The Moeraki Boulders Explained

The Moeraki Boulders are grey spheres resting on Koekohe Beach in New Zealand. Coastal erosion freed them.

About one-third measure 0.5 to 1.0 metre across. The remaining two-thirds span 1.5 to 2.2 metres in diameter.

Calcite cements fine silt and clay into solid rock. The outside rims contain up to 20% calcite.

Large cracks called septaria radiate from a hollow core. Brown and yellow calcite layers fill these fractures over time.

These concretions formed in Paleocene marine mud. A single 2-metre boulder required 4 to 5.5 million years.

Some Moeraki Boulders grew over millions of years.

Māori legends describe the boulders as eel baskets washed ashore from the sailing canoe Āraiteuru. Eel baskets are rarely this heavy.

Walter Mantell sketched the beach in 1848. His drawing shows more boulders than remain today.