To look at this landscape in central New
South Wales, it's hard to imagine that this was once the floor of a vast and
deep ocean. The only land you would have seen 450 million years ago would have been a chain of volcanic islands, stretching into the distance. The incredible story of how this ocean floor became land and how eastern Australia was formed is held in the rocks, that form the hills, that lie behind me Central New South Wales is in the
eastern part of this ancient continent the rocks that tell us this story lie in
several distinct belts running north-south, they form the bedrock of a calm land now, but there's evidence here of a tumultuous past. This area has always puzzled geologists. They've named these unique rocks the Macquarie Arc and new research shows they may hold the key to how eastern Australia was formed We're in the western part of the
Macquarie Arc and this is the Junee-Narromine Belt. As you can see the hills are low, the outcrop's not so good but there's still enough rock lying about the
surface and we can see what it is. These are some of the oldest rocks in the
Macquarie Arc and they're quite unusual They're high potassium calc-alkaline rocks That is, they're high in potassium, calcium and sodium. Under the hand lens I can see crystals set in a fine-grained matrix. This is a texture of a lava. These lavas once flowed all over the Macquarie Arc and they're unusual chemistry is a clue, they were not normal volcanoes. But the Macquarie Arc has never been normal. It has massive mineral deposits of copper and gold and limestone that preserves the evidence
for rich animal habitats It has long been a scientific challenge. How did this strange geological environment form Understanding its history is made even
more difficult because the Macquarie Arc is so unlike the rest of southeastern
Australia. Alongside and down to Victoria is deformed sedimentary rock with no
evidence of volcanic activity. At Pipers Creek we can see this sandstone
beautifully exposed. There's no mystery here. Thousands of layers were piled on top of each other in a deep ocean. So did these sandstones form at the same time as the volcanic rocks and if so how could these two totally different
geological environments have existed side-by-side? Dr. Richard Glen from the Geological Survey of New South Wales has assembled a new group of Australian geologists to try and answer these questions. When geologists first saw these rocks they found it hard to explain exactly how they got here and how they formed. Volcanic rocks in a sea of sandstone. So it really is an intriguing problem and solving it, finding the answers to it I think is going to give us some ideas about the
origins of southeastern Australia. So to crack the puzzle we put together a team
of fantastic geologists each of whom is an expert a specialist in their own field. Together with Dick Glen key members of this team will explain their new discoveries and what this means for the history of the Macquarie Arc. The team's volcanologist is Dr. Carol Simpson. In fact we have actually identified two quite contrasting types of volcanoes that existed in the Ordovician. Palaeontologist Dr. Ian Percival used fossils to accurately date the rocks and to reconstruct animal habitats. A lot of pieces are missing but
palaeontology certainly assists people to put the jigsaw back together. Geochemist Professor Tony Crawford and colleagues compared the chemistry of the
ancient volcanic rocks with modern examples. So we spent a lot of time looking at and analyzing rocks from modern settings to compare older
associations such as the Macquarie Arc By comparing the old rocks with modern
geological environments in other parts of the world the team could interpret
the ancient history It was clear this story was of worldwide interest so Dick Glen called a conference and invited international experts who could give the Australian scientists a different perspective. One of the experts is Professor John Dewey. He helped to revolutionise geology from the 1960s onwards Professor Dewey pioneered the use of plate tectonics to explain the behavior of the ancient Earth. His knowledge can help us untangle
the origins of the Macquarie Arc. The evolution of plate mosaics is actually
very very complicated the boundaries and at triple junctions and they produce the most incredibly complicated geology and we see all over all over the globe. The power of plate tectonic theory is that it explains how both modern and ancient rocks form. Dr. Yamirka Rojas-Agramonte a has studied the edges of a Caribbean tectonic plates she has a unique insight into how grinding plates can create unusual rocks both in the past and now. Because the mechanisms,
well the history, is always the same Older and younger, it will always be the same. So what were the mechanisms that created the Macquarie Arc it's possible to read the story of these ancient rocks in their chemical and physical makeup it's like the rock has its own DNA Rocks never lie basically they may confuse you you may find it very hard to extract the truth
