Six or seven million years ago Central and Western Victoria was covered by a broad basalt plain created by the lava flows of ancient volcanoes such as the Bacchus Marsh Volcano which had erupted some 45-50 million years prior. During the Pliocene age Victoria’s 200 or so conical scoria volcanoes gradually emerged across these basalt plains, beginning as a small vent and rapidly growing into steep sided mountains. Towards the end of this six million year period, about 470,000 years ago, Mt. Franklin was born in a fiery eruption, rapidly growing to stand next to Mt. Tarrengower and Mt. Alexander to tower over the surrounding rolling plains of Central Victoria. Although considered by volcanologists to be extinct, the last eruption of Mt. Franklin may have occurred as late as 5,000 years ago and there is evidence that the local aboriginals were witness to it as they enshrined the story in their oral histories and myths. It seems likely that this would have been the eruption that breached the conical wall of the volcano and thus opened the interior to easy access.
Mt. Franklin is classed as a prominent, breached conical scoria cone with a deep crater. The breach in the south-eastern side of the crater is most likely the result of late-stage lava flow breaking through the lower part of the cone and earlier flows were predominantly to the north and the west. Mt. Franklin is a major megacryst site with some of Victoria’s largest known examples of megacrysts of augite and an orthoclase. The coarse ejecta exposed around the summit also includes red and green olivine and megacrysts of high-temperature and orthoclase (to 7 cm long) and augite (over 9 cm long). Lumps of Ordovician sedimentary and granitic bedrock also occur in the ejecta and small basalt blocks contain cores of crazed quartz. On the western slope is a parasitic scoria mound referred to as ‘Lady Franklin’ but which remains officially un-named. The summit of Mt. Franklin is 635 m high, beginning from a base of 185 m above sea level, making the cone 450 m tall, and covered by extensive pine plantations whilst the interior of the crater is currently an arboretum containing many exotic species including an impressive stand of Californian Redwoods that were planted after a bushfire denuded the area.
I would like to acknowedge Alanna Moore and Geomantica magazine for many of the details in this essay- see http://www.geomantica.com/geom25.htm for more. Also, for the official Parks Victoria notes on Mt Franklin go to http://www.parkweb.vic.gov.au/1park_display.cfm?park=59 to download the PDF.
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