Welcome to the third in our series on volcanic activity in Australia. This post will clean up the remaining volcanic fields and provinces in southern Queensland and then work our way south through New South Wales (NSW). Unlike northern Queensland, activity in this region is not recent. Much of it positively ancient, though with the caveat that activity is generally younger the farther south you go in NSW. The leucitite volcanics are in the western portions of NSW. Finally, south eastern Queensland hosts the massive remains of the Tweed volcano (think of an eroded Hawaiian island volcano that does not suffer flank collapse landslides into the surrounding ocean).
Volcanic activity in this portion of Australia is generally tens of millions of years old. Lava field provinces are the most widespread. There are the remains of massive shield volcanoes. Leucitite basalts are found to the west and are not especially widespread. The youngest lavas in this region are at Bundaberg, Gin Gin and Childers dating as recent as 381,000 years old.
New South Wales
New South Wales was originally founded as a penal colony. The state of Queensland lies to its north, Victoria to the south, and South Australia to the west. The Tasman Sea (and a long portion of the Great Barrier Reef) lies to its east.
Original size of the state was over half of the Australian mainland. Its original extent contained a number of island territories including New Zealand. Most of the territories were detached during the 19th Century to form separate British colonies and other states and territories of Australia.
The national capital of Sidney is in a national enclave (not unlike Washington DC in the US). 5 million of the 7.8 million inhabitants in the state live in Sidney.
Most of the population is concentrated along the temperate and wet coastal plain. Temperatures along the coastal plain range from cool temperate in the south to subtropical in the north. Things get dry and hot west of the Great Dividing Range, where ranching and agriculture dominate. There are numerous forests in NSW and mining mainly west of the range.
Agriculture is spread generally in the eastern two thirds of the state. Tourism is a growing economic activity. Finally there over 780 national parks and reserves in the state.
Southern Queensland Volcanics
Bundaberg and Boyne
There are several smaller volcanic fields along the Burnett River in southern Queensland. The river flows roughly NE for a couple hundred kilometers. These fields include the Bundaberg field on the coast, Gin Gin, 5.1 – 2.7 Ma some 50 km up river to the SW, Stony Range perhaps 30 km NW of Gin Gin, Childers some 30 km to the SE of Gin Gin, Monto another 60 km up river to the SW, and Boyne another 75 km up river to the SW.
Lavas in the Bundaberg, Gin Gin and Childers fields were erupted in six short periods of volcanic activity starting 60 Ma. Composition of the erupted lavas changed over time. Eruption locations seem to be controlled by existing fault lines in the region. Volcanic activity began 60 Ma and continued sporadically until fairly recently. Most recent dated lavas are 381,000 years old. Basalts, pyroclastics and trachytes are found among the several fields.
Glass Houses (Glasshouse Mountains)
Glass Houses are a group of dome-shaped hills rising from the coastal plains north of Brisbane. Mount Beerwah is the highest of ten volcanic plugs in the Glass Houses. It was formed 26 Ma. The original mountain is thought to be up to three times higher than the higher of two peaks of Beerwah’s 556 m. Remaining plugs, domes, sills and dikes are generally trachytes. Earliest lavas appear to be basalts. And there are rhyolites in the region, though no recognized pyroclastics. Erupted volume is around 25 km3.
Bunya and Main Range
Main range includes Bunya Mountains in a belt close and parallel to the Great Divide. Volcanic rocks cover 4,900 km2 with an estimated 1,000 km3 magmas erupted. There are two volcanic formations in the southern Main Range, the Governors Chair Volcanics and the Superbus Basalt. Maximum total thickness near 900 m of lavas with some pyroclastics. Lower formations date 26 – 24 Ma, emplaced over 5 Ma. Most was erupted 24 – 22 Ma, with the younger eruptions being sporadic. Total volume of the Main Range is 840 km3.
The Governors Chair is a basaltic shield volcano with 50 km diameter elliptical base. The volcano contains rhyolitic and trachytic domes. Erosion has uncovered numerous plugs, dikes, and domes of the plumbing system. The Superbus basalts overlie the remains of the Governors Chair and cover 1,600 km2. Younger volcanic rocks tend to be more alkaline. Youngest volcanic rocks date 21 – 18 Ma.
The Bunya Mountains are the eroded remains of a basalt shield volcano, maximum diameter of 60 km. Maximum total thickness of the lava pile exceeded 600 m. Its basalts are not evolved.
There are basalt lava flows dating 55 Ma and 47 Ma. The older formation is up to 150 m thick. Basalts are interlayered with sediments. Drilling has identified a sequence of basalts up to 300 m thick.
Focal Peak is a recently identified eroded shield volcano 60 km WNW of the Tweed / Mount Warning volcano. Its presence was inferred from the presence of eruptive rocks in the Lamington Group, the Albert Basalt and the Mount Gillies Volcanics. The volcano has been named Focal Peak and dates around 23 Ma.
