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The Physical Geography Thread

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  • El Siglo wrote: »
    Remember, glaciers are extremely heavy things (hint hint...;)). The sea-level we have now is a result of two things; glacial isostacy or isostatic rebound and eustacy (sea-level rise). Due to the weight of the ice (especially in the north of Ireland where one of the ice sheets advanced from) actually had enough weight to 'push down' as a manner of speaking the 'land level' (i.e. the actual physical mass of Ireland) hence sea-level appears higher but it's a result of isostacy and not eustacy (i.e. that sort of explains Fig 2). What happens and this is where it gets really confusing is which takes place faster or slower; isostacy or eustacy and in some cases they cancel each other out or we see drops or rises in sea-level. It gets a bit hairy and my explanation probably does it no justice, but I'm currently in the lab and my neck is killing me!:D

    Don't worry, I'm by no means not an expert on the subject! Glad you said "geological ignorance", some people don't even consider that kind of research to be geology, would you believe!:pac:

    Thanks for the information El Siglo. Sounds very complicated - i'd hate to try and figure out the maths for isotacy vr's eustacy vr's glacial melt and all. I must get myself a good textbook on Geology.




  • Anyone help with out with a bit of clarification please?
    I've a geog exam tomorrow - 2nd yr 3rd level. I'm just a bit confused about the closure of the Iapetus ocean, do I have this right?

    It closed during the late silurian / early Devonian period; during the Ordovician period there was a period of volcanic activity and these formed island arcs in the Iapetus. it was the subduction of the ocean floor causing the island arcs to collide with the laurentia continent causing the caledonian orogeny.

    Does any of that make sense.... help please




  • angeldaisy wrote: »
    Anyone help with out with a bit of clarification please?
    I've a geog exam tomorrow - 2nd yr 3rd level. I'm just a bit confused about the closure of the Iapetus ocean, do I have this right?

    It closed during the late silurian / early Devonian period; during the Ordovician period there was a period of volcanic activity and these formed island arcs in the Iapetus. it was the subduction of the ocean floor causing the island arcs to collide with the laurentia continent causing the caledonian orogeny.

    Does any of that make sense.... help please

    Hi,

    You're pretty much spot on. The Ordovician volcanics only occur south of the Iapetus Suture, on what would have been Avalonia.

    The ocean closed (due to subduction) and Avalonia crashed into Laurentia, with the join being the Iapetus Suture. It runs from Limerick to Louth, approximately. The trend of the Caledonian Orogeny is parallel to that (SW-NE).

    Good luck.

    Iapetus_1.jpg




  • thank you so much for that, I think I'm working myself round in circles!




  • I think I have learned the meaning of two new terms here isotacy and eustacy. Isotacy is the effect of the weight of ice pushing the land mass down and eustacy is the effect of rising sea levels?

    I need to get a rough idea what sea levels were like in Ireland between the first and fifth centuries AD. Were they higher or lower than today, if so then roughly by how much?
    Thanks.


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  • slowburner wrote: »
    I think I have learned the meaning of two new terms here isotacy and eustacy. Isotacy is the effect of the weight of ice pushing the land mass down and eustacy is the effect of rising sea levels?

    I need to get a rough idea what sea levels were like in Ireland between the first and fifth centuries AD. Were they higher or lower than today, if so then roughly by how much?
    Thanks.

    You kind of have it. Isostasy is just the reaction of the land mass to extra weight, so it can be upwards or downwards. It happens slowly, which is why most of Northern Europe is still on the way back up since the end of the last ice age.

    Sea levels between 1st and 5th centuries would have been much the same as they are now. There was a warming period later than that, but it's not as if it would have had any kind of significant impact on sea level. I don't have any figures for this, but I really don't think there can have been an appreciable difference between then and now.




  • angeldaisy wrote: »
    thank you so much for that, I think I'm working myself round in circles!

    You're welcome. How did it go?




  • You kind of have it. Isostasy is just the reaction of the land mass to extra weight, so it can be upwards or downwards. It happens slowly, which is why most of Northern Europe is still on the way back up since the end of the last ice age.

    Sea levels between 1st and 5th centuries would have been much the same as they are now. There was a warming period later than that, but it's not as if it would have had any kind of significant impact on sea level. I don't have any figures for this, but I really don't think there can have been an appreciable difference between then and now.

    That's a very contentious point and I've had many arguments over that one. There's evidence that sea-level was reasonably lower than present, definitely in the north coast areas where you see the greatest isostacy taking place (where the rise in sea-level is negated by the fact that isostacy is of a much greater magnitude between 1.3 - 3 mm per annum). I wouldn't say that sea-level was terribly lower than present (not like the LGM), but I wouldn't be inclined to say that the 1st to 5th century and now are the same.

    Sorry if I sound like a bit of a dick, I don't mean to be at all.




