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I'm curious if we see other unconformities related to other Ice Ages. If receding glaciers scraped away the earth and led to this big of a gap, shouldn't this h

I'm curious if we see other unconformities related to other Ice Ages. If receding glaciers scraped away the earth and led to this big of a gap, shouldn't this have happened again when the glaciers receded after more recent ice ages, like in the Pleistocene?

cbkeller 5y ago
Great question -- and yes! Pleistocene glaciation in the northern hemisphere has been a lot shorter (so far) than the Cryogenian glaciations, but it is probably not a coincidence that the outline of Canada's "Precambrian shield" basically matches the outline of the Laurentide ice sheet (check out Figure 5 of the 2019 paper [1]). We're probably only talking about scraping off no more than a couple hundred meters of sedimentary rock formerly covering the shield, but that's about what you'd expect. More broadly, as one author noted long before us [2] it turns out that most of the places on Earth where there is a lot of Precambrian crystalline basement exposed at the surface today (i.e., where later sedimentary rocks have been scraped off, one way or another) were glaciated either recently or in the Late Paleozoic Ice Age [3], which hit much of Gondwana (see also Fig S16 here [4]). More recently, another group of researchers using thermochronology in Antarctica [5] found evidence of several kilometers of exhumation during the Late Paleozoic Ice Age, as well as perhaps 1-2 km during the last ~35 Myr of Cenozoic glaciation (n.b., Antarctica has been glaciated for a good bit longer than we've been having ice ages in the northern hemisphere). [1] https://www.pnas.org/content/116/4/1136/tab-figures-data https://www.pnas.org/content/116/4/1136/tab-figures-data and see also Fig S16 in [4] [2] https://pubs.geoscienceworld.org/gsa/gsabulletin/article/83/4/1037/7516/Deep-Erosion-by-Continental-Ice-Sheets https://pubs.geoscienceworld.org/gsa/gsabulletin/article/83/... [3] https://en.wikipedia.org/wiki/Late_Paleozoic_icehouse https://en.wikipedia.org/wiki/Late_Paleozoic_icehouse [4] https://www.pnas.org/content/pnas/suppl/2018/12/26/1804350116.DCSupplemental/pnas.1804350116.sapp.pdf https://www.pnas.org/content/pnas/suppl/2018/12/26/180435011... [5] https://doi.org/10.1016/j.epsl.2018.10.044 https://doi.org/10.1016/j.epsl.2018.10.044 (see especially Fig. 7)
- User23 5y ago
  I'm going to be a bit fanciful here, but it's a serious question. What implications does this have, if any, for the possibility of human civilizations older than currently known being completely erased by the ice? Is that plausible? How much rock would have to get scraped and pulverized into future magma to wipe out all evidence of a civilization?
  - cbkeller 5y ago
    To start with the last part > How much rock would have to get scraped and pulverized into future magma to wipe out all evidence of a civilization? it would take a lot. Even in the snowballs, when we had ice on every continent, there are still plenty of basins (mostly at the continental margins) where syn-glacial sediments are preserved; that is in part how we know the glaciations happened. While we have maybe 1/5th as much sedimentary rock volume per unit time prior to the end of the unconformity, that still leaves a lot! At the time of the Cryogenian, both the fossil record and DNA-based molecular clocks suggest we didn't have multicellular animal life until after at least the first (Sturtian) glaciation. And we're talking basically just sponges (porifera) at first. Of course, it's not impossible we could have another snowball in the far future (probably unlikely for several reasons, but never say never), and the question of what that would do to the record of modern human civilization is an interesting one. The short answer is "I don't know", but I think it would be hard to erase all traces without something a good bit more severe than the erosion that produced the Great Unconformity.
    - User23 5y ago
      I’m under the impression that we’re still in the quaternary ice age. What makes a snowball impossible now? The division of the ocean in two that restricts convection and the large polar continent that provides a convenient place for ice sheets to increase the Earth’s albedo both suggest, to my admittedly limited understanding, that a severe glaciation isn’t out of the cards. I ask not to challenge but to learn. Edit: I can’t remember the name right now, but there are also a bunch of solar cycles that affected past glaciations. I’d be grateful for information on that too.

