He is standing alongside Waterworks Road, Dynnyrne, looking down at a dark grey coloured rock, a small sheet of exposed dolerite. The surface of the rock is smooth as it is fine grained. As he strikes it with his hammer a sharp sound returns. This is the “clink factor” - an indication of an extremely hard rock.
Most of us are content to pocket the odd weathered stone or misshapen rock as memorabilia from a great bush walk or special sojourn. But for geologist David Leaman the trinket of choice is nearly as big as Tasmania.
David’s life work has been the study of dolerite, the rock Tasmanians think of when they think of home.
The rock Charles Darwin called Greenstone dominates the surface of Tasmania and is the basis for its magnificent scenery, though the landforms may now be so familiar as to have become commonplace. The verdict may still be out on how and when it came to be but David has made it his life’s journey to decipher the past and lift “the curse” on the unique rock that makes Tasmania.
It is close to the island's densest rock, weighing in at three tonnes per cubic metre. “It is more than a hard rock, it’s a tough rock,” David said. “Feel is what you do with this rock. On the clink factor, the ring will tell you how much it’s going to cost. Only two other rocks in Tasmania are as hard, conglomerate found in the west coast ranges such as Mt Lyell and the quartzites of the Southwest. These rocks generally don’t pose a problem for people because they aren’t in your backyard. Unlike dolerite.
“It is a rock that requires respect and understanding. You just can’t assume anything. The dolerite and I have come to an understanding lately. I let it win and occasionally, it lets me have a win.”
David does know this rock intimately as it has slowly revealed itself to him. If you offer David four hours, you’ll get 185 million years in return. Spending any time in his company inspires a sense of wonderment about these formations only occurring elsewhere in Antarctica and Southern Africa, but are most accessible, right here in our backyard.
Born in Hobart and educated at the University of Tasmania he grew up on the very rock he later agreed to research. Employed by the Geological Survey of Tasmania as a “mapper” amongst other things he eventually established a consultancy. For many years he led field excursions and taught specialised courses back at the University. David has also written a few books that encompass this passion bringing his knowledge into everyday living.
Tanned legs framed by hiking boots and Snowgum shorts are bared to the elements on this May morning. David is tall and toned belying his 60 years. Bright eyed and smiling, clearly being outdoors in this “big backyard” agrees with him regardless of the weather.
David deals with time frames most of us can only imagine. (He has been quoted as describing the intrusive part of the Jurassic Period as a “blink of an eye”). We are used to dealing with periods that span a few lifetimes and this is where some of the real issue lays when dealing with dolerite. How can a planner or developer be expected to embrace the possibility of a 1 in 1000 year seismic event and its flow on effects? That today may very well be the day?
David recalls a bush walk where he gently cautioned his group away from a cliff’s edge, saying, “enjoy the view but don’t become part of it”, only to be met with a very pale faced response from one of the group. She had once been standing on a coastal column with some friends when it gave way but saved herself by hanging onto the root system now exposed in front of the freshly revealed dolerite cliff face. No one was killed but it is a reminder that we live on an ever-changing platform and future events are fairly unpredictable.
First described in 1807 by French scientist and mineralogist, Rene Just Hauy, dolerite is derived from the Greek word meaning “deceptive”, for the difficulty of discriminating its constituents. Interestingly, Dolosus, of Latin origin, means crafty, cunning and deceitful. The rock was branded complicated right from the start.
This is the rock of Tasmanians. It’s the rock we get homesick for – the icon that has put the State on the “natural landscape” map. But how does such a beautiful phenomenon possibly cause so many problems? The answer is written in stone and, like anthropologists deciphering ancient hieroglyphics, David interprets these hidden messages from the angles, colours and “clink factors” to gain three-dimensional images of intrusions.
In very simple terms there is a factor that causes farmers, engineers and miners to pull their hair out and curse the skies when they encounter dolerite. It is variability and this is the key to unlocking “the curse”. “You can’t apply standard engineering measures to dolerite”, he said confidently while standing at the base a benched rock face further up the road. He has been called to many a face and asked, “How stable is this?”. Generally this is not a good question. It all depends….on composition and structure.
In Tasmania our dolerite dates back 185 million years to the Jurassic Period when Australia including Tasmania, Antarctica, Africa, South America and India formed the super-continent Gondwana. In depths of up to 25 to 50 kilometres in the upper mantle, temperatures at melting point caused volumes of molten rock to be ejected towards the surface. Magma then rose along the old crustal fractures resulting in devastation and the transformation of the earth’s surface. This was a violent time seismically. The intruding dolerite lifted and separated, but it didn’t change the original distribution of the rock it drove through. These intrusions of dolerite occurred at all scales of up to 500 metres in thickness that not only caps most of our high ground but also forms the magnificent coastal cliffs of the Tasman Peninsula – the main difference between Cape Hauy and Mt Wellington is sea level.
