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Moeraki Boulders

From Wikipedia, the free encyclopedia

Boulders of varying sphericity broaching the Otago coast
The boulders at sunrise

The Moeraki Boulders (officially Moeraki Boulders / Kaihinaki) are unusually large spherical boulders lying along a stretch of Koekohe Beach on the wave-cut Otago coast of New Zealand between Moeraki and Hampden. They occur scattered either as isolated or clusters of boulders within a stretch of beach where they have been protected in a scientific reserve. These boulders are grey-colored septarian concretions, which have been exhumed from the mudstone and bedrock enclosing them and concentrated on the beach by coastal erosion.[1][2][3][4] Especially in recent years, the boulders have been a popular tourist attraction.[3][5][6]

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Transcription

[Traditional Māori music (14secs)] [Narrator] You would think it unlikely that a series of rocks along a beach should become a tourist attraction, but the spherical Moeraki Boulders are a natural phenomenon that has intrigued people for centuries. Over fifty of these strange spheres lie along a sandy beach near the fishing port of Moeraki. Māori call them Te Kai Hīnaki -- eel pots. Locals call the boulders Hooligan's gallstones. Others have named them giant gob-stoppers, aliens' brains, the bowling balls of giants, or even the Stonehenge of New Zealand. Whalers named the place 'Vulcan's foundry'. The boulders grew in a pile of mud some 60 million years ago. They are called concretions -- lumps of sediment bound together by a mineral cement. Imagine dropping a lump of glue onto sand -- the resulting clump would be a concretion. At Moeraki the glue was calcite which was probably created when a marine animal began to rot. The calcite hardened around the sediment and as more was added it grew just like a massive pearl. The largest boulders weigh 7 tonnes and are 2 metres across. They are among the world's largest concretions and they took around 4 million years to grow. Their roundness is not because they have been tumbled in the surf, as many believe; but simply because they grew evenly in all directions -- forming a perfect sphere. The boulders only recently landed on the beach -- they formed in a layer of mudstone which was lifted above the sea about 15 million years ago. Breaking waves released them from the cliff and they rolled down to the beach. Today some boulders can be seen half-freed in the cliffs. According to Māori stories, many centuries ago, Māori arrived here from their South Pacific ancestral home of Hawaiki. They made the long ocean voyage in canoes carrying people and cargo. Unfortunately one of these canoes, the Ārai-te-uru, was wrecked in a fierce storm on Shag Point or Matakaea, just south of the Moeraki Boulders. The reef there is said to be the remains of the canoe. Just before the canoe was wrecked the travellers threw the food baskets of kūmara and gourds overboard to lighten the load. The baskets washed ashore at Moeraki where they petrified into what we now know as the Moeraki Boulders. Some of the less regular stones along the beach are said to be kūmara. Photographs from the nineteenth century show many more boulders on the beach than are there today. Many smaller boulders were taken as souvenirs or garden ornaments. One even made it to Australia. In 1938 a very large Moeraki boulder was hoisted onto a truck and taken to Otago museum to be preserved 'for all time'. But within 25 years it began to break up while its companions on the beach remained intact. In 1971 as concern mounted that names were being carved into the surface of the boulders and that rock collectors were using explosives, the boulders were given legal protection as a scientific reserve.

Description

The most striking aspect of the boulders is their unusually large size and spherical shape, with a distinct bimodal size distribution. Approximately one-third of the boulders range in size from about 0.5 to 1.0 metre (1.6 to 3.3 ft) in diameter, the other two-thirds from 1.5 to 2.2 metres (4.9 to 7.2 ft). Most are spherical or almost spherical, but a small proportion are slightly elongated parallel to the bedding plane of the mudstone that once enclosed them.[1][3][4]

Neither the spherical to subspherical shape or large size of the Moeraki Boulders is unique to them. Virtually identical spherical boulders, called Koutu Boulders, are found on the beaches, in the cliffs, and beneath the surface inland of the shore of Hokianga Harbour, North Island, New Zealand, between Koutu and Kauwhare points. Like the Moeraki Boulders, the Koutu Boulders are large, reaching 3 metres (10 ft) in diameter, and almost spherical. Similar boulder-size concretions, known as Katiki Boulders, are also found on the north-facing shore of Shag Point some 19 kilometres (12 mi) south of where the Moeraki Boulders are found. These concretions occur as both spherical cannonball concretions and flat, disk-shaped or oval concretions. Unlike the Moeraki boulders, some of these concretions contain the bones of mosasaurs and plesiosaurs.[3]

Similar large spherical concretions have been found in many other countries.[citation needed]

Composition

A cracked boulder displaying its hollow interior

Detailed analysis of the fine-grained rock using optical mineralogy, X-ray crystallography, and electron microprobe has determined that the boulders consist of mud, fine silt and clay, cemented by calcite. The degree of cementation varies from being relatively weak in the interior of a boulder to quite hard at its outside rim. The outside rims of the larger boulders consist of as much as 10 to 20% calcite because the calcite not only tightly cements the silt and clay but has also replaced it to a significant degree.[1][4]

The rock comprising the bulk of a boulder is riddled with large cracks called septaria that radiate outward from a hollow core lined with scalenohedral calcite crystals. The process or processes that created septaria within Moeraki Boulders, and in other septarian concretions, remain an unresolved matter for which a number of possible explanations have been proposed. These cracks radiate and thin outward from the centre of the typical boulder and are typically filled with an outer (early stage) layer of brown calcite and an inner (late stage) layer of yellow calcite spar, which often, but not always, completely fills the cracks. Rare Moeraki Boulders have a very thin innermost (latest stage) layer of dolomite and quartz covering the yellow calcite spar.[1][3][4]

