The Moeraki Boulders (officially Moeraki Boulders / Kaihinaki) are unusually large

spherical A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ce ...

boulders In geology, a boulder (or rarely bowlder) is a rock fragment with size greater than in diameter. Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely massive. In c ...

lying along a stretch of Koekohe Beach on the wave-cut Otago coast of

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...

between

Moeraki Moeraki is a small fishing village on the east coast of the South Island of New Zealand. It was once the location of a whaling station. In the 1870s, local interests believed it could become the main port for the north Otago area and a railwa ...

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 Coastal erosion is the loss or displacement of land, or the long-term removal of sediment and rocks along the coastline due to the action of waves, currents, tides, wind-driven water, waterborne ice, or other impacts of storms. The landwa ...

.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.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. Forsyth, P.J., and G. Coates, 1992, ''The Moeraki boulders''. Institute of Geological & Nuclear Sciences, Information Series no. 1, (Lower Hutt, New Zealand)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. Especially in recent years, the boulders have been a popular tourist attraction.C. Dann and N. Peat, 1989, Dunedin, North and South Otago. GP Books. Wellington, New Zealand. .Mutch, A. R., 1966
''Moeraki Boulders''
in A. H. McLintock, ed., An Encyclopaedia of New Zealand. Government Printer, Wellington, New Zealand.

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 in diameter, the other two-thirds from . 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. 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 The Hokianga is an area surrounding the Hokianga Harbour, also known as the Hokianga River, a long estuarine drowned valley on the west coast in the north of the North Island of New Zealand. The original name, still used by local Māori, is ' ...

, North Island, New Zealand, between Koutu and Kauwhare points. Like the Moeraki Boulders, the Koutu Boulders are large, reaching in diameter, and almost spherical. Similar boulder-size concretions, known as Katiki Boulders, are also found on the north-facing shore of

Shag Point Shag Point / Matakaea is a headland and township in East Otago, New Zealand. It is located close to State Highway 1 nine kilometres to the northeast of Palmerston, at the southern end of a long open bay known as Katiki Beach. The point itsel ...

some 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

mosasaur Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Maastricht on ...

s and plesiosaurs. Similar large spherical

concretion A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular ...

s have been found in many other countries.

Composition

Detailed analysis of the fine-grained rock using

optical mineralogy Optical mineralogy is the study of minerals and rocks by measuring their optical properties. Most commonly, rock and mineral samples are prepared as thin sections or grain mounts for study in the laboratory with a petrographic microscope. Opti ...

X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

, and

electron microprobe An electron microprobe (EMP), also known as an electron probe microanalyzer (EPMA) or electron micro probe analyzer (EMPA), is an analytical tool used to non-destructively determine the chemical composition of small volumes of solid materials. It ...

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. 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 Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

covering the yellow calcite spar. The composition of the Moeraki Boulders and the septaria that they contain are typical of, often virtually identical to, septarian

concretions A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular ...

that have been found in exposures of

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

rocks in New Zealand and elsewhere. Smaller but otherwise very similar septarian concretions are found within exposures of sedimentary rocks elsewhere in New Zealand. Similar septarian concretions have been found in the

Kimmeridge Clay The Kimmeridge Clay is a sedimentary deposit of fossiliferous marine clay which is of Late Jurassic to lowermost Cretaceous age and occurs in southern and eastern England and in the North Sea. This rock formation is the major source rock for Nor ...

and

Oxford Clay The Oxford Clay (or Oxford Clay Formation) is a Jurassic marine sedimentary rock formation underlying much of southeast England, from as far west as Dorset Dorset ( ; archaically: Dorsetshire , ) is a county in South West England on the ...

England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...

, and at many other locations worldwide.Scotchman, I.C., 1991, ''The geochemistry of concretions from the Kimmeridge Clay Formation of southern and eastern England'', Sedimentology. v. 38, p. 79-106.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.

Origin

The Moeraki Boulders are

created by the cementation of the

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pal ...

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, in diameter, are estimated to have taken 4 to 5.5 million years to grow while 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.Pearson, M.J., and C.S. Nelson, 2005
''Organic geochemistry and stable isotope composition of New Zealand carbonate concretions and calcite fracture fills''
, New Zealand Journal of Geology & Geophysics. v. 48, p. 395-414.

Documentation and oral tradition

Local Māori legends explained the boulders as the remains of eel baskets,

calabash Calabash (; ''Lagenaria siceraria''), also known as bottle gourd, white-flowered gourd, long melon, birdhouse gourd, New Guinea bean, Tasmania bean, and opo squash, is a vine grown for its fruit. It can be either harvested young to be consumed ...

es, and kumara washed ashore from the wreck of Arai-te-uru, 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. In 1848,

Walter Mantell Walter Baldock Durrant Mantell (11 March 1820 – 7 September 1895) was a 19th-century New Zealand naturalist, politician, and land purchase commissioner. He was a founder and first secretary of the New Zealand Institute, and a collector of moa ...

sketched the beach and its boulders, more numerous than now. The picture is now in the Alexander Turnbull Library in

Wellington Wellington ( mi, Te Whanganui-a-Tara or ) is the capital city of New Zealand. It is located at the south-western tip of the North Island, between Cook Strait and the Remutaka Range. Wellington is the second-largest city in New Zealand by metr ...

References

External links

Moeraki Boulders
* * Biek, B., 2002
''Concretions and Nodules in North Dakota''
North Dakota Geological Survey, Bismarck, North Dakota. * Hokianga Tourism Association, nd

Pictures of cannonball concretions similar in size and origin to Moeraki Boulders. * Irna, 2006

* Irna, 2007

* Mozley, P.S., 1999

Lite Geology, v. 14, p. 1-3 (Winter 1995) * United States Geological Survey, nd

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