The Moeraki Boulders on the Otago Coast

Johnny de Silva

On a recent visit to New Zealand, while travelling to Dunedin on the Hampden/ Palmerston Road – route 1, we visited the Moeraki Boulders. A short walk down a path and steps and I was on the sea beach. I was certainly not prepared for what lay on the sea shore ahead of me.


On the shore, scattered like giant marbles, were these near spherical boulders. Were they real? Were they man-made? If they were real how were they formed? All these thoughts ran through my mind while I got about photographing the collection.

There were those that were pretty large part buried in the sand but pretty round above ground. There was at least one small one that was spherical and perfectly smooth, much like a bowling ball but with cracks. There were those that had split asunder and looked like a monkey puzzle fruit would look when disassembled. All had a yellow tinge with distinct signs of rust colour in the cracks. Some had little barnacles on them. Some had eroded within the cracks on them and looked pretty surreal. 

It would appear that the boulders were once part of the sea cliff which through erosion exposed them and they ultimately dropped onto the beach.


Boulders 4

Extracts from Wikipedia

“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.”

“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.”

“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.”

“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.“

“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.

The Moeraki Boulders are not to be confused with stromatolites, fossilised examples of which can look similar.”

A NOTE FROM JOHNNY: there are similar boulders elsewhere in New Zealand and in certain other countries. In one spot on the North Island there are boulders that are very round and much bigger than the ones I have written about

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