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Moeraki Boulders, Koekohe Beach, Otago coast, New Zealand

Moeraki Boulders, Koekohe Beach, Otago Coast, New Zealand

Large spherical concretions, similar in size and shape to the Moeraki Boulders have been found elsewhere in the world. For example, large spherical concretions as large as 3 metres (10 feet) in diameter are along the Cannonball River within Morton and Sioux Counties, North Dakota. Large spherical concretions as much as 4 to 6 metres (12 to 18 feet) in diameter occur within sandstone outcrops the Frontier Formation in northeast Utah and central Wyoming. Similar somewhat weathered and eroded giant spheroidal concretions, as large as 6 metres (18 feet) in diameter, are at Rock City in Ottawa County, Kansas. Smaller spherical concretions are found on the shore Lake Huron near Kettle Point, Ontario, where they are known as "kettles".
As determined by detailed analysis the fine-grained rock using optical mineralogy, X-ray crystallography, and electron microprobe, the boulders consist mud, fine silt and clay, cemented by calcite. The degree cementation varies from being relatively weak within the interior a boulder to quite hard within its outside rim. The outside rims the larger boulders consist 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 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 Meoraki Boulders, and in other septarian concretions, remain an unresolved matter for which a number possible explanations have been proposed. These cracks radiate and thin outward from the centre the typical boulder and are typically filled with an outer (early stage) layer brown calcite and an inner (late stage) layer yellow calcite spar, which ten, but not always, completely fills the cracks. Rare Moeraki Boulders have a very thin innermost (latest stage) layer dolomite and quartz covering the yellow calcite spar.

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