How do oceans affect the Earth's crust?

[zapatos]

12 minutes ago, beecee said:

No of course not. I was simply trying to add more information. Tetonic action is of course more due to the extreme pressures and temperatures deep within the Earth's crust and the convectional magma currents.

Hydraulic fracturing causes small earth quakes, even up to magnitude 4, and the wastewater disposal from fracking caused a magnitude 5.8 earthquake. 

https://www.usgs.gov/faqs/does-production-oil-and-gas-shales-cause-earthquakes-if-so-how-are-earthquakes-related-these

Makes me wonder if tides could indeed affect tectonics.

[Kurious12]

On 5/13/2022 at 12:43 AM, joigus said:

Google: "why doesn't Venus have plate tectonics?"

Wow. loads of good info here, thanks for all the great answers, I've learned a lot here. I will indeed google this question about Venus, thanks.

[exchemist]

21 hours ago, joigus said:

CaO₂ is a peroxide, actually. Just to be precise.

Not that it matters, but nobody has mentioned CaO2 apart from you. My post referred to CaO.  

19 hours ago, sethoflagos said:

But eventually this reaction reverses as the high pressure form of serpentine (antigorite) breaks down at ~600 C into forsterite, enstatite and water.

OK but I was quoting that reaction to make my point that there does not have to be evolution of hydrogen, as you were previously suggesting.

Regarding the separate issue of thermal stability of these hydrated minerals, you will know better than I. However the link that @joigus provided speaks of water retention at depths of up to 200km. That seems to be what they mean by "deep" in the context of water recycling in tectonic processes. So evidently they think serpentine-type minerals can survive for a while at such depths -  if the subducted slab is descending fast enough (which seems to be the key variable their paper is all about). They seem to associate "deep" retention of water with water retained beyond the island arc, i.e. not entirely returned to the surface via island arc volcanism.

When you say 600C, what pressure are you assuming? As the thermal decomposition involves release of water, the equilibrium will lie further in favour of the hydrated mineral under high pressure.   

 

[joigus]

30 minutes ago, exchemist said:

Not that it matters, but nobody has mentioned CaO2 apart from you. My post referred to CaO.  

Sorry. Wires got crossed with another conversation.

[sethoflagos]

1 hour ago, exchemist said:

OK but I was quoting that reaction to make my point that there does not have to be evolution of hydrogen, as you were previously suggesting.

You are absolutely right that most of the water involved is absorbed during the alteration of forsterite the magnesium component of olivine. And olivine is mostly forsterite. But there is always >~8% fayalite in olivine, so in that sense there is always hydrogen produced during serpentisation. I'm just paraphrasing the commonly held understanding of the process eg (from https://en.wikipedia.org/wiki/Serpentinite)

Quote

Formation and petrology[edit]

Serpentinization is a form of low-temperature metamorphism of ultramafic rocks, such as dunite, harzburgite, or lherzolite. These are rocks low in silica and composed mostly of olivine ((Mg²⁺, Fe²⁺)₂SiO₄), pyroxene (XY(Si,Al)₂O₆), and chromite (approximately FeCr₂O₄). Serpentinization is driven largely by hydration and oxidation of olivine and pyroxene to serpentine minerals, brucite (Mg(OH)₂), and magnetite (Fe₃O₄).^[8] Under the unusual chemical conditions accompanying serpentinization, water is the oxidizing agent, and is itself reduced to hydrogen, H₂. This leads to further reactions that produce rare iron group native element minerals, such as awaruite (Ni₃Fe) and native iron;^[9] methane and other hydrocarbon compounds; and hydrogen sulfide.^[10]

