More volcanics

I seem to be on theme here with all things volcanic. So to continue...

Last week we explored the Limestone Coast of South Australia, the area south of Mount Gambier. I'll write more about the coast in the next blog but in the meantime it's hard to ignore the area's volcanic history because the two main mountains stand out from the plain. Mt Gambier and Mount Schank are the youngest volcanoes in Australia and are defined as dormant rather than extinct. They, and other volcanoes in the area, would have been quite a sight when active and there are local Aboriginal stories about the eruptions (thought to be about 5000 years ago).

Mount Gambier (left) and Mount Schank
as seen from Cape Northumberland near Port MacDonnell.

In 1800 Lieutenant James Grant, on board the Lady Nelson, thought he spotted four islands but as the ship moved closer the islands evolved into two capes he named Northumberland and Banks and two mountains he named Gambier and Schank.

Mount Schank as seen from Mount Gambier

Mount Schank

Blue Lake, Mount Gambier

The Blue Lake holds water because the crater is below the water table whereas the craters on Mount Schank is above the water table and dry. Scientists have worked out why the water in the Blue Lake turns to a brilliant turquoise blue early in summer each year and if you're interested I suggest you follow this link. 

Both mountains are maars, the result of the rising magma hitting shallow groundwater. Violent and spectacular. If another eruption were to occur in the area in the future it would cause widespread damage up to 100 km from the site.

Harman's Valley and Mt Napier

Mt Napier erupted over several decades about 8000 years ago. The lava flowed into valleys and over the landscape forming a shield around the volcano itself. Scientists can date the lava flow that filled Harman's Valley with carbon 14 isotopes of the peat that formed in a swamp after the lava filled a valley. 

Harman's Valley can be easily seen from a lookout on the road near Byaduk. And there are excellent information boards there as well. How else would I have known about isotopes!!

Lava blisters

It looked very different 8000 years ago. The local Aboriginals would have seen the lava slowly filling the valleys and spreading over the landscape in waves through several decades of eruption. 

We now call the volcano Mt Napier (more about that in the next blog) but I went to see the blisters, the tumuli, that form on the surface of a lava flow when gases build up beneath the surface. The dome roof collapses and the sides remain. The blisters (on private property along Old Crusher Road) are excellent examples apparently and much bigger than I expected..

There are no information boards at the site but at the Harman's Valley site nearby there are several that explain everything very well. The tumuli site is not far off the main road but the signposts do not entice the tourists so it can be very easily go unnoticed.