Mt. Warning is Australia's most prominent volcanic plug. It is all that is left of the eroded core of the Tweed Shield Volcano, Australia's largest central volcano, which formed about 22 million years ago and straddles the New South Wales-Queensland border. The igneous complex is elliptical, being 8 km maximum east-west diameter, and 5.5 km maximum north-south diameter. The structure we now see is a huge erosion caldera exposing the eroded remains of a large magma chamber. The trachyandesite plug (with minor plugs and radiating dykes) which is the central part of Mt. Warning rises to 1156 m, penetrating the volcano's outer ring dykes of syenite, as well as the surrounding gabbro and monzonite intrusion (itself intruded by the syenite) that now forms the plug's outer slope. The gabbro represents the oldest exposed rocks of the igneous complex. The ring dyke (intruded into a ring fracture) is generally less than 30 m thick and forms a steep-sided ridge. Alkali rhyolite (comendite) and trachyte dykes occur near the summit. The original vent would have been about 1940 m above sea level and over 30 km across. Thick rhyolite and basalt lava flows and pyroclastics were also associated with the volcano, extending up to 54 km away from the eruption centre.
Copyright © Australian Museum, 2004