Dolerite is both intrusive and an igneous rock, meaning it has cooled and crystallised from high temperature magma. Minerals as they cool form structures. In a regular dolerite profile, elemental composition, crystal size and texture are considered and the compositional range may include rocks such as gabbro, pegmatite, granophyre and glass. These are rocks in their own right but they are hanging out together in an intrusion which has structural form requiring them to be called dolerite. Follow?
When confronted, dolerite as its crafty and cunning name suggests, can present itself as hard and stable or clay like and unstable, and a broad spectrum of everything in between. It can disguise itself using different colours depending on its composition and this is determined by where it appears in an overall intrusion. The hydrology or the ability to retain water and weathering, chemical or physical, also impacts on the rock’s stability. A smooth stable exterior may also disguise a shattered or corroded core as the result of shaking due to earthquakes.
Engineers slam into it. Farmers sink into it or plough it. Nothing is ever easy. Miners are tricked into thinking they’ve lost their seams but really they’re just on the other side of the intrusion and maybe a bit higher up. Constructions built on the not so solid dolerite product’s crack. It is as perplexing as it is at times beautiful - one minute appearing like Mt Wellington, the other like a confused mosaic of rock and clay as it does along Waterworks Road.
So why do any of these factors matter? Since around two third of the State is covered in dolerite it would pay to have some greater understanding of its schizoid nature. “All of these aspects should really be considered before undertaking any activity,” David raises an eyebrow and continues, “but this rarely happens.”
“Construction of roads, like the Southern Outlet exemplify our misunderstanding and misplaced confidence. The rock turned out to be more difficult to handle than the designers expected and the budget for the project was son expended. Dolerite can do this if we don’t appreciate its properties and take the time to examine the particular needs of each place and project,” he said.
Facing yet another portion of rock face David sets a quiz on its composition. “Yes,” he says looking very chuffed, “to an untrained eye you would think that this is a conglomerate of some sort however, this is what I call ‘debris’”. David buries his pick into the surface with ease. He laughs and says, “This is the ‘clunk’ factor, and represents unstable, moist rock and the soft stuff can be used but without care presents another factor often overlooked before constructing.”
David relays a joke from his study days. The image is that of two engineers standing proudly alongside a recently constructed rather tall and elaborate circular tower. The quote reads something along the lines of, “Did you survey the ground before you built that mate?’. Of course the building in question is the infamous tower of Pisa.
The more you read, the more you’ll want to call on David’s expertise the next time you’re building something you want to have the same street number for more than the next 50 years.
Locating dolerite below the surface seems to require some sort of a divining rod although geologists tend to use slightly more scientific methods such as seismic refraction, magnetic models, electric resistivity, and the gravity method to an area in the hope of gaining an insight into this buried treasure.
Seismic velocity: the “clink” factor technique can measure sounds reverberating at up to 5000 metres per second through a stable piece of rock. Rock returning this type of reading is considered good quality rock and though the going will be tough for engineering at least you have some understanding of what you are up against. Anything below about 2500 metres per second is not considered stable enough for use in construction - this is not a question of hardness it is an issue of stability and this is another story all together.
Electric currents help to locate the differing dolerite qualities and detect fractures through differences in resistance. Responses indicate whether the rock plate below is a solid quality piece or if it is just covering a rusting core of dolerite.
During the cooling, rock with a “high clink” factor retains a magnetic field within its iron content and this sends a strong message aiding the geologist in locating plates that are often invisible from the surface. Temperature magnetises dolerite and, during the cooling process after intrusion, the rock takes on the magnetic field of where it comes to rest at that time. Two things can impact on this memory. If the exposed dolerite is hit by lightning it will reset its magnetic field and memory, as will weathering or rusting of the rock. This destroys its memory field as its character is lost making this form of detection harder to employ as a single method.
The methods quoted here can yield the everyday information about the rock – its condition, quality, use, etc but a fourth method, the gravity method, is the one which finally cracked the secret about intrusional shapes and their patterns.
Back at Waterworks Road, David finds a piece of weathered high “clink factor” dolerite. The surface is pitted and the crystals are obviously larger than the earlier grey piece. David looks at this piece as you would look at a good glass of red and says, “This rock has a higher content of feldspar, so we know we’re moving towards the centre of the ‘bucket’ in this intrusion”.
The “bucket” is a quick term David uses to describe the variability in the composition that arises during the cooling process of intrusions. The elements order themselves vertically and horizontally and this is helpful to understand when you are looking for answers from the outside in. For many years the debate as to the origin and make-up of dolerite ensued. Was it volcanic or intrusive? “In the end it was Darwin’s observations on texture and mineralogy and further microscopic work that finally convinced others, ceasing the debate in 1900,” he says.
