The Mount Elephant visitor centre is open from 1pm to 4pm on Sundays. The site can still be accessed on foot from the front pedestrian gate at other times.
The John and Millie Borbridge track starts at the Visitor Centre car park and is a 2 km walk to the peak, followed by a 3 km loop around the crater rim. The trail is steep and not suitable for prams, wheelchairs, or those with limited mobility. On a clear day there are stunning 360° panoramic views of the surrounding plains and lakes.
While it's an interesting walk and a great view up on the cone, the earthcache does not require you to walk up Mount Elephant and none of the waypoints are on the property.
THE QUESTIONS:
*Wpt 1.) Front gate: S37° 56.970' E143° 12.525'
1.) Inspect the scoria in the stone fence at the front gate, it is local to the site. Describe the rock you see. Include in your description colour, texture etc. Describe whether the rocks have a smooth or jagged appearance? Are they rounded or blocky in nature?
2.) Having inspected the rock at the front gate and read the lesson below, speculate as to why a scoria cone can have such steep sides?
*Wpt 2.) S37° 57.669' E143° 11.274' Mt Elephant Rd: Quarry Scar (do your photo here)
3.) Cast your eye over the slope from this viewpoint. Why do you think the trees have been planted below the quarry scar as opposed to above it? Do you think the slope where the quarry scar is will ever be successfully conserved or rectified? Give your reasons.
Then post a photo of you at the Wpt 2 (Quarry scar) location with your log, (please do not show the subject of Q 1&2 in your photos). Of course, if you do not want to appear in the photo, a personal item in the photo is enough proof of your presence. You may log the cache as soon as you submit your answers to us via messenger.
Logs without accompanying answers sent or without a photo uploaded may be deleted without notice.
THE LESSON:
The Western Victorian Volcanic Plains, also known as the Newer Volcanic Province (NVP), is the third largest volcanic province in world. Formed by volcanoes over the last 6 million years, it covers an area of 23,000 square kilometres. The most recent eruptions were around 7,000 years ago, hence the name of the Newer Volcanics Province (NVP). Over 400 volcanic sites have been documented within the NVP (last accurate count was 437). The volcanic activity produced extensive basalt flows forming a thin veneer, on average the depth of the lava is about 60 metres, and covers much of western Victoria. Many of the volcanic features might be mere bumps, this particular feature not!
Photo: View of Mount Elephant walking up from front gate. The breach in the cone is visible from this angle. A quarry scar is visible at the centre of the photo, that is where the visitor carpark is.
Welcome to Mount Elephant....a steep sided, conical breached scoria cone. It is the largest scoria cone in Victoria, rising 240m above the surrounding plain. The crater is 90m deep and the cone is 1.3km across the base. Scientific testing at Mount Elephant by Curtin University (WA) has put its eruption dates at about 550,000 years ago. The area for several kilometres around the cone consists of “stony rises” of more solid basalt of the same age and come from this eruption vent.
Since European settlement Mount Elephant has been privately owned by the Eldridge family. In December 2000 it was purchased from them by the Trust for Nature and the local community. The long term aim is to revegetate the area and promote local tourism with a focus on the area's volcanic past.
A key feature of Mount Elephant is its steep sided slopes. A scoria cone consists of a build up of loose pyroclastic debris, including ash, lapilli (small stones), and volcanic bombs around a volcanic vent. The eruptions are volatile and violent and often only occur over a geologically short period of time. This loose, fragmented material creates a steep slope entirely dependent on the ability of material to lock together in a relatively stable manner. The angle of the slope is referred to as the Angle of Repose.
Angle of Repose is a term used to describe the stability of any sloped surface. By definition it is the angle (relative to the horizontal plane) of the steepest stable slope a pile of material can maintain and be reasonably stable without the pile sliding, slipping or collapsing. Factors like particle shape, smoothness, moisture content, and any cohesive or electrostatic properties can influence this angle, which indicates the steepness of a pile of granular material before it slumps. There is a mathematical formula attached to this angle, but this earthcache isn't about complex maths...if you want to see the angle of repose in action just take a container of salt, sugar or rice and pour it continuously onto a horizontal surface into a pile and watch what happens! You will get a conical pile where the sides slip if the slope becomes too steep for the material to hold its stability. Material with a low angle of repose forms flatter piles than material with a high angle of repose. Most of us would predict the rice will have a lower angle of repose than salt or sugar due to the polished smoothness of its outer layer. Whereas sugar and salt grains are jagged and angular in nature and will lock together a bit like a jigsaw allowing their angle of repose to be steeper before they slip.
