My science/technology-related thoughts, sometimes controversial, sometimes can be based on limited knowledge base, logic can be non-perfect as well. I develop my vision in iterations. Don't take this blog as an attempt to convince anybody in anything.
Each post in this blog reflects my level of understanding of Tectonics of the Earth at the time the post was written; so, some posts may not necessarily be correct now.

27 May, 2011

Broken Subduction To Cause Earthquake And Tsunami.

   Imagine an oceanic (Pacific) plate is being subducted under a continental (Asia) plate from east to west. The  oceanic plate is not totally uniform. The most notable anomalies in my opinion are divergent boundaries, active or deactivated.

   The anomalies along the most stressed part of the subduction zone may lead to the rupture along the fragment of the subducted divergent boundary. The divergent boundary is a very thin, not yet fully solidified, highly irregular, and, therefore, weak juncture between plates.

Important is that the stress within the fragment is compression / bending. The compression is caused by distant divergent boundary pushing the oceanic plate under the continental plate. The bending stress is due to the oceanic plate moving down. The oceanic plate serves as a gigantic spring to store the energy a distant divergent boundary develops under Moon/Sun induced deformations, the energy to be released in the form of the rupture. How would a compressed / bent plate broke in normal life? There will be a rupture and the ridge of the rupture would go off perpendicular to the surface.

   In our case the ridge of the rupture would go up and west. Taking into account that the fragment on the west of the rupture is more molten than the fragment on the east,  the motion will be even more to the west. This would produce tsunami to west and anti-tsunami to east.

   The dynamics of the rupture should have a prehistory which, in my opinion, can be recognized prior to the rupture. Some of the detectable properties could be:
- usual set of divergent boundary features: numerous "molten" anomalies (higher seismic wave speed) within the future rupture line, thin crust, very thin viscous layer, magma is quite high here.
- sequences of the moderate deep earthquakes by which magma propagates through the crust (See "Intraplate Magma Transport. The Concept Of Moving Transition Zone."
http://divergent-boundaries.blogspot.com/2011/05/inter-plate-magma-transport-mechanism.html )
- pre-displacement under the force by the distant divergent boundary, as the zone gets hotter on average and therefore "less solid".
- oceanic water properties change due to magma having reached surface of the crust (probably could affect atmosphere as well).


   The described mechanism is heavily based on the assumption that the oceanic floor spreading mechanism is fueled by magma solidifying in the crust ruptures caused by Moon/Sun induced crust deformations (see "Divergent Boundaries Spreading Mechanism By Magma Solidifying In The Crust Ruptures Caused By Moon/Sun Induced Crust Deformations." http://divergent-boundaries.blogspot.com/2011/05/divergent-boundaries-spreading.html ).
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reposted from http://sukhotinsky.blogspot.com/
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22 May, 2011

Formation Of Mountain Ridges by Broken Process Of Subduction Of A Tectonic Plate.

   Generally accepted theory on Himalaya/Tibet formation is the collision between two continents. Tremendous forces are believed to had applied to the continents to form the mountain system. But, can other mechanism be suggested, the mechanism to demand less energy/forces to create the mountain system? Let's try to suggest one.

   When an oceanic plate is getting subducted, what would happen to a divergent boundary on the subduction? The divergent boundary is a very thin, not yet fully solidified, highly irregular, and, therefore, weak juncture between plates. The bending stress of subduction would just cut loose the subducting chunk of the plate. The loose chunk would pop-up along the viscous border of the continent to form a mountain ridge. Other divergent boundaries and, probably, other irregularities in the plate would add more ridges to the continent.

   I'd like to think, "The Ring OF Fire" systems of ridges and some other systems were created by the mechanism described above. To check the concept let's take Himalaya:
   1. Himalaya system is quite close to the point of start/collapse of The Ring Of Fire (see "10. Persistent location of the point of start/collapse of The Ring Of Fire " http://divergent-boundaries.blogspot.com/2011/05/continental-formation-and-evolution_18.html ) Thus, we would expect all the oceanic crust to go here mainly from the center of The Ring Of Fire, that is from South East. So, the boundaries' butt ends, I'd expect to look in South East direction.
   2. The deep anomalies should align in the same SE or NW direction.
   3. Magnetic anomalies of mountains along a ridge should maintain similar alignments.
   4. The ridge system should look paradoxically young, as exposed are only newly formed boundaries' butt ends.
   etc etc.

