Monday, 17 October 2011

'Recent' rhyolite: The Nimbin Rhyolite at Minyon Falls

The rhyolite forms a rugged range around the valley
If you are familiar with the northern rivers you would be aware of grand waterfalls in Nightcap National Park. The grandest (in my opinion) are the Minyon Falls which drop Repentance Creek around 100metres into the gorge below. I remember when you used to be able to stand at the very top and jump over the streams to cross but the National Parks and Wildlife Service of N.S.W. have stopped access (for obvious safety reasons) except at a constructed viewing platform.

Minyon Falls are spectacular. Geologically they represent thick units of rhyolite known as the Nimbin Rhyolite erupted during the later phases of the tweed volcano during the period known as the Cenozoic which was centred on the nearby Mount Warning. Underlying the rhyolite is basalt and andesite of the Lismore Basalt which appears to be from the earlier main phase of eruption from the volcano. At Minyon Falls the Nimbin Rhyolite is greater in thickness than the height of the falls themselves. It mainly shows massive units of rhyolite lava inter-collated with units of volcanic glass (obsidian) darker, but still of similar composition to the rhyolite.

Rhyolite is the volcanic equivalent of granite (which forms underground). It is fine grained due to quick cooling due to its volcanic nature which stops crystals from becoming very large. Rhyolite is silica rich which means that minerals like quartz and feldspar are abundant and other minerals such as olivine that is commonly be present in some of the basalts nearby are absent. The high silica content makes the lava thick and viscus and therefore gas bubbles are commonly trapped in the lava and banding of the lava flows becomes more frequently observed. The composition of rhyolite often leads to violent eruptions which are represented by ash and volcanic glass which can form thick layers themselves (some of these glass layers are present at Minyon Falls too).

If you are fit enough for a big walk at the base of the Minyon Falls are unusual structures which show how viscus the lava can be. Brittle-ductile structures are evident to the trained eye in this area. Smith (1996) identified these as essentially these are structures which show that when the lava was flowing the lava had become almost solid with many small faults mixed in with folding and flow banding of the lava.

Minyon Falls with the rhyolite cliff visible
Fresh rhyolite lava is a hard, erosion resistant rock and for this reason is why we have rugged ranges surrounding the central core of the Tweed Volcano at Mount Warning. The highest portions of the volcano including the rhyolite have been eroded away from the area now occupied by the Tweed Valley. Most of the volcanic rock in the valley has been eroded right down to the much older Paleozoic aged rocks of the Neranleigh Fernvale Group. The creeks that start in the ranges such as Repentance Creek have slowly cut back the face of the rhyolite cliffs as the velocity and power of the waterfalls slowly breaks grains from the rocks and creates cracks that break off in large rock falls.

Are you in Northern Rivers? It might be worth climbing Mount Warning to see the shape of the Tweed Valley and the remnants of the shield volcano in the cliffs seen all around the edge of the valley. Or maybe a trip into the Nightcap Ranges to Minyon Falls. Have a look at rocky creek beds to see exposed rock and many structures.

Note: There are two large areas of rhyolite in the Northern Rivers. These are the Nimbin Rhyolite of Cenozoic age discussed in this post but there is also rhyolites within the Chillingham Volcanics which are much older and are probably the basal units of the Mesozoic Ipswich Basin.


*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.
*Smith, J.V. 1996.Ductile-brittle transition structures in the basal shear zone of a rhyolite lava flow, eastern Australia. Journal of Volcanology and Geothermal Research V72
*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. Hi Rod and thanks for alerting me to your new blog. I have put up post -

  2. Hi Rodney, according to Cotter (in an earlier paper)there are variations within the Nimbin Rhyolite. He suggests that grain size is critical in the preservation of these long standing "nick points" or large water falls such as Minyon. The finer grained rhyolite is less susceptible to erosion and therefore the nick point is preserved. Valleys containing coarser grained rhyolite do not generally contain waterfalls of any significant size. This rhyolite is not differing in composition but has undergone a slightly different cooling history. Mechanisms for grain size distribution may have something to with the distribution of overlying (but mostly gone) Blue Knob Basalt. What do you think?

  3. Hi anonymous,
    I have not read the earlier work of Cotter. I assume you are referring to his honours thesis at SCU? I will have to go to the uni and look it up. The only published work I have seen that relates to the geomorphology and crystal size is Smith and Houston (1995) listed in the blog post above. Smith and Houstons work does not directly imply a mechanism for waterfall formation etc But I think your concept is not contradictory and certainly very plausible. I'll have to spend some more time up that way as well. It certainly sounds like something that would at the very least make a good future blog post or even a good uni level research topic.