Geology of Tasmania - Macquarie Island

Macquarie Island

Macquarie Island is politically part of the state of Tasmania, but comes from a very different geological context. It has formed as part of the oceanic crust and mantle was buckled upwards. It is the only place in the world where a complete section of oceanic crust is exposed above water in the place it was formed. The rock composing the island was formed at the ridge along the boundary of the Australian Plate and the Pacific Plate in Eocene times 12 to 9 million years ago. Spreading from the ridge became less perpendicular (ESE-WNW), more oblique (SE-NW) and eventually almost parallel to the ridge (NNE-SSW). The plate boundary is now entirely a transform fault a few kilometers to the west of the island. This left fracture zones and spreading fabric in the rock. The Geomagnetic reversals leave a magnetic anomaly trace in the rock. Transpression on the plate boundary has deformed the oceanic crust in the vicinity to make the Macquarie Ridge Complex, raising Macquarie Island out of the water. It is studied to understand seafloor spreading and transform faults, and hydrothermal alteration of the undersea floor. Most of the south of the island consists of sub oceanic basalt layered between Globigerina ooze. The part north of Langdon Point and Ballast Bay consists of serpentinite derived from gabbro, troctolite, and peridotite (dunite, wehrlite, and harzburgite). This was formed in the deep crust and mantle.

The two different rock zones are separated by the Finch-Langdon fault zone. It consists of seven segments of faults, subsidiary faults and splays. The fault is a transform fault with a corner at the spreading ridge. South of the fault on the west coast is breccia interbedded with the basalts. The breccia matrix is mud, and the stones consist of basalt, dolerite, and gabbro. The southern end of Bauer Bay has a talus of breccia 140 m thick. On top is greywacke and chert. Many other faults cut the rock due to stress from the transform, and uplift. Some of these have scarps that dam lakes.