Of all continents, Australia is the least geologically active. The landscape is ancient, including some of the oldest rocks in the world. The only mountains are on the eastern edge of the continent. The B59 road across the Blue Mountains from Windsor to Lithgow is one of my favourite drives, with spectacular views (a head for heights is helpful). The steep rock sides and deep canyons form a seemingly impassable barrier to the vast lands further west. Southward, the Tidbinbilla mountains are ruggedly beautiful but can be cold. A Koala bear was shivering in the rain half way up a small tree. A platypus was playing in a large pool, coming within meters of us on this grey day. This ancient land has a mountainous edge.
Volcanoes are signs of youth and seem out of place on this continent. But they are here, and some are surprisingly young. Even today a new eruption is not impossible. Like the mountains, the volcanoes are all on the eastern side of Australia, in a series of chains running north to south, starting 500 km of the northeastern coast (the Maer islands) to Tasmania in the south. The total length is 4400 km. That is a sizable sequence! It closely follows the Great Dividing Range, the third longest continental mountain chain in the world. There are also two chains of submarine volcanoes further to the east, off the coast.
Not surprisingly, the cause of these volcanic chains lies underground. Australia is the world’s fastest continent. Continental drift is moving it north (10 degrees east of north, to be precise) at break-neck speed, 7cm per year. While speeding along, the continent has been traversing a broad hot ‘spot’ underneath the continent, and the chain reflects the movement of the Australian plate over this region. The northern edge of Australia reached the hot spot (or warm region) perhaps 40 million years ago . At the current time the warm region is located near Tasmania. While Australia drifted over it, magma generated by the warmth (including some crustal melt) found weaknesses in the crust through which it managed to erupt. The result was a series of lava fields and volcanoes, stretching across eastern Australia.
Unlike most mountain ranges, the Great Dividing Range is not caused by collision or subduction. In fact it isn’t really a mountain range at all. The entire area was uplifted by as much as one kilometer. The eastern edge of the uplifted region sloped towards the sea. Rivers on this slope caused erosion, and turned the slope into a steep edge which was slowly forced back into the land. Climb up the steep slope, and you will find that what is a mountain from one side, is an escarpment from the other. The uplift started around 80 million years ago but different parts of the range do not all have the same age. The current edge of the escarpment was reached about 3 million years ago: it has been fairly stable since. Volcanoes occur both on the escarpment and on the eroded plains, but only one lava flow, a young one dated at 3 million years, comes over the escarpment. The largest of the volcanoes was probably Mount Tweed with its peak Mount Warning, on the coastal plains. It was between 2 and 3 km tall and had a volume of 4000 cubic kilometer but it has been eroded down to a remnant. One has to be amazed, and echo Lady Macbeth: Who would have thought the old man to have had so much blood in him? Mount Warning is not a volcanic neck, but rather a solidified magma chamber, dug out by erosion which removed the softer rock surrounding it.
Why do the volcanic chains follow the Great Divide? There is no close connection between them. In spite of its speedy drift, Australia is not subject to great tectonic stress. But there is some compression acting on it, in the east-west direction, caused by New Zealand. The centre and west of Australia have very thick crust and lithosphere and are hard to push. Any fault lines caused by the compression are therefore located on the younger, eastern side. (Over geological times, Australia grew its crust mostly from west to east.) It may be that the volcanic chains follow compressional fault lines, perpendicular to the stress. The faults weaken the crust and allow magma to push its way up. We should not assume that each chain segment necessarily shows a separate mantle hotspot. The condition of the crust plays a role. A small weakness can act as a chimney, funneling the heat towards it and up. In a way, the Australian volcanoes are like the chimneys of Mary Poppins.
The thickness of the lithosphere also acts as a barrier. Gaps in the chains have been attributed to places where the lithosphere is too thick. This keeps the warmth deeper down, under too high pressure to allow for decompression melt. The highest melt fractions occur where the lithosphere is thinnest. The lava fields may have a slightly different origin: there are suggestions that they come from edge convection, material traveling sideways along the bottom of the plate. This would explain why the newer volcanic region is some distance away from the main track.
Where did the warm region or broad hot spot come from? That is unclear, but a clue may be found in the fact that the two submarine chains, almost a thousand kilometers away, show exactly the same age gradient as the ones on the continent. Even more striking, volcanic mounts in different chains at the same latitude have the same age. It seems that the spot is stretched out along an east-west line. A warm, linear region sounds like the remains of an ancient spreading ridge.
The most recent eruptions have been at the end of the chains, in Victoria, forming a volcanic plain stretching westwards from Melbourne. Lava has erupted from nearly 400 locations. The most recent eruption was an explosive one, on the western edge in a region called ‘newer volcanics’. It was from Mount Gambier, and happened around 5500 years ago. The eruption or sequence of eruptions formed a series of ‘maars’ (lakes) of which the Blue Lake is the most photogenic. It hosts fresh water stromatolites. Nearby,, a little south of Mount Gambier, the ash cone of Mount Schank could even be a little younger, at 5000 years although aboriginal legends suggest the eruption at Mount Schank was earlier than the one(s) at Gambier. Other volcanoes in this region, such as Mount Napier, are dated to 33,000 years ago. These eruptions were small. Volcanism here has been intermittent but is not extinguished. Future eruptions are certain. But with an average time between eruptions of order 10,000 years, it may not happen any time soon.
