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Tectonic plate movement versus Pangea


hu??
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I have be wondering a long time about the idea of having 1 super continent (Pangea) and 1 huge ocean about 300 million years in the past. It always sounded a bit unreal to me. Let me explain why.

Let me first make clear: i am not putting the tectonic plate movement up to discussion! The theory about that is sound: i assume the geologic science is correct and that the tectonic plates forming the continents as we know now, originate from 1 tectonic plate about 300 million years ago (as detailed in https://en.wikipedia.org/wiki/Pangaea).

The part of the theory i doubt is the distribution of water around the planet being the same as in the present, giving rise to 1 main landmass and the remainder of the world being 1 big ocean. The reason i doubt this, is because the geologists seem to forget that having 1 landmass on 1 side of the globe means that the center of gravity is no longer the same as in the present: it shift to that landmass, causing the ocean to flow to the landmass, causing the center of gravity to shift even more to the landmass, etc, until a balance is new balance found.

The redistributed water will partially drown the Pangea landmass, and the sea-level on the other side of the world will drop, giving rise to other (big) landmasses.

I could not find any reference to a different center of gravity 300 mln years ago versus the current distribution.

Any ideas about this?

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46 minutes ago, hu?? said:

I have be wondering a long time about the idea of having 1 super continent (Pangea) and 1 huge ocean about 300 million years in the past. It always sounded a bit unreal to me. Let me explain why.

Let me first make clear: i am not putting the tectonic plate movement up to discussion! The theory about that is sound: i assume the geologic science is correct and that the tectonic plates forming the continents as we know now, originate from 1 tectonic plate about 300 million years ago (as detailed in https://en.wikipedia.org/wiki/Pangaea).

The part of the theory i doubt is the distribution of water around the planet being the same as in the present, giving rise to 1 main landmass and the remainder of the world being 1 big ocean. The reason i doubt this, is because the geologists seem to forget that having 1 landmass on 1 side of the globe means that the center of gravity is no longer the same as in the present: it shift to that landmass, causing the ocean to flow to the landmass, causing the center of gravity to shift even more to the landmass, etc, until a balance is new balance found.

The redistributed water will partially drown the Pangea landmass, and the sea-level on the other side of the world will drop, giving rise to other (big) landmasses.

I could not find any reference to a different center of gravity 300 mln years ago versus the current distribution.

Any ideas about this?

Yes. You need to take into account the principle of isostasy.https://www.britannica.com/science/isostasy-geology

The continents are less dense than the oceanic crust in which they float.  There is no disturbance to the centre of gravity when they move around, any more than the centre of gravity of your cold drink shifts, if you move all the ice cubes to one side.  

Furthermore if, as you propose, the ocean level were to drop, in the era of Pangaea, no significant land masses would be revealed, because all the continental crust is part of Pangaea. All the rest would be oceanic crust, at a depth of several km below sea level.   

Edited by exchemist
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24 minutes ago, exchemist said:

Yes. You need to take into account the principle of isostasy.https://www.britannica.com/science/isostasy-geology

The continents are less dense than the oceanic crust in which they float.  There is no disturbance to the centre of gravity when they move around, any more than the centre of gravity of your cold drink shifts, if you move all the ice cubes to one side.  

Furthermore if, as you propose, the ocean level were to drop, in the era of Pangaea, no significant land masses would be revealed, because all the continental crust is part of Pangaea. All the rest would be oceanic crust, at a depth of several km below sea level.   

Thanks for the info. I didn't know the floating-ice cube principle was valid for tectonic plates. Sounds plausible when the earth crust indeed has a lower density than the magma directly below and does not have a significant 'stiffness' (is that the correct terminology). However, the isostacy principle for tectonic plate mechanics seem to have the state of a hypophesys, not that of a scientific theory.

So, I am not completely convinced, considering:

1. looking up the density of magma (2.18 and 2.80) and the average density of the earth crust (2.83).

2. solidification of a fluid (magma) tends to increase the density (i know there are some exceptions such as water or iron below certain pressures). This seems to confirm (1).

