Are we heading for an Ice Age

[AstralTraveller (David)]

Originally Posted by JimmyH155

40,000 years???? I thought it was more like 12,000 years ago the last one, and on average one occurs ev 10,000 years so we are 2,000 years overdue
I hope so because I want to sell up here in Brissie - should get lots of refugees from the southern states, and I can double my asking price

Actually the period is about 100,000 years over the last 5-600,000 years. Prior to that the 40,000 year period dominates the record. The last clacial maximum (ie the last ice age) peaked at about 20,000 years ago. The 12,000 year date is for the end of the Younger Dryas event which was a widespread (global??) cooling thought to be caused by the sudden release of fresh meltwater into the North Atlantic Ocean which shut down the gulf stream (thus freezing Europe) and also shut down the 'great conveyer'.

[AstralTraveller (David)]

Originally Posted by renormalised

You're correct, Les. Glacial valleys have a characteristic "U" shape and not a "V" shape.

The 'V' for river 'U' for glacier is not completely correct. As far as I know glaciers do erode V valleys into U valleys, and straighten them (glaciers don't meander). Half Dome in Yellowstone would be the text book example. However rivers can produce both V and U profiles. The controls are many, varied and debated in the literature.

[renormalised (Carl)]

Originally Posted by AstralTraveller

The 'V' for river 'U' for glacier is not completely correct. As far as I know glaciers do erode V valleys into U valleys, and straighten them (glaciers don't meander). Half Dome in Yellowstone would be the text book example. However rivers can produce both V and U profiles. The controls are many, varied and debated in the literature.

Yes, rivers can produce U shaped profiles but that's not usually the case in most instances. By and large, they produce V shaped profiles. The glaciation in Oz was more of a small icecap type of cover rather than mostly individual valley glaciers. The morphology of the Australian Alps and the surrounding areas is indicative of this. Plus you also have to remember that this ice was deposited on top of an already existing erosional surface (a peneplain). Actually, the whole of the GDR, as it is now, is a fairly recent landform...only began to form around 20-25Ma when the eastern margin of the continent underwent epeirogenic downwarping. The peneplain I speak about is easily seen, if you know what you're looking for. Go to the top of Koscuisko and look out over the surrounding peaks and off to the horizon. You'll see they're relatively flat lying and that you can trace out a surface running from peak to peak. That's the old peneplain...it used to be at (or close to) sea level once

Actually, the whole eastern seaboard is one side of a rift valley...the other half is the Lord Howe Rise, New Caledonia (on the Norfolk Rise) and New Zealand. The floor of the valley is the Tasman Sea. It hasn't quite opened up completely as it's nearly joined to Oz at a point just off Rockhampton.

Just as an aside, the Coral Sea is a failed rift, where the tail of PNG began to rift away from Oz (off the Queensland Plateau) around 15Ma but never completely split.

[multiweb (Marc)]

Hey Carl, as you seem to know your stuff, are we fairly safe from earthquakes in Australia?

[renormalised (Carl)]

We don't get them like they do in Indonesia or NZ, however, we do get them every now and then. Most of the earthquakes in Oz are of the intra-plate variety...they occur where old faults do a bit of shifting around due to stresses building up in the plate. There are a number of old rift and fault zones in Oz that occasionally shift about. Most of the earthquakes we get are of moderate size (2.5-4 on the Richter Scale) but we do get the odd big guy every now and then. We get the smaller earthquakes all the time, it's just that you don't feel them as they're too small.

[multiweb (Marc)]

Originally Posted by renormalised

We get the smaller earthquakes all the time, it's just that you don't feel them as they're too small.

So if we get a lot of small one that release the "pressure" it's fair to assume we're less likely to cope a big one?

[renormalised (Carl)]

Australia normally gets a mag 7 quake about once every 100 years. All those smaller quakes smaller than 2.0 don't do much except tell you the plate is moving. In some cases, they can build up stress in the plate. They can relieve stress as well, it all depends on the rocks it occurs in and the geometry of the faulting.

[AstralTraveller (David)]

Originally Posted by renormalised

Yes, rivers can produce U shaped profiles but that's not usually the case in most instances. By and large, they produce V shaped profiles. The glaciation in Oz was more of a small icecap type of cover rather than mostly individual valley glaciers. The morphology of the Australian Alps and the surrounding areas is indicative of this. Plus you also have to remember that this ice was deposited on top of an already existing erosional surface (a peneplain). Actually, the whole of the GDR, as it is now, is a fairly recent landform...only began to form around 20-25Ma when the eastern margin of the continent underwent epeirogenic downwarping. The peneplain I speak about is easily seen, if you know what you're looking for. Go to the top of Koscuisko and look out over the surrounding peaks and off to the horizon. You'll see they're relatively flat lying and that you can trace out a surface running from peak to peak. That's the old peneplain...it used to be at (or close to) sea level once

Actually, the whole eastern seaboard is one side of a rift valley...the other half is the Lord Howe Rise, New Caledonia (on the Norfolk Rise) and New Zealand. The floor of the valley is the Tasman Sea. It hasn't quite opened up completely as it's nearly joined to Oz at a point just off Rockhampton.