from them but basically that is the the ultimate the ultimate data source the rocks themselves. The makeup of the Wilga rocks implicate a volcano as a player in the story but what sort of volcano and what sort of setting. Volcanologist Dr. Carol Simpson found two types of volcanoes and she realised the Ordovician environment wasn't dry land The first type was a volcano that built up as a cone under water and that's been built up by a successive pile of lava flows including some pillow lavas which are quite bulbous shaped features. When magma cools quickly in water it forms these pillow shaped lavas, a sure sign of
an underwater environment and this is important information for
the Macquarie Arc story the pillow lavas are the first evidence that the volcanoes developed in an ocean. That volcano probably may have just got to see a level but probably didn't really form a large island most of the rocks seem to have been deposited under the sea. ..and the plume will be much less
yellow and then here it goes big right towards the submarine Carol found that the submarine lavas were building a chain of volcanic islands slowly rising from the seafloor but when she examined the area in more detail Carol found other rocks amongst the lavas that suggested the involvement of much
shallower water. What we see is the material that's been eroded and washed down sloped back into the sea as a very thick sequence of rocks and rubble. Carol realized that the volcanoes had been attacked by weather and waves breaking down volcanic ash and lava into sand and boulders this material thenspread around the volcano forming volcanic sediment. Imagine at this time the string of volcanoes was well established in this deep ocean and
material was being constantly eroded from these volcanic edifices and you can see lumps of the material that eroded off the islands dumped into shallow water some of these pebbles are quite well rounded and they've been abraded in
that shallow storm water part of the environment The more that volcanic sediment spread the more this helped to build shallow
sea floor around the island chain and this was to have a profound effect on the environment When there's large areas of shallow sea well something completely different becomes possible similar to what you see in shallow oceans today and that is life. These uplifted limestone strata are full of the evidence of this life and to understand what this means for the
Macquarie Arc I spoke to palaeontologist Dr. Ian Percival In the Macquarie Arc there's an excellent record of trilobites there's also some of the world's earliest corals and I've also looked at some mollusks in particular nautiloids and gastropods. Using these fossils Ian can place the rocks into the Earth's timeline this shows the Macquarie Arc formed in
the Ordovician period which spanned 45 million years ending about 443 million years ago but the fossil species give Ian even more information. You can also use fossils to examine and analyse palaeoecology which is how these animals in the Ordovician lived. Working out the age of rocks is not the only use of fossils by looking at the type of fossil like this very beautiful
brachiopod and the rocks that it was found in which
in this case is in that hill just behind me geologists can have a pretty good
guess at what the ancient environment was like Specific brachiopods are associated with specific depths you can find shallow water brachiopods that are only in the intertidal and shallow subtidal zones as
distinct from deep water brachiopods which exists on the shelf edge and slope. Fossils like these gave the Macquarie Arc team two important bits of information the age of the rocks and clues about the environment the limestone had formed in shallow waters some animals had even created reefs confirming that the volcanoes had risen to form islands but what about the age
of the sandstones away from the volcanoes in the much deeper water what helps here are the partial remains of a free-swimming animal Well conodonts have hard parts, microscopic hard parts which are tooth-like in their appearance, but they are believed to have not existed so much in the mouths of the animals but in their gullets The reason the conodont teeth are important is because they're the only parts of the animal to be fossilized and they provide very accurate ages. In many instances you can go to a very precise zonation of the order of less than a million years. When the conodonts died and became incorporated in the seafloor sediments they preserved the age of those rocks forever. Now these small eel-like creatures lived and died in shallow or deep areas so that means that their fossil remains can give us the age of rocks formed in very different depths from the shallow water limestones that formed around volcanoes to the vast area of sandstone that was deposited away from the Macquarie Arc in the deeper water. The fossil conodonts proved that these rocks are exactly the same age This means the Macquarie Arc volcanoes and the sandstones were part of the same ocean at the same time This was the ancient Pacific Ocean. Based on previous research on the sandstones the team knew a large continent was not far away. This was the supercontinent Gondwana, which included Central and Western Australia As Gondwana was quickly wearing down its rivers carried
