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


Emily Finch, Monash UniversityRoad tripping with a geologist is a little different. While you’re probably reading road signs and dodging roadkill, we’re reading road cuttings and deciphering the history of the area over the previous millions — or even billions — of years.

Geology has shaped the Australian landscape. In Victoria where I live, for example, the western plains are pockmarked by Australia’s youngest volcanoes, while the east of the state has been pushed up to form the mountains of the Great Dividing Range.

Along the southern margin of the state are fossilised braided rivers, relics of when Australia drifted away from Antarctica. Evidence of this event extends into Tasmania, where dolerite, a rock that signifies this rift, looms in enormous columns over Hobart from Mount Wellington.

This probably won’t surprise anyone who knows me, but I have rocks peppered around my house that I’ve collected on my travels. Every time I look at them, I not only think about how the rocks were formed, I’m also reminded of the trip when I collected them.

With international and even state borders set to remain closed for a while longer, this is the perfect time to take a great Australian road trip, become a rock detective, and build up your rock collection while you’re at it.

To help you get started, I’ve listed five rocks any great Australian rock collection should have.

Green, volcanic crater
The crater of an erupted volcano near Mount Gambier in Victoria.

1. Mantle xenoliths

Western Victoria

The youngest rocks in Australia are those that erupted out of Australia’s youngest volcano in Mount Gambier, South Australia, 4,000 to 8,000 years ago. That volcano is the culmination of an enormous field of volcanoes that span central and western Victoria.

Read more:
Photos from the field: the stunning crystals revealing deep secrets about Australian volcanoes

In western Victoria, the volcanoes were formed from magma that ascended from the Earth’s mantle — the layer between the Earth’s core and crust. While the magma was rising, it tore off chunks of the surrounding mantle rock and transported it to the surface. We can find these chunks of the mantle — or mantle xenoliths (xeno = foreign, lith = rock) — in cooled lava today in western Victoria.

At first, these rocks look like any other piece of black or brown basalt, but then you turn them over or crack them open and there’s a blob of bright green rock staring back at you. The mantle rock inside is comprised mainly of olivine, which is a green mineral, and some black/brown pyroxene.

Green rock blob encased in black rock
Green mantle xenolith (xeno = foreign, lith = rock) encased in cooled basaltic lava from Mount Shadwell, Victoria.
Dr Melanie Finch, Author provided

Mantle xenoliths are a great place to start your rock collection because not only will they be your very own piece of Earth’s mantle, but you can find them yourself through a bit of fossicking around some of the volcanoes in western Victoria.

2. Meteorites

The Nullarbor Desert, South Australia and Western Australia

The Nullarbor is a desert plain region which straddles the border of South Australia and Western Australia.

The dry environment is ideal for preserving meteorites that fall to Earth, and the light colour of the limestone country rock and lack of vegetation means the black and brown meteorites are easier to see.

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

Even if you don’t have a great eye for spotting meteorites hiding in plain sight, you can do as the geologists do and use a magnet on a stick to help you. Most meteorites are iron-rich, so wandering around with a magnet hovering over the surface is a good way to pick them up.

Thousands of meteorites have been found in the Nullarbor, some up to 40,000 years old.

3. Metamorphic rocks

Broken Hill, New South Wales

You’ve probably heard of Broken Hill because of the large silver, lead and zinc mine there. But the geological conditions that created the ore deposit around 1.7 billion years ago also made some beautiful rocks.

A visit to Broken Hill’s Albert Kersten Mining and Minerals Museum will demonstrate the vast array of unusual minerals found in the region, some of them described for the first time at this locality.

If you’re seeking your own chunk of Broken Hill’s geological history, Round Hill is the place for you. Just a short way out of the town centre, you’ll find beautiful red garnets surrounded by patches of white minerals (quartz and feldspar).

A geologist holding a rock with various colours
A large garnet from the Broken Hill region.
Professor Andy Tomkins, Author provided

These rocks started out as sand and mud, and record the history of being buried and heated to over 700℃ deep below the Earth’s surface. This process caused the rock to start melting and created the striking stripey, garnet-rich rocks we find there today.

