Volcanic activity occurred in the Mount Gambier area in recent geological time. The eruptions took place along the edge of the main zone of activity centred on southern and western Victoria. Mount Gambier and the Blue Lake crater complex, at around 5000 years old, is one of the better preserved examples of its type in Australia.
The craters of Mount Gambier are explosion structures called maars, which consist of a rim made of ejected material, resting directly on Gambier limestone. In most cases, the eruptions were along lines of crustal fissuring, starting with small flows of black basalt lava. The shallow regional water table led to interactions with the hot rock and eventually to the rapid accumulation of steam. Activity changed as the lava flows were followed by explosion craters and blowholes, from which thick deposits of ash and scoria were ejected.
The main features of the crater complex can be seen from the many scenic lookouts, which show the relationship of the volcanic deposits to the Gambier Limestone. A thick layer of tuff (volcanic ash and cinders that have become compacted or cemented and rock-like) and tephra (fragmental volcanic material that has been explosively ejected) form the Mt Gambier and Blue Lake craters which can be seen today. Within the ash are basaltic clasts and volcanic bombs.
The track from the Lions Lookout to the Tower Lookout follows the edge of the crater rim. On one side is the drop into the crater. Then look outwards the flat plains of the south east. The walking track cuts through volcanic ash layers. There are many questions you could ask yourself as you stroll up the hill. What is the colour of the ash? Is it soft or hard? Are there small glassy fragments that glint in the sun? Are there round spongy bombs? Are there clasts of darker basalt materials? What size are the clasts? Are there layers of limestone in the tuff?
Dreamtime legends of the Booandik Tribe tell of the giant Craitbul and his family who wandered around the southeastern region looking for a place to settle and live in peace. They camped and made ovens at Mount Muirhead and Mount Schank but were frightened away from both sites by the moaning voice of a bird spirit. Moving to Mount Gambier they managed to escape from the spirit and there they lived for a long time. Again they made an oven, but one day water came up from below and put out their fire. They made others, until they had four, which are now the craters of Mount Gambier.
Visit the listed coordinates and make your observations. You will need to answer the following eathscience questions. Please provide your answers to the cache owner in a timely manner, i.e. less than a fortnight.
-What type of volcanic activity formed Mount Gambier?
-Describe the size of dark volcanic bombs that you can see?
-What is the colour of the ash matrix?
-Are there layers of limestone in the tuff?