Dr Vince Morand’s connection to the Australian Alps goes back about 160 million years. As a senior geologist with Geoscience Victoria, (which sits within the Department of Primary Industries), he has a better grasp than most of the likely scenario of events which created the alpine landscape we’re familiar with today.
We have this greater understanding, thanks to geologists like Vince. These people go out into the field to gather samples as part of mapping projects. Or they may spend time in laboratories heating rocks to melting point to study their behaviour. Or they enter data to generate models that produce scenarios which can be studied within human time frames. From these activities (and many more), we have an idea of how the Australian Alps were formed, and as it always seems to be the case, Gondwanaland is a good place to begin.
“The majority view is that about 160 million years ago, a super continent – Gondwanaland – consisting of South America, Africa, Antarctica, India, New Zealand and Australia began to break apart. A single land mass surrounded by ocean, it began to break into sections and these moved away from Antarctica and from each other.”
While this is not necessarily new news to many, the underlying process is probably not that well publicised. “The earth’s upper rocky layer – the lithosphere – is about 150 kilometres thick, floating on a layer known as the asthenosphere. About 550 kilometres thick, the asthenosphere is a very thick fluid, like viscous honey.”
Vince goes on to explain that within this layer, there is movement – an impossibly slow-to-imagine convection ‘boil’. It’s this force that theorists believe powered the Gondwanaland break-up, and continues to have influence to present day. Under Gondwanaland’s lithosphere, a pulse of hot material from the asthenosphere (magma) would reach up, heating the lithosphere’s rocky crust. At this point two things happened: the rock would expand and, in its expanded form being lighter (same mass but taking up a greater volume), literally float higher on the asthenosphere below it, thus causing the flat low-lying surface of the continent to be raised up along the region that was to become the Australian Alps.
In our case, around 100 million years ago these uplifted plains were pulled apart to form a rift valley – the sticky asthenosphere exerting its force, steadily widening the rift valley until, in the case of the eastern coast of Australia, it broke apart forming Zealandia, a ‘new’ continent. “At that point, eighty million years ago, Zealandia was above sea level, but now it sits mostly submerged to the east with only New Caledonia, New Zealand and a handful of small islands sitting above ocean level.”
This theory is supported by looking closely at the form of the Australian Alps. As Vince notes, our Alps are very different from the ragged peaks for example, of the active Andes of our Gondwanaland cousin, South America. Instead, the Australian Alps are described by geologists as passive margin mountains, formed along the edge of the rift valley, as sections of crust were dropped down along faults and newly formed watercourses eroded steep valleys as they flowed towards the sea.
Together this has produced a quite different alpine horizon, one where it’s difficult to determine the highest peak from amongst a series of plateaus broken by deep ravines. “Looking at the shape of the Australian Alps, they are a series of big plateaus. Mount Bogong, the highest peak in Victoria for example, has a flat top with steep valleys all around – it’s essentially an uplifted plain.”
So when you next survey the alpine landscape, take a moment to think of that now extinct rift-ridge, running along the sea-bed in the midst of the Tasman Sea, which played a key role in the formation of our passive margin mountains.
More information can be found on geology and soils in the Australian Alps education kit.
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