Geology of Mount Elephant, and the
Quarries
Scoria cones, stony rises and older
basalt:
Mount Elephant is a steep sided volcanic scoria cone. It was formed
about 20,000 years ago and is one of the highest volcanoes in Victoria,
rising 240 metres above the surrounding plain. The area for several
kilometres around consists of “stony rises” of more solid basalt of
about the same age. This emerged from numerous vents and flowed in many
directions as molten rivers of rock until they finally cooled. This
dammed the old creek lines to form the numerous lakes in the district.
There are similar areas of stony rises around the other volcanic cones
in the district.
In turn these stony rises sit on a much older basalt plain bordered by
Melbourne, Ballarat, Warrnambool, and westwards to Coleraine. These
plains formed at various times over the last 20 million years.
Mount Elephant is a site of National Significance and listed on the
Register of the National Estate (AHC Database 22 July 2002).
Characteristics of the mount include its conical form, steep sided,
crater, and access to sections of the ejecta in the quarries. “It is
the best example of a breached scoria cone in Victoria and possibly
Australia”. (Rosengren 1994)
Types of basalt:
The cone of Mount Elephant consists of scoria, blocks and bombs of
solid larva with common megacrysts of granite and olivine. There are
occasional lumps of the limestone layer through which the larva
erupted.
Basalt as in the stony rises comes from the earth’s core at about
1800*C and normally cools quite rapidly on the surface. It is normally
quite dense and hard. Basalt is quarried for bluestone houses and
bluemetal for concrete and railway line ballast.
Scoria from mount Elephant has come as hot liquid through a layer of
water or wet limestone. The steam made the solid rock frothy and the
expanding steam threw it into the air, so it cooled with the bubbles
inside, and is quite light.
Occasionally some rocks were thrown out while spinning rapidly. The
outer layer is basalt and shaped like a football with a point at each
end. The centre is often hollow and lined with green crystals of
olivine. These “volcanic bombs” can be from 2 to 20cm in diameter.
The scoria was used for a number of commercial purposes, including road
surfaces and building materials. The reason it was useful was that it
formed a solid foundation but was free-draining. It was easy to mine
and crush to a suitable size and to transport.
The quarries:
There are two quarry scars on Mount Elephant. The one on the northern
flank was used for railway ballast in about 1911 and has long since
been disused. The large scar on the western flank consists of two
quarries. The obvious one running up the slope was privately owned and
is now terminated. There has been partial restoration of topsoil at the
northern and southern ends, but the centre section has proved too
unstable to cover. Trees were planted in 1985 at the base of the
northern scar in an attempt to reduce the effect of the wind dislodging
scoria from the cliff face.
The second hidden quarry is below ground level at the base of the scar.
It is owned by Corangamite Shire, and is still licensed but not
operating. Modern quarrying techniques ensure that the slope is always
manageable, the quarry is out of public view, one face is rehabilitated
before another face is opened, and the quarry operator must hold the
rehabilitation cost in a bond until the work is completed. These
conditions did not apply at the time of these quarries.
Various reports have suggested rehabilitation plans for these quarries,
and these are being considered in the overall management plan of the
Mount.