Let’s step back in time to the Cambrian period, around 560 million years ago, when Australia was part of Gondwanaland, the massive ancient super-continent made up of what are today Australia, South America, Africa, India and Antarctica. At this time the continent was on the other side of the Earth, above the equator and drifting slowly southwards. The region that we call Victoria today was a deep ocean with the continental shelf lying far to the west, around where the Flinders Ranges are now. The oldest part of Australia is the West Australian plateau or “Shield”. There is amazing antiquity of rocks in some regions there. Some have been dated at 4,200 million years old, which is nearly as old as the planet itself!Returning to Victoria, The deep ocean was punctuated by some volcanic islands – much like the Pacific Ocean today. These volcanoes form the oldest known rocks in Victoria. They are dense, greenish rocks called “Greenstone”, much prized by the Aborigines as the raw material for axes, which were traded over vast distances. One notable outcrop is Mt William near Lancefield.
For the next 150 million years or so, the deep ocean gradually filled up with sediments from the erosion of the shield. The next phase of volcanism was in the Silurian period, around 430 million years ago. For the next 150 million years there was intense volcanic activity, with some truly huge and explosive events. Around Melbourne today, the ring of hills you can see are some remnants of this activity. Mt Macedon, the You Yangs, Mt Dandenong and the Healseville area are all big lumps of granite. Granite is lava that didn’t made it to the surface (Basalt is the stuff that did). So all these hills were once underground and the hard rock has been revealed by erosion of the surrounding softer sediments. A lot of erosion, you say, but even at a rate of a millimetre a year, over a million years, that’s a kilometre.
The landscape that we are familiar with today is from the most recent phase of activity that began with the break-up of Gondwanaland -the separations of Australia and Antarctica, and Australia and New Zealand, from some 80 million years ago, and continuing to the present day. There have been many lava flows across Victoria in this time, forming the Victorian Volcanic Plain, which extends from the Yarra River to across the South Australian border. Most of the stuff that we live on today is around 5 million years old, though the youngest volcanoes are only around 7,000 years old and are considered dormant rather than extinct. We may well have another phase of volcanism at any moment – wouldn’t that make life exciting. Aborigines certainly lived with volcanoes. An axe was found buried in a layer of volcanic ash at Tower Hill near Waarnambool.
There are many volcanic features around: Scoria cones like Mt Elephant, Cones with lava cap like Mt Kororoit, Lava Shields like Mt Cotterall, Maar lakes – huge craters that have exploded leaving a hole that has filled up with water – like Lake Corangamite, and lava tunnels near Hamilton,to name a few.
Naturally, volcanism is not the only force that has shaped our landscape. Apart from the sedimentary rocks laid down by erosion pretty well continuously, there was a massive ice sheet of continental proportions lying on the region in the Permian period some 380 million years ago (one of several occasions when there has been a “snowball” Earth). You can see evidence of this at Werribee Gorge near Bacchus Marsh. There have also been smaller glacial events in the area associated with the last Ice Age, that ended some 8,000 years ago.