3. at a glance, measured variations in in the earth gravitational field do not seem to support the isostacy hypophesys (https://en.wikipedia.org/wiki/Gravity_of_Earth )

Is there any reason that solidification of magma would a decrease in density or does the earth crust somehow have a different composition than the magma below?

 

 

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21 minutes ago, hu?? said:

So, I am not completely convinced, considering:

It is not suprising that you have reached some untenable consclusions since you have some flawed assumptions.

 

1 hour ago, exchemist said:

i assume the geologic science is correct and that the tectonic plates forming the continents as we know now, originate from 1 tectonic plate about 300 million years ago

Since we discovered 'techtonic plates' (do you know what they are ?) there has never been a proposal of only 1 plate !

The largest plate of all has almost no land on it.

All the plates are very ancient.

 

The plates do not float on magma. If you read exchemist carefully you will see he said that

1 hour ago, exchemist said:

The continents are less dense than the oceanic crust in which they float.

Both of these are  solid.

2 hours ago, hu?? said:

I have be wondering a long time about the idea of having 1 super continent (Pangea) and 1 huge ocean about 300 million years in the past. It always sounded a bit unreal to me. Let me explain why.

You need to a go a long way further back in time than Pangea to understand the development of the present arrangement.

We know the plate have been there for a very long time and that they have moved about (principally by rotation , not as popularly described by translation).

What we don't have yet is a well confirmed theory of how the motion is driven, although there are of course several promising hypotheses.

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1 hour ago, studiot said:

It is not suprising that you have reached some untenable consclusions since you have some flawed assumptions.

Hmm, you are suggesting this forum must not be used by ignorant people to seek knowledge but only for asking questions for which one already knows the scientific acceptable answer?

That is quite a certain way to stop scientific progress...

Please enlighten me, i do not not know what my flawed assumptions are.

1 hour ago, studiot said:

Since we discovered 'techtonic plates' (do you know what they are ?) there has never been a proposal of only 1 plate !

of course not. I did never say that.

1 hour ago, studiot said:

The largest plate of all has almost no land on it.

All the plates are very ancient.

so, what does that have to do with the earth gravitational field?

1 hour ago, studiot said:

 

The plates do not float on magma. If you read exchemist carefully you will see he said that

Both of these are  solid.

You need to a go a long way further back in time than Pangea to understand the development of the present arrangement.

We know the plate have been there for a very long time and that they have moved about (principally by rotation , not as popularly described by translation).

What we don't have yet is a well confirmed theory of how the motion is driven, although there are of course several promising hypotheses.

 

I am not doubting tectonic plate movement or the idea that the current land distribution comes from breaking up the  the latest 'supercontent'. I am just doubting the assumed relative water distribution with it, based on, for me, quite solid reasons. Considering i am not an expert on this i might very well be wrong. That is the whole reason for posting topic: finding out where i went wrong in my reasoning! So what are the incorrect assumptions?

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24 minutes ago, hu?? said:

Hmm, you are suggesting this forum must not be used by ignorant people to seek knowledge but only for asking questions for which one already knows the scientific acceptable answer?

I don't see where that was suggested

24 minutes ago, hu?? said:

 

of course not. I did never say that.

" i assume the geologic science is correct and that the tectonic plates forming the continents as we know now, originate from 1 tectonic plate about 300 million years ago" sounds a lot like you were, in fact, saying that

 

24 minutes ago, hu?? said:

so, what does that have to do with the earth gravitational field?

Why must it? It's not clear at all that you are discussing the earth's gravitational field.

You're worried about the center-of-mass shifting; what you could do is see how much it might shift. The deepest part of the ocean is ~10 km, and most land is under a few km above sea level. Compare this to the radius of the earth, being ~6400 km.

IOW, the amount of mass represented by the continents is small compared to that of the earth. If you rearrange the masses on the surface, what might change slightly is the wobble of the earth's rotation, which is something we already observe on shorter time scales. Freezing and melting of ice sheets causes this, too. 

https://www.geospatialworld.net/blogs/why-the-earth-wobbles/

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16 minutes ago, hu?? said:

Hmm, you are suggesting this forum must not be used by ignorant people to seek knowledge but only for asking questions for which one already knows the scientific acceptable answer?

