

Spennemann, Dirk H.R. (1998). Essays on the Marshallese Past Second edition. Albury:
URL: http://marshall.csu.edu.au/Marshalls/html/essays/t-glossary-test.html

CONTACT:
Dirk H.R. Spennemann, Institute of Land, Water and Society, Charles Sturt University, P.O.Box 789, Albury NSW 2640, Australia.
e-mail: dspennemann@csu.edu.au


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