GEOLOGY AND GEOMORPHOLOGY OF MOUNT BUNINYONG
Prepared for: Great Dividing Trail Association, Mount Buninyong Walk, 25th October, 2020
Author: Stephen Carey, Federation University, Ballarat; Additional Notes: Barry Golding
Context
These notes were prepared for an 11km Great Dividing Trail Walk from Buninyong Botanical Gardens to the Mount Buninyong summit and return. They are being shared more widely for anyone interested in undertaking a similar walk independently. We strongly recommend you obtain a copy of ‘Goldfields Track: Walk or Ride Guide’ published by GDTA. Please note that the alternative route to the summit via the ‘South Walk’ is not marked on the GDTA Guide but is reasonably well signposted.
Mt Buninyong is one of the largest volcanic edifices in the Newer Volcanic Province of western Victoria and south-eastern South Australia. Occurring in the Central Highlands, it is a landmark that is visible from the Grampians to the west and from a substantial portion of the Victorian Volcanic Plains (VVP) in the Otway Basin. The Peak Finder app identifies more than 250 (theoretically) visible peaks from the Mount Buninyong, 745 metre summit including Mount Baw Baw in Gippsland.
The shape of Mt Buninyong in the landscape is referred to as its geomorphology. The discipline of geomorphology encompasses the landscape processes that modify Mt Buninyong’s shape, such as soil development and slope failure. The formation of Mt Buninyong was by a variety of volcanic processes, whose study is a branch of geology called volcanology. To understand Mt Buninyong as a feature of the landscape, we need to consider its volcanology and geomorphology.
Geomorphology
Mt Buninyong is known as a composite lava and scoria cone. This is because it consists of both lava and scoria. The scoriaceous component is built up into a volcanic cone which is breached on the north-western side. The cone rises to a height of 745 m above sea level and has local relief of over 200 m. The flanks of the cone slope at angles up to about 35°. This is the angle of repose of loose scoria at which the latter could be supported without collapse at the time of eruption.
Covering a much larger area than the scoria cone are lava flows that emanated from the same site. One flow that is older than the cone extends to the south-east to Clarendon while another is younger than the cone and reaches westward to Buninyong township. It was the eruption of this younger flow that was responsible for the breaching of the scoria cone and opening of the cone to the north-west.
The Clarendon flow, meanwhile, had a profound effect on the geomorphology of the area it covers. The lave flowed down the valley of a forerunner of Williamsons Creek and blocked the drainage. The newly formed basalt (bluestone) was much more resistant to erosion by water than the older rocks and sediments on either side. Accordingly, new drainage lines, called lateral streams, were eroded into the older material to right and left of the basalt flow, with the modern Williamsons Creek and Back Creek being the result. Lateral streams are associated with many lava flows in the Central Highlands.
Volcanology
The scoria cone of Mt Buninyong was produced by an explosive eruption, whereas its associated lava flows are the result of much quieter, effusive eruptions. The difference between an explosive eruption and an effusive one is commonly the proportion of gas in the erupting magma (molten rock). The Clarendon and Buninyong flows had little gas – except for the initial stage of the Buninyong flow’s eruption which breached the scoria cone – and cooled to form coherent bluestone. Similar bluestone is a common material in early colonial buildings and gutters.
A large component of gas in magma increases the pressure that drives eruptions. A modest amount of gas may result in the formation of vesicular basalt (bluestone with numerous gas bubbles), but more commonly causes the magma to “fragment”, that is, the magma separates into blebs that are supported by the gas. When fragmented magma is erupted, the gas pressure sends it skyward in an eruption plume. As the plume mixes with cool air, the magma blebs may cool rapidly to form products called tephra. Tephra can be classified according to the size of the volcanic fragments, as follows: ash, <2 mm; lapilli, 2-64 mm; blocks and bombs, >64 mm. Mt Buninyong’s tephra is dominated by lapilli, as is evident from exposures in road cuttings below the fire tower.
Rapid cooling of the tephra means that most particles are themselves made of very fine crystals. In fact, in some cases, cooling may have been so fast as to preclude formation of a crystal structure, and natural glass is the result. A small proportion of the tephra is derived from the fracturing of rocks far below the surface of the earth, including from the mantle, below the Earth’s crust. At Mt Buninyong, mantle-rock fragments dominated by the green mineral, olivine, are sometimes found. Such fragments, especially from tephras of the VVP, have been critical in deducing the nature of the upper mantle.
Geology is an historical science, and it is important to determine the relative age of geological materials and events. Geochronology is the branch of geology that seeks to assign numerical ages to materials and, by inference, events. The variety of techniques that may serve to date particular materials is now immense, with very sophisticated methods and equally sophisticated instrumentation now enabling dating of materials that could not be dated before. In the case of Mt Buninyong, recent work proposes an age of about 200,000 years (200 ka). This most likely makes Mt Buninyong the youngest volcano in the Central Highlands other than Mt Franklin (Larnebarramul), near Daylesford (≤130 ka). It also means that Mt Buninyong is one of a number of cones and craters in the Central Highlands and the VVP that testify to an increase in volcanic activity in the Newer Volcanic Province between about 200 ka and 100 ka.
References
MATCHAN E., L., PHILLIPS, D., TRAINE, E., & ZHU, D. (2018) 40Ar/39Ar ages of alkali feldspar xenocrysts constrain the timing of intraplate basaltic volcanism. Quaternary Geochronology 47, 14-28.
OOSTINGH, K. F., JOURDAN, F., MATCHAN, E. L., & PHILLIPS, D. (2017) 40Ar/39Ar geochronology reveals rapid change from plume-assisted to stress-dependent volcanism in the Newer Volcanic Province, SE Australia. Geochemistry, Geophysics, Geosystems 18, 1065-1089, doi: 10.1002/2016GC006610.
ROSENGREN, N. (1994) Eruption points of the Newer Volcanics Province of Victoria: An inventory and evaluation of scientific significance. National Trust of Australia (Victoria) and Geological Society of Australia (Victorian Division).
Track Notes
Most of our 11km walk route is up and back to the summit on the southern end of the ‘Eureka Track’ section of Goldfields Track. Map 2 in the Goldfields Track: Walk or Ride Guide published by GDTA, (pages 34-35) covers and interprets our walk route starting from the Buninyong Botanical Gardens, within the eastern Buninyong township area past Gong Reservoir (created in 1850) and over Hastie’s Hill. Map 1 (pages 32-33) covers and interprets our walk route from the edge of Buninyong township to the summit, but does not include the ‘South Walk’, which we take to walk south of the peak before climbing up to the fire tower from the east. Our descent and return is mostly back via the walk route shown in the Goldfields Track Guide along many dry stone wall lanes, aside from part of the ‘Crater Walk’ including Blackberry Lane (which is marked in the Guide).
Vegetation & Land Status
The Mount Buninyong Scenic Area (90 hectare) retains excellent examples of tall, relatively mature, messmate stringybark forest and tussock ground cover with a very limited understorey. The Wathawurrung traditional owners called it ‘Buninyong’, alluding to its shape from a distance similar to a ‘bent knee’. The area was set aside as a Public Park in 1866, the same year the Buninyong Botanical Gardens were gazetted. The road to the top was completed in 1926. The current four level, steel fire observation tower, with public viewing platform on Level 3 was built in 1979.
Barry Golding 