New Zealand’s Spheroidal Moeraki Boulders

May 24, 2024


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Photographer: Ray Boren
Summary Author: Ray Boren

Resembling gigantic lithified soccer footballs, New Zealand’s striated gray Moeraki boulders (top photo) lie scattered across South Island’s Koekohe Beach, north of the city of Dunedin. It's as if primordial Titans had been playing an epic match… and forgot to retrieve some of their game balls when the contest ended. 

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Geologists, however, tell us that these patterned mudstone globes, as shown in the closeup photograph above, taken on February 14, 2008, are actually septarian concretions that formed via gradual mineral cementation of particles on the muddy marine floor during the Paleocene, 56 to 66 million years ago. Today most of the boulders — about four dozen of them, though more were carted away years ago — are bathed by waves of the South Pacific, as in the photo at right from the same visit, with a balancing tourist atop one for scale. Other boulders are still eroding from the oceanside embankment in which they were long buried.

The boulders vary in size and sit alone or in clusters. Some are shattered, looking like huge broken eggs. The most massive are about 6.6 feet (2 meters) in diameter and weigh several tons. And each one may have required 4 to 5.5 million years to form. The concretions eventually began to fracture, creating cracks called septaria, from a Latin term for “partitions.” Pale-yellow and brown calcites, and some dolomite and quartz, gradually filled these veins, apparently as groundwater percolated through the fissured mudstones.

New Zealand’s Maoris have their own traditions about Moeraki’s boulders: The spheres, a legend explains, are petrified remnants of water gourds, baskets and fishing net set adrift when a double-hulled canoe, the Araiteuru, carrying migrating ancestors, wrecked during a voyage to Aotearoa, often translated as “the land of the long white cloud” — today’s New Zealand.

Koekohe Beach, South Island of New Zealand Coordinates: -45.3453, 170.8263

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