I tend to take a more regional view of volcanic activity and am drawn into trying to figure out how things work, how they come to be, and at some level where they are going next. One of the places in the world with volcanic activity we haven’t covered is Australia.
At first, it seemed that a relatively simple post would adequately review what is on the continent. Problem is that Australia has a long, relatively complex volcanic history. It even has relatively recent onshore volcanic activity, so the sheer volume of things discovered during research expanded the scope of the post significantly. I will break it into a series of several posts not unlike what I did with Antarctica.
Recent volcanic activity (less than 35 Ma years old) in Australia is confined to the eastern part of the continent and a pair of parallel offshore seamount chains to the east. From north to south, these states are Queensland, New South Wales, Victoria and Tasmania.
The entire region is referred to as the Eastern Australian Volcanic Zone. It stretches 4,000 km north and south and is up to 500 km wide, mostly parallel with the eastern Australian coast line. The zone is well separated from plate boundaries and stretches almost to New Guinea. Total volume erupted is around 20,000 km3. Past activity throughout the zone dates from 70 Ma to as recent as 4,300 years ago.
Though the region has concentrated locations of volcanic activity, that activity is sporadic, with some activity continuous throughout a well-defined period of time. Other locations have had 5 – 10 Ma between eruptive episodes.
While hotspot models have successfully demonstrated general progression of activity from north to south, particularly in the Tasmantid and Lord Howe seamount chains to the east of Australia, onshore volcanic activity is more complex. For instance, youngest volcanism has taken place on either end of the continental volcanic zone in north Queensland and in Victoria to the far south.
The most recent volcanic activity on the Australian continent are Mount Gambier and Mount Schank, cinder cone eruptions and maars erupted some 4,300 years ago. These are part of the Newer Volcanic Field in Victoria on the southeastern corner of the continent, a typical intraplate basaltic volcanic field with monogenetic volcanic centers, maars and lava flows.
Lava field activity in Queensland to the north is sporadic, voluminous (nearly 200 km3 for the largest field / province), and relatively recent (as recent as 13,000 years old).
Older volcanic activity is generally concentrated along roughly parallel strings of massive central volcanoes and lava fields on the eastern part of the continent.
There are three types of volcanic structures: central volcanoes (generally silica-rich trachytes and rhyolites built on basalt bases), lava fields (effusive basalts), and four leucite suites (low silica, high potassium basalts).
Typical active lifetime of a volcanic center is 3 Ma, similar to that of a typical Hawaiian Islands volcano.
One note on nomenclature. Different nations use the language differently even when (nominally) speaking the same language (English in this case). One example is the use of the term “pyroclastic cone” in technical papers by Australians that typically refer to what are called scoria or cinder cones in other parts of the world.
The best source for researching past volcanic activity in Australia is a book entitled “Intraplate Volcanism: In Eastern Australia and New Zealand”, published in 1989 compiled by RW Johnson. Some excerpts from the 408 pp book are available in Google Books preview. The used book itself is available from Amazon for a mere $842 US for hardcover or $41 US paperback. Well worth your time to read. No complete online pdf of this exists that I was able to find.
There was a round of increasingly breathless articles perhaps a decade ago suggesting that Australia was overdue for a volcanic eruption. The articles quoted Professor Bernie Joyce pointing out that the chances of a future eruption were more a matter of when rather than if. The media instantly turned that observation into a series of relatively hair raising articles on the possibility of volcanic eruptions in the Atherton Volcanic Field in northern Queensland and the Newer Volcanic Fields in Victoria. Like persistent scaremongering about the next great eruption from Yellowstone, this sort of statement contains an element of truth if for no other reason than recently active volcanic fields have not been inactive long enough to demonstrate conclusively there will be no future volcanic activity. https://www.brisbanetimes.com.au/national/queensland/volcano-eruption-overdue-20090921-fwxm.html
Location of volcanic activity in Australia at its most basic is related to northward movement of the Australian Plate over time. The Australian Plate is moving north 75 km / Ma.
