Tuesday 30 July 2013

Blog update #5

Wow! 50,000 page views earlier today. Admittedly, about 20% seem to be bots.  I could never have thought that in less than two years, I should get so many visits to my blog. I certainly hope those visitors find what they are looking for or at least something of interest when reading my blog.

I've been very busy the last few weeks and will continue to be in the weeks coming. This means that I may be a little slow in getting new blog posts up. I normally aim to get one up a week but this time frame might double in the near future. There is no shortage of material to discuss, I'm not sure there ever will be, but simply having the time to discuss the material is my biggest limitation.

In the coming months, there is a few subjects that I'd like to post on. These are gold in the Orara River area, granites of the New England Batholith and touch on recent theories about the development of the Texas-Coffs Harbour Orocline/mega-fold.

In the mean time, thanks to all my readers, regular or just those that have stumbled by. I appreciate your comments and questions and I try to get decent answers to all of them, so please keep commenting. If you have a detailed question, a picture or information I might be interested in, please email me. My email address can be found by clicking on the "About This Blog" tab at the top of the page and scrolling down.

Thank you too to my wife Beck who has edited and proof read many of my latest posts. The clarity of language and ease of reading has certainly increased dramatically since I have received her help.

Saturday 20 July 2013

It's a Demon of a Fault

Many people have requested that I do a post on the Demon Fault. I've struggled to put something together because structural geology is not one of my strong points and secondly because there was so little published information about it, except for some specific papers in the 1970’s. Thankfully, a few months ago Babaahmdi & Rosenbaum (2013) published a detailed paper summarising what was known in the 1970s, presenting how the fault appears, how it seems to have developed and how it may fit into the development of eastern Australia. It is worth noting that Gideon Rosenbaum from The University of Queensland has been the major researcher on New England structural geology for the last 5 years. If it was not for him and his student’s research we would be struggling to understand some of the basic features of the older rocks of the region including the Demon Fault.

Large faults are usually have quite distinctive landscape features. The Demon Fault is mainly a transverse type fault (movement on the fault horizontally rather than vertically) which displays very obvious topographical features.  Transverse faults often form valleys, where the rock of the fault has been broken down into what is called gouge or rock-flour. Gouge is very weak material. It is easily eroded and rivers often preferentially follow the route of the fault carving out the gouge into deep valleys.  The presence of deformational features in the surrounding rock can give an indication of how deep the fault was when it was active.

The Demon Fault is a prominent feature because it is evident from a series of valleys from near the Queensland Border to Dorrigo. At the Queensland end it is partly obscured by the Cenozoic Main Range Volcanics and in the Dorrigo area the end is obscured by the Cenozoic aged Ebor Volcanics. Geological maps of the area show a nice linear feature with obvious truncation of pre-existing geological units. Aerial photos also show the fault up nicely with streams preferentially flowing along the trace of the fault and contrasting with the rugged forested mountains surrounding it. I’ve never taken a photo of any part of the Demon Fault but a nice photo taken from an aeroplane can be found here: http://www.panoramio.com/photo/35890361

The Timbarra River has followed the Demon Fault creating linear valley
http://www.panoramio.com/photo/35890361 (used with permission)

Korsch et al (1978) observed that the Demon Fault had displaced several geological units including intrusions of the Bungulla Monzogranite (now known as the Rocky River Monzogranite), Dumbudgery Granodiorite and Newton Boyd Granodiorite as well as the Drake Volcanics. The fault was interpreted as a dextral strike-slip fault (a fault where the eastern side had moved south relative to the western side). Korsch et al (1978) calculated that the fault had displaced these units 17km which is substantial in Eastern Australia. Dating of the displaced granite intrusions provides a possible maximum date of within Triassic period (249-232 million years). The nature of deformation features adjacent to the faulting indicates that the fault was shallow and/or was created in a brittle environment. Badaahmadi & Rosenbaum (2013) speculate that the timing of the faulting may actually be similar to that of faulting and extension in the earth’s crust that formed the Ipswich and Clarence-Moreton Basins (more about this in future posts).

