Northern Rivers Geology

Here are some common quotes about Mount Warning:

"World Heritage listed Mount Warning (Wollumbin) is the remnant central plug of an ancient volcano."

"The Mount Warning volcano was a huge shield volcano."

"Considered the central magma plug, Mt Warning and a system of ring dykes, being extremely hard rock, have resisted erosion, and dominate the valley landscape."

"Mt Warning, Wollumbin, the cloud catcher, is the basalt plug of the world's largest and oldest extinct volcano. "

"Now, Mt. Warning is the first place that that the sun hits at sunrise… the highest point in New South Wales….almost the highest in Australia!"

These are quotes typical of tourist and even educational resources. They are quite definite and the comments makes sense, mostly. There are also some points of view that I espoused for a long time... Except aspects of each of the quotes are technically wrong and in some cases completely wrong. Like my post on the "erosion caldera" something that is technically incorrect has become general knowledge. It is a little pedantic of me, but it is one of my hobby horses... so what is technically wrong with the quotes above?

Interestingly, Stevens et al (1989) and earlier authors noted that the rock composition of the intrusions that make up Mount Warning (the Mount Warning Complex) is different from most of the lavas (The Lamington Volcanics) that exist in the region. It is also slightly older than most of the lavas. Geologically speaking the age difference is not huge at only about 2-3 million years, but still significant enough.

It is apparent from Smith & Houston (1995) and other authors that much of the rhyolite lavas that remain of the Lamington Volcanics were not erupted from the central area now the site of Mount Warning but from vents on the flanks. Given the coverage of the mafic components (the Lismore Basalt, for example) it is more difficult to identify any vents.

An idea has been raised by Cotter (1998) which questions the volume of lava that was erupted from the Tweed Volcano. It is known that the Palaeozoic aged meta-sedimentary rocks of the Beenleigh Block, called the Neranleigh Fernvale Beds and the Mesozoic aged Chillingham Volcanics and Clarence Moreton Basin were not domed upwards by the underlying magma except a little around the Mount Warning Complex itself. However, other areas such as the nearby slightly older Focal Peak Volcano have been lifted by the Cenozoic aged volcanism. But in the case of Mount Warning, Cotter (1998) felt that lithology, the remnants of the rhyolitic lavas, the pre-existing Chillingham and Alstonville Volcanics was the main control on the geomorphology, not as suggested by others the volcanism that formed the shield volcano itself.

The idea suggested by Cotter (1998) has significant implications for the size of the Tweed Volcano. The volcano is considered the biggest by far of its age in eastern Australia. It appears likely that the extent of the shield volcano is not as great as originally thought. The underlying Chillingham Volcanics would have been an existing mountain range and therefore reduced the thickness of the Tweed volcanic pile and the Alstonville Basalts would have reduced the southerly extent. I think that when you add to this the idea that the rhyolite units have erupted away from Mount Warning, but instead from flanks on the volcano, the volume of lavas from the Tweed Volcano may actually be more in keeping with the other intra-plate volcanoes in Eastern Australia. It was also possible that before it was eroded into the present shape (which implies a central shield type volcano) it may have looked more irregular than we imagined...

But don't get me started on the comments about the biggest volcano in the world and the highest point in New South Wales!!! What were these people thinking?!

...but does any one want to talk down something that was presumed to be huge, just to something large? Emotionally, many (including myself) have an emotional attachment to the beauty and wonder of the Tweed Volcano, sometimes it is hard to take a step back and consider it is not quite as fabulous as originally thought, but what we see is still stunning... and it is still very, very big. To put that in perspective I think that even the small volcanoes in the region are stunning. We don't need to exaggerate something for it to inspire us.

Bibliography/References:

*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.
*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.
*Stevens, N.C., Knutson, J., Ewart, A. & Duggan, M.B. 1989. Tweed. In Johnson, R.W. (ed). Intraplate Volcanism in Eastern Australia and New Zealand. Cambridge University Press.

It is very popular to refer to the Mount Warning area as the Mount Warning erosion caldera or Tweed Shield erosion caldera. Many sources indicate that it is the biggest erosion caldera in the world. For example Bigvolcano or good ol' wikipedia use the term. There are some very informative books by top class geologists such as Rocks and Landscapes of South East Queensland by Warwick Willmott also use the term. It is certainly an imposing volcanic influences landscape, but what is an erosion caldera anyway.

