The Geological Development of a Young Island

Although Macquarie Island was first discovered by Captain Hasselburgh in 1810, the first geologist to study the island was Leslie Russell Blake, a member of the Australian Antarctic Expedition, who spent more that two years on the island from 1911 to 1914.

From the time the Australian National Antarctic Research Expedition (ANARE) set up a permanent station on Macquarie in 1948, there have been a considerable number of geologists keen to carry out detailed research into the origins of this tiny speck of land in the Southern Ocean.

Macquarie Island is geologically unique in a number of ways. Among subantarctic islands, it is the only one to be totally oceanic in origin and is a rare example of uplifted oceanic crust. It does not have large scale sedimentation and has not been built up from actual volcanic activity. It is also now thought that the island has been subjected to little or no glaciation during its development.

It all started some 12 million years ago. Basalt magma from deep inside the earth flowed from a fissure on the seabed at the abutting edges of two large tectonic plates, the Indian/Australian Plate, to which Macquarie Island belongs, and the Pacific Plate. A third tectonic plate, Antarctic, abuts the other two plates some 650 kilometres south of the island. Geologists now indicate that the Pacific Plate is being pushed, in an anticlockwise direction, under the Indian/Australian Plate at 2-3 centimetres a year. Consequently, the Indian/Australian Plate is riding up vertically over the Pacific Plate at 1 - 2 millimetres a year.

As a result of these complicated movements and pressures, the Macquarie Ridge complex has been growing upwards. Trending NNE in its northern part and curved towards the SE at its southern end, it runs from the New Zealand coastal shelf off the west coast of South Island to the triple plate junction at about 61šS, 162šE.

When the Macquarie Ridge had risen more than 2.5 kilometres from the sea floor, it broke through the ocean surface where it has since been subjected to the effects of the very strong winds and huge waves that are generated in the vast Southern Ocean. In spite of these eroding forces the island is now some 200 - 400 metres high. The date at which Macquarie Island broke through the sea surface is currently being debated by geologists. Some suggest 700,000 years ago while others think that it could be from 80,000 to 300,000 years ago.

Running parallel with and 40 km to the east of the Macquarie Ridge is the Macquarie Trench which is some 5,500 metres deep. This deep trench breaks through the ridge line nearly 100 kilometres to the south of the island and continues southwards as the Hjort Trench to about 60šS.

The age of the rocks which form the island have been calculated by geologists. Palaeomagnetic studies of the rocks suggest ages of 27 - 30 million years while Radiometric measurements yield ages of 9.7 to 11.5 million years. The age of the rocks is still extremely young in geological terms when you consider that the Australian mainland is mostly formed from rocks which are over 1,000 millions years old!

One of the rock types on the island, Harzburgite, suggest to geologists that the uplifting of Macquarie Island has been approximately 10 kilometres, as this rock type is normally formed 5 km below the ocean floor. (Selkirk, Seppelt & Selkirk)

P E Williamson (1987) considers that the detailed structure and stratigraphy of the extrusive rocks of the island are complex and poorly understood. Duncan and Varne (1987) suggest that the new maps and geological data show that the island is composed of a number of fault bounded blocks derived from different layers of the oceanic lithosphere. This implies that all the igneous rocks could have been formed at much the same time. The northern part of Macquarie Island is formed mainly of intrusive igneous rocks, whereas the remainder, south of a line Sandy Bay - Bauer Bay, is formed mainly of extrusive volcanic rocks and associated sedimentary rocks.

There were reports of earthquakes from sealers who were working on the island from time to time, the first on 31 October 1815. The measurement of earthquakes in the area of Macquarie Island were initially made in Australia and New Zealand. Since 1950, the Bureau of Mineral Resources has been operating a seismograph on the island itself so that much more accurate data has been collected.



Bathymetric map of Macquarie Island showing earthquakes
after Jones & McCue

Maps of the earthquake epicentres show a series of quakes following the line of the Macquarie Ridge from New Zealand to its junction with the Antarctic Plate and it can be expected that there will be an earthquake greater than magnitude 7 every ten years. The analysis of earthquake data since 1964 indicates that some of the events are very shallow and that the foci of the major earthquakes are under the Macquarie Ridge rather than in the Macquarie Trench.

This data tends to confirm the suggested plate movements explained above.


Dr. B. D. Goscombe and J. L. Everard of Mineral Resources of Tasmania carried out a detailed geological survey of Macquarie Island during 1994-96. The geology has been published in the form of 1:50,000, 1:25,000 and 1:10,000 geological maps in 1998 and the data is available in digital format.

There are many unanswered questions about the formation of Macquarie Island and much research remains to be done.

For further geological information please visit the following web site:-

Parks and Wildlife

Antarctic Division

Glyn Roberts
April 2000.

References:

Selkirk P.M., Seppelt R.D. & Selkirk D.R., 1990, "Subantarctic Macquarie Island, Environment and Biology", Cambridge University Press.

Williamson P.E. 1988, "Origin, Structural and Tectonic History of Macquarie Island Region", Papers & Proceedings of the Royal Society of Tasmania, 122(1).

Duncan R.A. & Varne R. 1988, "The Age and Distribution of the Igneous Rocks of Macquarie Island", Papers & Proceedings of the Royal Society of Tasmania, 122(1).

Jones T.D. & McCue K.F. 1988, "The Seismicity and Tectonics of the Macquarie Island Ridge", Papers & Proceedings of the Royal Society of Tasmania, 122(1).

Pemberton M., Davidson G. & Burdon J. 1998," Macquarie Island, a New Arrival", Parks & Wildlife Service Publication, Hobart.

Mineral Resources Tasmania 1998, "Geology of Macquarie Island, Maps".