Science Nugget: 12⚛ Moeraki Boulders

Speherical Beach Oddities

Wander down to New Zealand’s Koekohe Beach on the Otago coast and you’ll see what look like gigantic petrified coconuts scattered in the sand. These are the famed Moeraki Boulders, known affectionately by some locals as ‘giant gobstoppers’ or ‘alien’s brains’. About 50 of these near-spherical boulders line the beach, with some measuring over 3 metres in diameter and weighing several tonnes.

The boulders are what geologists call concretions, which are cemented masses of mineral deposits, often rounded, that form within a matrix of sedimentary rock. Typically harder than the surrounding rock, concretions can be exposed as shoreline erosion removes the enclosing layers. They’re characterised by hollow cores and unusual cracks lacing their surface. These cracks, called septaria, have been filled in with calcite, quartz, and dolomite, giving them a distinct yellow-brown line pattern.

Native Polynesians of New Zealand, called the Maori, have their own mythological explanation for how the stones came to be. According to legend, the rocks are bottle gourds, eel baskets, and other wreckage washed ashore from the Āraiteuru, a large waka, or sailing canoe, that sank off the coast of Shag Point.


‘Wait! So is this a hypothesis, a theory, or a fact?’

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