Geology Naurus

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The geology of Naurus is limited to an emerged seamount Eocene / Oligocene age that sits in the southern Nauru Basin of the oceanic crust of the western Pacific . Its top is covered by a Pliocene coral reef . The island was known for its once very rich phosphate stocks ( Nauruit ), which are now as good as depleted.

description

Satellite image of Nauru. The arched Anibare Bay to the east is easy to see . The dark spot in the southwest is the Buada lagoon .

The surface oval, potato-shaped seamount elongated to the northeast (6 kilometers in length and 4.5 kilometers maximum width) with an emerged surface of 21 square kilometers and a coastline of 30 kilometers is a former extinct submarine volcano , which is located at about 4,300 meters Sea depth rises from the ocean floor of the southern Nauru Basin. It forms part of the Pacific plate and is about 56 kilometers south of the equator. The Pacific plate is moving northwest at this point at a rate of 104 millimeters per year.

The seamount initially dips at an angle of 45 ° to a depth of 500 meters, but then flattens out to 34 to 31 ° (the 3000 meter isobath is already 8 kilometers from the island) and then gradually drains towards the basin. A 60-meter-high submarine edge girdles the island almost completely at a distance of 350 meters, its water depth is between 120 and 180 meters. A second, also parallel to the coast, 80 meters high, follows in 300 meters water depth, especially in the south and northeast of the island. Numerous grooves emanate from it.

At its peak, the Seamount next to the Pliocene Riffkern carries a modern, living, visible at low tide, Pleistocene / Holocene fringing reef ( English fringing reef ). Only smaller passages break through the 150 to 250 meter wide fringing reef, the intertidal surface of which is 7.4 square kilometers down to the 200 meter isobath . Behind the beach there is usually a beach wall and a 150 to 300 meter wide, flat coastal strip (English bottomside ) covered with sand or coarse alluvium at 0 to 10 meters above sea level follows inland , which is quite fertile. The coastal terrace is of Holocene origin. It usually closes with a distinctive, up to 30 meter high edge of the terrain towards the interior of the island (English topside ). Here appears the original reef, up to an average of 500 meters thick, which can reach down to 2000 meters sea depth and at its highest point on Command Ridge in the southwest of the island now reaches a height of 71 meters. It has been dated to 5 to 0.3 million years ago ( Pliocene to Pleistocene ). Its reef limestone was dolomitized by the sea water with the addition of magnesium .

Mass movements and tectonics

The Anibare Bay in the east of the island was created by a giant, facing southeast landslide in radially symmetrical volcanic buildings, leaving a concave arc in the beach line. The demolition was carried out in a canyon-like manner at a water depth of 250 meters over a slightly arched floor surface that is up to 2500 meters wide. The sides of the canyon, which is up to 400 meters deep and running in a northwest-southeast direction, are very steep at 80 °. Another, but much weaker, trailing edge with an associated, relatively shallow canyon is on the opposite side of Anibare Bay in the northwest sector.

Other minor mass movements have also taken place on the seamount, recognizable by the incised grooves, especially in the northeast and southeast, as well as several sediment bulges in the deeper area.

Further arched, more or less coastal breaks can be seen in aerial photographs in the southwest and northwest of the island. Straight breaks run through the island from northwest to southeast with offsets in the meter range, to be seen in the cart fields in the interior of the island.

The raising of the original reef, which was accompanied by karstification and the creation of fracture systems, is seen as the trigger for the mass movements. The exact point in time has not yet been clearly established, but is likely to be in the Pleistocene (<1.6 million years).

The Nauru seamount thus represents a tsunami risk, as comparable mass movements can be repeated.

Eustatic movements

The Buada lagoon

The tip of the Nauru seamount in the Paleogene was 60 meters below today's sea level. During the Miocene , the seamount with its 500 meter thick reef cap was raised by around 70 meters. The result was erosion and karstification of the reef with the formation of limestone columns up to 20 meters high. The reef limestone was affected by erosion to a depth of 55 meters below sea level, and small caves and sinkholes formed in the rock next to the limestone columns. The last ice age probably also contributed to the karstification, as the sea level was around 100 meters lower 15,000 years ago. Afterwards the atoll was flooded again and a shallow water lagoon emerged by means of lime dissolution and falls - the slightly brackish, on average 1 to 2 meters deep Buada lagoon .

