Tags
Australia, Cadia, Copper Hill, Japan, Ordovician, plate tectonics, Silurian, Skarn Mineralisation, Subductio, The Death Of The Dragon, Turning Japanese, volcanic island arc
Story by Warrigal Mirriyuula
It was with some happiness that I looked into the Arms the other day; first time for a long time and there was Lehan’s piece about the movie she’d seen. It all sounded a bit familiar and then I remembered.
I read that book, in translation of course, back in 1979. According to the note I compulsively scribbled on the first page, I purchased the book in Adelaide at The Third World Bookshop. Sadly that august institution has disappeared but the book remains on my shelf. It survived the house fire and the culling that went on afterwards when better books went west, I suspect mainly due to its geophysical theme and geomorphological underpinnings. I do like a good geology yarn.
So why is it that Japan rocks and rolls and Australia doesn’t?
The answer is simple. Japan sits almost on top of a triple convergence where three of the major tectonic plates that make up the crust of our planet meet. At this triple plate boundary the differing geodynamics of the plates are constantly jostling each other in an attempt to relieve the strains and pressures that build up as they are driven about the surface of the planet by the vast heat engine below. They want nothing more than to go about their business unrestrained but on all sides they are held in dynamic tension and every now and then one or another of them just seem to reach a point where they’ve had enough, and lets go and we get the recent Japanese quake and tsunami. The same thing happened in Aceh back in 2004. It’s the plate boundaries that spell trouble.
Australia sits smack bang in the middle of its plate; and it’s a pretty big plate, covering about 130 degrees of longitude and 65 degrees of latitude. Those troublesome convergent boundaries are a long way off shore.
You could say that the Indonesian Archipelago, New Guinea and New Zealand are to Australia what Japan and The Phillipines are to Asia. These countries are all on or near plate boundaries and all experience high levels of vulcanism and earthquakes. Indeed Indonesia and New Zealand are home to two of the biggest volcanic risks on the planet. The Toba Supervolcano and the Taupo Supervolcano.
The reason is simple. You simply can’t move such vast slabs of lithosphere about without creating huge amounts of internal heat and pressure and that heat and pressure are at their most intense at the plate boundaries, and it’s all got to go somewhere. The most common way heat and pressure are released is up, through the necks of volcanoes, and the slipping, sometimes catastrophic slipping, of faults already activated by eons of strain.
The vulcanism is also easily explained. As these thick slabs of rock collide it is not uncommon for one of them to be pushed under the other in what is called subduction. As the subducting plate is pushed deeper down into the mantle, a lower zone of plastic rock, it is subjected to increasing high pressures that raise the temperature of the subducting plate. Moreover, the subducting plate is gradually squeezed dry of the water contained in the rock and its interstitial spaces. This dehydrating of the plate does two things.
Firstly the migration of all that water makes the rock above the plate less dense and increases the temperature in the overlaying plate. This leads to melting and the plume of relatively less dense, very high temperature melt so created begins its rise to the surface by cracking and eroding the overlying material and incorporating it in the melt. Eventually the plume has so fractured and deformed the overlying slab that it breaks through in the form of an eruption. Think Mount Pinatubo, Vesuvius, Mount St Helens or any number of Andean volcanoes.
The second thing this process achieves happens at great depth and involves the percolation of superheated mineral saturated water through the cracked overlying plate. These mineralised waters are the beginnings of our mining industry with respect to metals such as copper, gold, silver, lead, zinc and many others.
In Australia these ancient geological processes enriched the western goldfields, the Broken Hill lode, the untold and as yet mostly untapped wealth of the Lachlan Fold Belt including the Cadia gold mine at Orange, and parts of Victoria and Tasmania.
But it takes millions of years, sometimes hundreds of millions of years for the overlying rock to be uplifted and worn down to expose these zones of mineralisation.
The gold and copper at the Cadia mine went through two primary periods of mineralisation; the first in the Ordovician nearly 500MYA and another, later during the Silurian some 60 million years later. At this time Australia was still part of Gondwana and what we now know as the east coast of Australia hadn’t formed. It was all under a shallow equatorial sea. Offshore from the then coast was an arc of volcanic islands above the then edge of the Australian plate as it subducted the paleo Pacific plate. It’s waited since then for the growth of Eastern Australia, continental extension and then compression, a long period of deposition, then uplift, and finally erosion, until a group of hard working, hard handed Cornish men began pulling the copper ore from the ground in the 1860’s, just a few years after The Copper Hill deposit at Molong had commenced sporadic operations and earning the right to claim the Copper Hill deposit as the first working copper mine in the colony.
So you see today’s Japan is just like that ancient Australian arc of volcanic islands, and in time it too will see a similar fate, but I doubt it will ever sink as Lehan’s movie and my book suggest. What is more likely, though it will take perhaps 100MY to come into being, is that Australia will scrape Japan off the map after ploughing its way northward through the Western Pacific at about 10-20mm/y and finally parking itself up beside the Asian landmass, creating another Himalayan sized range in the process. Back behind that range Japan will be just another scrambled terrane making up the suture sewing the next supercontinent together. They’ll be mining the deposits that are being laid down deep below Japan as we speak. That’s if we’re still here and still mine minerals.
Window Dresser's Arms, Pig & Whistle 