The Warrumbungles are a group of eroded volcanic cores and dykes, forming a weird landscape of sharp, narrow peaks and rounded domes. The Warrumbungle National Park covers 19 651 hectares, and its more interesting central portion (containing the 'Grand High Tops') is about 5 by 4 km. The original volcanic area, an almost circular shield volcano (now deeply dissected), occupies about 1 200 km2. The centre of the Park is located about 45 km south-west of Coonabarabran, north-western New South Wales.
The Warrumbungles are largely made of trachyte rock, erupted about 13 - 17 million years ago in Cenozoic times. Large volumes of trachytic pyroclastics were erupted (now tuff and breccia) as a blanket over the surrounding countryside, together with formation of lava domes and short, thick trachyte flows.
The viscous magma being fed to the volcanoes eventually blocked many of the vents, and new vents would have emerged. Eventually the eruptions changed, and lava spread out as thick flows, burying earlier pyroclastics and domes as a wide shield developed. Over time, the erupted magma continued to be less viscous, and trachyte eruptions were replaced by basaltic flows (hawaiite and mugearite) which became thinner and longer, until all activity ceased around 13 million years ago.
The composite shield volcano would have reached about 50 km across, consisting of a large complex central vent and many parasitic vents, domes and dykes on the shield flanks. Erosion has cut through the volcanic pile and removed most of the later volcanic deposits, exposing rocks of the earlier volcanic episodes. The eroded volcanic cores and dykes form a picturesque grouping.
Belougery Spire forms an irregular plug-like structure, and Belougery Split Rock forms rounded domes. Bluff Mountain (250 m high), made of coarse-grained trachyte, forms the largest lava dome of the Warrumbungle Volcano.
One famous feature is the Breadknife, a dyke which forms a narrow trachyte wall intruding pyroclastics and rising steeply into the air. It is traceable for over 600 m southward to Crater Bluff, its probable eruptive source.
Copyright © Australian Museum, 2004