4. Banded iron formation

Western Australia

Banded iron formation is a layered sedimentary rock mainly comprised of alternating bands of chert (a sedimentary rock made of quartz) that’s often red in colour and silver to black iron oxide. It is the main host of iron ore, and can be found in several regions in Western Australia.

The Hamersley Province in the northwestern part of Western Australia has the thickest and most extensive banded iron formations in the world. They are about 2.45 to 2.78 billion years old.

Red and brown bands along a rock face
Banded iron formation at Forescue Falls, WA.
Graeme Churchard/Flickr, CC BY

Geologists believe they formed on a continental shelf, where thick continental crust extends out into the ocean and then drops away to oceanic crust.

Banded iron formation is exciting because it no longer forms on Earth today, meaning it records an ancient process that we no longer see happening.

It is thought to have formed in ancient oceans, which were starting to increase in oxygen content at the time. It records the chemical input of these oceans, as well as sediments from the continent and volcanoes on the ocean floor.

5. Dinosaur fossils

Central and western Queensland

Oh to have been in Queensland 100 million years ago! Judging by the fossils found in parts of the state, it would have been a cornucopia of dinosaur activity.

From an unlikely duo of dinosaurs in a 98-million-year-old billabong in Winton, to fossilised evidence of a dinosaur herd at Lark Quarry, Queensland is the place to go to peer back in time to the Mesozoic Era between 252 and 66 million years ago.

And if you’re really lucky, you might even have dinosaur bones on your property, like the huge, long-necked sauropod discovered just this year on a Queensland cattle farm.

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

When building your Australian rock collection, remember to check first if fossicking is allowed in the area. When you find an interesting rock, your state or territory geological survey might be able to help with identifying it.

Happy hunting!

Read more:
How to hunt fossils responsibly: 5 tips from a professional palaeontologist

The Conversation

Emily Finch, Beamline Scientist at ANSTO, and Research Affiliate, Monash University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Satellites reveal melting of rocks under volcanic zone, deep in Earth’s mantle

File 20170705 9733 ityqvm
Mount Ngauruhoe, in the foreground, and Mount Ruapehu are two of the active volcanoes in the Taupo volcanic zone.
Guillaume Piolle/Wikimedia Commons, CC BY-ND

Simon Lamb, Victoria University of Wellington and Timothy Stern, Victoria University of Wellington

Volcanoes erupt when magma rises through cracks in the Earth’s crust, but the exact processes that lead to the melting of rocks in the Earth’s mantle below are difficult to study.

In our paper, published today in the journal Nature, we show how it is possible to use satellite measurements of movements of the Earth’s surface to observe the melting process deep below New Zealand’s central North Island, one of the world’s most active volcanic regions.

Rifting in the Taupo volcanic zone

The solid outer layer of the Earth is known as the crust, and this overlies the Earth’s mantle. But these layers are not fixed. They are broken up into tectonic plates that slowly move relative to each other.

It is along the boundaries of the tectonic plates that most of the geological action at the Earth’s surface occurs, such as earthquakes, volcanic activity and mountain building. This makes New Zealand a particularly dynamic place, geologically speaking, because it straddles the boundary between the Australian and Pacific plates.

The central region of the North Island is known as the Taupo volcanic zone, or TVZ. It is named after Lake Taupo, the flooded crater of the region’s largest volcano, and it has been active for two million years. Several volcanoes continue to erupt regularly.

The TVZ is the southern tip of a zone of expansion, or rifting, in the Earth’s crust that extends offshore for thousands of kilometres, all the way north in the Pacific Ocean to Tonga. Offshore, this takes place through sea floor spreading in the Havre Trough, creating both new oceanic crust and a narrow sliver of a plate right along the edge of the Australian tectonic plate. Surprisingly, this spreading is going on at the same time as the adjacent Pacific tectonic plate is sliding beneath the Australian plate in a subduction zone, triggering some of the major earthquakes in the region.

Sea floor spreading results in melting of the Earth’s mantle, but it is very difficult to observe this process directly in the deep ocean. However, sea floor spreading in the Havre Trough transitions abruptly onshore into the volcanic activity in the TVZ. This provides an opportunity to observe the melting in the Earth’s mantle on land.