That is quite a certain way to stop scientific progress...

When I read studiot's post, he seems to suggest your conclusions were adversely affected by mistaken assumptions. He does NOT seem to suggest that ignorant people should stay away from the forum, or that one should only ask questions one knows the answers to. In fact, he includes some information to help dispel some of the ignorance you admit to having about the subject. Ignorance is NOT stupidity, it's simply a lack of knowledge about a specific subject.

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2 hours ago, hu?? said:

Thanks for the info. I didn't know the floating-ice cube principle was valid for tectonic plates. Sounds plausible when the earth crust indeed has a lower density than the magma directly below and does not have a significant 'stiffness' (is that the correct terminology). However, the isostacy principle for tectonic plate mechanics seem to have the state of a hypophesys, not that of a scientific theory.

So, I am not completely convinced, considering:

1. looking up the density of magma (2.18 and 2.80) and the average density of the earth crust (2.83).

2. solidification of a fluid (magma) tends to increase the density (i know there are some exceptions such as water or iron below certain pressures). This seems to confirm (1).

3. at a glance, measured variations in in the earth gravitational field do not seem to support the isostacy hypophesys (https://en.wikipedia.org/wiki/Gravity_of_Earth )

Is there any reason that solidification of magma would a decrease in density or does the earth crust somehow have a different composition than the magma below?

 

 

As @studiotpoints out, the oceanic lithosphere and the mantle beneath are both solid, at least on a human timescale (though the upper mantle is more plastic than the crust). To understand plate tectonics you need to appreciate that even solids can deform plastically and flow under the influence of pressure and temperature, on geological timescales, that is, over millions of years.

Forget magma in this context. Magma is formed only in a few pockets, deep in the crust or in the top of the mantle, where locally the temperatures is hot enough, and/or the pressure is low enough, and/or the melting points of some of the minerals at that location are low enough that partial melting can occur. Magma is an incidental byproduct of tectonic processes. 

If you read the link I gave you on isostasy you will see it is about less dense continental crust finding an equilibrium depth in the denser oceanic crust and upper mantle in which it effectively "floats". Even though both are solids, on geological timescales, they will find gradually find their equilibrium level. It is the fact that these islands of lighter minerals have formed (largely due to the fractionation that goes on in volcanism) which gives us the continents. As with ice in water, because the density difference is small, the continental blocks extend to a far greater depth, below the top of the oceanic crust that forms the ocean floor, than the bit that pokes up from the ocean floor to give us dry land. (I find on Wiki density figures for continental crust of 2.8 and for upper mantle of 3.3.)

As for your remarks about gravitation and isostasy, the link you provided does not seem to support your claim that measurement of gravitation is at odds with the hypothesis of isostasy. Can you point to something specific in this link that supports your claim? 

 

  

Edited by exchemist
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21 hours ago, hu?? said:

The theory about that is sound: i assume the geologic science is correct and that the tectonic plates forming the continents as we know now, originate from 1 tectonic plate about 300 million years ago

I see I did incorrectly use the tectonic plate terminology here. Sorry for the confusion.

16 hours ago, exchemist said:

Forget magma in this context. Magma is formed only in a few pockets, deep in the crust or in the top of the mantle, where locally the temperatures is hot enough, and/or the pressure is low enough, and/or the melting points of some of the minerals at that location are low enough that partial melting can occur. Magma is an incidental byproduct of tectonic processes. 

 

Yeah, i wondered about that but could not find any info. Thanks.

17 hours ago, swansont said:

You're worried about the center-of-mass shifting; what you could do is see how much it might shift. The deepest part of the ocean is ~10 km, and most land is under a few km above sea level. Compare this to the radius of the earth, being ~6400 km.

I am not worried at all. Just questioning. 

The relative radius of the earth is irrelevant: e.g. a 50 meter center of mass shift would cause a huge variation in local sea-levels.