Ok. Let us start somewhere definite. A geological dictionary definition. Lets just look at caldera: A large circular crater left after the collapse or explosion of a volcanic cone. Now, lets look at erosion: The wearing away of rocks or other materials by the action of water or ice or wind.

So, Adding the term erosion to the front of the word caldera implies this: a landscape formed through the actions of wind or water or ice but also simultaneously formed through the collapse or explosion of a volcanic cone. Hopefully, you agree that this is possibly misleading. We have two different formation concepts equally applicable at the same time. What gives? How can the same feature form at the same site twice? once by erosion and once from the collapse of a magma chamber.

Mount Warning was once the centre of a large volcanic cone called the Tweed Volcano. The centre of the Tweed Volcanic cone was a crater which may have collapsed or exploded as some stage to create a larger caldera. But this process is not definitely known because if this caldera actually existed it has since been eroded away to reveal the valley systems that we see today.

I think is is becoming obvious that there has been some sort of mistake in the development of the name erosion caldera. So, why use this term? A short answer is that most geologists tend not use this term, unless informally to illustrate the grand nature of some valleys that are formed in the remnants of large volcanos. That is not to say that some geologists don't mistakenly use the term anyway, I mean even geologists are human!

My suggestion, is not to use the term erosion caldera at all since it often results in confusion on the mechanism for the formation of thing it is actually used to describe.

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.

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.

References/Bibliography:

*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.

I think I will start by my first blog by asking a rhetorical question.

How big was the Mt Warning/Tweed Volcano? It is certainly not a question that just jumps into ones head unless you love geology!

The traditional view by respected geographers such as Cliff Ollier is that Mt Warning is a remnant of a huge shield volcano that erupted during a time that was known as the Cenozoic (or Tertiary). The extents of the shield was from Evans Head in the south to Mount Tamborine in the North to Mount Lindsay in the West to somewhere out to sea to the east. It must have covered around 7 000 square kilometres in area and been almost 2 000m high. Ferrett (2005) gives its height as 2000metres and a diameter of about 100km. It was big. But I think it is wrong. Well, at least partly wrong.

The funny thing about scientific discovery is that once one is made once something is finally thought to be understood, contradictory information is seen as too hard to deal with. It is a kind of scientific inertia. Especially once the general public think something is true. For example, I've heard again and again that the Great Wall of China is the only man made object to be seen from space (it cannot be seen; whereas cities, irrigation channels, farmland and other objects are seen commonly). This is true too for Mount Warning/Tweed Volcano.

I've been able to find some 'forgotten' (but not long forgotten) research recently that I think turns things on its head. These are:

Masters research from Southern Cross University (when they had a geology department) by Cotter (1998) (the only online reference I can find is here but there is a copy of his thesis in the archives of Southern Cross University, which you can read under supervision only!), A journal article by Duggan and Mason (1978) here and another journal article by Smith et al (1998) here.

Can you put it all together?

I even think that Duggan and Mason (1978) are a little generous with the Lismore Basalt. I think that more of what they called the Lismore Basalt (from Mount Warning/Tweed Volcano) is actually Kyogle Basalt (from Focal Peak/Mount Barney). This makes the extent to the west much less. At a push Smith et al (1998) show that there are no Cenozoic basalts exist in the Evans Head area. But most significantly Cotter shows a even more:

1. basalts between Evans Head and Alstonville are different compositionally from the Lismore Basalt and are probably part of the Chillingham volcanics and therefore they are Mesozoic aged (much, much older than the Tweed Volcano.
2. the land form would have directed lavas away from the south and
3. most of the basalt in the Lismore/Alstonville area is likely to be twice the age at around 40 Million years and definitely would not have been part of the hot spot volcanism that formed the Mount Warning/Tweed Volcano around 23 million years ago.

This all shows that a lot of the recent volcanic geology of the area needs to be reviewed (Is there a 'Alstonville Basalt'). Was the basalt around Alstonville actually similar to basalts in the New England tablelands (such as the Maybole Volcanics) which were associated with the formation of the Tasman ocean? What were the southern extents of the Lismore Basalt after all?


References/Bibliography:

*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.
*Ferrett, R. Australia's Volcanoes. New Holland Publishers 2005.
*Learned Australasian Volcanology Association, 1998. Lava News, December 1998.
*Smith, J.V., Miyake, J., Houston, E.C. 1998. Mesozoic age for volcanic rocks at Evans Head, Northeastern New South Wales. Australian Journal of Earth Sciences V45