Volcanics are mostly located in roughly parallel strings with oldest volcanoes to the north and the youngest to the south. Each string preserves 10 – 12 Ma of volcanic activity over its length. This layout of activity suggests that volcanic activity in Australia is related to the passage of the Australian Plate over a hot spot with the magma finding its way to the surface through weak spots in the continental craton. Given the large area and widespread coverage of multiple lines of volcanoes, I would suggest that perhaps we are talking about a mantle plume head rather than a single discreet hot spot.
The Australian Plate collided with the Ontong – Java Plateau some 26 – 23 Ma. That collision introduced an eastward offset in the tracks of seamount chains. It also coincided with a brief period of reduced northward plate motion as the plate was briefly deflected westward. The Plate resumed its northward motion roughly the same time that subduction began along the Trobriand Trough. This collision provides strong evidence that impact into large oceanic plateaus can contribute to rapid changes in plate boundaries and plate motions on a global scale.
A recently identified chain of volcanoes slicing from north to south across eastern Australia is described as the longest chain of volcanoes on any continent. This is the 2,000 km long newly named Cosgrove volcanic track. It dates 33 – 9 Ma and erupts leucite, a basalt high in potassium, thorium and uranium. The scientists believe the chain is connected to the actions of a single hot spot under Australia during its move north. The problem with the hot spot theory is its large gaps between individual volcanoes that erupted chemically similar lavas. Theory behind this is that there is a direct relationship between the continental thickness and the ability of hot spot magmas to break through to the surface. In this case, that thickness is 130 km. Less than 110 km, and we have standard intraplate basaltic volcanic activity. Gaps exist when the lithosphere is thicker than 150 km. Low volume leucites erupt in intermediate thicknesses – 110 – 150 km. This finding may help volcanologists model how mantle plumes and hot spots interact with continental crusts.
A 1998 paper by Sutherland proposed a plume model for recent volcanic activity in Australia and a timeline for future activity. Queensland to the north and the Newer Volcanic field in Victoria to the south are both recently active volcanic fields on the continent. The Newer field in Victoria is leaving the vicinity of its underlying thermal plume system / low velocity zone. This means that volcanic activity will be decreasing over time. Queensland is now moving toward an underlying thermal plume system. This means that the already significant volcanic activity will not slow down or stop. The paper suggests that the sporadic activity may continue for the next 5 – 10 Ma.
Australia is home to at least four self-contained and part of a fifth Large Igneous Provinces (LIPs) active over the last billion years. One is associated with an extinction event. These include the Warakuma LIP, 1,070 Ma in west central Australia; the Gairdner LIP, 825 Ma, 105,000 km3 in south central Australia; Kalkarindji LIP, 510 Ma, 150,000 km3 in NW Australia (extinction event); the Whitsunday LIP, 132 – 95 Ma, 2,200,000 km3, off the east coast of Australia; and just a bit of the Ferrar LIP stretching from Oz to South Africa during the breakup of Gondwana, 180 Ma, 15,000 km3 of sills in Tasmania.
Dr. Franco Pirajno of the Geological Survey of Western Australia published the August 2007 LIP of the Month entitled “Alkaline intraplate volcanism in eastern Australia” that describes Australian intraplate volcanism in terms of a LIP. Although the widespread, sporadic and long lived activity does not fit the formal definition of a LIP, this perspective is useful to describe what is going on. http://www.mantleplumes.org/WebDocuments/Davies2014.pdf
The short-lived volcanics (3 Ma) throughout the Eastern Australia Volcanic Zone has been described as fingers of magma (finger plumes) that rise from the main body of the underlying low velocity zone, interact, mix, and / or crystallize to some extent with the surrounding mantle and crust before erupting through zones of tensional stress.
Eastern Australia Volcanic Zone (EAVZ)
This overall region is divided into three types of volcanic provinces: Lava field; central volcano; and leucitite. The central volcano provinces are predominantly basaltic and include felsic lava flows and intrusions. They are centered on a central vent or cluster of vents. The leucitite provinces contain mafic, high-K minor intrusives. They are chemically and spatially distinct, being on the western part of the EAVZ. The lava fields are exclusively basaltic with abundant vents that are well dispersed with widespread lava flows.
