Monday, 23 January 2012

More on the Tweed Volcano

I had the pleasure in obtaining a copy of a University of New England honours research thesis by Howden (2009) a week ago. For one thing, I'm pleased to see that there is still some research being conducted on the Tweed Volcano and Focal Peak Volcano, despite the state of our Country's university science faculties these days. Howden has put a great deal of effort into distinguishing between the mafic rocks of the volcano (basalts) including some detailed petrographic and geochemical analysis. One of the points of interest to me is the attempt to distinguish between the Blue Knob Basalt and Lismore Basalts, sadly, the work undertaken by Cotter (1998) was unavailable (lost to the world until recently) to her. This would have clarified some issues which were difficult to resolve in her thesis.

Previous authors such as Duggan & Mason (1978) noted that there appeared to be very little (if any) distinction between the Blue Knob and Lismore Basalts except for their apparent stratigraphic location. Duggan & Mason (1978) determined that the Blue Knob Basalt appeared to overlay the Nimbin Rhyolite and the Lismore Basalt under it. However, Duggan & Mason and other authors such as Smith & Houston (1995) suggested a possibility that the Blue Knob Basalt could actually be inter-collated with rhyolite flows indicating that it was possible that the basalts were really just occasionally interrupted by flows of the Nimbin Rhyolite.

Howden (2009) has through comprehensive geochemical and petrological study of the Lamington Volcanics demonstrated that the only way to distinguish between the two basalt units was on the basis of phenocryst size with the Blue Knob Basalt showing larger grains of plagioclase feldspar. In the absence of any other geochemical or petrological distinguishing characteristics this shows a very uninspiring difference between them, I would suggest, insufficient to say that they were in fact different.

Because of the absence of significant differentiating features it is likely that the Blue Knob Basalt is really just the Lismore Basalt which continued to erupt at various times with intervening periods of large rhyolitic eruptions of the Tweed Volcano. This means that this can be confirmed if flows of basaltic lava can be identified between rhyolite. In Queensland the equivalent of the Nimin Rhyolite, the Binna Burra Rhyolite shows intercollated flows of Hobwee Basalt (the equivalent of the Lismore Basalt). The plagioclase phenocryst grain size difference probably just reflects slightly different magma residence periods in the magma chamber becoming more obvious at the volcano became older. This is also demonstrated as the Hobwee Basalt in Queensland shows the upper flows have larger phenocrysts.

Slowly we are gaining a clearer picture of our present day landscape and the mechanisms that made it. Sometimes difference between the way we think they have occurred and they way we later find out seems quite minor, yet the implications are significant in understanding how the landscape actually behaves under the ground. The small areas of 'Blue Knob Basalt' were thought to be a last spurt of eruption of the Tweed Shied Volcano (either centred on present day Mount Warning, or other vents on the flanks of the volcano), I think that Howden (2009) has presented us with enough evidence how to say that the way the volcano formed included two different types of lavas (basalt and rhyolite) erupting at essentially the same time.


*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University.
*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.

*Howden, S. 2009. An Evaluation of Mafic Extrusives Spatially Assoicated with the South-Western Aspect of the Tweed Shield Volcano, BSc(Hons.) thesis, University of New England, Armidale.
*Smith, J.V. , Houston, E.C. 1995. Structure of lava flows of the Nimbin Rhyolite, northeast New South Wales. Australian Journal of Earth Sciences V42(1) p69-74.


  1. I'm not very knowledgable on geology Rod but enjoyed reading this as I grew up near Blue Knob. A very beautiful area of our country.

  2. In the town of Lismore itself two types of basalt are strongly suggested by the markedly different weathering characteristics producing two distinct soil types; one is a red krasnoszem, the other is a chocolate loam with small felspathic grains in a clay matrix. The souce rock for the latter contains large ( often 5 - 10 mm ) grains of feldspar. It crops out on Girards Hill and at the southern end of the Leycester Creek railway bridge.

    1. Hi Anonymous,

      Thanks for letting us know that. I have noticed the difference in some of the soils which are apparently on the same geology. I will have to get a sample this weekend and make a thin-section out of it to see if there is any petrographic differences. Knowing this area it is probably difficult to tell, even the geochemistry might only show a small difference (e.g. Cotter 1998 showed that the Chromium and Zircon contents may be diagnostic for different basalt sources, whereas nearly everything else is similar)

  3. It would be useful to look at Ted Houston's preliminary work for his PhD wherein I think he looked at the suitability of Blakebrook basalt for engineering use (concrete, roadbase, etc.) as the large phenocrysts make the rock weak in compression. There was perhaps some variation in the basalt being quarried at Blakebrook. The phenocrysts are quite vitrious, flat sided and transluscent with a yellow/green tinge when freshly exposed. (can't remember felspar ID clues unfortunately).

    1. Hi again!
      I understand that Ted Houston was at Southern Cross University for some time, was this where he did his PhD? I cannot seem to find any reference to his work in the SCU library, I might have to try Lismore City Council and see if they have a copy... Thanks for your comments, sorry they are not automatically published, any posts greater than 3 months old are not automatic to stop spam. If you wish please feel free to email me