The eastern flank of the volcano is marked by the Albert Basalt, thickest basalt at 440 m thick. It thins to the east and underlies early basalts of the Tweed volcano. There are rhyolitic lavas in the Hillview and Mount Lindesay areas. These include pyroclastics. There is a suggested caldera centered on Focal Peak similar in shape to Hawaiian calderas based on slope of underlying rocks centered on a 6.5 km diameter central plug. There is a wide range of rhyolite types.
The Tweed volcano is a 100 km diameter shield volcano, heavily eroded, centered on Mount Warning, the remaining volcanic plug. The youngest lavas date at 20 Ma. There is a 30 km diameter heavily eroded caldera that separates the central core of the volcano from its surrounding flanks. Present coverage of Tweed lavas is 4,000 km2. Original coverage may have been as extensive as 7,000 km2. Maximum thickness of those lavas is 900 m. They flowed all the way east into the Pacific Ocean. The original height of the shield is estimated at 1,900 m. Original volume of erupted lavas is estimated at 3,200 km3. Total volume of Tweed was in the vicinity of 4,000 km3.
The volcano grew in three phases during its active lifetime 24 – 21 Ma. First phase was mafic basaltic lavas. Middle phase was rhyolitic unit. All of this was overlain with a final mafic lava sequence. There are also rhyolitic dikes identified late in its active phase.
Surrounding volcanics are referred to as Lamington Volcanics (or Lamington Group). They are considered to be the first lavas erupted from Tweed.
Tweed is one of the largest calderas on the planet. Unlike most other calderas, this one was created neither by subsidence nor cataclysmic eruption. Rather, this caldera was formed by erosion over the last 20 Ma. Today, it is 40 km in diameter. What remains of the volcano is the central plug, Mount Warning, and a series of ring dikes around the circumference.
As initial eruptive products were basalts, followed by pyroclastics, erosion of the shield went pretty fast. The pryoclastics are topped with more basalts (trachyites?), which protected them from erosion. The rim of the caldera to the west is a semi-circle of vertical cliffs, 1,100 m high. They’re known by the aboriginal name – Wollumbin. There are also secondary vents and dikes exposed by erosion.
The central plug, Mount Warning, is made of three lava types: gabbro at its base, syenite for most of the mountain, and trachyandesite near the top.
New South Wales
The Belmore Volcanic Province is an eroded volcanic structure active 21 – 19.5 Ma. It covers 140 km2 with an original volume around 14 km3. It is unusual among eastern Australian volcanic provinces in that it lacks prominent basalts (although basalts are present), is very small. Its lavas are more chemically similar with volcanic activity in Victoria than along the eastern volcanic zone of Australia. Sutherland, et al suggest it may represent residual magmas from the neighboring Tweed volcano. Original lavas were trachytes. There are minor rhyolites. It appears the magmas were evolved before eruptions began and did not produce the underlying basalt shield seen in neighboring Tweed and Ebor. It is an unusual field in that trachytes are the dominant erupted lavas.
Nandewar is another shield volcano that was built mainly 18 – 17 Ma. Earliest lavas were basalts that evolved to pyroclastic rocks and tuffs. Final phases of the eruption produced trachyte dikes, plugs and ring dikes. These date around 17 Ma. Later lavas are typically on the southern part of the volcano. There appear to be two distinct phases of rhyolite eruptions separated by perhaps a million years. Total eruptive volume is 600 km3.
Central and Doughboy
This is another belt of volcanic rock measuring 240 by 70 km. It is west of Ebor. The two provinces are geologically similar and geographically close. Lava flows are typically thin and flat, forming tablelands following erosion over tens of millions of years. These basalt flows are up to 150 m thick. Central appears to have erupted in two phases, 34 – 32 Ma and 22 – 19 Ma. Doughboy basalts date 45 – 42Ma.
Ebor is the remains of another eroded central volcano. There is little field and lab data available on the province. Extensive erosion has removed much of the original shield. Lavas closely resemble the tholeiitic andesites of the nearby Tweed vlcano. There are also basalts and rhyolites present. Total erupted volume was 270 km3. It was built 19 – 18 Ma.
Warrumbungle volcano is a well eroded 50 km diameter shield. The central part of the volcano has multiple plugs, dikes, and domes. It appears that the volcano was constructed much like other central volcanoes in NSW, with a basalt foundation followed by pyroclastics as the magma source evolved. Final eruptive products appear to be trachytes which form the plugs, dikes and sills. Total volume of the volcano is 400 km3. It was active 17.1 – 13.6 Ma.