Lake Taupo is the caldera of the region’s largest volcano.
NASA/Wikimedia Commons, CC BY-ND

In general, volcanic activity happens whenever there is molten rock at depth, and therefore the volcanism in the North Island indicates vast volumes of molten rock beneath the surface. However, it has been a tricky problem to understand exactly what is causing the melting in the first place, because the underlying rocks are buried by thick layers of volcanic material.

We have tackled this problem using data from Global Positioning System (GPS) sensors, some of which form part of New Zealand’s GeoNet network and some that have been used in measurement campaigns since 1995. The sensors measure horizontal and vertical shifts in the Earth’s surface to millimetre precision, and our research is based on data collected over the past two decades.

Bending of the earth’s surface

The GPS measurements in the Taupo volcanic zone reveal that it is widening east-west at a rate of 6-15 millimetres per year – in other words, the region, overall, is expanding, as we anticipated from our previous geological understanding. But it was surprising to discover that, at least for the past 15 years, a roughly 70-kilometre stretch is undergoing strong horizontal contraction and is also rapidly subsiding, quite the opposite of what one might anticipate.

Also unexpectedly, the contracting zone is surrounded by regions that are expanding, but also uplifting. Trying to make sense of these observations turned out to be the key to our new insight into the process of melting beneath the TVZ.

We found that the pattern of contraction and subsidence, together with expansion and uplift, in the context of the overall rifting of the TVZ, could be explained by a simple model that involves the bending and curving of an elastic upper crust, pulled downwards or pushed upwards by an underlying vertical driving force. The size of the region that is behaving like this, extending for about 100 kilometres in width and 200 kilometres in length, requires this force to originate nearly 20 kilometres underground, in the Earth’s mantle.

This diagram illustrates a patch of suction stress along the axis of the underlying upwelling mantle flow beneath the Taupo volcanic zone.
Simon Lamb, CC BY-ND

Melting the mantle

When tectonic plates drift apart on the sea floor, the underlying mantle rises up to fill the gap. This upwelling triggers melting, and the reason for this is that hot, but solid, mantle rocks undergo a reduction in pressure as they move upwards and closer to the Earth’s surface. This drop in pressure, rather than a change in temperature, begins the melting of the mantle.

But there is another property of this upwelling mantle flow, because it also creates a suction force that pulls down the overlying crust. This force comes about because as part of the flow, the rocks have to effectively “turn a corner” near the surface from a predominantly vertical flow to a predominantly horizontal one.

It turns out that the strength of this force depends on how stiff or sticky the mantle rocks are, measured in terms of viscosity (it is difficult to drive the flow of highly viscous or sticky fluids, but easy in runny ones).

Experimental studies have shown that the viscosity of rocks deep in the Earth is very sensitive to how much molten material they contain, and we propose that changes in the amount of melt provide a powerful mechanism to change the viscosity of the upwelling mantle. If mantle rocks don’t contain much melt, they will be much stickier, causing the overlying crust to be pulled down rapidly. If the rocks have just melted, then this makes the flow of the rocks runnier, allowing the overlying crust to spring back up again.

We also know that the movements that we observe at the surface with GPS must be relatively short lived, geologically speaking, lasting for no more than a few hundred or few thousand years. Otherwise they would result in profound changes to the landscape and we have no evidence for that.

Using GPS, we can not only measure the strength of the suction force, but we can “see” where, for how long, and by how much the underlying mantle is melting. This melt will eventually rise up through the crust to feed the overlying volcanoes.

This research helps us to understand how volcanic systems work on a variety of time scales, from human to geological. In fact, it may be that the GPS measurements made over just the last two decades have captured a change in the amount of mantle melt at depth, which could herald the onset of increased volcanic activity and associated risk in the future. But we don’t have measurements over a long enough time period yet to make any confident predictions.

The ConversationThe key point here is, nevertheless, that we have entered a new era whereby satellite measurements can be used to probe activity 20 kilometres beneath the Earth’s surface.

Simon Lamb, Associate Professor in Geophysics, Victoria University of Wellington and Timothy Stern, Professor of Geophysics, Victoria University of Wellington

This article was originally published on The Conversation. Read the original article.

China: The red Rocks of Mount Gongga

The link below is to an interesting article on the algal growth on rocks over the slopes of Mount Gongga in China.

For more visit: