The volcanic features in the Camperdown district were formed within the last 20,000 years making them some of the youngest of their type in Victoria.
While 20,000 years may seem a long time in our terms, it is a relatively short period in geological history. The basement rocks of the district are mostly marine sediments laid down in the later part of the Miocene Period roughly 15 to 20 million years ago.
Camperdown was established at the foot of Mount Leura and Mount Sugarloaf which are actually part of a much larger volcanic complex known as the Leura Maar. A maar is an uncommon landform comprising a broad, roughly circular, flat floored volcanic crater with steep inner walls and a low surrounding rim built of fragments of rock material blown out of the crater during eruptions. The word, Maar, is derived from the German language and is the name given to lakes in volcanic craters of this type in the Eifel district of Germany. In Australia, most maars occur in the southern part of Victoria's Western District and there are more than 30 in the area between Colac and Warrnambool. Lake Purrumbete and Tower Hill are nearby examples of other maars.
Mt Leura and Mt Sugarloaf near Camperdown
The Leura Maar is a shallow oval shaped depression about 2.5 kilometres long, 1.7 kilometres wide and up to 50 metres deep which originated from a series of major volcanic explosions, possibly 22,000 years ago. Some of the eruptive material was thrown high into the air, but most surged rapidly across the ground surface as a dense cloud of steam, gas and rock fragments. The Leura Maar may have been formed in just a few months.
Inside the Leura Maar, there are many younger eruption points marked by craters and mounds of scoria. These are said to be nested inside the maar and Leura therefore is a nested marr These younger volcanic features include Mount Leura, Mount Sugarloaf, the deep crater which separates the two mountains, and several smaller craters and mounds in the southern part of the maar.
Although not as explosive as the activity which produce the maar the growth of Mount Leura and Mount Sugarloaf would have provided a grand spectacle, similar to the present day activity of volcanoes on the island of Hawaii.
They began as vents on the floor of the maar from which glowing fountains or explosive bursts of lava were ejected Gases escaping from the lava caused it to break into angular, gravel sized fragments called scoria which fell back to the ground and heaped up around the volcanic vent forming the high, steep cones. The scoria including large blocks and smooth surfaced lava bombs that were hurled into the air and partly cooled before crashing back to the ground to be buried in the scoria.
Mount Sugarloaf is basically a steep conical accumulation of scoria rising as a high point on the same crater rim as Mount Leura. It formed as a result of perhaps sever, months of lava fountaining from the same point in the crater continuously building an ever increasing pile of scoria. Scoria volcanoes can achieve height quickly and Mount Leura could have been built in less that 20 years.
The major volcanic features include the single scoria cones of Mount Noorat Mount Meningoort, Mount Elephant (the largest scoria cone in Victoria), and Mount Myrtoon, and the multiple eruption complexes forming the Wiridgil Hills and Red Rock. These are all older than Mount Leura, but are still regarded as geologically you features being much less than one million years old.
The numerous lakes in the Camperdown district, like the mountains, are related to volcanic activity. Some, such as Lake Purrumbete, Lake Bullen Merri and Lake Gnotuk lie in volcanic craters or maars and are fed largely by underground water. The larger lakes including Lake Colongulac and Lake Corangamite (Victoria's largest lake) occupy depressions which are partly a result of overlapping lava flows that have blocked or diverted streams.
These depressions are now internal drainage basins with little river inflow or no outlet. Evaporation has concentrated salts in the lakes and some are now much saltier than sea water. The size and water quality of these lakes varies in response to long term climatic changes, and there is clear evidence that Lake Corangamite was once much more extensive.
Lake Bullen Merri and Lake Gnotuk are parts of an uncommon volcanic landform known as a maar. A maar is a broad, roughly circular, flat floored volcanic crater with steep inner walls and a low surrounding rim built of fragments of rock materials blown out of the crater during eruptions. The word, Maar, is derived from the German language and is the name given to lakes in volcanic craters of this type in the Eifel district of Germany. In Australia, most maars occur in the southern part of Victoria's Western District and there are more than 30 in the area between Colac and Warrnambool. Lake Purrumbete and Tower Hill are nearby examples of other maars.
The simple, circular shape of Lake Gnotuk shows it to be a single, small maar, while the clover leaf outline of Lake Bullen Merri suggests it is made of two maars which have joined together. After the eruptions ceased, the craters became choked with rock debris and the takes developed. Some of the water is direct run oft from the crater slopes, but most is a result of underground water from the water table leaking into the craters. Without an outlet, evaporation causes the lakes to become saline. Lake Bullen Merri is brackish, but Lake Gnotuk is twice as salty as sea water.
Research into fossils and sediments from the floor of the lakes shows the salinity to have changed considerably over the last 10,000 years. The bottom of both lakes is about the same level (80 metres above sea level), but Bullen Merri has a steep cone shaped floor and is more than 60 metres deep while Lake Gnotuk has a flat floor and a depth of less than 20 metres.
Although Lake Bullen Merri overflowed into Lake Gnotuk in the mid 1800s, the level of both lakes has fallen considerably in the last 100 years due possibly to climatic changes. Former high water level shoreline cliffs can be clearly seen as parallel benches around the north western rim of Bullen Merri.
Lake Purrumbete is a freshwater lake east of Camperdown. The 45 metre deep lake is located in an almost circular maar 2.5 kilometres in diameter.
Volcanic sediments on the edge of lake
deformed sediments resulting from impact of material ejected from volcano
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