Over 90% of the original shield volcano has been eroded, removing most of the softer eruptive products. Crater Bluff is a trachyte volcanic plug. Bluff Mountain is a trachyte lava dome. The Breadknife is a 90 m high, 4 m thick trachyte dike. It was extruded through surrounding pyroclastics. There are four types of trachytes in the volcano. There is a broad belt of basalt outcrops to the SE of the remains of the shield.
Lakes formed as the volcano eroded supported a thriving diatom population. These died and deposited their skeletons on the lake floors, forming thick deposits of diatomaceous earth. The Chalk Mountain Diatomite Mine mines these deposits.
Indigenous peoples occupied the Warrumbungles for at least the last 5,000 years. Its name in their language is thought to mean “crooked mountains.” There is a national park centered on the eroded shield.
Liverpool Range is the largest volcanic province in NSW. Current basalt coverage is 6,000 km3, with an estimated erupted volume of 4,000 – 6,000 km3. Basalts were erupted in two phases 40 – 38 Ma and 35 – 32 Ma. Two distinct magma sources may have existed for the two groups of lavas. There are three types of eruptive events: massive basaltic lava flows, columnar layers that appear to be slowly cooled lava flows, and massive units of highly altered basalts.
Walcha is a lava pile up to 600 m thick near the crest of the Great Divide. Original coverage in the 100 – 200 km2 range. Total volume remaining today around 20 km3. Activity dates are similar to lavas from Barrington.
Remains of either a basaltic lava field or shield volcano active 61 – 53 Ma. Heavily eroded, with perhaps a sixth of the original 700 km3 erupted lavas remaining. Location coincides with a basement uplift and subsurface magnetic anomalies.
Estimated original diameter of the shield volcano is 20 km active some 16 Ma. Total volume estimated at 40 km3. Original volcano erupted basalts. Remaining plugs, domes, dikes and sills are trachytes. There are also rhyolites present. Lavas have large coverage but limited thickness. There may have been multiple vents involved.
Lansdowne volcanics are associated with the Comboyne shield volcano. This is another heavily eroded shield volcano with residual volcanic structures of plugs, dikes, domes and sills. There are national parks and four nature reserves located around Comboyne rich in threatened species of plants and animals.
There are three prominent trachyte plugs called Big Nellie, Little Nellie and Flat Nellie, or collectively The Nellies. The Comboyne Plateau and the neighboring Bulga Plateau are outlying residuals of what is called the Great Escarpment, an erosional feature that has migrated to the west in NSW over the last 16 Ma.
Dubbo dates 14 – 12 Ma. It is highly eroded and erupted basalts and trachytes. Evidence of at least two ancient active episodes before the most recent basalts were erupted. This lava field province is relatively small at only 8 km3.
A well preserved shield volcano active 13 – 11 Ma. Estimated volume is 50 km3. Trachyte is the most abundant rock in the core and basalts dominant on the flanks. Flanks are best preserved to the NW and SE of the volcano. There are pyroclastic beds and extensive flows of lava to the SE extending over 20 km. The central zone of the volcano is complex with an intricate pattern of domes, dikes and plugs. There are rhyolites present and abundant pyroclastics in the central zone.
The Sydney province included widely spread volcanic rocks and intrusions in the Sydney Basin. This includes diatremes. Basin area is 20,000 km2. Rocks are geochemically similar basalts erupted sporadically 57 – 20 Ma.
Southern Highlands, Grabben Gullen, Abercrombie, and Kandos
Kandos is located in the Sydney Basin. Southern Highlands is centered on the western margin of the basin. Grabben Gulen and Abercrombie are to the west of the basin. All four provinces are associated with a broad area of uplift related to the Great Divide and are basaltic lava field provinces.
Southern Highlands and Kandos were both active in two periods. Although activity at Southern Highlands lasted from 54 – 30 Ma, the two most active periods were centered on 45 and 35 Ma. Activity at Kandos started very early 190 – 170 Ma. Its most recent basalts were produced 50 – 45 Ma.
Grabben Gullen and Abercrombe are the most recently active provinces, with basalts as recent as 14 Ma. Abercrombe erupted some 200 km3 throughout its lifetime.
Erosion in the region has left a variety of necks, dikes, sills, flat-topped mesas, diatremes and domes. These provinces are relatively small in coverage or erupted volume.
Monaro, Snowy Mountains and South Coast
The Monaro Volcanic Province covers 4,200 km2 and originally contained some 630 km3 of lavas and minor pyroclastics. The main part of the province dates 57 – 34 Ma. There have been over 65 volcanic plugs identified in the field. The area has multiple intersecting fault lines which appear to have provided weak areas in the underlying bedrock allowing the magma to reach the surface. All known plugs lie on one or more known faults or lineaments. Lava dikes and plugs are found at all levels of the lava pile. There have been possible lava lakes identified as have maar remains. Volcanism was lava field type eruptions rather than central volcano. It was sporadic in three distinct pulses with long pauses in between. Total erupted volume was 480 km3.