16 hours ago, exchemist said:

As for your remarks about gravitation and isostasy, the link you provided does not seem to support your claim that measurement of gravitation is at odds with the hypothesis of isostasy. Can you point to something specific in this link that supports your claim? 

My reasoning is as follows: When isostacy is the only mechanism creating deviations from an idealized, smooth Earth, than the earth gravitation should be the same everywhere (except close to large density variations). If you look at the picture, mountainous areas seem to have higher gravitation. As i see it, this contradicts isostacy being the only factor. Of course, this could be an artefact of the way it is measured, it could be a local fenomena only, etc.

 

Anyways, I found a reference that the lithosphere has a different chemical composition than the asthenosphere (i learned this terminology here ;) ). My assumption was that those were more or less the same. So this different composition allows for a difference in density and make isostacy on a global scale a valid mechanism.
Thanks for the info. I learned a lot.

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1 hour ago, hu?? said:

 

I see I did incorrectly use the tectonic plate terminology here. Sorry for the confusion.

Yeah, i wondered about that but could not find any info. Thanks.

I am not worried at all. Just questioning. 

The relative radius of the earth is irrelevant: e.g. a 50 meter center of mass shift would cause a huge variation in local sea-levels.

My reasoning is as follows: When isostacy is the only mechanism creating deviations from an idealized, smooth Earth, than the earth gravitation should be the same everywhere (except close to large density variations). If you look at the picture, mountainous areas seem to have higher gravitation. As i see it, this contradicts isostacy being the only factor. Of course, this could be an artefact of the way it is measured, it could be a local fenomena only, etc.

 

Anyways, I found a reference that the lithosphere has a different chemical composition than the asthenosphere (i learned this terminology here ;) ). My assumption was that those were more or less the same. So this different composition allows for a difference in density and make isostacy on a global scale a valid mechanism.
Thanks for the info. I learned a lot.

Exactly. The interesting thing - to my mind - is that this difference in composition may have arisen as a result of fractionation, due to volcanism. The lower melting minerals would tend to become concentrated in magma, which then tends to rise (due to lower density) and erupt - or form intrusions. I have read that some studies have shown the continents appear to have grown in size over geological time, due to this process. It is even possible that all the continental masses formed progressively by this mechanism, from an initially more homogeneous mineral composition of the very early earth. 

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5 hours ago, hu?? said:

The relative radius of the earth is irrelevant:

No, it is not. The large radius means the mass is quite large, the effect of movement of a small mass is going to be small.

5 hours ago, hu?? said:

e.g. a 50 meter center of mass shift would cause a huge variation in local sea-levels.

How would it do that?

 

edit: keep in mind that this shift is happening over the course of millions of years

 

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22 hours ago, hu?? said:

Hmm, you are suggesting this forum must not be used by ignorant people to seek knowledge but only for asking questions for which one already knows the scientific acceptable answer?

That is quite a certain way to stop scientific progress...

Please enlighten me, i do not not know what my flawed assumptions are.

Are you asking us or telling us ?

 

 

4 hours ago, hu?? said:

I am not worried at all. Just questioning. 

The relative radius of the earth is irrelevant: e.g. a 50 meter center of mass shift would cause a huge variation in local sea-levels.

Needless to say the Earth-Moon system is very complicated and has developed of more than 4,000 million years.

Like swansont I would like to see some calculations to back that up, but remember you need to consider the Earth-Moon system to calculate water levels.

 

On 12/27/2021 at 12:21 PM, hu?? said:

Thanks for the info. I didn't know the floating-ice cube principle was valid for tectonic plates. Sounds plausible when the earth crust indeed has a lower density than the magma directly below and does not have a significant 'stiffness' (is that the correct terminology). However, the isostacy principle for tectonic plate mechanics seem to have the state of a hypophesys, not that of a scientific theory.

So, I am not completely convinced, considering:

1. looking up the density of magma (2.18 and 2.80) and the average density of the earth crust (2.83).

