Geology

Oceanic Crust
Paleomagnetic surveys indicate that the oceanic crust below Nauru was formed about 132 million years ago (Early Cretaceous). A clockwise rotation of 8 of the crustal plate has occurred around Nauru with a general movement northward by the island itself of 12° 3 since its formation.

Vulcanism
Nauru is similar to Ocean Island (Banaba) in that it is an isolated volcano rising from the ocean floor, 4300m below the surface with the peak still submerged but close enough to the surface to allow coral growths to form, eventually becoming an island through uplift.

The instability of the crystal plate caused the volcanism which created Nauru as well as Ocean Island which continue to drift to the northwest at a rate of 25mm per annum. This vulcanism which created the volcano on which Nauru wits was probably caused by the changed in the direction of the movement of the Pacific plate as evidence by the 110° bend the Hawaiian-emperor chain which took place 41 million years ago. Gravity and Magnetic surveys estimate that Nauru was formed between 29 and 47 million years ago.

As with other volcanic cones in the Pacific, it is probable that the base rock in the cone that formed Nauru is basalt.

Earth tremors take place infrequently and since the volcano is supposed to be extinct, may warrant further investigation.

Central Plateau
The inland plateau is capped with phosphate several metres thick beneath which karrenfeld and karst limestone have been eroded producing pinnacles of up to 24 metres in height and a maze of caves underneath. The last emergence of the island was about 15,000 years ago during the last ice age when sea level was 100 metres lower than at present.

Phosphate
Phosphate has three sources – marine, igneous and guano (bird droppings).

1. During a period of submergence, the limestone is thought to have become dolomitized through the action of sea water. The most commonly held theory regarding the formation of marine-based phosphate is that organic debris (plant and animal matter as well as faecel pellets) in highly fertile tropical waters decayed in the seawater to form phosphate sediments. Uplift or changes in sea-level brought the phosphate deposits to the surface.

2. Igneous deposits are derived from rocks that melted in the earth’s crust, flowing upwards and solidifying as intrusions. The actual geological origin of these deposits remains obscure.

3. The guano theory is based on phosphatic solutions leaching downwards from bird droppings, reacting with the underlying rock formations to form phosphate rock which over time weathered into soft friable rock and soil.

The source of Nauru’s phosphate has not yet been established beyond doubt but evidence seems to support the belief that it is of marine origin. The vast quantities of marine fossils including a giant Nautilus about a metre in diameter strongly supports the marine connection. Chemical analyses carried out on phosphate rich specimens seem to indicate that Nauru phosphate was derived from upwelling cool, deep water currents rich in phosphate which caused phosphatisation of organic remains as well as direct precipitation of cellophane to form phosphate pellets. Under high magnification, Nauruan phosphatic pellets show an inner core of calcareous algal material enclosed by thin concentric rings of collophonic phosphate which grew under the rolling action of marine currents.

The plateau is covered with Arcalcum diphosphate to a depth of 24 metres.

Limestone
Dolomitised limestone which forms the bulk of the island is found to a depth of 55m below sea-level. It is believed to have been formed by the progressive magnesium enrichment of calcareous rock over an extended period of time. Consequently, on Nauru, it is found mainly in aged rocks dating from the Miocene epoch. Quartenary molluscs have been found near the plateau surface.

Minerals
Quartz, feldspar and mica are not found because there is no granite on Nauru as the sedimentary deposits completely cover the volcanic base rock. However, volcanic rocks thrown up onto the beach during severe storms were collected and highly prized as they were the only stones that could be heated without exploding and were therefore used for traditional underground cooking. High levels of zinc and cadmium were found in the phosphate a feature of sedimentary deposits it probably originated from selective element assimilation from sea water by marine organisms.

Coastal Terrace
It appears that this platform was the result of erosion by wave action on the steep coral and limestone cliffs after uplift which took place 15,000 years ago. Over thousands of years, a coastal terrace was formed and coral pinnacles which dot the landscape give evidence to this theory. As storm debris and decaying organic matter built up this area above high tide level, vegetation continued the leveling process and man completed it. The coral and limestone foundation of the coastal terrace could even be anywhere between 5 and 20 million years old.

The upper layers of the coastal terrace is composed of aragonitic coral of fairly recent origin. Radiocarbon dating from the drill core of the storm ridge at Anabar places its formation in the Holocene epoch, 2,700 to 2900 years ago. (With aging and subsequent compression, aragonite is transformed into calcite.) It is of far too recent origin to be anything more than cemented storm deposits. There is still plenty of evidence of this cementing taking place on the beach even today.

Coral Platform
A coral platform or fringing reef surrounds the island varying in width from 100 metres at Orro in Aiwo to nearly 300 metres at Ewa with the widest platform generally in the northern half of the island and appears to be the section of the coastal terrace which is still within reach of wave action. It is completely covered a high tide to a depth of about 1 metre except for the coral pinnacle remnants which stand like sentinels on the reef. The tallest are found at Anibare and rise to about 10 metres above the reef platform. The densest concentration of pinnacles is found at Anibare also.

On investigating the crevices and hollows in these taller pinnacles, phosphate soil and rock can be still found, further supporting the erosion theory.

Since coral dies when it has been out of water for only a few minutes, the reel platform itself is dead except for the hollows which are always filled with water, even at low tide, and the seaward edge which is actively growing. The life in this reef is best seen at night and it literally blooms like a garden.

Beach

A golden sandy beach separates the reef and coastal terrace. The beaches have undergone a lot of change in the past thirty years, some of it caused by the continuing growth of the coral but some of it affected by the activity of man.
      
Up until the 1960s, Nauru was surrounded by beaches of golden sand. On top this sand, high tide waves frequently left deposits of shingle formed from broken bits of coral and shell whose sharp edges had been worn down. The amount left on the beaches depended on the size of the tide with more deposition at King Tide. Slabs of coral averaging 30cm in length and 20cm in width with a thickness of about 5cm are also found on the beaches on the eastern side of the island.

In recent years, some areas of beach especially at Anibare Bay, have lost their sand and pitted coral platform underneath has been exposed. The sandy covering has been denuded by sucking action of channels which have been blasted in the reef, to give motorized boats easier access, without the construction of groynes to prevent erosion. At both ends of the runway, extension work has brought changes to the deposition of sand and now that the work is complete, it remains to be seen what further effect the new coastal configuration will have on the beaches.

Information drawn from the Jacob, Maggie(1995). Nauru Geography;Nauru Island - A general Background. Nauru Studies Section, Curriuclum Office, Nauru Department of Education: Yaren, Nauru. October 1995.  

Last modified: 21/11/2006
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