   The ridge systems of the described nature, I would expect, could be found far from a continent's borders as The Ring Of Fire is developing many cycles, adding more area to the continent on each cycle.
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reposted from http://sukhotinsky.blogspot.com/
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18 May, 2011

Continental Formation And Evolution Revisited.

1. Appearance of the plates. 
   Liquid Earth cools enough to let the stuff with the highest melting temperature to solidify. The denser than magma stuff is moving down to form Earth's solid core, the less denser stuff is popping up to solidify on the surface.

2. Appearance of divergent boundaries. No subduction yet.
   The solidified stuff on the surface is getting deformed by Moon. Moon induced deformations are giving birth to the divergent boundaries ( see http://divergent-boundaries.blogspot.com/2011/05/divergent-boundaries-spreading.html ). And we assume that plates are moved by the forces that originate within the divergent boundaries. The legacy concept of asthenosphere is not used, plates now own their viscous bottom. The process of subduction is not started yet anywhere within the plates.

3. Subduction started.
   The compression between plates is getting greater and ridges over the boundaries are getting higher. At some point, most probably near Equator, a newer, therefore thinner and denser plate started subducted by older, thicker, and less denser plate. It would be natural to expect Moon to trigger the process in East-West direction, that is, the "first" subducted plate to move from East to West. The subducting plate disturbs the bottom viscous layer of the upper plate pushing the roll of viscous stuff ahead. Plates subducting from different directions are creating a lens of viscous stuff under this construct of plates.

4. Proto-cratons creation.
   The process of subduction is very intense at this point. The plates with melting temperature higher than current temperature of magma are getting semi-molten. The semi-molten stuff under the plate is getting penetrated by newer subducting plates. The newer plates have lower melting temperature. Eventually they may get molten. And trapped within the much more viscous stuff they would look like huge anomalies with higher seismic waves speed surrounded with the less molten stuff of lower seismic waves speed. The resulting thick constructs may, therefore have less dense and much more solidified bottom than its middle layer. This effectively would lead to almost absence of the so called "asthenosphere" layer on the bottom of the construct. I'd like to call the constructs proto-cratons.

5. "The Ring Of Fire" sets around the cluster of proto-cratons.
   The cluster of proto-cratons gets too thick to maintain subduction within the cluster. The Ring Of Fire - the circle of subduction sets around the cluster.

6. Two types of divergent boundaries.
   Now and on two types of divergent boundaries are spreading the solidified magma:
-  First type, divergent boundaries that move plates towards the subduction (directly or indirectly by pushing other boundaries  towards the subduction zone).
- Second type, divergent boundaries within the cluster of proto-cratons. These boundaries move deep rooted proto-cratons. And they do the job against counter-acted subduction process. Thus, boundaries of this type develop higher ridges to gain greater force than the ridges of boundaries of first type.

7. Some types of mountains.
- Type 1. A divergent boundary creates a ridge. Then the boundary deactivates for some reason, say, the plates breaks along another fault line.
- Type 2. A divergent boundary (probably deactivated at this time) reaches subduction zone. It's hard to imagine how the ridge would go under the upper plate. Rather as the ridge interior is still semi-molten, the ridge would break along its line. The loosen chunk on the continent's side would rotate and expose its sharp edge on surface. the incoming plate would again try to subduct under it. The loosen chunk after the rotation can produce high mountain as it can go as deep as hundreds kilometers.
- Other types. A convergent and other types of boundaries between plates are beyond the scope of this posting.

8. Tibet/Himalayan creation.
   So, the creation of Tibet/Himalayan would take:
- a number of divergent boundaries on the ocean floor;
- subducting the floor against the continent to create a number of high ridges;
- pressing the messed plates with the incoming continent from the other side of the floor.
- a divergent boundary moving the continent off;
- repeating from the beginning as many times as needed;
 - placing some cratons around the area to not let the subducted plates leave the region in the molten state; the locked molten stuff will be pushing the Plato even higher.