There was interaction between erupted basalts and lakes creating hyaloclastites and pillow lavas in places. Basalts originally flowed down and blocked river and streambeds in the region. As they did so, the volcanic pile progressively blocked the drainage to the south and east creating substantial lakes along former tributaries to the south and west. Volcanic pile long axis is generally WNW – ESE. The surrounding streams re-cut their former valleys.
Lava flows were at least 240 m thick, perhaps up to 400 m in places. They were erupted onto surrounding countryside with relief of 300 m. Individual lava flows are up to 30 m.
Snowy Mountains is a basalt lava field province active 23 – 18 Ma and 24 km3.
South Coast is a very small basalt lava field active 32 – 27 Ma. It erupted perhaps 2 km3 of lavas.
As I put together this series of posts, one of the puzzling things was the grouping of leucitites, leucite-bearing basalts as a separate category of eruptive product. Although this type of basalt is scarce, it is found worldwide. It is low in silica and weathers very easily. It gets it name from the light color of mineral crystals contained in it. Chemistry of these basalts are very similar throughout Australia. The string of leucitites is what was identified as the longest string of continental volcanoes a few years ago in Australia.
Most of the western leucitites are located within a 1,300 km2 region in Western NSW. Total erupted volume is less than 3 km3. They can be found at the following locations:
Byrock, 17 Ma, is a single isolated central volcano. Leucitite. Leucitite basalts cover 8 km2. There are additional valley-filling basalts NE at El Capitan and to the west of Byrock. This is part of the eastern Australian leucitite suite, stretching from Byrock in the north to Cosgrove, Victoria in the south. Dates of this portion 16.8 – 6 Ma. There are three main outcrops. Bye Hill, lava mound 5 km2 rising 35 m above surrounding plains. Also have two smaller disconnected outcrops in the N and NE. Main outcrop of leucitite has been cut by a quarry.
Lake Cargelligo erupted 14 – 10 Ma. It is a single isolated central volcano located SE of Lake Cargelligo in central NSW. Its lavas are also found near Bergargo Hills, Flagstaff Hills, Bygalorie and Condoblin.
Condoblin erupted 14 Ma. It is a single isolated central volcano.
Finally, El Capitan is a 40 m high ridge capped with basalt NE of Cobar, NSW. It dates in the same range as Byrock.
As discussed in the prior two posts, volcanic activity in eastern Australia is driven by the passage of the continent over low velocity zones in the underlying mantle.
There are two other local tectonic features of interest. One is the observation that most volcanic activity in this part of the continent is tied to magma intrusion through weak spots created by fault lines. Not all of these are visible at the surface. The presence of some subsurface fault lines has been inferred by location of volcanic activity. There is also uplift in places along the Great Divide. Although volcanic activity is associated with uplift in places, it is not exclusively limited to areas of uplift. Rather uplift is simply another factor that seems to determine the location of volcanic activity along with the presence of local fault lines.
The other tectonic feature of note is the Great Dividing Range or the Great Divide. This is a range of highly eroded mountains formed some 300 Ma when Australia collided with parts of South America during the formation of Gondwana. Think of the Appalachian Mountains in the US as an analog.
It is not a single mountain range. Rather, it consists of multiple mountain ranges, uplifted regions, plateaus, and escarpments. Erosion is gradually working its way west into the Great Divide. Typical height of the highlands is 300 – 1,600 m above sea level. The name Great Dividing Range comes from the division of the watershed along its ridge. On one side, water flows tend to move generally westward, on the other, waterflow tends to go eastward toward the Pacific Ocean. Think of the Continental Divide along the Rocky Mountains in the western US as an analog.
Final tectonic note is that the continental shelf along most of NSW is among the smallest along any continental boundary.
Like we saw in northern and central Queensland, volcanic activity in southern Queensland and NSW is generally divided between central volcanoes and lava fields. Most of the central volcanoes appear to start with a large outpouring of basalts that evolve to rhyolites / pyroclastics over time. End sequence caps the structure with more basalts and more erosion resistant trachyites in the residual plugs, domes, dikes and sills. This sequence and its subsequent erosion has left some spectacular landforms.
The lava fields show a different evolution, with widespread basalt lava flows initially filling eroded valleys before spreading far and wide over surrounding relatively flat terrain. Like the central volcanoes, remaining plugs, dikes, domes and sills created at the end of the eruptive sequence have trachytes present, though there are not significant amounts of rhyolites or pyroclastics. Surprising, at least to me is the presence of a number of diatremes in the Sydney basin.
The erosion of the Great Dividing Range and these fields have created widespread tablelands, lava-topped mesas as erosion works its way westward.
The final post in this series will take a look at volcanic activity in Victoria and Tasmania. As always, not an expert. Simply an interested hobbyist.