[HN Gopher] The Great Unconformity: Research points to glaciers ...
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The Great Unconformity: Research points to glaciers being the
culprit
Author : cbkeller
Score : 38 points
Date : 2022-01-30 19:52 UTC (3 hours ago)
(HTM) web link (www.vice.com)
(TXT) w3m dump (www.vice.com)
| ruined wrote:
| it's notable that the missing time period is longer than the
| entire time period since, including basically all of known
| evolution. immediately prior to the discontinuity, there is only
| very simple life, and immediately after, life is significantly
| more complex, but also highly constrained, as after a mass
| extinction.
|
| i just like to think about this sometimes. it brings me a deep
| sense of peace
| codesnik wrote:
| peace? why?
| cbkeller wrote:
| Geologist here -- happy to answer any questions about this! The
| underlying paper [1] and the older one it builds on [2] are both
| open access. The rj-MCMC for the time-temperature inversion used
| an existing program from the thermochron community, but we made
| most of the figures in Julia.
|
| [1] https://doi.org/10.1073/pnas.2118682119
|
| [2] https://doi.org/10.1073/pnas.1804350116
| mynegation wrote:
| Thank you! So I get it that glaciers eroded miles of rock from
| the surface. But where is all that material? Was it moved to
| the mounds elsewhere? (where?). Was it ground into gravel, dust
| or silt and washed into the ocean?
| cbkeller wrote:
| Ground up and washed into the ocean -- and then ultimately
| subducted and made into new magmas!
| throwawaylinux wrote:
| Noob question but are these layers in general built up from
| sediments eroded mostly from higher ground, volcanic activity,
| or both (or something else)?
|
| In any case, if glaciers rapidly eroded kilometers of rock on a
| continent-wide or global scale, could evidence in similar
| layers be found for all that sediment when it is deposited by
| the glaciers? And if so, might that look different expected by
| the other theory (tectonic activity)?
| cbkeller wrote:
| 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)
|
| [2] https://en.wikipedia.org/wiki/Sedimentary_basin
| vrdabomb5717 wrote:
| 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 wrote:
| 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
| 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/2018/12/26/180435
| 011...
|
| [5] https://doi.org/10.1016/j.epsl.2018.10.044 (see
| especially Fig. 7)
| h2odragon wrote:
| Alternate theory: Earth was assembled by some subcontractor who
| skimped on construction costs where they thought no one would
| notice.
|
| This accounts for the absence of slood.
| 74B5 wrote:
| Let us thank the free market for his miraculous creation, amen.
| neffy wrote:
| But Slartibartfast designed every single one of those Glacial
| Fjords personally!
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