During serpentinization, large amounts of water are absorbed into the rock, increasing the volume, reducing the density and destroying the original structure.^[11][12] The density changes from 3.3 to 2.5 g/cm³ (0.119 to 0.090 lb/cu in) with a concurrent volume increase on the order of 30-40%.^[13] The reaction is highly exothermic and rock temperatures can be raised by about 260 °C (500 °F),^[12] providing an energy source for formation of non-volcanic hydrothermal vents.^[14] The hydrogen, methane, and hydrogen sulfide produced during serpentinization are released at these vents and provide energy sources for deep sea chemotroph microorganisms.^[15][12]

 

1 hour ago, exchemist said:

Regarding the separate issue of thermal stability of these hydrated minerals, you will know better than I. However the link that @joigus provided speaks of water retention at depths of up to 200km. That seems to be what they mean by "deep" in the context of water recycling in tectonic processes. So evidently they think serpentine-type minerals can survive for a while at such depths -  if the subducted slab is descending fast enough (which seems to be the key variable their paper is all about). They seem to associate "deep" retention of water with water retained beyond the island arc, i.e. not entirely returned to the surface via island arc volcanism.

Yes, some intriguing ideas here. Most of the quoted research is dated recently and some seems to be a bit dismissive of the standard sequence of metamorphic grades and facies that I'm familiar with from a different century(!)

If they are suggesting that the slab is subducting so quickly that the water content (whatever its form) is unable to rise fast enough to escape being dragged down, then I'd be more than interested to know what form it actually is in. Some exotic new hydrated mineralogy would be fascinating. Free water even more so. Unfortunately the authors do not say.

I'm not suggesting these ideas are impossible, and in support, the material returned deeply emplaced at constructive plate margins (typically gabbro) always has a bit of hydrated material in it in the form of 2-3% hornblende. So there must be some water content deep down.

3 hours ago, exchemist said:

When you say 600C, what pressure are you assuming? As the thermal decomposition involves release of water, the equilibrium will lie further in favour of the hydrated mineral under high pressure.   

 It's one of the standard equilibrium shifts for eclogite facies metamorphism which I simply accept as read. I guess it must be 2-3 GPa or so.

But your point is well taken.

It's the sort of question I ask myself, and I wish I could get my hands on the thermodynamic data for these materials to plot out the relevant phase equilibria for the benefit of both of us. It's got to be out there somewhere,

 

 

 

[beecee]

On 5/15/2022 at 8:20 AM, zapatos said:

Makes me wonder if tides could indeed affect tectonics.

Probably a minimal effect.

I did find this though..............................

https://www.sciencedirect.com/science/article/pii/S0012825220302257#:~:text=If the tidal forces influence,of the baselines of Figs.

Tidal modulation of plate motions:

 

Abstract:

While mantle convection is a fundamental ingredient of geodynamics, the driving mechanism of plate tectonics remains elusive. Are plates driven only from the thermal cooling of the mantle or are there further astronomical forces acting on them? GPS measurements are now accurate enough that, on long baselines, both secular plate motions and periodic tidal displacements are visible. The now >20 year-long space geodesy record of plate motions allows a more accurate analysis of the contribution of the horizontal component of the body tide in shifting the lithosphere. We review the data and show that lithospheric plates retain a non-zero horizontal component of the solid Earth tidal waves and their speed correlates with tidal harmonics. High-frequency semidiurnal Earth's tides are likely contributing to plate motions, but their residuals are still within the error of the present accuracy of GNSS data. The low-frequency body tides rather show horizontal residuals equal to the relative motion among plates, proving the astronomical input on plate dynamics. Plates move faster with nutation cyclicities of 8.8 and 18.6 years that correlate to lunar apsides migration and nodal precession. The high-frequency body tides are mostly buffered by the high viscosity of the lithosphere and the underlying mantle, whereas low-frequency horizontal tidal oscillations are compatible with the relaxation time of the low-velocity zone and can westerly drag the lithosphere over the asthenospheric mantle. Variable angular velocities among plates are controlled by the viscosity anisotropies in the decoupling layer within the low-velocity zone. Tidal oscillations also correlate with the seismic release.