9. "The Ring Of Fire" cycle approx 300 million years. Or a time to cast away stones, and a time to gather stones together (Ecclesiastes 3:5).
   The Ring Of Fire changes its shape as the divergent boundaries within the cluster of proto-cratons expand the cluster area. When the cluster gets all the surface, The Ring Of Fire collapses. There are no place on Earth to cheaply (in terms of energy) allocate the solidified stuff produced by divergent boundaries. So, new subduction line develops. And the process repeats from the step 3 as described above.
   Let's calculate the cycle of The Ring Of Fire, that is, the cycle the continents would get cast away and then gathered together. If we assume that opposite sides of The Ring are approaching each other at 15 sm/year, we would get roughly 40000km/15sm = 40 million meters / 0.15 meter ~ approx 300 million (years).

10. Persistent location of the point of start/collapse of The Ring Of Fire between cycles.
   It was mentioned on step 3 that it would be natural to expect Moon to trigger the process of first subduction in East-West direction.  It would also be natural to expect that the initial subduction line won't be able to move from West to East due to Moon's tidal activity. Also it can't move from East to West as the subduction has already built the continent on the West of it. In other words, the point of start of The Ring Of Fire will be rather close to the point of its collapse in the sense it will be the Eastern border of the continent again. With each cycle the continent would add more ridges to its Eastern border.
   This way The Ring Of Fire with each its cycle is adding more and more ocean ridges to Tibet/Himalayan cluster, and subducting more and more stuff under/inside the cluster's body.
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reposted from http://sukhotinsky.blogspot.com/
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10 May, 2011

Intraplate Magma Transport. The Concept Of Moving Transition Zone.

New concept of Plate proposed in the post "Plate Tectonics. Thinking Out Of The Sphere." immediately raises the question,  -  Is Transition Zone stable? What factors affect Transition Zone propagation? Can Transition Zone reach surface?

One of the specific features of the Transition Zone is that above it the solid body can respond with ruptures on the deformations, and below the Transition Zone the deformations can not cause ruptures. It was suggested that very this feature is responsible for magma propagation (see the post "Magma Transportation On The Temperature Gradient").

Now we are ready to think of Plate as a complex system of a) elastic solid body of very irregular shape and density b) viscous matter with very irregular viscosity/density/temperature/local_speed beneath/inside the solid body. If we accepted that plates are driven by divergent boundaries, then we don't need to group the semi-solidified/semi-molten matter beneath the plates into single wold-wide global class; we don't need to decouple the class from their solid hosts to invent the mechanism that drives plates.

Of course, the above is based on the "Magma Transportation On The Temperature Gradient" mechanism. The mechanism yet to be proved experimentally. The successful experiment would lead to relevant model of  Continental Formation and Evolution.

Quick search reveals that the closest in this direction were research works on shear-driven upwelling. Probably, if not limited by the paradigm of asthenosphere as separate world-wide layer, they could reach even further.

Plate being a complex system can develop deformations in its solid layer not only along lines, but also it could develop deep localized deformation induced by Moon/Sun. Such repetitive deformations can propagate Transition Zone up to the surface to form and feed a volcano.
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reposted from http://sukhotinsky.blogspot.com/
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06 May, 2011

Plate Tectonics. Thinking Out Of The Sphere.

A problem with plates "floating" on the asthenosphere.

Wegener thought that the continents were moving through the earth's crust. Now the mostly accepted point is that continents do not move through the ocean floor. Rather, continents and oceanic floor form solid plates. And the plates move over the asthenosphere, the highly viscous layer of the upper mantle of the Earth.

But, really, don't you see the problem with the approach: a plate floats over the viscous layer, which floats over just liquid magma? We know some plates reach as far as 700km and even greater depths, and their bottom is not flat. How would such complex structure manage to flow over the highly viscous layer? The viscous stuff is not glued to anything, it would just go with the plate over liquid magma.
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Why not to suggest that both parts - solid and viscous parts are drifting over magma?

Wegener suggested that continents drift through the earth's crust, later it was corrected that entire plates "float" on the asthenosphere. Why did the science stop at this point? Why was not it suggested that both parts - solid and viscous parts are drifting over magma? I could imagine two reasons:

- Nobody risked to insist on such "revolutionary" step remembering how Wegener was treated with his much milder suggestion. Taking the step would mean that a plate owns its underlying viscous stuff and moves with it. This would effectively discard the legacy concept of asthenosphere as a physically consistent world-wide viscous layer.

- Science needed explanation how plates move. The underlying viscous stuff was magically decoupled from its upper solid host and it was suggested that the decoupled layer can transform the  interaction with underlying magma into the force to move the plate.  
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Avoiding postulates. 

