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How do oceans affect the Earth's crust?


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The oceans it would seem have a lot of weight that bare down on the Earth's crust, what effect if any do this have on the crust? Also, did the oceans play a role in the creation of the Earth's tectonic plates?

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24 minutes ago, Kurious12 said:

The oceans it would seem have a lot of weight that bare down on the Earth's crust, what effect if any do this have on the crust? Also, did the oceans play a role in the creation of the Earth's tectonic plates?

I can't think of a specific effect of the pressure of the oceans on the oceanic crust, but I have read that water may play a very important role in the convection cycle responsible for plate tectonics. My understanding is that many hydrated minerals are mechanically relatively weak (e.g. "soapy" minerals like serpentine) and that "wet" rocks can get a degree of lubrication from the presence of water as they slide past one another in faults. So it may be that water facilitates the convective motion.

I have also read that the volcanism behind subduction zones is at least partly due to the water entrained by the descending slab of lithosphere, which is chemically altered by it, producing lower melting point minerals which expand and rise towards the surface, as "diapirs" of magma.

I suspect it's a big subject, actually. I'm not a geologist, I'm afraid. 

 

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A direct effect of oceans on the underwater landscape is making rocks lighter and thus the underwater mountains taller, canyons deeper, and walls steeper then those above water.

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4 hours ago, Kurious12 said:

The oceans it would seem have a lot of weight that bare down on the Earth's crust, what effect if any do this have on the crust? 

The total mass of the oceans slightly depresses the equilibrium level of the oceanic plates, thereby slightly increasing the underlying mantle pressure sufficient to raise the less dense continental plates by an (approximately) equivalent degree.

4 hours ago, Kurious12 said:

Also, did the oceans play a role in the creation of the Earth's tectonic plates?

 The causal factors for the onset of plate tectonics, and its timing are currently major open questions.   

As @exchemisthas noted, it seems entirely plausible for the oceans to have supplied some chemical and lubricative effects to facilitate the process. As they still do.

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Just to add to what other members have said.

It's perhaps interesting to use Venus as a comparison: a planet that should have had plate tectonics --it has very active vulcanism, has the right size, etc-- but doesn't.

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

As stated above, the role of water as lubricant in the subduction zones is thought to be of critical importance. It also leaks into the mantle, so the oceans are ever so slowly being depleted.

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10 minutes ago, joigus said:

Just to add to what other members have said.

It's perhaps interesting to use Venus as a comparison: a planet that should have had plate tectonics --it has very active vulcanism, has the right size, etc-- but doesn't.

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

As stated above, the role of water as lubricant in the subduction zones is thought to be of critical importance. It also leaks into the mantle, so the oceans are ever so slowly being depleted.

Are they being depleted, though? Volcanism returns water to the surface, after all. I would have expected that there would be equilibrium, over some sort of long term cycle. 

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4 minutes ago, exchemist said:

Are they being depleted, though? Volcanism returns water to the surface, after all. I would have expected that there would be equilibrium, over some sort of long term cycle. 

It's a minor effect in comparison to volcanism, only noticeable at the time-scale of hundreds of millions? billions? of years. A part of the water gets recycled to the atmosphere as you say, but a small fraction is incorporated as hydrous minerals, from what I know. I think this is the original find:

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019GC008232

Does that check with you? I'm very interested in learning what you think.

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15 minutes ago, joigus said:

It's a minor effect in comparison to volcanism, only noticeable at the time-scale of hundreds of millions? billions? of years. A part of the water gets recycled to the atmosphere as you say, but a small fraction is incorporated as hydrous minerals, from what I know. I think this is the original find:

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019GC008232

Does that check with you? I'm very interested in learning what you think.

This is a really interesting article. Thanks for posting. I'm going out now but will read it when I get back. But from initially scanning it, it looks as if you are quite right: they do seem to suggest a gradual net absorption, with a cycle superimposed on it. 

 

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12 hours ago, Kurious12 said:

The oceans it would seem have a lot of weight that bare down on the Earth's crust, what effect if any do this have on the crust? Also, did the oceans play a role in the creation of the Earth's tectonic plates?

~At last someone has posted a scientific thread. +1

 

The oceans have 'a lot of weight'  , but do they ?

