KANAWINKA GEOPARK

South Western Victoria and South Eastern South Australia

 

 

 

 

LANDSCAPE FEATURES BUILT UP BY VOLCANOES

STONY RISES, LAVA FLOWS

Along the Volcanoes Discovery Trail between Colac and Camperdown, unusual mounds and depressions jumble over the flat plains. Known as the Stony Rises, these mounds of jagged rocks rise ‘like petrified waves in a storm’, 10 to 15 metres above ground level. Stony rises are also found at Byaduk, and other locations in the Western District.

How did stony rises form?

Very runny lava from Mount Porndon (Grid Ref. 7T) and Mount Eccles (Grid Ref. 6K) spread radially over these areas, forming broad sheets or shields of lava. As tongues of lava broke out from the partly-congealed lava sheets, the surface sagged and collapsed. Fingers of lava at the outermost edges of the flow cooled in their original shape. They can be seen at the north-eastern shore of Lake Corangamite.

SHORELINE CLIFF

Semi-circular Bridgewater Bay, near Portland, is part of a sea-filled volcanic crater and offers a spectacular glimpse into the anatomy of a volcano. In cross-section at the cliff-face you can view multiple layers of ash and lava deposits, as well as the lava plug which filled the feed-pipe below ground level.

LANDSCAPE FEATURES EXCAVATED BY VOLCANIC PROCESSES

LAVA CAVES

 Byaduk Caves In this region the lava flows were able to travel long distances without spreading laterally, and some flows reached the sea at Port Fairy. These long fingers of lava cooled at the surface while hot lava inside kept running. Eventually the interior lava drained away, leaving hollow lava tubes or tunnels. Weathering and erosion in some places caused the ceilings to cave in, allowing human access to the interior.

LAVA-BLOCKED LAKES Lava flows respected no boundaries and frequently poured into existing lakes, reducing their size, and flowed across rivers, blocking their passage to the sea. The resulting lakes have no natural outlets, although subsequent water movement has worn channels into the lakes.

Lakes Corangamite, Beeac, Cundare, Colac and other smaller lakes were once part of a much larger lake that stretched from Camperdown to Winchelsea, now shrunken by climate change and lava flows.

LAVA-FILLED RIVERBEDS

In the Mount Napier/Mount Eccles district, molten lava flowed into and filled existing river valleys. The Harman’s Valley area now looks like a ‘river’ of basalt rock varying from 200m to a kilometre wide. No surface streams have yet formed on this young volcanic feature and all drainage occurs through underground channels in the basalt.

ARE THESE VOLCANOES DORMANT (sleeping) OR EXTINCT?

What causes the volcanic activity in this region? It is possible that Australia is passing northwards over several ‘hotspots’, which may help to explain the siting of lava flows found down the eastern edge of the continent. Some of the youngest volcanoes in this string are on the Victorian and South Australian volcanic plains.

Dating of volcanic sediments in this region tells us that the most recent phase of volcanic activity began about 6 to 7 million years ago and continued periodically until 6 to 8,000 years ago. Most of the large scoria cones were probably built around 20-25,000 years ago.