An insight into Macquarie Island's more subtle qualities
The brief to the two University of Tasmania geologists currently working on Macquarie Island - 'go bushwalking for a year'. The last Zodiac bounced awkwardly off the shore at Buckles Bay heading to the Aurora Australis, anchored about a km or so from the shore, leaving 18 expeditioners to spend the next year on the island.
Thus marked the beginning of a year long mission for Andy and Bronwyn – to conduct a comprehensive ground magnetic survey of Macquarie Island, and to establish and maintain four seismometers at different locations around the island. By measuring the magnetic variations in the rocks on the island, and the seismic events that regularly occur here, we can gain a clearer understanding as to the geological history of the island, and the nature of the oceanic plate boundary on which the island occurs. This is part of a research project run by Michael R, Garry D (University of Tasmania), Jeff K (Duke University, USA), and Carol F (USGS) to understand the anatomy and hydrothermal patterns in ocean crust. The work is being logistically supported by the Australian Antarctic Division.
Andy on Caroline Point, looking back towards Caroline Cove and Mt Hasswell
Photo: B. Kimber
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Bron stands on the Brothers Fault above Green Gorge. This fault extends for almost half the length of the island, and often appears as a distinct ridge and scarp along the coast
Photo: A. Wakefield
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Walking on Macca is often not all that easy. Winds commonly gust above 50knots, and blowing snow reduces visibility
Photo: A. Wakefield
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When commenting on Macquarie Island, many people speak of the amazing abundance of wildlife and spectacular scenery. Most are unaware of the island's unique geology, the primary reason behind its World Heritage Area status. The most abundant rock type on the island is basalt, one of a sequence of rock types found on the island that together form 'oceanic crust'. Macquarie Island is special because it is the only known ophiolite (entire cross section of rocks making up the oceanic crust) still in its original ocean basin. The fact that it has been rotated and tilted so that it protrudes above the ocean provides geologists with a unique opportunity to study the full ophiolite sequence without having to descend to the bottom of the ocean.
Steep hills are the usual on this island. Many faults are still active, producing substantial fault scarps such as this one. Can you spot the geologist??
Photo: A. Wakefield
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Bron and Andy grin as they contemplate the easy descent into Carrick Bay. Scree slopes such as this are a good find, as they provide rapid access to the bays along the coast.
Photo: Expeditioner
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Four seismometers have been installed in a small hole close to 4 huts around the island. The data recorded on these has to be downloaded every 19 days, making for a number of mad dashes across the island throughout the year!
Photo: A. Wakefield
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Andy and Bron spend much of their time walking on the island with a magnetometer strapped to their back. They carry a handheld GPS to record their track, and at the end of each day they download the magnetic and GPS data onto the laptop, and plot up the total magnetic intensity (TMI) on a geological map of the island. The aim is to walk evenly spaced lines of data all over the island down to the smallest spacing possible given the time allowed. The basalts that make up the lower two thirds of Macquarie Island are in general poorly exposed, so we aim to use the magnetic information to better determine the basic geometry and age relationships in this section. This will in turn feed back into ideas on how this sort of oceanic crust actually forms. The finished product, a map of the TMI over Macquarie Island, will provide further clues as to the nature of the oceanic plate boundary above which Macquarie Island is situated.
Changes in TMI are caused by variations in magnetite abundance, which is determined by the hydrothermal (hot fluids) history of the rock. This history can be reconstructed in two main ways from the magnetic data:
- when oceanic crust is infiltrated by hot seawater, magnetite forms – this is indicated by big positive magnetic anomalies (high TMI), and
- when hot metal-bearing water courses through basalts it destroys the original magnetite – indicated by negative magnetic anomalies.
The exact processes involved in this destruction and formation of magnetite is currently a widely discussed topic, and it is hoped that information gleaned from the research occurring here on Macquarie Island will help geologists find mineral deposits in similar rocks on mainland Australia.
Andy on the plateau, carrying the magnetometer. Snowfall completely transforms Macca into a wintery wonderland
Photo: B. Kimber
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Bron follows the Brothers Fault into Green Gorge, on the east coast
Photo: A. Wakefield
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Macquarie Island is surrounded by a sharp apron of poorly charted shallow rocks, thus there is no ship-borne data set, such as detailed bathymetry, that reached the Macquarie Island shore. In the second phase of this magnetics work, The University of Tasmania hopes to get a magnetic data set using AAD helicopters that will link the regional magnetic data with the detailed magnetic data on the island that Bron and Andy are acquiring. Then, for the first time, we will be able to see how the island geology connects to the rest of the surrounding ocean floor, an area of current uncertainty.
Written by geologist Bronwyn Kimber