The angle of repose for a Scoria Cone is quite high, generally between 28° and 40°, with a common gradient range of around 30° to 35°. This angle determines the typical shape of the cone, where the deposited scoria particles stop sliding and come to rest. Factors like particle size, temperature, the presence of water, and whether the scoria is welded or stuck together can influence the angle and lead to variations in the slopes. This steep angle makes scoria cones inherently unstable. On occasion, the slopes during deposition can become too steep causing minor avalanching. As a slippage event settles this maintains the achievable angle of repose for the material.
Mount Elephant’s angle of repose is generally between 25 and 32 degrees.
Influence of the Angle of Repose on the formation of a Scoria Cone
Diagram: the influence of the Angle of Repose on the formation of a Scoria Cone shown in cross section, only one-half of the cone is shown.
The diagram illustrates the four major stages in the development of a scoria cone and shows how the angle of repose affects both the steepness of the outer slope as well as the inner slopes of the crater.
Stage 1: laying the foundations, a low-rimmed pyroclastic ring forms as the vent violently bursts into action shooting ejecta material into the air at an angle between 75 and 90 degrees. The ring is composed of scoria-fall beds and ballistically placed blocks and bombs.
Stage 2: the ring reaches the angle of repose for unconsolidated clastic material (loose, un-cemented fragments of rocks of various sizes); slumping and avalanching of scoria occurs and the outer slopes of the cone are covered in talus.
Stage 3: the original rim is destroyed as the talus built up on the edge of the rim goes beyond the angle of repose and collapses, avalanching either down the inner slope towards the crater or down the outer slope creating an sharper, much more defined rim peak;
Stage 4: the talus apron at the base of the cone reaches the ballistic limit of the ejecta. The size and shape of a scoria cone depend on the stage reached as the eruption ended.
The crater may fill with a lava lake and for lava to then spill out of the crater cementing the talus and stabilizing the slope.
(Adapted from McGetchin et al ., 1974.)
Photo: View of one of the Quarry Scars on Mount Elephant.
There were lava flows associated with Mount Elephant in the latter part of its eruption cycle which is why the cone is not intact, having been breached on one side. However, the majority of the flows came from underneath the cone. Scoria is light weight and often too weak to support a lava flow from the crater and over the rim. The lava flows were vented at the base of the cone and the cone would literally “float” on the flow.
Scoria is a valuable resource to humans so Mount Elephant has been quarried in the past. Evidence of the quarries are visible at the base of its cone. While the upper slopes, rim and crater and the southern flanks of the volcano are intact there are two obvious major quarry scars on Mount Elephant.
The one on the northern flank (visible from the front gate) was used for railway ballast in about 1911, the material proved too brittle for the intended purpose and the quarry was quickly abandoned. This site is where the cone was breached and lava subsequently flowed down the side, consolidating the material in the slope. Hence, the old quarry face here is stable. The carpark for the park’s information centre is here, built on the old flow.
The large scar on the western flank (waypoint 2) consists of two quarries. The most obvious one, is the crudely cut scar running up the slope. This was a privately owned quarry. Look up to the rim, you can see here it has not been breached, no lava flowed over these slopes. The lava flows from this side of Mount Elephant emerged from underneath the cone. This old quarry was cut into very unstable material. The individual pieces of the scoria cone on this side of Mount Elephant lack strong cohesive bonds, meaning they don't stick together well, making them prone to breaking apart under stress and more prone to erosion by wind and water, further weakening the structure. This early quarry rivals the worst examples of disfigurement of a major and significant volcanic structure of the Newer Volcanics Provence. 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 but human intervention may never be able to stabilize or repair the slope. 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.
Resources:
https://mountelephant.weebly.com/australian-heritage.html
https://sciencenotes.org/cinder-cone-volcano-formation-characteristics-eruption/
https://publishing.cdlib.org/ucpressebooks/view?docId=ft6v19p151&chunk.id=d0e12460&toc.id=&brand=ucpress