Macquarie Island is one of Australia’s subantarctic islands and is situated half way between Tasmania and Antarctica. Geologically speaking it is considered to be a very special place and not just by seals- in 1997 it was listed as a World Heritage Area. The island is 34 km long and up to 5 km wide and consists of a long, north-south trending plateau, 200-350m above sea level. The highest point is Mt Hamilton at 433 m.
Map of Macquarie Island (from Parks and Wildlife, Tasmanian Government).
The reason that geologists love Macquarie Island is that it is composed of ophiolites – yes those oceanic crust and mantle rocks. It is part of the Macquarie Ridge, which lies at the junction of the Pacific and Australian plates. It is thought that about 40 million years ago, sea floor spreading started in the area. However sometime since this divergent boundary changed into a transform boundary. Consequently spreading stopped and things started to be pushed back together. The newly formed oceanic crust was squeezed up until it emerged as an island about 600,000 years ago. Since then it has continued its rise at about 0.8 mm per year.
Yes there are ophiolites at other places in the world (eg Cyprus), but the ones on Macquarie Island are unique because, unlike the others, they show very little deformation. Therefore it is a great place to study the products of seafloor spreading.
Dr Nathan Daczko, a Sydney University graduate, worked on Macquarie Island during the summer of 2000. He took the following photos.
East coast of Macquarie Island
South-west coast of Macquarie Island
Pillow basalt - these are the basalts extruded onto the
oceanfloor like toothpaste. The edge of the hot magma is quenched by water,
causing the lava to solidify as ever lenthening tubes or 'pillows'.
Layered Olivine Basalt - Basalt is a dark, fine grained,
extrusive igneous rock composed predominently of plagioclase and pyroxene
with up to 5% olivine.
