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Island Arcs and Depressions

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					If we look carefully at the map of Pacific ocean floor, we shall be impressed by a system of zones –
those of Aleutian islands , Kuril – Kamchatka islands , Japanese zone , Izu-Bonin islands, Mariana
islands, Ryukyu islands , New Hebrides, Tonga islands.
From geophysical point of view they have the following characteristic features:
1. Islands and depressions/trenches go in parallel, as a united system. On both sides of the their
   major part, they are flanked by ocean earth’s crust.
2. Islands arcs and depressions coincide with zones of characteristic gravity anomalies.
3. Isostatic equilibrium is disturbed in depression area.
4. Points of deep-focal earthquakes are concentrated in the area of island arcs. Depressions are
   characterized with concentration of near-located earthquake foci, showing a tendency to
   replenish the system along it’s entire length
5. Volcanoes are concentrated in a front on island arcs.
6. Thermal flow reaches maximum in arcs and minimum in depressions.
7. Island arcs resemble zones at state of orogeny.

An widespread position in geophysics, at the moment, assumes that these areas are
subduction zones of the ocean crust (born from median ridges ) below the continental crust.
Such model can not clarify the way of formation and the typical features 5 and 7.
Features 4 and 6 are partly explained.
At the same time the model requires introduction of descending convectional flows — something
rather artificial and disputable.
Moreover, the subduction model is introduced primarily for compensation of the effect of earth’s
crust expansion within the medium ridges, and to avoid the model of “Expanding Earth” . This is
artificial conception, which converts mobility from global to local phenomenon.
Accumulated observations are adjusted to a priory idea, which has come into being by another
reason.
From physical point of view, the motive forces of subduction are not reliable. The remarks of
V.V. Belousov are rather strong.

The subduction model can not explain the exceptionally high tectonic activity in the Japanese
island zone during the year 1989, as there had not any adequately increased activity neither in the
median ridges, nor in the area between ridges and depressions.
This should be said for every year from 1989 to the contemporary days.

That is why, it is reasonable to search for other models to explain these exotic tectonic formations.

The isostasy theory (1 , 2 ) assumes that the Earth’s crust is composed by separate blocks floating
in the mantle like the icebergs in the ocean.
The icebergs represent pieces of ice, broken away from glaciers, floating freely in the ocean water
due to a positive balance between buoyancy force and their weight.

Existence of icebergs is possible because of two facts:
1. Dynamic equilibrium between heating and cooling processes in the ice block.
2. The ice density is lower than this of water:
– Because of inverse factor of volumetric thermal expansion of the water around the solidus and
below it.




– Because of accumulation of fresh /rainy or snowy/ water in the glaciers which is less dense then
the saline water. I.e. chemical separation takes place during crystallization.
The iceberg thickness is of the order of hundred of meters, with hydrostatic pressure of
surrounding water reaching tens of atmospheres. Within these frames the water density remains
constant.

When transferring by analogy considerations onto the Earth’s crust, we should bear in mind some
differences:
1. It seems natural that the Earth’s crust reflects dynamic equilibrium between cooling and heating
   processes in the surface layer of the Earth.
   A. The processes of cooling are the following:
       a/ Direct heat transfer from the crust to the air and the water in the boundary layer. The water
          will cool more effectively and more steadily then the air, because the bottom water has
         higher     and undergo less variations then the ground air layer.
     b/ Heat ejection from the deep parts of the crust through water and gases. The high
         hydrostatic pressure, the higher viscosity of the ocean water as compared to this of air
         strongly impede this process and it is less expressed in the ocean crust.
     c/ Phase transitions.
     d/ Radiant emanation. A nearly impossible process for the ocean crust and a limited one for
         the land crust. The upper crust layer is cold for such process.
  B. There are also several processes of heating:
     e/ Heat transfer from the mantle to the crust by heat-conductivity. It can be assumed, that this
         process is identical with the both types of Earth’s crust.
     f/ Heat exchange during intrusions and volcanic activity – also identical for the both types of
        the crust.
    g/ Absorption of radiation from the mantle to the crust – also identical for the both types of the
        crust. In general, it should be assumed that the heating processes in the both types of crust
        are approximately alike, while the processes of cooling are quite different.

2. If we presume a free floating of the blocks in the mantle /isostasy/, we have to acknowledge a
   positive balance between buoyancy force and the block’s weight. Therefore the block density is
   lower than that of the mantle. This is confirmed by the seismology.
   What is the reason for the lower density of the hard crust ?!
   There are several possible reasons: – gravity separation. – Chemical separation during
   crystallization. – Inverse factor of volumetric thermal expansion around the magma solidus.
   Unlike the ice, consisting of only one substance, the Earth’s crust is a complex conglomerate of
   substances and the problem about its factor of volumetric thermal expansion is quite
   complicated. The experience of some metallurgical enterprises with slag containing
                     ,                  ,                                    shows a picture identical
  with water in qualitative respect.

The possibility that the individual layers of the Earth represent approximately one and
the same substance in various states, suggests a great potential for development of our
notions.
That is why it is interesting to discuss the case when the third reason appears as
decisive one.




