6 ms·

The Great Unconformity: Research points to glaciers being the culprit

87 points by cbkeller 5y ago

h2odragon 5y ago
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.
- neffy 5y ago
  But Slartibartfast designed every single one of those Glacial Fjords personally!
- 74B5 5y ago
  Let us thank the free market for his miraculous creation, amen.
ruined 5y ago
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 5y ago
  peace? why?
  - morphle 5y ago
    Read his book or watch Feynman explain [1][2] for one of many answers. Pondering these questions and learning about them gives joy, excitement, peace of mind and is my great hobby. So much so that I recently purchased 21 hectare of mountain for $9000 and emigrated there so I can study erosion (geology), wildlife, flowers and forest (biology and evolution) and dig for fossils in my own garden. And still read science and hacker news online. Feel free to come and visit, either camping or residing in a tiny house on my mountain[3]. I'll play science and nature guide in the Sierra de Cazorla, Segura and other mountain ranges. [1] Richard P Feynman - The Pleasure of Finding Things Out https://www.youtube.com/watch?v=7XA_NVn7XnE https://www.youtube.com/watch?v=7XA_NVn7XnE [2] Fun To Imagine with Richard Feynman https://www.youtube.com/watch?v=P1ww1IXRfTA https://www.youtube.com/watch?v=P1ww1IXRfTA [3] https://en.wikipedia.org/wiki/Sierra_del_Mugrón https://en.wikipedia.org/wiki/Sierra_del_Mugrón
    - codesnik 5y ago
      To have your own mountain just to dig it up and ponder about stuff! What an usual idea. I'll remember your comment, thank you. Who knows, maybe I'll pass by sooner or later.
- morphle 5y ago
  Maybe we could explore this thought together. It could lead to new research.
cbkeller 5y ago
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 https://doi.org/10.1073/pnas.2118682119 [2] https://doi.org/10.1073/pnas.1804350116 https://doi.org/10.1073/pnas.1804350116
- vrdabomb5717 5y ago
  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)
snidane 5y ago
Glaciers and big rocks? Obligatory Randall Carlson reference https://youtu.be/A2PzfWur6pY https://youtu.be/A2PzfWur6pY

87 points by cbkeller 5y ago

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.

neffy 5y ago
But Slartibartfast designed every single one of those Glacial Fjords personally!
74B5 5y ago
Let us thank the free market for his miraculous creation, amen.

ruined 5y ago

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 5y ago
peace? why?
- morphle 5y ago
  Read his book or watch Feynman explain [1][2] for one of many answers. Pondering these questions and learning about them gives joy, excitement, peace of mind and is my great hobby. So much so that I recently purchased 21 hectare of mountain for $9000 and emigrated there so I can study erosion (geology), wildlife, flowers and forest (biology and evolution) and dig for fossils in my own garden. And still read science and hacker news online. Feel free to come and visit, either camping or residing in a tiny house on my mountain[3]. I'll play science and nature guide in the Sierra de Cazorla, Segura and other mountain ranges. [1] Richard P Feynman - The Pleasure of Finding Things Out https://www.youtube.com/watch?v=7XA_NVn7XnE https://www.youtube.com/watch?v=7XA_NVn7XnE [2] Fun To Imagine with Richard Feynman https://www.youtube.com/watch?v=P1ww1IXRfTA https://www.youtube.com/watch?v=P1ww1IXRfTA [3] https://en.wikipedia.org/wiki/Sierra_del_Mugrón https://en.wikipedia.org/wiki/Sierra_del_Mugrón
  - codesnik 5y ago
    To have your own mountain just to dig it up and ponder about stuff! What an usual idea. I'll remember your comment, thank you. Who knows, maybe I'll pass by sooner or later.
morphle 5y ago
Maybe we could explore this thought together. It could lead to new research.

cbkeller 5y ago

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 https://doi.org/10.1073/pnas.2118682119 [2] https://doi.org/10.1073/pnas.1804350116 https://doi.org/10.1073/pnas.1804350116

vrdabomb5717 5y ago
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)

snidane 5y ago

Glaciers and big rocks? Obligatory Randall Carlson reference https://youtu.be/A2PzfWur6pY https://youtu.be/A2PzfWur6pY