5 Rocks any great Australian rock collection should have, and where to find them

It's a little unusual to go on a road trip with a geologist. We're reading road cuts and interpreting the history of the place over the last millions — or even billions — of years while you're presumably reading traffic signs and dodging roadkill.

The Australian terrain has been formed by geology. The western plains of Victoria, where I live, are pockmarked by Australia's youngest volcanoes, whereas the east of the state has been pushed up to form the Great Dividing Range's mountains.

Fossilized braided rivers run along the state's southern border, remains of when Australia moved away from Antarctica. Dolerite, a rock that represents this rift, towers in massive columns over Hobart from Mount Wellington, providing evidence of the catastrophe.

I have rocks strewn about the house that I've collected on my trips, which will come as no surprise to anyone who knows me. Every time I look at them, I'm reminded not just of how the rocks were made, but also of the trip I took to collect them. With international and even state borders expected to remain closed for the foreseeable future, now is the ideal time to embark on a fantastic Australian road trip, train as a rock detective, and add to your rock collection.

To get you started, I've compiled a list of five rocks that every excellent Australian rock collection should include.

  1. Dinosaur fossils

Queensland's central and western regions

Oh, to have lived 100 million years ago in Queensland! It would have been a veritable feast of dinosaur activity, based on the fossils discovered in various sections of the state. Queensland is the place to go to peep back in time to the Mesozoic Era between 252 and 66 million years ago, from an unusual pair of dinosaurs in a 98-million-year-old billabong in Winton to preserved remains of a dinosaur herd at Lark Quarry. You might even have dinosaur bones on your property if you're very lucky like the massive, long-necked sauropod discovered earlier this year on a Queensland cattle farm.

The Australian Age of Dinosaurs Museum in Winton, Queensland, is home to the largest collection of Australian dinosaur fossils. (Note: not a real dinosaur.) Shutterstock

The Australian Age of Dinosaurs Museum in Winton, Queensland, is home to the largest collection of Australian dinosaur fossils. (Note: not a real dinosaur.)

2. Mantle xenoliths

Western Victoria

The youngest rocks in Australia are those that erupted 4,000 to 8,000 years ago from Australia's youngest volcano, Mount Gambier in South Australia. That volcano is the apex of a massive field of volcanoes that stretches across central and western Victoria.

The volcanoes in western Victoria were generated by magma rising from the Earth's mantle, which is the layer between the core and the crust. As the magma rose, it ripped fragments of the surrounding mantle rock away and carried them to the surface. These mantle fragments, known as mantle xenoliths (Xeno = foreign, lith = rock), can still be found in cooled lava in western Victoria. 

These rocks appear to be ordinary black or brown basalt until you turn them over or crack them open, revealing a blob of vivid green basalt looking back at you. The green mineral olivine and some black/brown pyroxene make up the majority of the mantle rock inside.

Mantle xenoliths are a terrific location to start your rock collection because they are not just your own piece of Earth's mantle, but you can locate them yourself by fossicking near some of western Victoria's volcanoes.

The crater of an erupted volcano near Mount Gambier in Victoria. Shutterstock

3. Meteorites

The Nullarbor Desert, South Australia, and Western Australia

The Nullarbor is a desert area that runs between South and Western Australia. The dry climate is perfect for conserving meteorites that fall to Earth, and the light color of the limestone country rock, combined with the lack of vegetation, makes it easier to spot black and brown meteorites.

Even if you don't have a keen eye for identifying meteorites hiding in plain sight, you can use a magnet on a stick to assist you, like geologists do. Because most meteorites are iron-rich, stumbling around with a magnet hovering over the ground is a good way to find them.

Thousands of meteorites, some as old as 40,000 years, have been discovered in the Nullarbor.

A black meteorite standing out against the white limestone of the Nullarbor Plain. Professor Andy Tomkins

4. Banded iron formation

       Western Australia

A layered sedimentary rock called a banded iron formation is made up of alternating bands of red chert (a sedimentary rock made up of quartz) and silver to black iron oxide. It is the main source of iron ore in Western Australia, and it may be found in numerous regions.

The Hamersley Province in Western Australia's northern corner boasts the world's thickest and most widespread banded iron formations. They are estimated to be between 2.45 and 2.78 billion years old.

They originated on a continental shelf, where thick continental crust extends out into the ocean and then lowers down to oceanic crust, according to geologists. The banded iron formation is fascinating because it no longer occurs on Earth, implying that it documents an ancient process that we no longer observe. It is assumed to have originated in ancient waters as the oxygen content began to rise at the time. 

It captures the chemical input of these oceans, as well as continental deposits and ocean floor volcanoes.

                                               Banded iron formation at Forescue Falls, WA. Graeme Churchard/FlickrCC BY

5. Metamorphic rocks

      Broken Hill, New South Wales

Because of the massive silver, lead, and zinc mine in Broken Hill, you've probably heard of it. However, the geological circumstances that gave rise to the ore deposit roughly 1.7 billion years ago also produced some stunning rock formations.

A visit to the Albert Kersten Mining and Minerals Museum in Broken Hill will reveal the region's huge assortment of rare minerals, some of which are discussed for the first time at this location. 

Round Hill is the place to go if you want to acquire a piece of Broken Hill's geological heritage. Beautiful red garnets surrounded by spots of white minerals can be found within a short distance from the town center (quartz and feldspar).

These rocks began as sand and mud and have been buried and cooked to temperatures of around 700°C deep beneath the Earth's surface. This caused the rock to melt, resulting in the remarkable striped, garnet-rich rocks that we see today.

                                                     A large garnet from the Broken Hill region. Professor Andy Tomkins

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Jo Adetunji
Editor, The Conversation UK

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