3. In the case of Earth’s crust, the pressure is high enough to cause compaction of the substance –
   an important difference with the case of an iceberg.
Thus, the fact of existence of the Earth’s crust in general, as well as some differences
between land crust and ocean crust, can be explained by the following reasons:
-Thermodynamic field
– Isostasy
– Pressure field
These three reasons are mutually interwoven and condition each other during the process
of crust formation.

Taking the above said into consideration, let’s examine the following model:
A part of ocean Earth’s crust, isometric in plan, with characteristic dimension about 500 km, with
thickness of about 10 km, flat, overlaying the mantle, covered by water layer about 5 km. /Fig.1/




                                                                               Fig. 1




Supposing that:
– The object is small enough as compared to the Earth’s area and the plane-parallel geometry of
  the force’s fields is valid for it.
– The object is a monolithic entity, build in its whole volume of one and the same substance,
  isotropic by physical properties.
– The hard crust and the underlying mantle are built of one and the same substances which density
  decreases below the solidus temperature /the crust “floats” in the melt/. The solidus isotherm is
  a boundary between the crust and the melt.
– The water cools, and the melt heats the crust.
– The object is subjected over its whole width and thickness to an uniform bilateral stretching,
  increasing in time.

What kind of processes are likely to be held?

Let’s confine our considerations to the section        , parallel to the direction of stretching forces
    . At The moment        , when       is small as compared to the cohesion forces of the crust, the
object will be a flat plate. Isotherms and isobars represent parallel planes.
At the moment , when            becomes commensurable with cohesion forces, the mechanic of the
plate is complicated by stress field according to Hooke’s law. Isobars and isotherms bend. A part of
the weight of the middle areas is supported by the Hooke’s field and the isostasy is disturbed.
At the moment       , when      exceeds cohesion forces, the plate breaks apart.
A lot of variants are possible.
Let’s take this one, where points     and       are formed in the middle of the section         , and
the layer is being partly broken from below under       point, and from above under        point.
/Fig. 2a/




                                                    Fig. 2a

As a whole , the plate recovers its plane geometry. Thermal and baric fields get complicated: Heat
outflow increases under      point due to enlargement of the water/crust interface. Respectively –




isotherms go down in depth, and a “root” is formed below the layer. Phase transitions and gas
discharge are possible in the root zone. The gases are being accumulated around the root as
gas pillows. Additional buoyancy force appears from the root, which can be partly or completely
compensated by the residual stretching stress. Thus an area of disturbed isostasy and negative
gravity anomalies, diluted by the gas pillows, is formed below      point. Heat inflow from the melt,
having intruded into the breakage area, increases below   point. A zone of positive gravity
anomaly appears, a tendency to sinking can be expected as well, partly or totally compensatedby
the residual stress. With      increasing, the described up to here can be repeated several times.
The difference between the plate parts below the two points increases.
The root under      point rises. There is no process to compensate the loss of thickness under
point. If two points are closely situated, it is possible that the gas pillow moves from     point to
the breakage area under         point. The areas of the plate below     point, which have been cold
before, are heated. A high temperature gradient leading to additional stress appears. It is possible
in this condition that local areas with insufficient mechanical strength are formed. This strength
can be overcome by the accumulated gases which go out carrying away some melt and gases – a
submarine volcano arises! /Fig.2b/




                                                  Fig.2b



This moment is critical one!

A tremendous quantity of heat is led away through the lava and the gases.
Isotherms fall down abruptly. The solidus also falls down.
The hard portion does not possess mechanical strength because of the fast cooling.
Caverns from the gases and fractures remain.
At the same time, phase transitions take place in the melt with temperature above solidus, but
approaching to it /around the root/. New gases are liberated, which find easier their way out.
The zone under       point changes quickly.
A root with great depth appears.
The buoyancy force is enormous. It can not be overcome by residual stresses, and the newly
formed block /the part below        point together the root/ is subjected by isostatic uplift.
The old plate breaks apart.
An orogenic process begins.
This is already a very active zone.
Heat outflow is carried out, besides through heat transfer, but also through gases, vapor, lava. Such
carries have a great efficiency. An area of heat flow maximum is formed below           point.
Because of the slower and more uniform deposition, the root below         point is denser and
stronger then its neighbor. It is also smaller because of decreased heat outflow. The gases,
liberated around it, find easier their way through the structure below     point, which thus
takes a part of its neighbor’s heat outflow. In this way, the heat flow maximum below       is
sustained, while a minimum is formed below          point. A common area, from the point of view of
thermodynamic field, is formed below the two roots, characterized with active phase transitions and
gas liberation. The substance, in this area, will posses properties transient between those ones of
the mantle and the crust. It is this area that can be interpreted as “Wadati-Benioff zone”. /Fig. 2C/




The zone below      point is a part of a island. This one below      point – a part of depression
respectively.
The depression remains the single place for further breakage.
Besides        section, there are a lot of sections where couples of points analogous with
and      appear. The combination of formations, along with these sections, could make a picture
analogous with that one observed in the west part of Pacific ocean. Only should the process be in
different stages and the picture -variegated.

It is the   field that played the main part in the “screenplay” under consideration.
Earth expansion could be one of the possible reasons for the rise of such field.

If the resent island arcs are the beginning of new Cordilleras and Andes?




All three stages of the development of the zone.

Nikolay Kitov
nikolakitov@gmail.com

				
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Description: Non-subduction explanation of Vadati-Benioff zone. Evolution and state of the big ocean ridjes and trenches