  • El Siglo wrote: »
    That's a very contentious point and I've had many arguments over that one. There's evidence that sea-level was reasonably lower than present, definitely in the north coast areas where you see the greatest isostacy taking place (where the rise in sea-level is negated by the fact that isostacy is of a much greater magnitude between 1.3 - 3 mm per annum). I wouldn't say that sea-level was terribly lower than present (not like the LGM), but I wouldn't be inclined to say that the 1st to 5th century and now are the same.

    Sorry if I sound like a bit of a dick, I don't mean to be at all.

    No, you're not being a dick. I don't disagree that it was probably a bit lower, I suppose my point was that for the purposes of the question asked, it probably wouldn't have made much difference at all.




  • El Siglo wrote: »
    That's a very contentious point and I've had many arguments over that one. There's evidence that sea-level was reasonably lower than present, definitely in the north coast areas where you see the greatest isostacy taking place (where the rise in sea-level is negated by the fact that isostacy is of a much greater magnitude between 1.3 - 3 mm per annum). I wouldn't say that sea-level was terribly lower than present (not like the LGM), but I wouldn't be inclined to say that the 1st to 5th century and now are the same.

    Sorry if I sound like a bit of a dick, I don't mean to be at all.
    You raise my hopes and dash them all in the one breath.
    I had been expecting/hoping that sea levels might have been higher during this period.
    If they had been higher, a particular river I am interested in might have been navigable further upstream - this would have been exciting news.
    Ah well.


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  • No, you're not being a dick. I don't disagree that it was probably a bit lower, I suppose my point was that for the purposes of the question asked, it probably wouldn't have made much difference at all.

    No worries, I know what you mean now! :pac:
    slowburner wrote: »
    You raise my hopes and dash them all in the one breath.
    I had been expecting/hoping that sea levels might have been higher during this period.
    If they had been higher, a particular river I am interested in might have been navigable further upstream - this would have been exciting news.
    Ah well.

    Well actually who's to say that the river wasn't navigable upstream? If you're looking at the medieval warm period then it is quite possible because we would have had more rain (i.e. more energy means that clouds would be able to hold more water and then release more water) which may mean that there may have been raised water levels within rivers and possible larger water discharge. So if you have more water say within the fluvial system then there could have been deeper water further upstream. Also, the land level (depending on whether it's down south or north) may have been lower if it was down south, so there still might be a possibility for the situation that you're looking at. It's just a hypothesis, I could be completely wrong, you may go out and test for it. If I were you I'd speak to Robin Edwards (sea-level expert) in TCD and Conor Murphy in NUIM (he's a hydrologist, knows his stuff).




  • slowburner wrote: »
    You raise my hopes and dash them all in the one breath.
    I had been expecting/hoping that sea levels might have been higher during this period.
    If they had been higher, a particular river I am interested in might have been navigable further upstream - this would have been exciting news.
    Ah well.

    Unlucky, sb.




  • El Siglo wrote: »

    Well actually who's to say that the river wasn't navigable upstream? If you're looking at the medieval warm period then it is quite possible because we would have had more rain (i.e. more energy means that clouds would be able to hold more water and then release more water) which may mean that there may have been raised water levels within rivers and possible larger water discharge. So if you have more water say within the fluvial system then there could have been deeper water further upstream. Also, the land level (depending on whether it's down south or north) may have been lower if it was down south, so there still might be a possibility for the situation that you're looking at. It's just a hypothesis, I could be completely wrong, you may go out and test for it. If I were you I'd speak to Robin Edwards (sea-level expert) in TCD and Conor Murphy in NUIM (he's a hydrologist, knows his stuff).
    Thanks very much for the contacts El Siglo. Much appreciated. I'll send an email or two, one of these days.

    The river in question is the Avoca in Wicklow. It is very much a spate river, subject to rapid rises and falls in water levels. The descent of the river is precipitous in places.
    In high water, it is a raging torrent and in low water, it is far too shallow for the passage of most boats - you could easily cross the shallows in your wellies.

    Whether or not the topography made for a river of a different character in the late iron age/early medieval, is another question.
    The brute force of some of its floods are astonishing. I have witnessed maybe four or five significant floods on this river in recent years; each one had the capacity to alter the river's topography - add those together over a thousand or so years and there could indeed have been quite a bit of change to the structure of the river.

    That said, I suspect that the only likelihood of it being navigable further upstream than today, would be if higher tides caused freshwater to back up and become deep and 'slack' - if you know what I mean. Today the river is tidal for perhaps a couple of kilometres or so above the mouth at Arklow. I heard mention somewhere that it was tidal to a point some 8 - 10 km up to Woodenbridge - but I am unsure what period this was.




  • I was just reading this. I seen about the laurentian plate. I know that we and scotland were apart of it. Does that mean that the rock here is the same type found in north america for example the canadian shield and what caused us to break off anyway. Surely we can't just break off a plate can we?




