|Mt Napier along Harman's Valley. The two stages in this composite volcano can be seen; the lower, broad shield and the steep-sided scoria cone on top.|
Mt Napier is an extinct volcano south of Hamilton in the Victorian Western District. The mountain's volcanic history has been throughly studied and the sequence of volcanic incidents described. Access is available to the crater at the summit via a steep track.
Mt Napier erupted in two stages causing the distinctive profile. The eruption began as a shield volcano with fluid lava flowing out of vents relatively quietly and running downhill forming a low dome (or shield) centred on the eruption point.
This quieter stage was replaced by more violent eruption with magma being forcibly ejected in a fountain from the crater lake. Ejected lava formed scoria before it fell back to earth, or sometimes reached the ground as liquid lava which splattered onto previous splatters. Scoria formed a steep-sided, but fairly wide, cone on top of the initial lava shield. The cone was breached at some time towards the end of the eruption. The summit of Mt Napier is now accessible and the breached crater can be easily seen and walked around. On one side of the crater an outcrop of lava splatter remains.
|During the final eruptive phase, some lava ejected from Mt Napier did not harden into scoria but was still liquid when it fell back to the crater where it splashed on earlier fallen lava forming a splatter cone which can still be seen on the edge of the crater.|
|At the end of the eruption the crater wall of scoria was breached and the lava lake in the crater drained away. The breach in the crater wall can still be seen.|
|Mt Napier - page 2|
Byaduk Caves Lava Tubes
At least one of the flows from Mt Napier, along what is now Harman Valley, cooled unevenly leaving a stream of liquid lava flowing through solid lava. As the edges of this flow cooled a skin of lava on top of the flow merged with the sides forming a tube of cooled, hardened lava enclosing a stream of liquid lava. When the liquid lava flowed away a hollow tube remained; so much lava had flowed that the ground level had been raised and the tube was now underground.
The roof of some parts of the underground lava tube collapsed forming caves and sinkholes at Byaduk Caves. The tubes are up to 18 metres wide and 10 metres high. The exposed tube did not drain completely before the lava solidified so sinkholes do not provide access to a continuous hollow chamber.
|Sinkhole formed where a lava tube has collapsed at Byaduk. This one has vertical sides but at a nearby sinkhole fallen boulders form a ramp allowing access into the tunnel. The caves are a favourite site for ferns and bats.|
The age of Mt Napier's eruption is not entirely certain. Initial estimates were based on radio-carbon dating of peat under lava flows which were 7,500 years old. Allowing 500 years for peat to form gave an age for Mt Napier of 8,000 years, and Mt Napier has been described as Australia's youngest volcano. But a recent study using newly developed techniques to measure the effect of cosmic ray bombardment on lava at Byaduk Caves has suggested an age of about 37,000 years is more accurate for Mt Napier's eruption.
The walking track (1.5 km) to the top of Mt Napier starts at a car park in Menzel's Pit which is an abandoned scoria pit reached from the Hamilton to Port Fairy road via Murroa Lane and Coles track.
¶ "Mount Napier State Park and Byaduk Caves" - Volcanoes Discovery Trail leaflet.
¶ Volcano Discovery Centre, Penshurst.
¶ "Types of Volcanoes in Western Victoria" Compiled by Ken Grimes, Hamilton Field Naturalists Club, June 2005.
¶ "Cosmogenic 21Ne exposure dating of young basaltic lava flows from the Newer Volcanic Province, western Victoria, Australia" by D. Gillen, M. Honda, A.R. Chivas, I. Yatsevich, D.B. Patterson, P.F. Carr. in Quaternary Geochronology 5(2010) pp1-9.