EPOW - Ecology Picture of the Week

Maar Crater

Blue Lake Maar Crater
Mount Gambier, South Australia, Australia

Explanation:  Is this a sinkhole?  A mining pit?  No, it is a maar crater formed from explosive volcanic activity in recent geological time.  

We are in the town of Mount Gambier in South Australia, where this geological marvel is a prominent resident.  

A maar is a volcanic eruption that spews ash and basalt. This maar -- appropriately named Blue Lake -- was formed from two eruption activities, and is one of the most recent volcanic events in Australia.

On the crater inner wall you can see strata of deposits of the eruption outfall events:

  
The deposits form layers of ash and basalt from the various eruptions.  The layers are also evident in roadcuts in the region.  Luckily for the townspeople, this volcanic vent is no longer active.

The lake then formed when the groundwater seeped into the crater following the eruptions, and refilled to its natural level.  So from the rim, you can see how far down the water table resides below the surface.  

According to an informational signboard, the lake's bottom is flat and the lake on average is 230 feet (70 meters) deep.  Over the past 25 years, the water level has dropped 6.5 feet (2 meters).  The lake itself covers a surface area of 174 acres (70 hectares), and its circumference is 3.1 miles (5 kilometers).  This is no small puddle.  

A pumping station extracts water from the lake for local use.  At the current rate of usage, it takes 10 years for the volume of water in the lake to be replaced from the aquifer filtering through the surrounding limestone formation.

Acknowledgment:
    My deep appreciation to friend and colleague Dr. Trent Penman for hosting me on this wonderful exploration through southeast Australia.