Moeraki Boulders

The Moeraki rocks, also known as Kaihinaki or Moeraki Boulders, are very enormous spherical rocks located between Moeraki and Hampden along a section of Koekohe Beach on the wave-cut Otago coast of New Zealand.

They can be found sporadically along a beach where they have been conserved in a scientific reserve, either as solitary stones or in groups. These boulders are septarian concretions, which are gray in color. Coastal erosion has removed them from the mudstone and bedrock that surrounds them and concentrated them on the shore. The boulders have been an increasingly popular tourist destination, particularly in recent years.

An Explanation

The boulders’ extraordinarily huge size and spherical form, together with their peculiar bimodal size distribution, are their most noticeable features. The diameters of around one-third of the boulders range from 0.5 to 1.0 meters (1.6 to 3.3 feet), while the remaining two-thirds are between 1.5 and 2.2 meters (4.9 to 7.2 feet). The majority are almost spherical or spherical, although a tiny percentage are somewhat extended parallel to the mudstone’s bedding plane, which formerly encased them.

The Moeraki Boulders are not exclusive in either their massive size or their spherical to subspherical form. Found on the beaches, under the cliffs, and below the surface inland of the Hokianga Harbour shore in North Island, New Zealand, between Koutu and Kauwhare points, are almost similar spherical stones known as Koutu stones.

Similar to the Moeraki Boulders, the Koutu Boulders are nearly spherical and huge, reaching a diameter of 3 meters (10 feet). Approximately 19 kilometers (12 miles) south of the Moeraki Boulders, on the north-facing beach of Shag Point, are similar boulder-sized concretions known as Katiki Boulders. These concretions might take the form of flat, disk-shaped, oval, or spherical cannonball concretions.

Some of these concretions, in contrast to the Moeraki boulders, include plesiosaur and mosasaur bones.

Numerous other nations have produced enormous spherical concretions of a similar nature.

Contents

Using optical mineralogy, X-ray crystallography, and electron microprobe, a thorough examination of the fine-grained rock revealed that the boulders are made of mud, fine silt, and clay that is cemented by calcite.

A boulder’s degree of cementation can range from being somewhat weak within to being rather hard outside the rim. Because the calcite not only securely cements the silt and clay, but has also replaced it to a substantial degree, the exterior rims of the bigger boulders contain as much as 10 to 20 percent calcite.

Large fissures known as septaria, which radiate outward from a hollow core coated with scalenohedral calcite crystals, are rife throughout the rock that makes up a boulder’s mass. Numerous theories have been put up to explain the mechanism or processes that produced septaria in Moeraki Boulders and other septarian concretions, but the question remains unanswered.

An exterior (early stage) layer of brown calcite and an inner (late stage) layer of yellow calcite spar, which frequently but not always entirely fills the fissures, usually radiate and thin outward from the center of the typical boulder. The yellow calcite spar in rare Moeraki Boulders is covered with a very thin coating of quartz and dolomite, which is the last stage.

The Moeraki Boulders’ composition and the septaria they contain are similar to, and sometimes almost identical to, septarian concretions discovered in sedimentary rock exposures across the world including in New Zealand.

In various parts of New Zealand, exposures of sedimentary strata have smaller but generally quite similar septarian concretions. Similar septarian concretions have been discovered all over the world as well as in the English Clays of Oxford and Kimmeridge.

Starting Point

The Paleocene mudstone of the Moeraki Formation, which has been excavated by coastal erosion, was cemented to form the Moeraki Boulders. Near the Paleocene seafloor’s surface, in what was then marine muck, the bulk of the boulders began to develop.

Studies of their composition, particularly the amounts of magnesium and iron as well as stable isotopes of carbon and oxygen, provide evidence for this. Their spherical form suggests that mass diffusion, not fluid movement, was the source of calcium.

It is believed that it took 4 to 5.5 million years for the bigger boulders, measuring 2 meters (6.6 feet) in diameter, to form, while marine muck collected on the seafloor above them, ranging from 10 to 50 meters (33 to 164 feet).

Septaria—large fissures—formed in the concretions as they solidified. When a dip in sea level allowed new groundwater to seep through the mudstone surrounding these crevices, brown calcite, yellow calcite, and trace quantities of dolomite and quartz gradually filled them.

Oral tradition and documentation

The stones were interpreted by local Māori traditions as the remnants of calabashes, kūmara, and eel baskets that washed ashore from the wreck of Āraiteuru, a big sailing canoe. According to one tradition, the petrified hull of the wreck is represented by the stony shoals that stretch seaward from Shag Point, while the captain’s body is represented by a nearby rocky point. This folklore claims that the reticulated patterning on the rocks is made up of the fishing net remnants from the canoe.

Walter Mantell drew the beach and its larger-than-now stones in 1848. The image is currently housed in Wellington’s Alexander Turnbull Library.

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