There are big chemical and mechanical differences in the crustal materials of the ocean floors and the land masses.

Oceanic crust is largely basaltic rock of greater density than 'continental' crust which includes much lighter material.

Quote

Britannica

https://www.britannica.com/science/continental-crust

Composition

Continental crust is broadly granitic in composition and, with a density of about 2.7 grams per cubic cm, is somewhat lighter than oceanic crust, which is basaltic (i.e., richer in iron and magnesium than granite) in composition and has a density of about 2.9 to 3 grams per cubic cm. Continental crust is typically 40 km (25 miles) thick, while oceanic crust is much thinner, averaging about 6 km (4 miles) in thickness.

The effect of the different densities of lithospheric rock can be seen in the different average elevations of continental and oceanic crust. The less-dense continental crust has greater buoyancy, causing it to float much higher in the mantle. Its average elevation above sea level is 840 metres (2,750 feet), while the average depth of oceanic crust is 3,790 metres (12,400 feet). This density difference creates two principal levels of Earth’s surface.

 

OK, so the continental material is 2.5 to 3 times as 'heavy' as water would be overlying the basal platform surface.
Thus the oceans apply one half to one third of the load that is applied by continental material.

 

Secondly most of the planet volcanic activity takes place at the bottom of the oceans, since they cover so much more of the surface.
The ocean water dramatically reduces or prevents altogether the amount of solid material ejected into the atmousphere during eruptions.
The water also quenches the molten rock much more rapidly than the atmousphere.

As to the role of water in the origin of the Earth's plates, this is not known with any degree of certainty.
Several different theories have been proposed.
But we do not know when or how the water originally came to be there.
We now think that this happened very early in the history of Earth, less than half a billion years after its formation, and this time is being pushed back and back.
Our best guess is that plates formed shortly after the original semi-molten Earth cooled at the surface sufficiently to start crusting over.
Contraction cracking in this crust was then thought to have occurred as a result of that cooling.

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14 hours ago, Kurious12 said:

The oceans it would seem have a lot of weight that bare down on the Earth's crust, what effect if any do this have on the crust? 

The time-varying depression of the ocean bed and adjacent coast (from the tides) was a factor that entered into an experiment that measured the variation in pendulum clocks owing to changes in g caused by the moon

Analysis of Records made on the Loomis Chronograph by Three Shortt Clocks and a Crystal Oscillator.  Ernest W. Brown and Dirk Brouwer p584

https://www.semanticscholar.org/paper/Analysis-of-Records-made-on-the-Loomis-Chronograph-Brown-Brouwer/507ddf49e246d79933bd77cb871969994581465d

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Posted (edited)
10 hours ago, joigus said:

It's a minor effect in comparison to volcanism, only noticeable at the time-scale of hundreds of millions? billions? of years. A part of the water gets recycled to the atmosphere as you say, but a small fraction is incorporated as hydrous minerals, from what I know. I think this is the original find:

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019GC008232

Does that check with you? I'm very interested in learning what you think.

One issue I have difficulty in getting my head around here is the apparent assumption that 'hydrous minerals' accounts for all the lost water.

These hydrous minerals (such as amphiboles etc) incorporate -OH presumably sourced from a water molecule in which case one can ask, 'What happens to the missing hydrogen?'

The paper references serpentinisation quite extensively as a major consumer of oceanic water. Serpentisation is a highly exothermic reaction occuring in strongly reducing conditions where water becomes a primary oxidising agent, releasing large quantities of hydrogen. Again. Same question.

I'm not seeing a distinct water cycle as such here. I'm seeing separate oxygen and hydrogen cycles. 

My guess is that if one ignores the hydrogen cycle (as some seem to), one is likely to miss a good chunk of the returned water.    

Edited by sethoflagos
typo
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1 hour ago, sethoflagos said:

One issue I have difficulty in getting my head around here is the apparent assumption that 'hydrous minerals' accounts for all the lost water.

These hydrous minerals (such as amphiboles etc) incorporate -OH presumably sourced from a water molecule in which case one can ask, 'What happens to the missing hydrogen?'

The paper references serpentinisation quite extensively as a major consumer of oceanic water. Serpentisation is a highly exothermic reaction occuring in strongly reducing conditions where water becomes a primary oxidising agent, releasing large quantities of hydrogen. Again. Same question.