I'd like to suggest that there is no need to decouple plate's underlying viscous layer. Let a plate "own" its viscous layer and move with it. Let's suggest that plates are driven by divergent boundaries as was described in my previous posts. I don't see how the suggested magma transport mechanism contradicts lows of Physics. Though I must admit, right now I can't back my considerations mathematically or experimentally. A quick search reveals that the closest in this direction were research works on shear-driven upwelling. Probably, the proposed mechanism will get attention as well.

If proven that plates are driven by divergent boundaries as was described in previous posts, then we don't need to make highly controversial postulates that feed so well the opponents of theory of Plate Tectonics. For instance, the postulate that oceanic divergent boundaries are made by some mysterious hot magma jets, the jets that don't obey Coriolis effect, the jets that can pump magma up as high as many kilometers.

We don't need to postulate that plates are driven by some mysterious magma flows, and driven not directly, but rather by means of separate world-wide layer - Asthenosphere. Instead, we could assume that everything beneath the plate's upper layer belong to this plate. It's just a)solidified, b) semi-solidified magma or semi-molten another plate subducted under this plate.

Of course, the plate on its way can lose some of its less viscous stuff on its bottom by the process of magma recycling. But on average, due to the Earth being cooled, it gains more by the plates subducting process.

With the help of the theory outlined in previous posts we can explain the oldest parts of plates with the roots as down as many hundred kilometers. These parts had been subducting other plates at the beginning of the plates evolution. As magma is globally cooling, the melting temperature of subducting plates was higher than the temperature of magma of newer ages. The subducted plates just added to the bottom of the oldest parts of plates as a semi-melted stuff. As magma temperature was dropping, the underlying layers of the plates were increasingly solidifying.

A number of other postulates could be avoided as well, but, probably, it can be the subject of the next posts.
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New concept of Plate.

Probably, in the future it would not be a bad idea to rethink further usage of the concepts "asthenosphere" and "lithosphere", as the concepts don't fit well the proposed theory. The suggested new concept of Plate, I'd like to define the next layers:

1. Solid layer.
1.1 Upper solid layer - crust, can be investigated by mechanical tools.
1.2 Bottom solid layer, too hot to be investigated by mechanical tools.


2. Transition layer. Here seismic wave speed would undergo, say, 10% to 90% of its speed change.

3. Viscous layer.
3.1 Upper viscous layer, persistent to the plate on the course of, say, 10 million years.
3.2 Bottom viscous layer, participates in magma recycling process;


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reposted from http://sukhotinsky.blogspot.com/
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(edited Jun-17, 2011)

02 May, 2011

Continental Formation And Evolution.

If the concept of magma propagation in crust along temperature gradient caused by crust deformation is correct, then interesting perspective on Continental Formation is opening. Let's assume border forces are the main forces that drive continents. Then the next basic rules could be derived:
*****
- The main source of the force to move continents are ocean floor divergent boundaries.


- The main source of the force to break a continent apart are the divergent boundaries within the continent. These divergent boundaries later can start ocean floor spreading.

- The force a sub-ducting ocean floor is acting on the continent with, is much less than the force of non-sub-ducting floor. Therefore, the sum of vectors of forces acting on a continent would point the continent in the direction of its borders under which ocean floor sub-ducts.
*****


Let's go further:
*****
- If between two convergent boundaries one has to sub-duct, the more dense one would go down.


- The crust is made of the material that a) is lighter than underlying magma and b) have higher melting temperature than the temperature of the underlying magma.

- The sub-ducting crust may not necessarily melt under the continent, as current temperature of magma can be lower than the temperature of magma the old crust was formed at.
*****


The above rules lead us to a non-that-obvious mechanism of ocean floor / continent transformations:

1. Initial sequence: 
  - Very thin chunks of crust start to pop up all over the surface.
 

  - The chunks are drifted towards Earth poles by the Moon induced tidal waves.
 

  - All the surface is filled up with the chunks. Polar layers are thicker.
 

  - The chunks are merged into crust. Moon induced waves are braking the crust into big plates. The plates are getting thick enough to resist moon induced waves to overlap them. The underlying heat is blocked. Atmosphere and ocean are developing. Polar crust regions are the first continents as they are thicker and are made of lighter crust.

