Geologist Neil Phillips’ talk on the subject of Strathbogie Granite in Strathbogie Memorial Hall attracted an interested audience of approximately 70 people from across the plateaus and below.
Neil’s expertise was evident throughout. He introduced the rock types found, depicted granite distribution and identified the different minerals comprising Strathbogie granite across the Strathbogie Ranges. Of great interest was the discussion surrounding granite formation underground and the marvellous manifestations of it we see above ground.
What were some of the things we learned?
Granite is a hard igneous rock. The constituent minerals are quartz, feldspar, biotite mica and cordierite. In some areas there are also amounts of garnet and tourmaline. There are four identifiable Strathbogie granite plateaus: Tallarook, Highlands, Seven Creeks and Mt Samaria. At 100km long and 2,000 sq km, these make up the Strathbogie granite batholith, probably the largest area of cordierite rich granite in the world. This hard and large granite range forced a major deviation in the main route between Melbourne and Sydney, the Hume Highway.
How are granite tors formed?
Around 370 million years ago and 15-20km below the surface, temperatures of 850-900 Celcius melted a mixture of sedimentary rocks. Some were rich in clay. This hot magma rose to 1-3km below the surface, cooled and crystallised. Strathbogie granite was created.
Cracks formed in the cooling granite. Erosion removed some of the overlying surface material. Water penetrated the cracks, reacting with the minerals present, weakening and oxidising the rock. Some surfaces were returned to clay. As weathering continued, rocky edges and corners disappeared and the remaining granite rounded.
Over time, erosion continued to expose, and often separate, the underlying rock. It frequently emerged reshaped as tors.
There was much more to hear and many questions to be discussed.
Strathbogie Landcare and the wider community are very grateful that Neil made time to tell us the Strathbogie granite story. Neil is very grateful to the many private landholders who have assisted him in his research and field work.
[Read more about the general history of the formation of the Strathbogie Ranges landscape.]