The composition of the Moeraki Boulders and the septaria that they contain are typical of, often virtually identical to, septarian concretions that have been found in exposures of sedimentary rocks in New Zealand and elsewhere. Smaller but otherwise very similar septarian concretions are found within exposures of sedimentary rocks elsewhere in New Zealand.[7][8] Similar septarian concretions have been found in the Kimmeridge Clay and Oxford Clay of England, and at many other locations worldwide.[9][10]

Origin

The Moeraki Boulders are concretions created by the cementation of the Paleocene mudstone of the Moeraki Formation, from which they have been exhumed by coastal erosion. The main body of the boulders started forming in what was then marine mud, near the surface of the Paleocene sea floor. This is demonstrated by studies of their composition; specifically the magnesium and iron content, and stable isotopes of oxygen and carbon. Their spherical shape indicates that the source of calcium was mass diffusion, as opposed to fluid flow. The larger boulders, 2 metres (6.6 ft) in diameter, are estimated to have taken 4 to 5.5 million years to grow while 10 to 50 metres (33 to 164 ft) of marine mud accumulated on the seafloor above them. After the concretions formed, large cracks known as septaria formed in them. Brown calcite, yellow calcite, and small amounts of dolomite and quartz progressively filled these cracks when a drop in sea level allowed fresh groundwater to flow through the mudstone enclosing them.[1][3][4][7]

Documentation and oral tradition

Albert Percy Godber portrait among the boulders (circa 1925)

Local Māori legends explained the boulders as the remains of eel baskets, calabashes, and kūmara washed ashore from the wreck of Āraiteuru, a large sailing canoe. This legend tells of the rocky shoals that extend seaward from Shag Point as being the petrified hull of this wreck and a nearby rocky promontory as being the body of the canoe's captain. Their reticulated patterning on the boulders, according to this legend, are the remains of the canoe's fishing nets.[5]

In 1848, Walter Mantell sketched the beach and its boulders, more numerous than now. The picture is now in the Alexander Turnbull Library in Wellington.[5]

See also

References

  1. ^ a b c d e Boles, J. R., C. A. Landis, and P. Dale, 1985, The Moeraki Boulders; anatomy of some septarian concretions, Journal of Sedimentary Petrology, vol. 55, n. 3, p. 398-406.
  2. ^ Fordyce, E., and P. Maxwell, 2003, Canterbury Basin Paleontology and Stratigraphy, Geological Society of New Zealand Annual Field Conference 2003 Field Trip 8, Miscellaneous Publication 116B, Geological Society of New Zealand, Dunedin, New Zealand. ISBN 0-908678-97-5
  3. ^ a b c d e f Forsyth, P.J., and G. Coates, 1992, The Moeraki boulders. Institute of Geological & Nuclear Sciences, Information Series no. 1, (Lower Hutt, New Zealand)
  4. ^ a b c d e Thyne, G.D., and J.R. Boles, 1989, Isotopic evidence for origin of the Moeraki septarian concretions, New Zealand, Journal of Sedimentary Petrology. v. 59, n. 2, p. 272-279.
  5. ^ a b c C. Dann and N. Peat, 1989, Dunedin, North and South Otago. GP Books. Wellington, New Zealand. ISBN 0-477-01438-0.
  6. ^ Mutch, A. R., 1966, Moeraki Boulders in A. H. McLintock, ed., An Encyclopaedia of New Zealand. Government Printer, Wellington, New Zealand.
  7. ^ a b Pearson, M.J., and C.S. Nelson, 2005, Organic geochemistry and stable isotope composition of New Zealand carbonate concretions and calcite fracture fills Archived 2008-07-09 at the Wayback Machine, New Zealand Journal of Geology & Geophysics. v. 48, p. 395-414.
  8. ^ Pearson, M.J.; Nelson, C.S. (2006). "Organic chemical signatures of New Zealand carbonate concretions and calcite fracture fills as potential fluid migration indicators". New Zealand Petroleum Conference Proceedings. Crown Minerals Group, Auckland, New Zealand.
  9. ^ Scotchman, I.C., 1991, The geochemistry of concretions from the Kimmeridge Clay Formation of southern and eastern England, Sedimentology. v. 38, p. 79-106.
  10. ^ Hudson, J.D., M.L. Coleman, B.A. Barreiro and N.T.J. Hollingworth, 2001, Septarian concretions from the Oxford Clay (Jurassic, England, UK): involvement of original marine and multiple external pore fluids, Sedimentology. v. 48, p. 507-531.

Further reading

  • Brunsden, D., 1969, Mystery of the Moeraki and Katiki boulders. Geographical Magazine. v. 41, n. 11, pp. 839–843.
  • Klug, H., and R. Zakrzewski, R., 1986, Die Moeraki Boulders; Riesenkonkretionen am Strand auf Neuseelands Suedinsel (The Moeraki boulders; giant concretions of the beach of New Zealand's South Island) Schriften des Naturwissenschaftlichen Vereins fuer Schleswig-Holstein. v. 56, pp. 47–52

External links

45°20′42.99″S 170°49′33.82″E / 45.3452750°S 170.8260611°E / -45.3452750; 170.8260611

This page was last edited on 19 March 2024, at 01:31
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