Long Durational Natural Phenomenon
Existence of Moeraki Boulders
Duration: 4 to 5.5 Million years
The Moeraki Boulders are unusually large, spherical boulders lying either in clusters or alone along a stretch of Koekohe Beach on the wave-cut Otago Coast of New Zealand between Moeraki and Hampden. 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, two meters (6.6 feet) in diameter, are estimated to have taken 4 to 5.5 million years to grow while 10 to 50 meters (33 to 164 feet) of marine mud accumulated on the seafloor above them. After the concretions formed, large cracks known as septaria formed inside. 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 most striking aspect of the boulders is their unusually large size and spherical shape. A small portion of them are not spheres and are slightly elongated parallel to the bedding of the mudstone that once enclosed them.
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