An excellent place to see an Australian volcano is in the Warrumbungles National Park, New South Wales. It is a 7 hour drive from Sydney, starting with the Richmond to Lithgow road across the Blue Mountains and stopping for lunch in Mudgee. The undulating landscape along the way is Jurrasic sandstone (so much of Australia is build on sand). Coonabarabran, with its mini Back-to-the-Future clock tower, is a good place for dinner. After Coonabarabran a low mountain appears on the horizon and the road begins to rise. Eventually it tops a ridge 500 meters above the undulating plain, and drops into a valley beyond, surrounded by ragged ridges. (The Aboriginal word Warrumbungles means ‘crooked mountain’ – all this land once was theirs and their names and history are everywhere.) Go too far and the road, now unsealed, enters the never-ending plains of western New South Wales – best to turn back. The national park is partly grassland, partly wooded with a variety of gum tree species. The white gums make a striking contrast with the black cypress pines. The park has suffered extensive fire damage recently. Grass trees, seen at higher levels, are a curious combination looking like a stump in need of a haircut. Kangaroos are everywhere: they make driving hazardous in twilight by their unerring tendency to jump in front of the nearest car. In the evening, the booming sounds of emus can be heard.
15 million years ago, this was a shield volcano, 1 km high and 50 km across build on the sandstone. Unlike the much larger Mount Tweed, the rocks were formed in real eruptions rather than from underground solidification. The lava flows of the Warrumbungles amounted to 500 cubic kilometer, erupting over a long period of time, estimated at 2.5 million years. The old shield is now largely eroded away. The much harder dykes and volcanic plugs were more resistant to the Australian rain (when it rains, it pours) and now stick out above the rest: the innards of the volcano visible in the crooked ridges and mini-mountains. The most spectacular of these is the Breadknife: a rock 90 meter high, but in places only 4 meters thick. It is clearly the skeletal remains of a volcanic dyke. A footpath, difficult and steep, runs around the entire base. Climbing the knife is no longer allowed as even this hard rock can be fragile. A Japanese children’s tv program from the 1980’s, on the fictional adventures of a koala bear, plays beneath the Breadknife. Originally, this dyke was deep underground, and the koala bear would have been better off as a cave bear. The program perhaps missed a trick by not using the setting to show-and-tell geology.
The tallest mountain in the park is Exmouth, 1206 meters high, where some of the volcanic shield can still be seen. The view from the top is spectacular, a reward for a long and hot hike. The lava is seen to be stratified, with pyroclastic tuffs and breccias. The pyroclastic deposits can be recognized elsewhere in the park, including the area around the Breadknife. Trachyte is common in the central areas. The lavas lack primitive mantle components and crustal melt is indicated. Grey or white diatomites deposits, fossil algae covered in chalk, are found around the edges of the old volcano. There were lakes here during the volcanic era, perhaps heated by the activity. The chalk is now mined, for use in swimming pool filters. In Australia, people find a use for everything, even its fossils.
16 million years ago, the first flows of the emerging volcano were probably the thick trachyte lavas, erupting from a variety of vents over a larger area and forming a number of separate domes. The trachyte eventually gave way to more fluid lavas, with intermittent explosive events, which filled in the gaps between the trachyte domes. This formed the large shield. The youngest flows were the least viscous and travelled furthest, but were also the thinnest. The lava in this phase was more basaltic: hawaiites and trachyandesites. Erosion has removed the basalt and left the harder trachyte domes.
Australia’s volcanic heritage is hidden in its vast landscapes. Nothing here is what it seems. Mountains aren’t mountains, deserts turn into lakes at the whim of the weather, subtropical Sydney shares its seas with penguins. It is no surprise that here, volcanoes travel, perhaps following faults generated by pressure from New Zealand. Aboriginal legends, the longest memory on earth, may recall the most recent eruptions. One day there will be another eruption. There is life in the old lady yet.
Geological sites of NSW: http://www.geomaps.com.au/scripts/warrumbungle.php
An outline of the geology and petrology of the warrumbungle volcano: M. Duggan and J. Knutson, 1986.
Australia in time and space, R. Blewett, B. Kennett, D. Huston, 2012. Published in: Richard Blewett (ed.), Shaping a Nation: A Geology of Australia, Richard Blewett (editor), ANU ePress, Canberra, pp. 49-119.
Lithospheric controls on magma composition along Earthâ€™s longest continental hotspot track: D. Davies et al., 2015, Nature, 525, 511 (pay-walled, not accessible)
Short note on NDVP #1
Dear readers, originally we had planned for today as the date of publication but due to recent developments, we have been asked by a third party to hold publication for another two weeks. Our apologies for this delay!