2. The ocean plates with the help of Moon develop divergent boundaries. The divergent boundaries stress the crust globally, the most stressed direction is East-West on Equator. The beginning of sub-ducting process is inevitable.

3. The weakest point breaks (probably with the help of some asteroid). If not with the help of the asteroid, it would probably break on Equator, as Equator crust at this time is newest, thinnest, and most stressed.

4. The sub-ducted crust around the fault line creates a continent. The crust sub-ducting under the continent borders expands the continent.  The distant divergent boundaries push the crust to the continent. The crust is getting jammed on its way toward the continent. The structure of the resulting islands I'd expect to be heavily twisted. The island are bumped into the continent's plate adding to the plate's area.

5. The two old Polar continents are moving toward the new continent, but not too fast as the diverging in North-South direction is slower than in East-West direction on Equator. Soon they enter the continent's Ring Of Fire of sub-ducting process and bump into the continent's plate, having traveled not too far. The remnants of those Polar continent's, I'd like to think, can be found in Australia and North of Canada and Greenland.

6. The line of Ring Of Fire of sub-ducting process expands to embrace the hemisphere the continent belongs to. Then The Ring Of Fire shrinks and disappears within the other hemisphere. Now again Earth's crust has no place to easily sub-duct the extra area added by newly developed crust. The divergent boundaries stress the crust. The beginning of new sub-ducting process is inevitable. Loop to paragraph 3.

7. A number of cycles of the process had passed, I'd like to think, before Earth's crust reached its current state.
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Ridge Push, Or Bridge Over Troubled Magma.

Usually Ridge Push as a boundary force, is considered to be produced by the gravity effect. Hot mantle up-wells under the ridge, and this gives a topography-induced horizontal pressure gradient.

The problem with the approach is that I can't imagine the speed of the mantle upwelling to dynamically maintain heavy rock above the floor level at hundreds meters (if not kilometers). And, as Earth rotates, it is just impossible for mantle to up-well to sea floor perpendicularly to it. Besides, it's hard to imagine the nature of topography of the upwelling line.

But, why for two these phenomena not to reverse cause and consequence? Let "Magma Transportation On The Temperature Gradient" mechanism consume magma by lifting it up hundred meters. Fresh incoming magma would create hotter region under the ridge. The resulting bridge of solidified magma over the hot magma would push off the boundaries.

(Well, the post should, probably be titled "Troubled By Earthquakes Ridge Over Hot Upwelling Magma" :-)
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Magma Transportation On The Temperature Gradient.

Why do volcano eruptions correlate with earthquakes? Which of two of the types of events are the cause and which are the consequence? Probably, one would say, crust layers collide squeezing magma up. But, really, can the collisions produce enough pressure to pump magma that high. Have a look at The East Pacific Rise, just pure magma under the Ridge, just nothing that could resemble any kind of the pump mechanism. Nothing to get collided here at first glance. New crust is getting born here out of magma, and the newborn crust is getting spread out.

The suggested concept is the next: Powered by deformations, crust propagates magma in the direction of lower temperature by the means of earthquakes, the magnitude of the earthquakes depends on many factors, among them are: amplitude and frequency of the deformations, crust characteristics, magma temperature, the value of the temperature gradient.

On the example of The East Pacific Rise, it can be Moon/Sun induced deformations on the border between the plates. The mechanism can be the next:
- When crust is stretched, its ruptures are filled with magma, when the crust is getting compressed, it can not produce the same flow of magma back, as magma had lost its temperature on heating and melting the rupture borders. The more viscous magma would try to find easier ways to escape the pressure, by, say, creating new ruptures.

This way, I believe The East Pacific Rise is being built of solidified magma.
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How To Tell The Nature Of The Mechanism Of The East Pacific Rise.

There are a number of models of driving mechanisms behind plate movements, just a few to mention:

- Mantle convection model;
- Slab suction;
- Gravitational sliding;
- Tidal friction of the moon;
- etc;

The proposed in previous posts mechanism "Magma Solidifying In The Crust Ruptures Caused By Moon/Sun Induced Crust Deformations" is completely different to them. Let's see how it fits East Pacific Rise spreading.

First, under "Crust Deformations" in this case we have the process the boundary plates are getting closer/more distant with moon month cycle. Let's leave Sun and planets effects for next postings. Moon orbit is inclined toward Equator plane. Thus, Earth twice a month is more stretched on Equator, twice a month is less stretched. Because of that the distance between plates varies with the period.

