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The difference isn't active tectonics vs not. It's plate tectonics vs stagnant lid tectonics. The presence of water at many fault lines is essential to allowin

The difference isn't active tectonics vs not. It's plate tectonics vs stagnant lid tectonics. The presence of water at many fault lines is essential to allowing plates to stay separate and move relative to each other. The subduction and divergence is then essential to gas exchange between atmosphere and mantle that gives the system stabilizing inertia. It's unclear what came first on Venus, but it's pretty clear that high temperature, boiling water (a greenhouse gas!), and stagnant lid tectonics all provided positive feedback to each other, with the boiling off of water probably kicking the process into high gear.

dalyons 4y ago
Can you expand on the water at fault lines thing? Sounds interesting
- azernik 4y ago
  Silica's melting point is lowered when mixed with water, so oceanic faults can have actual molten magma lubricating and weakening the fault line. Otherwise, you'd have the stronger malleable solid mantle at the joint, and lose much more energy whenever the fault tried to move. Having at least some faults that slippery is necessary to keep the system from locking up.
  - dalyons 4y ago
    late but thank you! That is indeed super interesting
- dredmorbius 4y ago
  Subducting oceanic plates drag down with them a great deal of water. When that flashes to steam as it hits magma and the mantle, tremendous pressures are generated. Many volcanoes (though not all) are in this regard "steam engines", where much of the driving pressure is a consequence of subducted ocean water, and possible some innate crustal water, interacting with geothermal heat. I'm supposing this is why steam is a major factor in many volcanic eruptions. There's an explainer here: <https://www.nps.gov/subjects/geology/plate-tectonics-subduction-zones.htm https://www.nps.gov/subjects/geology/plate-tectonics-subduct...> As to the role in tectonics and plate movement, I'm less aware.
  - azernik 4y ago
    With tectonics, it's that the silica/water mixture melts at lower temperatures, so the melted hydrated rock lubricates tbe fault.
    - dredmorbius 4y ago
      Thanks, I suspected something like that. Reminds me of the Vajont Dam disaster, in which filling of a reservoir behind a newly constructed dam lead to catastrophic collapse of a mountainside in the Italian Alps, where layered sedimentary limestone separated by thin layers of clay became unstable as the clay was wetted and lubricated by the reservoir. Shortly before 11 pm on 9 October 1963, a two-kilometer-long landslide triggered a 250m (850 ft) megatsunami. That killed a number of engineers who'd gathered on top of the dam to observe its filling, as well as around 2,000 inhabitants of villages adjacent to and downstream of the reservoir, most notably Longarone which was virtually scrubbed from the map. The dam itself was virtually undamaged by the incident, save the topmost metre or so of nonstructural masonry. <https://en.wikipedia.org/wiki/Vajont_Dam https://en.wikipedia.org/wiki/Vajont_Dam> <https://yewtu.be/watch?v=lkGnnc8Ezlk https://yewtu.be/watch?v=lkGnnc8Ezlk>

dalyons 4y ago

Can you expand on the water at fault lines thing? Sounds interesting

azernik 4y ago
Silica's melting point is lowered when mixed with water, so oceanic faults can have actual molten magma lubricating and weakening the fault line. Otherwise, you'd have the stronger malleable solid mantle at the joint, and lose much more energy whenever the fault tried to move. Having at least some faults that slippery is necessary to keep the system from locking up.
- dalyons 4y ago
  late but thank you! That is indeed super interesting
dredmorbius 4y ago
Subducting oceanic plates drag down with them a great deal of water. When that flashes to steam as it hits magma and the mantle, tremendous pressures are generated. Many volcanoes (though not all) are in this regard "steam engines", where much of the driving pressure is a consequence of subducted ocean water, and possible some innate crustal water, interacting with geothermal heat. I'm supposing this is why steam is a major factor in many volcanic eruptions. There's an explainer here: <https://www.nps.gov/subjects/geology/plate-tectonics-subduction-zones.htm https://www.nps.gov/subjects/geology/plate-tectonics-subduct...> As to the role in tectonics and plate movement, I'm less aware.
- azernik 4y ago
  With tectonics, it's that the silica/water mixture melts at lower temperatures, so the melted hydrated rock lubricates tbe fault.
  - dredmorbius 4y ago
    Thanks, I suspected something like that. Reminds me of the Vajont Dam disaster, in which filling of a reservoir behind a newly constructed dam lead to catastrophic collapse of a mountainside in the Italian Alps, where layered sedimentary limestone separated by thin layers of clay became unstable as the clay was wetted and lubricated by the reservoir. Shortly before 11 pm on 9 October 1963, a two-kilometer-long landslide triggered a 250m (850 ft) megatsunami. That killed a number of engineers who'd gathered on top of the dam to observe its filling, as well as around 2,000 inhabitants of villages adjacent to and downstream of the reservoir, most notably Longarone which was virtually scrubbed from the map. The dam itself was virtually undamaged by the incident, save the topmost metre or so of nonstructural masonry. <https://en.wikipedia.org/wiki/Vajont_Dam https://en.wikipedia.org/wiki/Vajont_Dam> <https://yewtu.be/watch?v=lkGnnc8Ezlk https://yewtu.be/watch?v=lkGnnc8Ezlk>