Blue Lake - Mt Gambier - South Australia by ted Brattstrom

Canon PowerShot SX110 IS

The Blue Lake is one of several lakes in Mount Gambier - the city and the mountain. It provides the drinking water for the area and is refreshed through the limestone aquifer at its base.

This is an example of Maar volcanism. In this case, a limestone plain was gently erupted through to spread a layer of basalt/lava over the surface. After cooling, water went down the conduits, and when enough water met enough lava - KaBoom! A large ash and rock blast / eruption occurred forming the mountain.

Three distinct layers can be seen in the image - Lowest: Limestone, Middle: Basalt, Top: Ash

The other impressive action of this lake is a color change from grey-green to blue caused by a combination of algal/bacterial and chemical changes (which are linked, and seasonal)

Although this image looks blue to me, supposedly the true Blue is significantly different.