Now, let us ask some questions:

- Does the mechanism pump magma up?

Yes, vertical temperature/viscosity gradient in the gap between plates acts as a pump, less viscous magma is easier to get "sucked/pushed up" when the gap between plates increases. And after cooling the "more solidified" magma under the press of boundaries would try to find "easy" way, that is, partially to up-well. If the were no vertical viscosity gradient of magma between plates, the up-welling would not be significant. The magma up-welling depends on many factors, one of them is speed of boundaries spreading. The less speed, the higher the magma could be pumped up, Mid Atlantic Ridge can be an example, in my opinion.

-What signs specific to this concept would be left on/within crust, that can be measured experimentally?

1. I think, some semi-solidified regions could be left within the spreading crust, as the squeezed up magma would not flow uniformly. With proper instrumentals the anomalies could be found relatively easy, as their depth, I'd expect, would be less than 100 km.

2. The pumping up magma should produce specific earthquake pattern in correlation with month cycle.

3. Probably it would not be a bad idea to place precision sensors on both sides of the Rise to track their relative position. If monthly relation of the distance variation is found, that can be an evidence as well of the mechanism working on the ridge.

4. Probably, there should be some properties to be detected that depend on the crust tension.

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The difference between proposed concept and models mentioned in the beginning of this post is that the models operate with force that presumably acts on a plate, while the proposed concept deals with displacement, not with force. And variable displacement between plates have to exist according to lows of Physics, the question is - is it enough by amplitude to produce the detected effect of ridge spreading. Also it should be mentioned that the the speed of the spreading is greater closer to Equator regions, that correlates with the fact that Moon's induced variations of Earth surface are greater closer to Equator regions.


The proposed idea of magma transportation on vertical temperature gradient (with the help of Moon) seems to be new as well.
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Divergent Boundaries Spreading Mechanism By Magma Solidifying In The Crust Ruptures Caused By Moon/Sun Induced Crust Deformations.

This article is to formalize my previous article http://sukhotinsky.blogspot.com/2011/04/forces-behind-continental-drift.html.

Earth is getting colder. Upper parts of its molten layers have to solidify. Crust is not static. It undergoes deformations caused by Moon and Sun. Resulting ruptures have to be filled with magma, the magma within the ruptures gets solidified. Following deformations would further propagate ruptures to be filled with the solidified magma. A divergent boundary brakes the crust along the line where the crust experienced the most deformation.

If the crust was ocean crust then a mid oceanic ridge emerges, the divergent boundary that spreads crust. If that was continental crust then a continental divergent boundary emerges. The thickness of the divergent boundary increases until:
a) the boundary starts to spread crust. The boundary thickness maintains at the level just enough to spread the crust.
b) the boundary is thick enough to reduce local deformation to the level the crust forms new divergent boundary along new line. In this case deformations in old divergent boundary stop and the boundary deactivates.
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Forces behind Continental Drift. A concept of mid oceanic ridge self-spreading.

The key point of this article is that a mid oceanic ridge is self-spreading.

The oceanic crust is rather thin around a mid oceanic ridge. Moon/Sun induced deformations cause ruptures in the crust within the ridges. The ruptures are filled with magma, magma cools down and gets solidified. On the next half of Moon-cycle the crust would deform the opposite way, causing more ruptures on the opposite side of the crust.

The thickness of the mid oceanic ridge is self-regulated. The greater force is required to spread the crust, the slower it would move, the thicker the ridge will be. East Pacific Rise spreads the crust that will be sub-ducted in Pacific Ring of Fire, a relatively little force is required, thus the crust in the ridge is rather thin and the speed of the spreading is rather high. Mid-Atlantic Ridge has to spread not only ocean crust, but also the entire America, thus it is much thicker and much slower.

Crust sub-ducting in Pacific Ring of Fire takes a relatively little force from the mid oceanic ridges - the work is "done" by Moon/Sun. The sub-ducting crust and its environment are elastic, but not ideally elastic. When Moon deforms them, the crust would move slightly by the push force of mid oceanic ridge. But when Moon would restore the surface's shape, the crust won't return totally back, thus, the slight force from the ridge would sub-duct the crust in a small step.
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