There are many factors in understanding the Demon Fault. It is interesting to note that other authors have come up with different lengths of displacement including 30km in the northern part of the fault and 23Km in the central part. Badaahmadi & Rosenbaum (2013) have calculated that the northern part of the fault displaced 35km, in the centre by 25km and south by 19km. Some components of reverse faulting (where one side of the fault slid down and away from the other side) were observed. Additionally, it was noted that the Demon Fault did not appear to follow one big long line but instead had numerous splays (deviations, splitting, etc) especially in the south. Badaahmadi & Rosenbaum (2013) suspect that there may be two causes to the different lengths:
  1. Splays may have created movement of the fault which had a vertical component as well as horizontal.
  2. There may have been some fault reactivation of the northern part of the fault as recently as the Cenozoic era.
Both of these possibilities really need a discussion in their own right, rather than cursory mention. So, I’ll get back to these in a future post.  I also want to cover the significance of the Demon Fault in formation of the Texas and Coffs Harbour Oroclines which are incredibly large features that I’ve briefly touched on in an earlier post about the South Solitary Island.

*Babaahmadi, A. & Rosenbaum, G. 2013. Kinematics of the Demon Fault: Implications for Mesozoic strike-slip faulting in eastern Australia. Australian Journal of Earth Sciences. V.60
*Korsch, R.J., Archer, R. & McConachy, G.W. 1978. The Demon Fault. Journal and Proceedings, Royal Society of New South Wales. V111.

Sunday 14 July 2013

Are our volcanoes extinct?

Firstly, I've been a bit quiet on the blogging front for a couple of weeks. There has been a lot going on personally which has meant very little time for research or blog posts. I usually have a few scheduled posts up my sleeve for those times when I simply don't have the time... but as a measure of how busy I've been, even these have run out.

Having said all that, I must point out another interesting post by New England self-government advocate Jim Belshaw. It is interesting because it takes us back over a hundred years and shows us that we can sometimes have a little laugh about silly geological ideas from back then. But, it is important to know that miss-understandings of geology continue to this day, including a belief by some that Mount Warning (for example) might erupt again or that we are due for a magnitude 7.0 earthquake etc.

Tuesday 2 July 2013

Pacific Islands on holiday to the North Coast

I often find some stories in newspapers touch too lightly on the subject of geology. These articles are often quite limited in scope and generally indicate quite simplistic notions of natural processes. This morning when reading a local newspaper The Tweed Daily News, I came across one such article. A link can to the article can be found here. This article is interesting because it covers some surprising points, but as Dr Malcom Clark an Environmental Geochemist from Southern Cross University implies in the article, there is more to the story than just a once-off beaching of pumice on Kingscliff Beach.

Pumice is a highly vesicular (aerated) volcanic glass. It is created when super-hot, highly pressurized rock is violently ejected from a volcano, especially those found in volcanic island arcs which are near active subduction zones. The unusual foamy feel of pumice occurs because of simultaneous rapid cooling and rapid depressurization. During the eruption the air bubbles are frozen in the rock. The amount of air trapped means that pumice usually has the unusual property of a rock being able to float on water.

If we work backward in time from the Tweed Daily News article a story starts to emerge on how the pumice on the beach got there. The first thing to note is that there are no active volcanoes on the Australian mainland or close to the eastern Australian coast. So, the pumice must have been brought in from somewhere else. Pumice has been common on Byron Bay beaches for the last few weeks ever since winter storms gave a good battering the coast in June. But a large amount of Pumice was also observed on the Queensland sunshine coast in April following late summer storms and the tail ends of cyclones. The storms force floating materials like rubbish and pumice onshore. This gives a clue about movement. It has taken a month or two to travel down the east coast on prevailing currents such as the south moving Eastern Australian Current. But there are no active volcanoes in Queenland either.

