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Ah yes, quite right on the first part! The catch is that to be preserved, those sediments have to be deposited somewhere else on the continental crust, say in e

Ah yes, quite right on the first part! The catch is that to be preserved, those sediments have to be deposited somewhere else on the continental crust, say in epicratonic seas [1] or in subsiding basins [2]. If the sediments wash all the way off the continental shelf and onto the ocean crust, then they'll ultimately get subducted into the mantle! The catch with glaciers is that putting a lot of big glacial ice sheets onto the continents takes that water out of the oceans, and lowers sea level -- meaning a bunch of places where you could normally preserve sediments on the continents will be above sea level during the glaciations (AKA above "base level"), and more of those sediments will be washed off the continents entirely. There still some places where you can find the fossilized glacial till from these Cryogenian glaciations (which is in part how we know they happened), but it's basically only in the tectonic basins at the margins of the continent that were subsiding fast enough to stay below base level and not get eroded away. [1] https://en.wikipedia.org/wiki/Inland_sea_(geology) https://en.wikipedia.org/wiki/Inland_sea_(geology) [2] https://en.wikipedia.org/wiki/Sedimentary_basin https://en.wikipedia.org/wiki/Sedimentary_basin

throwawaylinux 5y ago
Thanks, great answer. So it's not that sediments in the ocean can't be found so much as they don't stay around that long you'd find enough of it to be able to account for that massive glacial erosion event, due to tectonic movement?
- cbkeller 5y ago
  Yeah, the oldest ocean crust on Earth today is only about 180 million years old, and the older it gets the colder and denser (and thus more prone to subduct) it gets. And when ocean crust subducts it generally (with some caveats) takes the sediments that have accumulated on top with it. So we still have ocean crust from when T. rex lived, for instance, but none from when trilobites lived, and definitely none from the time of these glaciations.
  - morphle 5y ago
    I didn't know that the colder and denser ocean crust gets it thus is more prone to subduct. Does that also imply the denser crusts subducts deeper towards the earth center or does the colder crust prevent it from going deeper? Or does the angle of friction of ocean crust with the underside of the continental crust dominate the angle of subduction deep into the mantle? How mindblowing to imagine. We should make a simulation of the entire earth history where you could follow the atoms and molecules from the birth in stars to the forming of the earth and then through forming of continents, erosion, sedimentation, subduction, eruption as lava, etc. etc. Of course we only know a partial history of a fraction of material in these longterm cycles but still, it would be a nice way for our brains to get a moving simulation of all that science learned about the Earth System History. It would beat Google Earth or a planetarium or universe simulator. You could watch the neighbourhood you live in and follow it back in time for as far as we know.
    - throwawaylinux 5y ago
      > How mindblowing to imagine. We should make a simulation of the entire earth history where you could follow the atoms and molecules from the birth in stars to the forming of the earth and then through forming of continents, erosion, sedimentation, subduction, eruption as lava, etc. etc. Of course we only know a partial history of a fraction of material in these longterm cycles but still, it would be a nice way for our brains to get a moving simulation of all that science learned about the Earth System History. It would beat Google Earth or a planetarium or universe simulator. You could watch the neighbourhood you live in and follow it back in time for as far as we know. Maybe this is the simulation.
      - morphle 5y ago
        You mean to say that this text that forms a simulation in the reader's brain is that simulation?
        throwawaylinux 5y ago
        I meant our universe is the simulation, but I like that one too.

throwawaylinux 5y ago

Thanks, great answer. So it's not that sediments in the ocean can't be found so much as they don't stay around that long you'd find enough of it to be able to account for that massive glacial erosion event, due to tectonic movement?

cbkeller 5y ago
Yeah, the oldest ocean crust on Earth today is only about 180 million years old, and the older it gets the colder and denser (and thus more prone to subduct) it gets. And when ocean crust subducts it generally (with some caveats) takes the sediments that have accumulated on top with it. So we still have ocean crust from when T. rex lived, for instance, but none from when trilobites lived, and definitely none from the time of these glaciations.
- morphle 5y ago
  I didn't know that the colder and denser ocean crust gets it thus is more prone to subduct. Does that also imply the denser crusts subducts deeper towards the earth center or does the colder crust prevent it from going deeper? Or does the angle of friction of ocean crust with the underside of the continental crust dominate the angle of subduction deep into the mantle? How mindblowing to imagine. We should make a simulation of the entire earth history where you could follow the atoms and molecules from the birth in stars to the forming of the earth and then through forming of continents, erosion, sedimentation, subduction, eruption as lava, etc. etc. Of course we only know a partial history of a fraction of material in these longterm cycles but still, it would be a nice way for our brains to get a moving simulation of all that science learned about the Earth System History. It would beat Google Earth or a planetarium or universe simulator. You could watch the neighbourhood you live in and follow it back in time for as far as we know.
  - throwawaylinux 5y ago
    > How mindblowing to imagine. We should make a simulation of the entire earth history where you could follow the atoms and molecules from the birth in stars to the forming of the earth and then through forming of continents, erosion, sedimentation, subduction, eruption as lava, etc. etc. Of course we only know a partial history of a fraction of material in these longterm cycles but still, it would be a nice way for our brains to get a moving simulation of all that science learned about the Earth System History. It would beat Google Earth or a planetarium or universe simulator. You could watch the neighbourhood you live in and follow it back in time for as far as we know. Maybe this is the simulation.
    - morphle 5y ago
      You mean to say that this text that forms a simulation in the reader's brain is that simulation?
      - throwawaylinux 5y ago
        I meant our universe is the simulation, but I like that one too.