cbkeller 5y ago

Great question -- and yes! Pleistocene glaciation in the northern hemisphere has been a lot shorter (so far) than the Cryogenian glaciations, but it is probably not a coincidence that the outline of Canada's "Precambrian shield" basically matches the outline of the Laurentide ice sheet (check out Figure 5 of the 2019 paper [1]). We're probably only talking about scraping off no more than a couple hundred meters of sedimentary rock formerly covering the shield, but that's about what you'd expect. More broadly, as one author noted long before us [2] it turns out that most of the places on Earth where there is a lot of Precambrian crystalline basement exposed at the surface today (i.e., where later sedimentary rocks have been scraped off, one way or another) were glaciated either recently or in the Late Paleozoic Ice Age [3], which hit much of Gondwana (see also Fig S16 here [4]). More recently, another group of researchers using thermochronology in Antarctica [5] found evidence of several kilometers of exhumation during the Late Paleozoic Ice Age, as well as perhaps 1-2 km during the last ~35 Myr of Cenozoic glaciation (n.b., Antarctica has been glaciated for a good bit longer than we've been having ice ages in the northern hemisphere). [1] https://www.pnas.org/content/116/4/1136/tab-figures-data https://www.pnas.org/content/116/4/1136/tab-figures-data and see also Fig S16 in [4] [2] https://pubs.geoscienceworld.org/gsa/gsabulletin/article/83/4/1037/7516/Deep-Erosion-by-Continental-Ice-Sheets https://pubs.geoscienceworld.org/gsa/gsabulletin/article/83/... [3] https://en.wikipedia.org/wiki/Late_Paleozoic_icehouse https://en.wikipedia.org/wiki/Late_Paleozoic_icehouse [4] https://www.pnas.org/content/pnas/suppl/2018/12/26/1804350116.DCSupplemental/pnas.1804350116.sapp.pdf https://www.pnas.org/content/pnas/suppl/2018/12/26/180435011... [5] https://doi.org/10.1016/j.epsl.2018.10.044 https://doi.org/10.1016/j.epsl.2018.10.044 (see especially Fig. 7)

User23 5y ago
I'm going to be a bit fanciful here, but it's a serious question. What implications does this have, if any, for the possibility of human civilizations older than currently known being completely erased by the ice? Is that plausible? How much rock would have to get scraped and pulverized into future magma to wipe out all evidence of a civilization?
- cbkeller 5y ago
  To start with the last part > How much rock would have to get scraped and pulverized into future magma to wipe out all evidence of a civilization? it would take a lot. Even in the snowballs, when we had ice on every continent, there are still plenty of basins (mostly at the continental margins) where syn-glacial sediments are preserved; that is in part how we know the glaciations happened. While we have maybe 1/5th as much sedimentary rock volume per unit time prior to the end of the unconformity, that still leaves a lot! At the time of the Cryogenian, both the fossil record and DNA-based molecular clocks suggest we didn't have multicellular animal life until after at least the first (Sturtian) glaciation. And we're talking basically just sponges (porifera) at first. Of course, it's not impossible we could have another snowball in the far future (probably unlikely for several reasons, but never say never), and the question of what that would do to the record of modern human civilization is an interesting one. The short answer is "I don't know", but I think it would be hard to erase all traces without something a good bit more severe than the erosion that produced the Great Unconformity.
  - User23 5y ago
    I’m under the impression that we’re still in the quaternary ice age. What makes a snowball impossible now? The division of the ocean in two that restricts convection and the large polar continent that provides a convenient place for ice sheets to increase the Earth’s albedo both suggest, to my admittedly limited understanding, that a severe glaciation isn’t out of the cards. I ask not to challenge but to learn. Edit: I can’t remember the name right now, but there are also a bunch of solar cycles that affected past glaciations. I’d be grateful for information on that too.