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 and see also Fig S16 in [4]
[2] https://pubs.geoscienceworld.org/gsa/gsabulletin/article/83/...
[3] https://en.wikipedia.org/wiki/Late_Paleozoic_icehouse
[4] https://www.pnas.org/content/pnas/suppl/2018/12/26/180435011...
[5] https://doi.org/10.1016/j.epsl.2018.10.044 (see especially Fig. 7)
> 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.
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.
1) While having continents at the poles makes it easier to have icesheets at all, it also makes it harder for them to grow into a full snowball. This is because
a) covering the continents with ice shuts down silicate weathering (and silicate weathering consumes CO2, so that's a stabilizing negative feedback)
b) the difference in albedo between water and sea ice is greater than the difference in albedo between land and ice. So if you can get cold enough to start making sea ice at the poles, you should get a stronger positive feedback of cooling -> higher albedo -> more cooling
During the Neoproterozoic, most or all of the continents seem to have been near the equator, so silicate weathering could keep going until sea ice reached the "point of no return" of the sea ice-albedo feedback [e.g. 1]
2) The biosphere is pretty different today than it was last time we had a snowball, and there is some reason to think that evolutionary developments like land plants and pelagic calcifiers may make the climate system more stable than it was 700 million years ago.
None of that is to say it's impossible though! The solid earth acts slowly, but it's a big lever, so hypothetically if you could somehow crank silicate weathering up high enough and volcanic degassing down low enough, you could probably still in principle reach the tipping point again.
For your last question, you are probably thinking of Milankovitch cycles [2] -- those are definitely going strong as well, though generally not strong enough to get us into or out of a snowball state.
Are you making a point by leaving human emissions of CO2 out of your list?
The other one I forgot to mention is that the sun is a bit brighter now than it was 700 Myr ago (by perhaps a few percent). Go back another two or three billion years to the Archean and the difference would have been bigger -- to the point that we have some trouble explaining why there weren't a lot more snowballs back then [1]
Up thread, there seems to be a question about human civilizations being erased by the ice.
A billion years ago.
And there was a polite reply, from said geologist, not a comment about timescales.
The most common solutions involve high concentrations of organic greenhouse gases like methane as well as high CO2, but it's always possible there are other possibilities that have not yet been considered.
As I understand according to the current estimates this not enough to avoid the cold Earth problem, but there are way too much uncertainty. But if we do not have way to read the brightness from geology alone, that can be an answer.