Just as an aside, the Coral Sea is a failed rift, where the tail of PNG began to rift away from Oz (off the Queensland Plateau) around 15Ma but never completely split.

Carl, this post nearly got me into trouble. I recognised the old myths and legends about peneplains and the late Tertiary timing of the Kosciusko Uplift but wanted to check a few things before I replied. So I asked a couple of blokes at morning tea about the history of the GDR and they tried to hand me a pile of undergrad essays on the Landscape Evolution of Eastern Australia to mark (it's that time of session) . I tried to claim that I am not qualified and they said I was qualified enough, so I said that I don't get paid enough to do that and they replied that I get more than the pg students who do plenty of marking. In the end they let me off.

The rifting of the Tasman Sea started at 85Ma and it is believed that the uplift which must have preceeded the rifting would have begun at about 100Ma. By about 50-60Ma the landscape was pretty much as we see it today. This is demonstrated by the distribution and age of basalt flows. In the southern Sydney Basin basalt eruptions flowed down into pre-existing valleys in a landscape with at least 250m of relief. These basalts have K-Ar ages of 45-50Ma. Since then there has been downcutting of 30-40m, though as little as 10m at some sites. (From memory, in analogous sites elsewhere in the region the downcutting since 50Ma may be as much as 100m, but that is still very little over that time span.) The basalt work was done around 1985 or earlier so it should be well known. I am aware that some 'authorities' are reluctant to accept this conclusion and that said resistance my have little to do with the dispassionate examination of the evidence. (Nuff said.)

Peneplains do exist but they have been invoked carelessly at times. Regarding the Sydney Basin they got knocked on the head in the late 1970s. Any near-planar surface in that Basin is a lithologically controlled erosional surface. One of the best papers I ever read was RW Young 1978 Australian Geographer. In this paper Bob firstly traces the origin and development of the idea and then proves that it is false. It seems that in ca. 1910 the famous American geomorphologist Andrews visited Sydney, but not the Blue Mountains. He commented that _if_ this area had developed in the same way as the Appalacians in the USA then the upper surface was a peneplain. Fair enough. Over time and by repeated retelling the possibility became a probability, became the accepted mechanism until finally evidence that contradicted the 'wisedom' was disgarded as faulty. When it was realised that some of the 'peneplain' surfaces were at different elevations faulting was invoked to explain that, even though no faults were ever mapped (they don't exist).

The academics I spoke to are willing to go further and state that there are no peneplains along the eastern seaboard. I thought I saw some in the New England area once but closer inspection made me realise that it was an illusion. I know what you mean about the peaks in the High Country of Vic being at about the same height but I am not convinced you can invoke a peneplain. For one thing they are all peaks - there are no planar remnants. I might look into that a bit further if I get a chance (and motivation.)

Below are a couple of refs that I found after a quick search. Obvious there are more but I haven't had time to find them.


Young, R. W. and McDougall, Ian(1985)'The age, extent and geomorphological significance of the Sassafras basalt, south-eastern New South Wales', Australian Journal of Earth Sciences, 32:4, 323 — 331

Young R. W. 1977. Landscape evolution in the Shoalhaven River catchment of southeastern New South Wales. Z. Geomorph. 21, 262-283.

[renormalised (Carl)]

The problems with just looking at the geology of one specific region is that you can get lost in the geology of that region, where different circumstances may have obliterated evidence for what you're looking for. If you look along the entire length of the GDR, you'll find in some parts where it's better preserved than at others, a planar surface (or remnants thereof) that sits at about 600m in altitude. In some parts it can be traced for many kilometres. The ranges just west of where I live is an example. It's the result of the late Tertiary uplift.

I'll have to go through all my old papers and dig up a few more, but I'll see what I can find on this. Might even see if I can contact some of my old lecturers and ask them. I know Dave Johnston lives in Hughenden (retired to dinosaur country!!!)...he wrote a book on the subject. Might buy it actually. Funny, old Dave....we used to call him "Ned" because he had a "Ned Kelly" beard One of the guys drew a cartoon with all the lecturers in it...sort of a caricature of them and their idiosyncrasies. Really funny

But regardless of all that, you know what really annoys me...Oz had some really great dinosaur fossils, unfortunately they're now 1500km away and 1000m down underwater!!!!!.

Also, it's a pity our mountains are so low. We don't have a "Mt Warning" anymore, except for its remnants. Would be good to see a 9500-10000 foot high mountain in Oz, even if it is a volcano.