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Comment by throwawaylinux | Hacker News Reader

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)?

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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 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

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?

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.

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.

> 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.

You mean to say that this text that forms a simulation in the reader's brain is that simulation?

I meant our universe is the simulation, but I like that one too.

Older ocean crust does indeed generally subduct at a steeper angle! Relatedly, if you have ocean crust that is relatively young and hot enough but is still colliding with continental crust, then you can get "flat-slab" subduction [1]

Once the crust gets deeper in the mantle, eventually metamorphic phase transitions become the most important factor. In some seismic images, it looks like subducted slabs make it all the way to the core-mantle boundary (where they may contribute to the source of new mantle plumes).

For sure, I would love to have that simulation!

[1] https://en.wikipedia.org/wiki/Flat_slab_subduction

To create that simulation I would need to team up with scientist who have enough credibility, reputation get the funding for programming that simulation (see my other comments on this thread). My part, the open source code would cost $25K to create. Collection and readying all the geology, all paleontology and part of biology science papers can partially be done with machine learning like Google's scholar search engine until you get a list of all relevant papers that humans must parse to find the timeline and spatial movements of rocks and materials we know about. A nice job for students.

Still, it would be a cheap research project with big science returns like the Hubble Ultra-Deep Field, Dark Energy Survey or Sloan Digital Sky Survey did for cosmology.

Thank you for your answers, I now have a few weeks of science papers to read and follow up on.

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