  • owenc wrote: »
    I was just reading this. I seen about the laurentian plate. I know that we and scotland were apart of it. Does that mean that the rock here is the same type found in north america for example the canadian shield and what caused us to break off anyway. Surely we can't just break off a plate can we?

    Yes, there are some very similar rocks between here and Newfoundland for example.

    Plates have changed, disappeared and grown throughout Earth's history. For example, at the moment, the Juan de Fuca plate is being subducted under the North American Plate, and it was part of a much bigger plate, most of which is now gone.




  • Ok,
    Lest finish the year with a tasty question, hopefully someone can provide an even tastier answer...

    Please tell me how Sea level rise from the Holocene is responsible for the formation of barrier island lagoons in South east Ireland.

    I know the barriers are sorted glacial deposits and that the lagoon is on the backside of them (not seawards).
    But how is sealevel linked to their formation?




  • Eh...the sea flooded the land between them and the coast? ;)




  • Eh...the sea flooded the land between them and the coast? ;)

    Woww!!!!! What an intelligent reply , you really know what you're talking about don't you?




  • Ok,
    Lest finish the year with a tasty question, hopefully someone can provide an even tastier answer...

    Please tell me how Sea level rise from the Holocene is responsible for the formation of barrier island lagoons in South east Ireland.

    I know the barriers are sorted glacial deposits and that the lagoon is on the backside of them (not seawards).
    But how is sealevel linked to their formation?

    I went through a thesis there from an American chap, and he has given probably one of the more understandable explanations regarding your question:
    Transgressive shorelines along the Atlantic coast of North America [still very applicable to Ireland] are dominantly influenced by relative sea level rise and a low contribution of sediments from rivers and streams draining into the area (Kraft, 1978). The balance between wave energies, tidal ranges and prisms, erosion rates, and antecedent topography are the driving forces in barrier island morphology and migration (Dalrymple et al., 1992; Harris et al., 2002). Under these conditions, if sea level rise and erosion rates are balanced, barriers may migrate more or less continuously landward as sea level rises. Any back barrier lagoonal facies would likely be eroded in this process (Reinson, 1992). Alternatively, if the rate of sea level rise exceeds erosion rates, barriers may remain in place as sea level rises to the level of the top of the dunes; then the surf zone may “jump” landward to establish a new shoreline, thus drowning the barrier in place (Sanders and Kumar, 1975). In this case, an entire sequence of transgressional lagoon facies may be preserved (Reinson, 1992).

    You might want to read on some of the stuff by JAG Carter in UU, J. Orford in QUB etc... They're experts on gravel barriers and will probably have a few articles in Marine Geology and Zeitschrift für Geomorphologie.




  • El Siglo wrote: »
    I went through a thesis there from an American chap, and he has given probably one of the more understandable explanations regarding your question:


    You might want to read on some of the stuff by JAG Carter in UU, J. Orford in QUB etc... They're experts on gravel barriers and will probably have a few articles in Marine Geology and Zeitschrift für Geomorphologie.

    Cheers that gives me a better idea , will look them up .


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  • Hey ,
    Is there any papers published on the flooding event in Ireland in 2011?

    Or storm/climate related flooding from rivers in Ireland... I can't find much in the google scholar search....




  • Hey ,
    Is there any papers published on the flooding event in Ireland in 2011?

    Or storm/climate related flooding from rivers in Ireland... I can't find much in the google scholar search....

    There was something in the last issue of Irish Geography regarding the 2009 Cork city flooding: "The Cork City flood of November 2009: Lessons for flood risk management and climate change adaptation at the urban scale", by Jim Jeffers.

    However, don't expect to see any articles in peer reviewed journals for some time on events that have happened in 2011. I mean the data still probably has to be collected from tide gauges and weir stations along with the meteorological data and and then it has to be analysed which could take quite some time and finally it is written up. All in all, for a good journal article you're talking a year of work, maybe more.




  • Hey ,
    Is there any papers published on the flooding event in Ireland in 2011?

    Or storm/climate related flooding from rivers in Ireland... I can't find much in the google scholar search....
    Some data here on river flows, if it's of any use to you.
    http://hydronet.epa.ie/introduction.htm

    or straight here for the data
    http://hydronet.epa.ie/hydronet.html




  • El Siglo wrote: »
    There was something in the last issue of Irish Geography regarding the 2009 Cork city flooding: "The Cork City flood of November 2009: Lessons for flood risk management and climate change adaptation at the urban scale", by Jim Jeffers.

    However, don't expect to see any articles in peer reviewed journals for some time on events that have happened in 2011. I mean the data still probably has to be collected from tide gauges and weir stations along with the meteorological data and and then it has to be analysed which could take quite some time and finally it is written up. All in all, for a good journal article you're talking a year of work, maybe more.

    Good stuff! That will do nicely


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