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| The steep slopes of Mount Elephant, a scorio cone, rising from the plain near Derrinallum. |
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| Photo by Graeme Kinross-Smith, 1999. |
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| Workers in a basalt or bluestone quarry at Footscray, on the edge of the lava flows. |
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| Date and photographer unknown. Image contributed by Melbourne's Living Museum of the West. |
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The volcanoes of Victoria's basalt plain take many different forms, from steep-sided cones to huge, low-lying craters with walls that rise so gently you hardly notice them.
About 400 separate eruption points have been identified, most of which erupted between 4.5 and 2 million years ago. Some are much younger, however. Mount Schanck, just across Victoria's border with South Australia, erupted only 4000 years ago if current dating is correct, while Mount Napier, near Hamilton in Victoria's Western District, last erupted about 7200 years ago. Both these volcanoes are considered dormant rather than extinct. Their eruptions would have been witnessed by the indigenous peoples of the plain to become part of their Dreaming.
Volcanoes are classified according to the matter they eject: lava, scoria, or superheated steam. Many of Victoria's volcanoes ejected more than one kind of matter so do not fit neatly into any single category.
Maar volcanoes
Maar volcanoes form when rising magma meets layers of water-saturated sedimentary rock below the ground. This produces super-heated steam, which explodes with the power of a small nuclear bomb, blasting magma and shattered rock high into the air. Such explosions can create craters several kilometres wide. The craters of maar volcanoes are below ground level, and are surrounded by rings of material known as tuff, which consists of fine volcanic ash mixed with the older bedrock. This ash may be blown several kilometres downwind of a volcanic crater to form a wide ring. Such tuff rings often rise up to 40 metres above general ground level, but from the outside the slope is almost imperceptible, with an angle of only 3 to 4 degrees. In Victoria, these slopes usually form on the eastern side of maar volcanoes because of the prevailing winds.
There are 40 maar craters in Victoria, mostly between Colac and Port Fairy. Many have lakes in their craters, including Tower Hill near Warrnambool, Lake Purrumbete and near Camperdown, and Lake Keilambete and Lake Terang near Terang.
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| Evening light over Lake Bullen Merri, the flooded crater of a maar volcano. |
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| Photo by Graeme Kinross-Smith, 1999. |
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Scoria volcanoes
The 200 steep-sided scoria volcanoes scattered across Victoria's basalt plain were true 'fire mountains' when they erupted. They were formed when magma interacted explosively with groundwater, blasting molten rock high into the air in spectacular and violent displays. The ejected material cooled before it hit the ground, forming fragments of frothy red or black rock called scoria. These fragments quickly settled around the vent, building cones with deep central craters. Many of Victoria's youngest volcanoes are scoria cones. Among them are Mount Elephant near Derrinallum, Mount Noorat near Terang, and Mount Fraser near Beveridge, north of Melbourne.
Scoria cones can grow very rapidly at a rate of up to 100 metres per day, depending on the amount of matter ejected. The 240-metre scoria cone of Mount Elephant, for example, was probably created in a single brief eruption. Most scoria cones form over periods ranging from a few months to a few years.
Scoria volcanoes can go misleadingly quiet after their initial eruption. Often a lake of lava forms in the crater and eventually bursts through the loosely packed rock that forms the side of the cone. There are many such breached cones across central and western Victoria, including Mount Franklin near Daylesford, Mount Eccles near Macarthur and Mount Shadwell near Mortlake.
Several of Victoria's best-known scoria cones have formed inside existing maar volcanoes. Mount Leura and Mount Sugarloaf near Camperdown, were created in this way, as was the Red Rock volcanic complex near Alvie. At Mount Noorat near Terang, the tuff ring of the maar volcano has been almost completely buried by a later scoria eruption.
Lava volcanoes
During the eruptions of lava volcanoes, molten magma, or lava, pours from vents in the earth's crust and flows across the countryside, engulfing everything before it. In Victoria such flows have cooled into dark grey basalt rock, forming the main expanse of Victoria's basalt plain. Flows from these volcanoes also created complexes of lava tunnels and stony rises. (See below.)
Prominent lava volcanoes in Victoria include Mount Bainbridge near Hamilton, Mount Blackwood north-west of Bacchus Marsh, and Mount Cottrell south of Melton. From the earliest stages of European colonisation in Victoria, the basalt from these lava flows was quarried for use in building and road construction.
Stony rises, lava tunnels, and floating islands
The volcanic region of Western Victoria has large tracts of broken country known as 'stony rises'. As one early settler described it, 'The basalt, instead of being spread out, as on the plains, is here reared up as in waves, petrified in their rise'. These formations were created when the surface of a lava flow cooled and solidified while the lava underneath remained fluid. In many places the molten lava flowed away altogether, to leave tunnels underneath the fresh skin of cooled basalt. In some places the brittle surface rocks warped and subsided, leaving the appearance of 'waves.' In other places the roofs of the lava tunnels caved in, leaving deep depressions which formed natural water catchments. Over time beds of peat moss built up in the depressions, and other vegetation took root. The Floating Islands between Colac and Camperdown arise from such a formation. The most accessible of Western Victoria's lava tunnels are east of Byaduk North on the Hamilton-Port Fairy Road.
Quarrying volcanic material: basalt, scoria and tuff
Basalt and other volcanic materials have been quarried on Victoria's basalt plain since the earliest days of European colonisation. In the nineteenth and early twentieth centuries basalt was used either as rectangular cut blocks called 'pitchers', or as the naturally occurring boulders known as 'floaters', to build private houses and public buildings, to line the banks of the Maribyrnong River, to surface many lanes and roads and to fence farming properties. Basalt slabs are still used in the twenty-first century to pave Melbourne's footpaths. Scoria and tuff, which are used as a surface materials in road and path construction, are also quarried from pits on the slopes of many volcanic cones, including Mount Leura, Mount Noorat, Mount Fraser, Mount Shadwell, Mount Elephant, Mount Eccles and Mount Napier.
What are your stories about the volcanic cones, crater lakes, lava flows and general geology of the plain?
Copyright Imagine The Future Inc. and Australian Film Commission, 2002.
Text by Merrill Findlay for ITF.
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