Macquarie Island is located in the south-west Pacific Ocean, between Australia and New Zealand, and officially belongs to the Australian state of Tasmania. The island is tiny, only 5 km (3.1 mi) wide and 35 km (21.7 mi) long, covering a total area of 128 km2 (49 sq mi). It is a unique place: it is the only sub-Antarctic island to be fully oceanic in origin, and it is the only known site on Earth where an ophiolite complex is presently undergoing formation in it’s original geological setting, actively exposing mantle rocks on the Earth’s surface.
An ophiolite is essentially oceanic lithosphere (crust and upper mantle) that has been emplaced upon continental crust and can be identified by a classic stratigraphic sequence that reflects the geological processes at work along a mid-ocean ridge. Ophiolites are special to geologists as they are places where the oceanic crust and upper mantle rocks can be seen on land – normally, these are only found in the deep ocean and along active spreading ridges (such as Iceland). These features are the main reason for Macquarie Island’s 1997 inclusion on the UNESCO World Heritage List, along with the gorgeous natural landscape and prolific wildlife such as seabirds and seals.
The island is actually an exposed section of the Macquarie Ridge which is a major geological feature of the Macquarie Fault Zone which runs from the Macquarie Triple Junction (the intersection of the Indo-Australian, Pacific and Antarctic Plates) up to New Zealand where it intersects with the famous Alpine Fault.
The Macquarie Ridge formed 12 mya when basalt lava erupted from a fissure between the Pacific and Indo-Australian plates. Subsequent uplift is due to the incipient subduction of the Pacific Plate beneath the Indo-Australian – although the fault is a right-laterally moving, transform fault (a fault between two plates which essentially just slide past each other) the overall anticlockwise motion of the Pacific Plate movement causes it to start subducting here at a rate of ~ 2-3 cm/yr instead. Edited from The unique geology of Macquarie Island