Moeraki Boulders, New Zealand


natural oddities
Photo: Karsten Sperling

Large spherical boulders -- some measuring 12 feet in circumference -- are scattered on Koekohe Beach on the east coast of New Zealand's South Island. They formed millions of years ago on the ancient sea floor, collecting and hardening sediment and minerals around a core such as a fossil or a shell similar to the way oysters form pearls.
They're not the world's only examples of what geologists call septarian concretions. You can also visit the Koutu Boulders near Hokianga Harbour on the northwestern coast of New Zealand's North Island, for example. Yet the Moeraki Boulders are some of the world's largest. The particulars of their origin and what caused the distinctive cracks inside them are still being studied.




The Moeraki Boulders are unusually large and 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. The erosion by wave action of mudstone, comprising local bedrock and landslides, frequently exposes embedded isolated boulders. These boulders are grey-colored septarian concretions, which have been exhumed from the mudstone enclosing them and concentrated on the beach by coastal erosion.[
Local Māori legends explained the boulders as the remains of eel baskets, calabashes, 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. In 1848 W.B.D. Mantell sketched the beach and its boulders, more numerous than now. The picture is now in the Alexander Turnbull Library in Wellington. The boulders were described in 1850 colonial reports and numerous popular articles since that time. In more recent times they have become a popular tourist attraction, often described and pictured in web pages and tourist guides.
A cluster of highly spherical boulders
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.
Large spherical concretions similar in size and shape to the Moeraki Boulders have been found in many other countries.

Credited from http://www.sunnyskyz.com and Wikipedia

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