weathered sand and mud to the continental shelf from here it spread far across the ocean floor this is how the sandstone layers of Piper's Creek formed If you look at a piece of this sandstone under a hand-lens remarkably you can still see individual grains of mica and quartz that were deposited on that ancient ocean floor 480 million years ago. With the knowledge from the sandstones the Macquarie Arc team could at last create a total picture of the Ordovician environment. A wide and deep ocean floor southeast of Gondwana covered by sandstone layers and further east again were the Macquarie Arc volcanic islands they were surrounded by shallow water limestone. The team realised that these environments have modern equivalents today like along the West Pacific Ocean and the similarities became even more striking when Carol Simpson discovered in the rocks of the Macquarie Arc evidence for a much larger and more violent type of volcano. It almost certainly was a large island so
it built above sea level and in building above sea level the nature of the eruptions changed from being fairly passive lava flows that just sort of flow away from the vent to a very explosive style of eruption. The more silica you have in your magma the more tendency there is that those
eruptions will go to be explosive. The explosions caused by buildup of
dissolved gases within the magma and they get to a critical point where the
pressure exceeds the actual pressure of say either a water column under water or
the air pressure above water and when that happens those gases start to expand
and that's what blows the magma apart. So like some of the islands you see around
the Pacific today. Carol's discovery adds more evidence
that the Macquarie Arc volcanoes were similar to those we see in the Pacific
and Caribbean oceans. Like the Sarychev volcano in the northern Pacific, they erupted explosively and probably formed in a similar island chain. But what causes this oceanic volcanic activity? This is where an understanding of plate tectonics can help. This shows that the Earth's surface is divided into more than seven rigid tectonic plates that change and move over time. Professor John Dewey explains what happens at plate boundaries. If two plates are pulling apart in one place they have to be converging in another place. Ocean plates are generated at mid-ocean ridges where hot magma rises to form new ocean crust. The seafloor spreads as new material is added to the edges of the ocean plates. But the opposite edge of a plate can be quite different. And then there are places are much colder where the plate comes along and descends asymmetrically beneath the
leading edge of another plate, we call that subduction. As the cold slab of ocean crust descends it takes water with it which lowers the melting temperature of mantle rock. This then melts to form magma the magma rises, some cools at depth to form intrusive rocks like granite but other magma reaches the surface and forms chains of volcanic islands called island arcs. Commonly on the leading edge of the overriding plate we have these volcanic arcs perhaps like Macquarie Arc here which is probably a good fossil example of one of these things at the edge of the Australian continent during Ordovician times. Modern island arcs are found in the Caribbean and right around the Pacific Ring of Fire. These curved island chains are born from parent Magmas as they rise from a subduction zone. So is this how the Macquarie Arc was created? The answer is hidden in the chemical makeup of the volcanic rocks. Like DNA the rocks chemical composition can reveal what type of parent magma that came from. Geochemist Professor Tony Crawford explains why magma compositions differ from one side of a plate to another. The processes are different the
source rocks in different tectonic settings are different so when they melt the magmas that are generated reflect those sources and processes. In subduction zones around the West Pacific for example we're having subduction of ancient altered ocean crust and it's putting water back into the mantle. The water helps the mantle to melt but it also brings new elements into the mix giving the magmas the chemical fingerprint of a subduction zone. Water takes with it a number of important species such as potassium and barium and rubidium - those potassium like elements. Now, as the mantle melts and the magma rises it carries the family chemical signature of a subduction zone. Analysing a rock that cools from this magma can reveal its origins. Some of the most revealing elements that Tony looks for are the so-called trace elements. They're rare and occur in minute amounts but they give the final clue to the rocks parentage. The trace element geochemistry of an igneous rock can reveal crucial information about the likely source of a magma and it's all done with this gear here this is an Inductively Coupled Plasma Mass Spectrometer. This machine is capable of measuring the concentrations of trace elements like strontium, neodymium