Cemented storm deposits on the limestone terrace could be dated with 2730 ± 60 years BP . They clearly show the renewed sea level rise during the Holocene, which was responsible for the final design of the terrace.

The lagoon sits in a depression in the terrain below 25 meters. It is surrounded by high altitudes over 50 meters, which probably trace the course of the former atoll rim. A smaller, now dry lagoon lies in the north near Ewa . Brackish water pools can be found at the foot of the terrain step in the northeast at Ijuw and Anabar . In the Moqua Caves in the southeast of the island there is even an underground lake, the Moqua Well .

Only the uplift that took place from the Pleistocene (from 1.6 million years ago) finally reached today's final height of 71 meters. It is believed that this last uplift, as well as on Banaba, is due to a deformation of the Pacific plate, which was triggered by collisions on the Ontong-Java plateau.

Sea level rise

Nauru is also threatened by rising sea levels , as the majority of its population lives near the coast. In the period from 1993 to 2009, the increase was 4.4 millimeters / year, significantly higher than the global average of 3.2 millimeters / year. A very strong El Niño in 1998 and 1999 even temporarily halted the increase.

Geological overview

The Nauru Basin, which is generally between 4000 and 5000 meters deep (5177 meters in the north, 4110 meters in the center and 4430 meters in the south) is regarded as the Magmatic Greater Province. It is characterized by significant flood basalts and intruded storage tunnels , which together can be up to 5500 meters thick (2230 meters in the north, 5464 meters in the center and 4288 meters in the south). About 550 meters of deep sediment lay on top of this - 115 meters of volcanic plastic and zeolitic claystones on the lying surface , followed by 150 meters of chert , chalk limestone and limestone, and 300 meters of limestone and radiolarian sludge and chalk limestone in the hanging wall.

The surface of the Nauru Basin is 800,000 square kilometers with a north-south extension of 1000 kilometers and an east-west extension of 800 kilometers. The underlying oceanic crust, which has not yet been drilled, was formed on the Upper Jurassic / Lower Cretaceous border . In the north of the basin the oceanic crust is 155 million years old (magnetic anomaly M 26 of the Oxfordian / Kimmeridgian border ), but tapers to 140 million years towards the southern edge (magnetic anomaly M 14 of the Berriasian / Valanginian border ).

The MORB- like flood basalts are genetically related to the neighboring flood basalts of the huge Ontong-Java plateau , which were formed during the Aptian around 120 million years ago. However, they are not absolutely pure MORB, but rather show geochemical deviations from ocean island basalts ( OIB ). The Ontong Java Plateau is much shallower with a water depth of 2000 to 3000 meters and girdles the Nauru Basin in the south and west. In the east and north, the Nauru Basin is bounded by the seamount chain of the Marshall and Gilbert Islands ( Marshall-Gilbert-Seamount-Chain ).

The Nauru Seamount also has a basaltic composition and was formed 35 to 27 million years ago (Upper Eocene , Priabonian to Oligocene , Chattian ). It is very likely located on a hotspot , which in turn penetrated 43 to 42 million years ago in the Middle Eocene ( Lutetian ) in connection with a major reconstruction in the configuration of the Pacific plate - recognizable by the noticeable kink in the course of the Hawaii-Emperor chain was. Possibly this is the Samoa hotspot .

The Nauru Seamount is isolated in the southern part of the Nauru Basin, which is around 4,300 meters deep, directly on the east-north-east trending magnetic anomaly M 14 , which is between 136 and 140 million years old (Berriasium / Valanginium border) and thus the age of the showing hidden oceanic crust here. It is about 300 kilometers to the southeast to the steeply rising eastern spur of the Ontong Java plateau. The next neighboring seamount, in addition to several smaller seamounts in the southwest, is Banaba, 290 kilometers away on the M 13 , which lies in an east- south- east direction and shares a very similar history.

