Mount Gambier's Blue Lake in southeast South Australia an enigmatic legacy of volcano from 28,000-6,000 years ago
Mount Gambier's Blue Lake in South Australia southeast had an almost overnight change in colour to a deep turquoise in early November.
The Blue Lake was one of four shallow volcanic crater lakes near Mount Gambier (the city named after the extinct volcano) on south-east South Australia’s Limestone Coast. Only two of the four lakes remained; Leg of Mutton and Brown dried up over 30 to 40 years as the water table dropped.
Dates for the volcano’s eruption varied from 28,000 years to 6,000 years ago – making it the most recent on the Australian mainland. Blue Lake’s average depth was 72 metres but a natural cave section could take its deepest point to 204 metres. The bottom of the lake, that supplied drinking water to the city of Mount Gamber, was 17 metres below the level of the city’s main street.
In 1985, cave diver Peter Horne made temperature and visibility studies and found freshwater sponge species and other invertebrates, along with stromatolite formations. South Australia government-owned utility SA Water approved another dive in 2008 that took core samples from the calcite silt on the lake bed.
Early each November, the lake's sombre blue during winter changed to an intense deep turquoise blue almost overnight. This colouring remains until late February, when it gradually changed. From late March, it returned to a distinct sombre blue. Cause of this phenomenon was still up for conjecture but likely it involved the warming of the surface layers of the lake during the summer to around 20 °Centigrade, causing calcium carbonate to precipitate out of the solution and enabling microcrystallites of calcium carbonate to form. This scattered the blue wavelengths of sunlight.
An obelisk beside the lake marked poet Adam Lindsay Gordon’s daring feat in 1865 when he made his famed leap on horseback over an old post and rail guard fence onto a narrow ledge overlooking the Blue Lake and jumped back again onto the roadway.
The 3.6 kilometre road and walking track around the Blue Lake gave access to many viewing points, the most popular being the underpass between the Blue Lake and the Leg of Mutton Lake.
Tours of the lakes include Dreamtime stories of the Boandik Aboriginal tribe that they were the work of the giant Craitbul. Every time he lit a campfire, it was doused by emerging underground water, leaving gaping holes in the ground.