I'm not seeing a distinct water cycle as such here. I'm seeing separate oxygen and hydrogen cycles. 

My guess is that if one ignores the hydrogen cycle (as some seem to), one is likely to miss a good chunk of the returned water.    

If you have oxides you don't have to generate free hydrogen. For example CaO + H2O -> Ca(OH)2.

A quick internet search yielded this: https://www.liquisearch.com/serpentinite/formation_and_petrology/serpentinite_reactions. which suggests several suites of reactions, some of which generate hydrogen and others not. I imagine free hydrogen could contribute to some of the apparently reducing conditions in some volcanism, e.g. when H2S is evolved. (The link also mentions possible reduction of carbon from carbonate to methane.  

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On 5/13/2022 at 6:31 AM, Kurious12 said:

The oceans it would seem have a lot of weight that bare down on the Earth's crust, what effect if any do this have on the crust? Also, did the oceans play a role in the creation of the Earth's tectonic plates?

Not sure if it has been mentioned yet, but the tidal buldge of the Ocean's when the Moon is overhead, is also actually slightly ahead of the overhead Moon, a result of the rotating Earth, and the fact that some of that energy is transferred to the Oceanic tidal buldge via friction.

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44 minutes ago, beecee said:

Not sure if it has been mentioned yet, but the tidal buldge of the Ocean's when the Moon is overhead, is also actually slightly ahead of the overhead Moon, a result of the rotating Earth, and the fact that some of that energy is transferred to the Oceanic tidal buldge via friction.

This is correct. It reminded me of a question from the astronomy class. Sorry, completely unrelated to the OP:

During a Solar eclipse, the Moon casts a shadow onto the Earth. Which way is this shadow moving during the eclipse, East to West or West to East?

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53 minutes ago, beecee said:

Not sure if it has been mentioned yet, but the tidal buldge of the Ocean's when the Moon is overhead, is also actually slightly ahead of the overhead Moon, a result of the rotating Earth, and the fact that some of that energy is transferred to the Oceanic tidal buldge via friction.

If you are going to consider this, it is true that there is a greater depth of water covering the sea bed at high tide.

But the 'strength' of the Earth's gravitational pull is reduced by the direct opposition of the Moon's gravitational pull, thus reducing the weight of that water.

 

Yes there is some transfer of energy but is it concentrated enough to cause tectonic action ?

Note it is thought that the plates preceeded the Moon in the history of the Earth, though both were quite early in that history.

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1 hour ago, exchemist said:

If you have oxides you don't have to generate free hydrogen. For example CaO + H2O -> Ca(OH)2.

A quick internet search yielded this: https://www.liquisearch.com/serpentinite/formation_and_petrology/serpentinite_reactions. which suggests several suites of reactions, some of which generate hydrogen and others not. I imagine free hydrogen could contribute to some of the apparently reducing conditions in some volcanism, e.g. when H2S is evolved. (The link also mentions possible reduction of carbon from carbonate to methane.  

But Ca(OH)2 (and the more relevant Mg form, brucite) thermally decompose, releasing their water while still at only modest depth.

For water to get beyond say 100 km depth, it has to find its way into a high density mineral that is stable at the elevated temperatures found down there. All the likely candidates I can think of are hydrogen depleted. 

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9 hours ago, sethoflagos said:

But Ca(OH)2 (and the more relevant Mg form, brucite) thermally decompose, releasing their water while still at only modest depth.

For water to get beyond say 100 km depth, it has to find its way into a high density mineral that is stable at the elevated temperatures found down there. All the likely candidates I can think of are hydrogen depleted. 

Fair enough, but the link I provided also includes reaction such as

3Mg2SiO4 + SiO2 + 4H2O → 2Mg3Si2O5(OH)4. 
 
 
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9 hours ago, sethoflagos said:

But Ca(OH)2 (and the more relevant Mg form, brucite) thermally decompose, releasing their water while still at only modest depth.

I'm out of my depth here --no pun intended. I'd heard that when temperature in the mantle goes down below 650º, water can start leaking into the deeper mantle and essentially disappear from the water cycle. If I understand correctly, the formation of hydrous minerals is essential for this removal.

