Geology


(Updated 26/4/2016)

Structure of the earth:
The earth formed about 4.5 billion years ago and has now settled to be a sphere with a core of mainly molten iron, the movement of which causes the earth's magnetic field.
Between the core and the crust is the mantle, which consists of mainly silicates.
The crust is between 50 and 150km thick and supports our land masses and oceans. Over the previous billions of years the land masses wandered over the surface, occasionally bumping into and sliding over each other, which forced up mountain ranges. One kind of volcano forms along these lines. These can cause massive eruptions and ash clouds around the "Pacific rim of fire"
Another kind of volcano forms generally under the sea in the crack formed as the continents drift apart. There is a line of these between Australia and Antarctica.
A third kind of volcano forms when the land mass passes over a "hot spot" in the crust. The plains basalts and volcanoes from Mt Widderin near Skipton to Tower Hill near Warrnambool were formed like this.
They are bringing contents of the upper mantle to the surface.

The plains basalt:
Between 4.6 and 1.5 million years ago the early land form was greatly altered by a series of basalt lava flows which came from possibly a large number of vents scattered down the valley between the Otways to the south, Central Highlands to the north, the Dandenongs to the east of Melbourne, and Casterton to the west. An area of 100km by 300km. The older of these basalts developed a forest cover and the younger basalts were treeless grassland plains when europeans arrived. They now carry
introduced pastures and shelter belts of sugargum eucalypts, cyprus and pines.

Forming the scoria cone and  stony rises.
Mount Elephant is a steep sided volcanic scoria cone. It was formed maybe 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.

It is thought that the eruption had several stages. There are several theories about the order of these stages. one is that the magma broke through the crust collecting water (from a layer of wet limestone) and gas. This made the molten rock froth up as it was ejected. The "frothy" rock was ejected in small lumps, solidified in the air and over many years formed the scoria cone.
When the frothing ceased a more liquid lava flowed out under the cone (which floated on it) and flowed for several kilometers in all directions before finally solidifying. This and other lava flows blocked many old water courses which then formed the many lakes in the district.
Parts of the floating scoria cone broke away in 2 places to form "breaches" in the cone. The sloping access track up the north side passes over one of these breaches. The bed of the car park is the surface of the liquid lava flow which caused the breach.

An alternative theory puts the lava flow first and the scoria cone second, being the youngest unit in the complex.

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 inclusions of granite and olivine. (Granite outcrops are common for 50 km to the northeast of Mount Elephant). 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 1200*C and normally cools quite rapidly on the surface. It is normally quite dense and hard.
 Occasionally some rocks were thrown out while spinning rapidly. The outer layer of these is basalt and shaped like a football with a point at each end. The centre often contains 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. It was used as early railway ballast but was too brittle to be durable.

Click here for a link to photos of rock samples from Mount Elephant, also to a page of links to more articles about the geology of scoria cones, and to a map of the lava flows from the base of the mount.

The quarries: (Click here for more details of 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.

Links to related sites:
Brief survey of types and ages of volcanoes in Victoria   This is in the Mount Elephant School study package.

How different volcanoes form    A great animation by museum vic.

VRO volcanic landform     describes the geology of the Mount Elephant.

VRO Mount Elephant        discusses formation of Mount Elephant.

History of volcanoes in Victoria     Article in The Age in 2008

Kanawinka Geopark                Introduces volcanoes and links to many other basalt features in western Victoria.

Western Volcanic Plains Survey by E.B.Joyce      Describes the basalt soil types around Mount Elephant. Very academic!

Penshurst Volcanoes Discovery Centre              Has many good links to "how volcanoes form" with videos and descriptions of other volcanoes in the district.  Very interesting!

History of the eruptions of the district                . How the larva flows  of Mt Vite Vite, Mt Widderin and Mt Elephant affect the route of the Mt Emu Creek.
                                                                        An interesting extract of a CD for sale from the Penshurst centre

Geological history of the district.                       This outlines more of the geology of the district than just the volcanic history.

Volcano types and how they form                    Powerpoint slide show for schools. Very good.

Click here                Excellent description by Museum Victoria of rocks and animals from precambrian to the present.

Click on the details below it opens 3 more pages of details