Many millions of years ago, Melbourne’s west was alive with volcanic activity. Due to this activity, Point Gellibrand in Williamstown is home to an intriguing volcanic phenomenon.
Victoria’s Basalt Plains
Victoria's basalt plain was created only yesterday in geological time, by lava spewed from the vents of hundreds of volcanoes across what geologists call Australia's Newer Volcanics Province. This Province covers approximately 15,000 square kilometres of south-eastern Australia. Its lava flows extend about 400 kilometres west from Melbourne to Mount Gambier, just beyond the South Australian border, defining a unique bioregion, one of 21 within the state of Victoria.
The lava flows of Victoria's basalt plain in south-eastern Australia meet both the crashing waves of the Great Southern Ocean and the more protected waters of Port Phillip Bay. This convergence of land and sea is marked by dark basalt boulders that are characteristically stained with lichen the colour of magma. Up to 95 per cent of the species that inhabit these marine ecosystems are found nowhere else on earth. They include towering forests of kelp, vast fields of sea grass, and coral reefs as colourful and ecologically diverse as those found in tropical waters.
The Lava Blister
At the rear of the shore platform is a raised, kidney-shaped basalt structure that has been interpreted as a lava blister. Lava blisters develop when fluid lava crosses a shallow body of standing water or swamp ground, causing the water to boil, and the confined steam pressure is sufficient to raise the surface of the lava into a closed domal form.
Photo by The Empire
The structure at Williamstown is a shallow elliptical open bowl, measuring 4.8m by 3.4m with a rim 30 to 40cm deep. If it is a truce lava blister, the upper part of the dome has been removed by weathering and erosion.
While there are many types of raised and uneven surfaces on lava flows, truce lava blisters are unusual in the volcanic rocks of Victoria. This formation is very rare and has fascinated geologists all over the world.
Rock Type
The Rocks on the Williamstown shore platform are Igneous. This is any rock made by the cooling of magma. Igneous rocks are divided into two types, this is where you’ll need to do some research.
To log this EarthCache, please take a photo of yourself and your GPSr standing near the Lava Blister (optional) and answer the following questions…
1. Describe the rocks that make up the Williamstown shore platform (colour, texture etc).
2. The rocks at this EarthCache are Igneous, but are they intrusive or extrusive?
Please e-mail your answers to mailto:empiregc@gmail.com before you log your find, logs with no email verification will be deleted as per EarthCache guidelines.
A photo can be taken when the tide is out and you can clearly see the blister, follow this link to the Board of Meteorology’s Oceanic Services Tide Predictions… http://www.bom.gov.au/oceanography/tides/MAPS/vic.shtml#form
References:
1. Imagine the Future, Redreaming The Plain by Merrill Findlay.
2. Victorian Resources Online (Department of Primary Industries), M1 Williamstown – Lava Blister.
3. GeoTech.org – Basic Geology.
