Macquarie Island is situated about 1,500 km south-south-east of Tasmania, half way between Tasmania and Antarctica at around 55 degrees south. The main island is approximately 34 kilometres long and 5.5 kilometres wide at its broadest point.
Macquarie Island provides evidence of the rock types found at great depths in the earth's crust and of plate tectonics and continental drift; the geological processes that have dominated the earth's surface for many millions of years. It is the only island in the world composed entirely of oceanic crust and rocks from the mantle, deep below the earth's surface. Macquarie Island probably began as a spreading ridge under the sea with the formation of new oceanic crust somewhere between 11 and 30 million years ago.
At some stage the spreading halted and the crust began to compress, squeezing rocks from deep within the mantle upward like toothpaste from a tube. As the ridge grew it eventually became exposed above the ocean's surface about 600,000 years ago. Thus, rocks normally only occurring deep within the earth's mantle have become exposed on the earth's surface.
Since Macquarie Island emerged, it has mainly been carved by marine processes such as wave action, unlike other subantarctic islands, which have been shaped by glaciers. The geodiversity of Macquarie Island provides the foundation for the landforms, soils, plants and animals occurring there. It is an island of unique natural diversity, a site of major geoconservation significance and one of the truly remarkable places on earth.
Around the shoreline there is a coastal terrace formed from a wave-cut platform now raised above sea level. Vast waterlogged areas on the coastal platform are heavily vegetated, forming a mire based on deep peat beds and known locally as "featherbed" from the sensation gained when walking over them. Old sea stacks testify to the continual uplifting of the island as they protrude through the peat beds, some of them now being several hundred metres from the existing coastline. Behind the coastal terrace, steep escarpments rise more than 200 metres to the undulating central plateau which has three peaks over 400 metres; the highest being Mt Hamilton at 433 metres.
The slopes from the plateau to the sea are most spectacular at the southern end of the island and along the west coast where the relentless pounding of the Southern Ocean has cut a myriad of rugged bays and coves, fringed with sea stacks and reefs. The plateau surface is dotted with innumerable lakes, tarns and pools, mainly of structural origin. Fluctuations in sea level and marine erosion have cut away the original escarpments leaving some lakes perched on the edge of the plateau, while others have been partially or totally drained.