and lead down to parts per billion. We found using that data that the Macquarie Arc rocks including the high calc-alkaline rocks almost certainly formed in an intra oceanic arc setting that is above a subduction zone of course but well away from any continental crust. The trace element signature has revealed the final piece of evidence for the team. The Macquarie Arc volcanoes had formed above a subduction zone and what's more Tony found no geochemical signature that continental crust was caught up with this melting. So Gondwana, which is continental crust, was still sitting away to the west. But the team found evidence
that the Macquarie Arc was changing by testing more and more rocks they discovered that the chemical composition of the ancient lavas had changed over time. and by slotting these different types of lavas into the fossil timeline it was clear that the arc's history could be
divided into several distinct phases just as animals evolved over time so had
the volcanoes challenging the scientists to explain why. Something weird happened in Phase 3 where there was a terrific pronounced swing in the composition of
the lavas from what they had been like The gradual change in the geochemistry
of the lavas is a sign to Tony of profound changes in the behaviour of the subduction system. That probably reflected some incoming seamounts causing local compression the magmas didn't easily get to eruption like they had been doing they were trapped deep in the volcanic system and different minerals crystallized forced compositional change on the magma and we had this unusual phase 3 magmatism. By the time of Phase 3, subduction had been happening for at least 30 million years and seemed unstoppable and those seamounts riding on the leading edge of the tectonic plate jammed up the subduction zone and changed the way that magma was created and released and then in Phase 4 the compositions of the magma changed in a way that affects us all today In Phase 4 the arc went crazy we don't really know what happened then but we think subduction restarted and poured out new arc lavas but then it stopped and this time for good. Amongst this chaos a different type of magma was forming it had more silica and potassium but most significantly for us now it brought with it new precious metals and that paved the way for the formation of the really rich gold and copper deposits that we see in the Arc today. The magma rose to within a few
kilometres of the surface concentrating gold and copper into huge deposits known
as Porphyry mineral deposits. Porphyry deposits like those found in the Cadia Valley are a member of a class of deposits that are broadly called
magmatic hydrothermal that they involve melting of large volumes deep down in
the earth crust, down into the mantle that material rises up and collects in
this huge batholithic chamber that's down about 6 to 10 kilometres This thing is very active and it's convecting and turning around in the process little little porphyry stocks come off the top and in exolving a hydrothermal fluid and
doing that is extracting all the copper and the gold that one finds in the magma
and it is going up following the porphyry intrusions up to about a
kilometre to kilometre below the Earth's surface where the rocks are fracturing and the metals are being deposited along with
quartz. Metals like copper and gold react with sulphur to form sulphide minerals. These new minerals are concentrated around the porphyry intrusions a favourite target for mining geologists Yeah these are actually some of the most
beautiful rocks in the district but I'm biased because this is the project that
I'm working on right now Malissa Washburn-Groom showed me these gold and copper sulphide minerals in drill core from the Ridgeway and Cadia mines. In this particular deposit the gold is associated with Chalcopyrite it's similar to pyrite but it's a bit brassier in colour but you do occasionally get free gold? we do that's most commonly found in quartz veins and deposits like Cadia Hill, Cadia East and Ridgeway associated with bornite which is a bright purpley blue sulphide mineral. But sometimes gold occurs with more iron-rich minerals. And this particular piece of core has magnetite which is an iron ore as well as pyrite which is an iron sulphide and the
magnetite is the dark mineral there isn't it? That's right and we can use simple field identification techniques such as a magnet, to find where this is in the core. It turns out that porphyry mineral
deposits are a characteristic feature of many modern island arcs. Dr. Rich Goldfarb from the United States Geological Survey travels the world
studying the distribution and origin of gold deposits. We caught up with Rich in
the mineral and fossil research lab of Museum Victoria. he explains where most modern porphyry deposits are found. Those are mainly found associated with young volcano and volcanic areas along the Pacific Rim such as the southwest Pacific from the Philippines down to Indonesia, they're