Phosphate degradation

The effects of phosphate mining on the former coral reef

Together with Banaba and Makatea , Nauru is one of the three large phosphate islands in the Pacific. Since its emergence around 300,000 years ago, the island has been used by sea birds as a nesting and breeding ground. Their phosphate-rich guano collected in depressions, several meters thick in places. Since the deposits, which had originally been estimated at 90 million tons, were on the surface of the reef, they could be extracted in open-cast mining. In the meantime (2011) the deposits are as good as exhausted, residual supplies are still available, but economically no longer worthwhile to mine.

The open-cast mine has caused major environmental damage, particularly through the removal of the soil layer and the removal of the ancestral vegetation, around 80% of the island's land surface has been transformed into a bizarre moonscape ( cart fields up to 15 meters deep ) and is as good as devastated. The marine life in the exclusive economic zone of Naurus was also badly affected by the phosphate mining. It is estimated that around 40% of marine organisms were destroyed by the input of silt and phosphate-containing wastewater.

literature

  • Hill, PJ and Jacobson, G .: Structure and evolution of Nauru Island, Central Pacific Ocean . In: Australian Journal of Earth Sciences . tape 36 , 1989, pp. 365-381 (English).
  • Jacobson, G., Hill, PJ and Ghassemi, F .: Geology and hydrology of Nauru Island . In: Vacher, HL and Quinn, T., Geology and hydrogeology of carbonate islands (Eds.): Developments in Sedimentology . Vol. 54, 1997, pp. 702-742 (English).

Individual evidence

  1. ^ Minster, JB and Jordan, TH: Present-day plate motions . In: Journal of Geophysical Research . tape 83 , 1978, pp. 5331-5354 .
  2. ↑ Dalzell, P. and Debao, A .: Coastal fisheries production in Nauru . South Pacific Commission, Noumea 1994.
  3. ^ Hill, PJ and Jacobson, G .: Hydrogeology and groundwater resources of Nauru Island, Central Pacific Ocean . In: Groundwater . tape 13 , 1988.
  4. ↑ Kroenke, LW, Wessel, P. and Sterling, A .: Motion of the Ontong Java Plateau in the hot-spot frame of reference: 122 Ma - present . In: Fitton, H. et al., Origin and evolution of the Ontong Java Plateau (Ed.): Geological Society, special publication . Vol. 229. Geological Society, London 2004, pp. 9-20 .
  5. ↑ M. Hussein et al .: Rise and fall of sea level in Nauru area . In: The South Pacific Journal of Natural and Applied Sciences . tape 28 , 2010, p. 63-68 , doi : 10.1071 / SP10007 .
  6. ↑ Kimihiro Mochizuki et al: Massive Early Cretaceous volcanic activity in the Nauru Basin related to emplacement of the Ontong Java Plateau . In: Geochemistry, Geophysics, Geosystems . Volume 6, No. 10 , 2005, pp. 1-19 , doi : 10.1029 / 2004GC000867 .
  7. ↑ Paterno Castillo et al .: Petrology and geochemistry of Nauru Basin Igneous Complex: large-volume off-ridge eruptions of MORB-like basalt during the Cretaceous . In: Initial Reports of the Deep Sea Drilling Project . tape 89 , 1986, pp. 555-576 .
  8. ^ Jacobson, G., Hill, PJ and Ghassemi, F .: Geology and hydrology of Nauru Island . In: Vacher, HL and Quinn, T., Geology and hydrogeology of carbonate islands (Eds.): Developments in Sedimentology . Vol. 54, 1997, pp. 702-742 .
  9. ↑ Dalrymple, GB, Lanpher, MA and Clague, DA: Conventional and 40 Ar- 39 Ar and K-Ar ages of volcanic rocks from Ojin (site 430), Nintoku (site 432) and Suiko (site 433) seamounts and the chronology of volcanic propagation along the Hawaii-Emperor chain . In: ED Jackson, I. Koisumi, et al. (Ed.): Initial reports of the Deep Sea Drilling Project . tape 55 . US Government Printing Office, Washington, DC 1980, pp. 659-676 .
  10. ↑ Dupon, JF, Bonvallot, J. and Florence, J .: Pacific Phosphate Island Environments Versus the Mining Industry: An Unequal Struggle . In: South Pacific Regional Environment Program . South Pacific Commission, Noumea CEDEX, New Caledonia 1989.