Warrumbungle National Park Environmental Education Centre

Telephone02 6825 4302

Emailwarrumbung-e.school@det.nsw.edu.au

Geology

Although no volcanoes are currently active in Eastern Australia, a broad strip parallel to the coast from North Queensland to Tasmania, including the Warrumbungles, has been affected by volcanic activity over the past 70 million years.

The central volcanoes at Cape Hillsborough and the Glasshouse Mountains in Queensland, the Nandewar Ranges (Mt Kaputar National Park), the Warrumbungles and Mt Canobolas in New South Wales and Mt Macedon in Victoria get progressively younger as you travel south. This probably indicates that as the crustal plate carrying Australia travelled north under the influence of continental drift, it passed over a fixed zone of melting or hotspot in the earth’s mantle, which periodically erupted forming mountains.

Before the volcano the area was probably not unlike the Pilliga to the north – wooded, undulating to flat sandstone country cut by shallow valleys and creeks. These sandstones were laid down during the Jurassic period (the age of the dinosaurs) about 180 million years ago, in the shallow freshwater lakes which covered most of the eastern part of Australia.

Initially, about 17 million years ago, there were scattered flows of basalt. These were followed about 16 million years ago by numerous explosive eruptions which built up thick piles of breccia and tuff. As time progressed the magma became less and less viscous, spreading further and further from the vents and forming thick trachyte flows such as Mt Exmouth and Siding Spring Mountain. In the final stages from 14 million years ago, the flows became thinner and longer and more basaltic.

Since the end of volcanism about 13 million years ago, erosion has cut through the volcanic pile, removing most of the later deposits and the softer rocks to expose the products of the early phases of the volcano, the hard trachyte domes, plugs, and dykes now visible as Belougery Spire, Crater Bluff, the Breadknife and Bluff Mountain.

For more detailed geological information see “The Warrumbungle Volcano” written by MB Duggan and J Knutson and published in 1993 by Australian Geological Survey Organisation.