Nature applies two main types of weathering techniques to dolerite – physical and chemical. One is easier to understand especially as it relates to the Tasmanian climate – dramatic temperature change. When dolerite is warmed then cooled quickly it can cause fresh fractures to occur or existing weaknesses to expand. This stimulates a “peeling” action and on a big scale entire columns, such as the organ pipes of Mt Wellington, can fall away. On a smaller scale faces of exposed weaker rock areas, such as along Waterworks Road, may lose pieces the size of a headstone.
Weathering processes also lead to mineral soil production and many plants are happy to place roots into crevices or fractures in the absence of other soil cover. Certain species of plants will be drawn to the chemical bi-products of the rusting dolerite for nutritional requirements whilst others will draw upon its water retaining abilities. Its variable character means it also supports a wide range of vegetation. Soils derived from dolerite present a large range of compositions and produce different families of clays that have varying water carrying capacities.
Pointing to an area up above the cliff face David said, “Sometimes you will see a well-vegetated area where there are also patches of dead trees. In drought times, this may point to areas where dolerite lay just below the surface and the rock’s water carrying capacity has been altered rendering it unable to support the life that usually draws upon it.”
This begs the question. How on earth does a hard rock hold water? Easy.
“Rocks are good for storage and the cracks are important for ecology.” David, now on a roll continues, “Cracks are only thought to penetrate to around 50 metres. Water storage is held in these cracks and can account for between two and five percent of the rock’s overall volume and is the driving force of an area’s ecology and hydrology. A river or stream’s character can be determined by the surrounding geological make-up. In drought times where the land yields its water to evaporation, the underlying rocks will also be drawn upon and this will affect the environmental flows of waterways that are fed by rock storage.
“As long as it rains again and the surface soils remain penetrable, the rock will be able to recharge itself with water and maintain its hydrology and support that of the surrounding ecology. Events that change the surface soil’s structure, such deep disturbance of clays as in some farming and forestry practices or high temperature bush fires that scorch the earth, can seal the surface in turn damaging the dolerite’s ability to elicit and store water. This in turn will change the overall hydrology of an area and eventually impact on environmental water flows. This highlights the importance of fire regime and land-use practices within Tasmania given the quantity of dolerite in our geological make-up,” David explains.
“Dolerite is magnetic and as demonstrated, can be magnetised. It is recommended that while bush walking in Tasmania, you watch your compass like a hawk. A northerly bearing can easily be spun around if you happen across an affected zone of dolerite lying below the surface”. He calls these “carrots” and draws a picture. “Carrots are places where the great intrusive feeders have left more iron minerals, or currents from lightening strikes have bled into the rock.”
Robert Brown, botanist and naturalist on board Flinders’ Investigator, recorded the first geophysical observation in 1804 having climbed the pinnacles, now known as the Organ Pipes, several times and noted that the “perpendicular rocks atop the ‘Table Mountain’ [Mt Wellington] affected his compass needle.”
David says, “Environmental processes modify or cloud the magnetism of the rock with time since nothing stays the same or constant.”
The same can be said for the Tasmanian weather and the natural Hobartian act of glancing towards Mt Wellington for an indication. People often comment on the mood of the rock or the way the cloud is moving across it utilising this magic mountain as a barometer. Hobart definitely feels very flat and ordinary when the mountain is completely shrouded in clouds and Mt Stuart becomes the tallest landform in view.
Now in a clearing with a view to Mt Wellington David explains, “The colouring in dolerite comes not from external elements but is yet again an aspect of its complex composition. It may preset itself in a range of colours from ochre through to black depending on the amount of iron oxidising through exposure and the internal water regime inherent in its make-up. This, coupled with the variety of lichens which select climatic, compositional and aspect niches complete the colouring process.”
Mt Wellington for example, appears red and brown and this implies that structures such as the Organ Pipes are rich in iron. They are also strong. How can a large exposed rock with a high content of iron stand so tall and apparently stable? It’s apparently all a question of time and relativity. Think back to the story of the falling column.
Dolerite is the end process of a series of dramatic geological processes and is no stranger in this landscape. This rock dominates Tasmania’s impressive scenery in the form of mountains, escarpments, the high columnar cliffs of the Tasman Peninsula and is possibly so familiar that its significant and powerful journey into being might have been left uninvestigated and unappreciated if not for David Leaman.
The afterward of David’s most recent book, “The rock that makes Tasmania”, says, “This book exudes passion: for the rock and with concern that we understand how to live with it. He believes everyone should have some familiarity with the geology of where they live and work…..The effect dolerite has had upon the development of Tasmania, both geologically and more recently, must be appreciated if we are to transform our future.”
The passenger seat floor is now littered with fresh samples from today’s outing. With a smile David passes on his sincere thanks for the interest in the rock and indicates, apparently, the dolerite will be most appreciative as well.
“I am convinced it has a wry smile whenever we state what we think we know, or write about it. I am sure it keeps the cuttings for cold nights.”
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Thursday, July 17, 2003