also found in the Russian far east as well as parts of Western North America
in the high Andes Is there any unifying process that can
explain this worldwide distribution of gold well specifically around the Pacific Rim? The unifying process is actually plate tectonics we, up till 25 years ago till we really started understanding plate tectonics we could not understand the distribution of gold but as our understanding of plate
tectonics grew we started to being able to understand the spatial and temporal
distribution of gold on the planet. It's now clear that subduction provides
just the right conditions for porphyry deposits to form that's why there are so many gold and copper deposits that are formed around the Pacific Rim in the recent past. The Macquarie Arc is just an ancient example of this process but the creation of mineral deposits in the fourth Phase of the Macquarie Arc is also a sign that events were reaching a climax plate tectonics it generates things but it also destroys them the squashing mangles things in the collision zones between the continents. Up to now subduction had been busy creating islands, mineral deposits and and habitats for life but subduction was
entering a new destructive phase and this was to have dramatic consequences
for the nearby Gondwana supercontinent It's very clear that in these island arcs
like the Macquarie Arc it is a very good mechanism for manufacturing continental
crust because in in the island arcs new igneous rocks come up in the arc and form lavas and things we call plutons at depth and those are adding volume to the crust from the mantle and then you slam one of those arcs into into a continent and you've added a big strip a new continent and that's happening at the present day in Taiwan and various other places. Dr. Yamirka Rojas-Agramonte a has studied a similar process in the dynamic Caribbean Ocean. In Cuba we have an island arc and the collision of this island arc against against the continent this is an arc-continent collision and this
you have in many other places in the world. But there were some important differences in the way the Macquarie Arc was deformed. In Taiwan and the Caribbean the
island arc is on the overriding plate and that's colliding with pieces of
continent on the subducting plate so as a result in the collision the arc just
rides over the continent Subduction pulls the nearby continent closer and
closer towards the island arc until they collide but the Macquarie Arc is an
unusual beast and there's a different type of collision and this is because the
arc and the continent were on the same plate separated by a wide sea 500 kilometres wide floored by several kilometres of sandstone what happened there was, the Palaeo-Pacific plate was subducting underneath the Gondwana
plate and the collision was caused by the locking of this plate boundary the plate boundary fused together after subduction had stopped but the Pacific plate was still moving and started pushing the Macquarie Arc westward so a collision between the arc and Gondwana was inevitable and the sandstone ocean was caught between them and without subduction the whole ocean
floor between the Macquarie Arc and East Gondwana was squeezed and deformed. and this whole shuffled together mess of
sandstone and island arc was sort of plastered against the Gondwana continent. And the evidence for this big
squeeze is still here the layers both tilted and folded are seen right across southeastern Australia from the coast and to inland areas like Piper's Creek and central New South Wales. All the layers on the seafloor were deformed and
compressed against Gondwana's eastern margin crushing and lifting the island chain and the sandstones as this seafloor rose to form new land the
Macquarie Arc was split into several belts. The immense forces of subduction
are both destructive and creative subduction created the Macquarie Arc
islands, but then it destroyed them but in this act it created eastern Australia We now have a much better understanding of how southeastern Australia rose out of the ocean and to me that's really a fascinating story. I think the story of the Arc is
important not just because the rocks gave us mineral resources and soils for
agriculture but because it gives us an understanding of this land and of the
evolution of all the continents. I always wanted to be a geologist
since I was a child because I like nature and I wanted to be out I like to
be taking rocks and look at the minerals, this I like. It's obsession it's a deep obsession. From me coming from the streets of New York getting outside
having someone pay me to run around the woods all day long now for 28 summers to
run around every part of Alaska it's just incredible. If you take a piece of rock and what it will tell you I mean what piece of a story this rock is going to tell you how old it is, what it has inside this is the most exciting for me and you can't be a good scientist unless it's fun for you I can't imagine doing science and have it be a drag I mean it has to be fun.