Bryan et al (2004) published an interesting article in Earth and Planetary Science Letters on pumice that was washed ashore all down the east coast of Australia in 2002. Here lies more of the answer. Bryan et al (2004) demonstrated that the pumice rafts were transported a vast distance across the Coral Sea and South Pacific Ocean, taking about almost a year to complete its trip on the prevailing currents and winds (the pumice was even blown backwards at one stage by a tropical cyclone). Surprisingly the 2002 Pumice landfall came from the Tonga area (North of an island and seamount chain that stretches to New Zealand called the Kermadec Islands), which is a long way away! Between the Kermadec Islands and Australia lies the Solomon Islands, Vanuatu and Fiji which all have active volcanic systems.  However, The pumice that washed ashore in 2002 was erupted in a submarine volcano (underwater) un-excitingly named Volcano 0403-091 from the Kermadec Islands and swept past all the other islands.

As for the current pumice landfall, in the last year there has been several eruptions of island arc volcanoes the Vanuatu islands, but significantly in July 2012 there was a major eruption of pumice from the vicinity of the Havre Seamount in the Karmadec Islands (Smithsonian Institute 2012). The time between eruption and East Australian landfall is interesting because it is similar to that for the 2001-2002 event discussed by Bryan et al (2004). More recently in 2012 an article was published (Bryan et al 2012) that demonstrated that rapid and long distance movement can be a frequent occurrence. So, maybe the pumice on our beach today this is just a little bit of history repeating – a bit of a pacific volcano on a holiday to the north coast of New South Wales.


Scott Bryan sent me this email yesterday. Being so informative I thought I should post it here.

Hi Rodney,
...I was actually at point lookout (nth Stradbroke) today collecting the pumice. The pumice is indeed from the Havre submarine eruption in the Kermadecs last year. There is a good summary of the eruption and discovery of the pumice rafts at the global volcanism program of the smithsonian institution (USA) at www.volcano.si.edu. 
This pumice is distinctive in being white when fresh; there is also a lot of grey/dark grey pumice at north stradbroke which is from tonga and the previous eruptions I have published on. It has been eroded out of the beach dunes.

The main influx along our shores began in mid-late march, continuing up to early May. There has been a bit of a break, but with the windy and wild weather this last weekend, some more pumice has come in, as well as probably reworked material (abraded and cleaned of attached biota) which seems to be what has washed up at Kingscliff. Newly washed up pumice will be covered in a black or dark green slime (Cyanobacteria) and be loaded with lots and large goose barnacles. You will also find on closer inspection, some molluscs, bristle worms (feeding on the barnacles), bryozoans, hydroids, anemones. Look up Denis Riek and his web page www.roboastra.com - he has taken some fantastic close ups of the pumice and biota found on it at Brunswick Heads.

This pumice has travelled about 3000 km in 8-12 months. We have observed it as far north as Heron Island.

Let me know if you need more info.

I would appreciate further reports of any new strandings as I have a Masters student beginning her research on this pumice and the attached biota. New strandings give us a temporal perspective as the biota mature and diversify with time and also begin recruiting species locally.


*Bryan, Scott Edward, S., Cook, Alex, Evans, Jason, Hebden, Kerry, Hurrey, Lucy, Colls, Peter, Jell, John S., Weatherley, Dion, & Firn, Jennifer (2012) Rapid, long-distance dispersal by pumice rafting. PLoS ONE, V7.
*Byran, S.E., Cook, A., Evans, J.P., Colls, P.W., Wells, M.G., Lawrence, M.G., Jell, J.S., Greig, A. & Leslie, R. 2004. Pumice Rafting and faunal dispersion during 2001-2002 in the Southwest Pacific: record of a dacitic submarine explosive eruption from Tonga. Earth and Planetary Science Letters V227.
*Smithsonian Institute 2012. Havre Seamount. Bulletin of the Global Volcanism Network. Smithsonian Institute. September 2012.