Geological time machines: looking back to take us forward

From the mines of Broken Hill to the remote icefields of Antarctica, the New Zealand Fiorlands to the Andean Margin in Chile and Argentina, Professor Geoff Clarke’s work as a metamorphic geologist takes him, literally, to the ends of the Earth.

Clarke has been studying the processes that have shaped our continent since its emergence. Examining the careful interplay between how new materials form, and how old formations move between the mantle and the surface, is really a quest to discover how the Earth came to be as it is today, and how it will look in the future.

The mineral composition of rocks changes in response to subcrustal conditions, with specific temperatures and pressures resulting in unique changes. To uncover subcrustal conditions, Clarke and team used rocks exhumed in a rapid manner, and have identified that molten rock moves through deep crustal environments by an unusual combination of partial melting, dyking and fracturing.

“Once continental crust forms it tends to remain above the mantle. It’s an asymmetric process: the centre of the Earth sends out low density material through volcanoes. Like foam on water, you can push it under but without pressure it floats back to the top. On the other hand, higher density oceanic crust is drawn back into the mantle when subduction slides one plate under another.”

Subduction exposes dense crust to elevated pressures and temperatures, changing mineral assemblage and leading ultimately to the formation of volcanoes and island chains. A key to understanding subduction is the amount of mineral bound water drawn below the surface. Great pressures rapidly eliminate porosity, pushing all other water up, creating a rock-water cycle. Less well known than the water cycle forming our weather, this cycle has controlled the nature of Earth’s surface.

The Andes are a prime example of the impact exhumation and subduction exert on the landscape, formed when plate movement in subduction zones inverted sedimentary basins, and lifted the thickened crust to new heights. Clarke, in collaboration with scientists from the USA and Chile, will explore in greater detail the formation of these mountains, with focus on the deep crust and tectonic evolution.

In the coming years Clarke hopes to develop new methods of mineral deposit detection. Matching data drawn from remote sensing with real life deposits of metamorphic rocks around Broken Hill will allow an efficient and less intrusive method of mineral exploration throughout Australia.

“The greatest challenges with geological research is finding a way to overcome the constraints of the geological time frame and the inaccessibility of the Earth’s core,” acknowledges Clarke. With the aids of remote sensing and modelling it is becoming increasingly easier to turn back the clock and delve to previously unreachable depths.