I understand @exchemist's example of CaO2 as simply an example that if you include oxides, you can account for this.

I've been looking for online credible literature about the subject, and I've found this:

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/hydrous-mineral#:~:text=The hydrous minerals like rock,water in a shallow sea.

 

Quote

Early studies of hydrous minerals in subduction zones tended to focus on a few minerals, with an emphasis on discontinuous dehydration reactions. Key phases were thought to be white mica (phengite) in the metamorphosed sediments, amphibole in the altered basaltic crust, and amphibole and phlogopite in the hydrated mantle wedge (e.g., Bebout et al., 1993, 1999; Moran et al., 1992; Tatsumi and Eggins, 1997; Tatsumi et al., 1983). Hydration in the residual mantle of the subducting plate was largely not considered.

 

Quote

Current work demonstrates that a wide variety of hydrous minerals can be stable in the slab and emphasizes the importance of continuous metamorphic reactions in natural systems that generally show extensive solid solution (Poli and Schmidt, 2002; Schmidt and Poli, 2003; see also Hacker, 2008). In metasediments, phengite (white mica with varying Mg and Fe content) is important geochemically, along with topaz-OH, and MgAl-pumpellyite. These phases may be stable up to 900 ᵒC and 7 GPa (Domanik and Holloway, 1996; Ono, 1998). If so, these phases have the potential to carry H2O and many associated trace elements beyond volcanic arc depths during subduction. Hydrous phases in the subducting basaltic crust that can break down at the depths encountered beneath volcanic arcs include amphibole, lawsonite, zoisite, chloritoid, talc, and possibly phengite.

I won't pretend I understand every argument there, but it seems as if we're at some point in a shift of paradigm here, and people are pushing the boundaries of the depths at which it's thought that this hydration can occur.

Am I reading this correctly?

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21 minutes ago, joigus said:

and people are pushing the boundaries of the depths at which it's thought that this hydration can occur.

Yes I think we are still well in information gathering and hypothesizing phase. +1
Information about high temperature, high pressure geomaterials is sparse.

One comment about the water is that we are (or should be) talking about seawater.
This, of course, contains many minerals and is not just the plain H2O that appears in the simplified chemical reactions.

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11 minutes ago, studiot said:

One comment about the water is that we are (or should be) talking about seawater.
This, of course, contains many minerals and is not just the plain H2O that appears in the simplified chemical reactions.

Good point.

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2 hours ago, joigus said:

I understand @exchemist's example of CaO2 as simply an example that if you include oxides, you can account for this.

 

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

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4 hours ago, exchemist said:

Fair enough, but the link I provided also includes reaction such as

3Mg2SiO4 + SiO2 + 4H2O → 2Mg3Si2O5(OH)4. 
 
 

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

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Posted (edited)
6 hours ago, joigus said:

I'm out of my depth here --no pun intended.

Me too. I'm no more than a hobby geologist but it has been a somewhat obsessive fascination since I was about 8! (a very long time ago)

One observation that strongly colours my view of this topic is that it must square with not only the surface geology we see around us, but also a very long term gradual trend of oxidation from the global reducing conditions of the earliest times due to photosynthesis. 

The banded iron formations around the world record oxygen fugacity being controlled by the oxidation of oceanic Fe II to Fe III in the Archean.

And to this day, there is still an iron oxidation front controlling oxygen fugacity - called the FMQ (fayalite-magnetite-quartz) redox buffer - now deep within the earth's crust.

3Fe2SiO4 + O2 = 2Fe3O4 + 3SiO2

Compare this with @exchemist's serpentisation reaction 1a)

3Fe2SiO4 + 2H2O → 2Fe3O4 + 3SiO2 + 2H2

... which can proceed when FMQ has exhausted all the free silica in its environment, and water becomes the favoured source of oxygen.

I'm all too aware that this picture is simplistic in the extreme, and maybe the second reaction is not favoured at some key limiting temperature, but it does raise a question in my mind about the stability of water in the low silica reducing environments found at depth.

Or perhaps I'm completely off-track, and the ocean is busy converting the lower mantle to topaz!

Edited by sethoflagos
typo
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23 hours ago, studiot said:

Yes there is some transfer of energy but is it concentrated enough to cause tectonic action ?

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.

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