The Basalt Columns are one of the most popular attractions on the Islands. Located on private land at Ohira Bay in the northwest this is an impressive outcrop of olivine basalt. The columns were formed from lava flows from the Southern Volcanics that erupted on land around 80 million years ago. The columnar-jointed structure has resulted from the way the lava cooled.
The remarkably well-organised and regular geometric patterns seen in the Basalt Columns exposed in northern Chatham Island are simply stunning, a marvel of nature.
They are located on private property in Ohira Bay on a south-facing coast of Petre Bay which takes the brunt of the prevailing southwest wind and ensures perpetual environmental vigour and dramatic seascapes.
The columns are actually vertical shrinkage structures. They are the product of the natural cooling of a ponded basalt lava flow which erupted 80 million years ago during the Cretaceous Period. Dinosaurs would have witnessed it, and no doubt our mammalian ancestors!
The eruption of the lava flow occurred on land in Zealandia some five million years after the separation of Zealandia from Gondwana. It flowed out of a fissure, about one kilometre long, and accumulated in a natural depression.
It is thought that tensional forces (due to extensional rift-related tectonism) cracked the Earth’s crust, depressurised the Earth’s mantle, and a small batch of basalt magma rushed to the surface.
Basalt lava is a very hot liquid, greater than 1,000 ⁰ C. When it stops flowing, it slowly cools, and as it does it becomes a solid made up of crystalline minerals i.e. rock. But, like all hot things, as they cool they shrink to occupy a smaller volume.
In a homogeneous fluid, and a quiet setting, cooling can be very consistent and results in a highly organised structure. Columnar jointing in volcanic rock formations (lava flows) is actually relatively common globally, including elsewhere in New Zealand, but rarely are they seen in such exquisite glory as in Ohira Bay. It is the dynamic sea in Petre Bay which keeps them fresh and clean.
Rounded cobble-sized bodies of greenish mineral masses are conspicuous within the basalt. The dominant mineral is olivine, a common mineral component within basalt. These curious crystal masses possibly represent the olivine-rich mantle rock which hosted the basalt magma chamber, or more likely a ‘crystal mush accumulate’ that grew and formed a layer in the bottom of the magma chamber, only to be coughed up during eruption as a solid by-product within the liquid basalt lava.
Dr Hamish Campbell
The area was once a quarry used by Moriori for tool making purposes. The pentagonal shape is represented in many symbols of the island including the design of the central room within Kōpinga Marae.
Ask your host to arrange a trip to the Basalt Columns.