The central and leucitite provinces form a hotspot trail with a well-defined age-latitude relationship. Ages generally decrease to the south, with the youngest felsic volcanism being the Macdon – Trentham Province. Oldest of these are 33.2 Ma decreasing to 7 – 6 Ma as activity moved south over time (as the plate moved north) over the underlying mantle plume / low velocity zone.
There have been some claims of a step-wise age-latitude plot. If an accurate description, magma from the sub-lithospheric hotspot source migrated into the lithosphere and is then carried along with the plate until the connection with the source is broken and the next province is charged with new magma.
In contrast, the lava field provinces show little correlation between age and latitude (northern provinces being older than southern provinces). Central volcanoes in New South Wales date 33 – 0 Ma. Lava field volcanic activity in Queensland and Victoria seem to be concentrated in two periods – 70 – 14 Ma and 5 Ma to recent.
Queensland volcanic centers are a combination of very young and relatively older central volcano dominated fields. These are listed roughly from north to south. Those that are parallel are listed from west to east.
Recent lava fields (provinces) active within the last 8 Ma include: Piebald, Cooktown, Atherton (most recently active in Queensland) , McBride, Walaroo, Mt Fox, Chudleigh, Broken River, Mingela, Nulla, and Sturgeon.
Mainly central volcano dominated fields are to the south of the previously listed lava field provinces were active 32 – 23 Ma. These include Mount Dukes, 32 Ma, Hillsborough, 32 Ma, Nebo, 28 Ma, Peak Range, 30 Ma, Hoy, Springsure, 27 Ma, Buckland, Bauhinia, Monto, Giri Gin, Mitchell, Main Range 26 – 23 Ma, Glasshouse, 24 Ma.
New South Wales Volcanics
New South Wales Volcanics are a mixture of central volcanoes and lava fields. They are generally younger than those in central and southern Queensland. These fields include Doughboy, Tweed, 21 Ma, Nandewar, 18 Ma, Ebor, 19 Ma, Wamrbungle, 17 – 13Ma, Liverpool Range, Comboyne, 16 Ma, Barrington, Dubbo, Canobolas, 12Ma, Kandos, Abercrombe, Southern Highlands, Grabben Gullen, Snowy Mountains, and Monaco.
Victoria is dominated by the relatively recent Newer Volcanic field. There are older volcanic fields to the east. These include South Coast, Eastern Victoria, Monaro, Mt Macedon – Trentham, 6 Ma. The monogenetic Newer Volcanic Field stretches from the central part of the state to the western border. It also stretches down to the southern coast.
Most of the volcanic activity in Tasmania is quite ancient. It had a massive intrusion of dolerite 183 Ma that covered a third of the island with 15,000 km3 of magma, mostly in the form of sills. It is the largest exposure of dolerite in the world (30,000 km2). The intrusion was part of the Karoo – Ferrar LIP, which also covered substantial portions of Antarctica, Argentina, and South Africa. Karoo erupted 3 – 4 million km3 of magmas as the supercontinent Gondwana split apart.
Volcanic vents were active 58 – 5 Ma. These released basalt flows large enough to fill surrounding valleys, forming a 750 m thick basalt plain. The eruptions also left cubic basalt columns and volcanic plugs.
The eastern side of Tasmania is a low earthquake zone. There is an area just off the NE coast that is thought to be a reactivating hot spot and potential future volcanic activity. This is seismically active.
For an ancient and remote continent, Australia has a remarkable history. Unlike most plates, there is almost no subduction driven volcanism in the last 50 Ma. Like neighboring Antarctica, it appears that volcanic activity is driven by the ability of underlying mantle plumes / hot spots or low velocity zones to move magma into areas of weakness in the overlying plate. Unlike relatively static Antarctica, the movement of the plate is so fast to the north that volcanic activity is sporadic, not unlike what we see with the Hawaiian Islands – a few million years of activity before the activity migrates to the next location. The central volcanoes are generally old and eroded. Lava fields are large and do not seem to be age constrained. The two newest fields are a lava field in Queensland and a monogenetic field in Victoria.