2. solidification of a fluid (magma) tends to increase the density (i know there are some exceptions such as water or iron below certain pressures). This seems to confirm (1).

3. at a glance, measured variations in in the earth gravitational field do not seem to support the isostacy hypophesys (https://en.wikipedia.org/wiki/Gravity_of_Earth )

Is there any reason that solidification of magma would a decrease in density or does the earth crust somehow have a different composition than the magma below?

 

 

Isotasy is a local effect.

Two matters you need to consider.

Firstly the earth's surface is curved not flat.
You can't fit one or even a small number of plates to the surface of a globe.
The direction of gravity is radia not parallel.

Secondly a comment on magma to amplify (not contradict) what exchemist has already told you.

The rocks from the surface to near the interface with the outer core (the core has at least two layers, a molten outer and (semi) solid inner) are generally solid.
Here the temperature is actually hotter than the surface of the Sun.
They are under enormous pressure, the deeper you go the greater that pressure, due to gravity.
Because of that pressure, they cannot melt even though some are 'above their melting point'.
 

When Earth movements relieve (some of) that pressure they melt and the pressure energy has to go somewhere so the rocks melt locally.
This is the source of magma. Ultimately it is powered by gravity.
Near the surface This magma breaks through at discontinuities and weaknesses to create the features of vulkcanism in general.

 

There is a great deal more to it than this but I will stop there, and await your response if any, as you did not respond to the last pointers of information I offered.

 

Edited by studiot
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1 hour ago, studiot said:

Are you asking us or telling us ?

 

 

Needless to say the Earth-Moon system is very complicated and has developed of more than 4,000 million years.

Like swansont I would like to see some calculations to back that up, but remember you need to consider the Earth-Moon system to calculate water levels.

 

Isotasy is a local effect.

Two matters you need to consider.

Firstly the earth's surface is curved not flat.
You can't fit one or even a small number of plates to the surface of a globe.
The direction of gravity is radia not parallel.

Secondly a comment on magma to amplify (not contradict) what exchemist has already told you.

The rocks from the surface to near the interface with the outer core (the core has at least two layers, a molten outer and (semi) solid inner) are generally solid.
Here the temperature is actually hotter than the surface of the Sun.
They are under enormous pressure, the deeper you go the greater that pressure, due to gravity.
Because of that pressure, they cannot melt even though some are 'above their melting point'.
 

When Earth movements relieve (some of) that pressure they melt and the pressure energy has to go somewhere so the rocks melt locally.
This is the source of magma. Ultimately it is powered by gravity.
Near the surface This magma breaks through at discontinuities and weaknesses to create the features of vulkcanism in general.

 

There is a great deal more to it than this but I will stop there, and await your response if any, as you did not respond to the last pointers of information I offered.

 

Surely magma production involves more factors than pressure reduction, doesn't it? I seem to recall reading that the magma that produces island arc volcanoes, behind subductions zones, is formed due to a combination of frictional heating of the subducted slab as it descends, combined with the formation of lower MP hydrated minerals, due to the entrained seawater in the slab. Diapirs of magma then rise from the descending slab and, where they reach the surface, volcanoes form. 

And at hot spot volcanoes, e.g. Hawaii's Kilauea, the magma forms due to unusually high temperatures close to the surface.

So it's an interplay of temperature, pressure and mineral melting points. 

 

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3 hours ago, exchemist said:

Surely magma production involves more factors than pressure reduction, doesn't it? I seem to recall reading that the magma that produces island arc volcanoes, behind subductions zones, is formed due to a combination of frictional heating of the subducted slab as it descends, combined with the formation of lower MP hydrated minerals, due to the entrained seawater in the slab. Diapirs of magma then rise from the descending slab and, where they reach the surface, volcanoes form. 

And at hot spot volcanoes, e.g. Hawaii's Kilauea, the magma forms due to unusually high temperatures close to the surface.

So it's an interplay of temperature, pressure and mineral melting points. 

 

I said it was both complicated and not yet fully understood.

New material and ideas are emerging all the time.

I didn't really want to mention 'plume' theory just yet either.

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