The Pressure Solution Page
(AKA "Pressure Dissolution," "Dissolution Creep," "Stress Induced Solution Transfer," "fluid-assisted grain boundary diffusion creep", "Diffusional Mass Transfer", DMT, etc., etc., etc.)
This page is a work in progress.
|. . . Constant volume deformation was an underlying but unspoken assumption in the debates [about the origin of cleavage]. . . . It now seems clear that the dominant mechanism for cleavage formation is pressure dissolution [pressure solution] removal of the original host rock.|
|From Davis and Reynolds, Structural Geology of Rocks and Regions|
Pressure solution is more important that even the quote suggestsat least for some areas of geology. This page explores the nature and origin of pressure solution and a bit about the history of how it has been interpreted by geologists in the past. I may think pressure solution is important, but when there isn't even an accepted name for it, one can bet that some uncharted territory is going to be crossed.
Originally this page was intended as a resource for pages describing geologic problems where pressure solution is key to an understanding. Those pages are here and here. The work done on this topic now suggests that it is more important than the examples it was supposed to support.
Pressure solution is documented in many kinds of rocks, especially carbonate rocks. The emphasis on this page (for now) is on phyllosilicate bearing rocks.
What is Pressure Solution
|Here is the short
version: To the right, is a fairly conventional graphic
representation of that action of pressure solution in the
context of lithification or induration of clastic rocks.
Compaction of grains causes mineral redistribution as
indicated in the cartoon.
In carbonate rocks stylolites (styolites) are ascribed to pressure solution. The photo below is of a classic carbonate stylolite. The main stress direction is interpreted to be vertical in this photo.
Pressure Solution and Rock Fabrics
The section above gives one little idea how pressure solution expresses itself in rocks. For that, a more basic course is required. The following graphics are taken from a Power Point lecture by Professor Pamela Jansma of the University of Arkansas for her Structure class. The original can be found here: http://cavern.uark.edu/~pjansma/structure_pdf/geol3514_17_foliations_06.pdf. I thought that this basic discussion of 'fabrics, foliations, and cleavages was particularly good because it recognizes the central role of pressure solution. That said, it also contains some of the central misconceptions of structural and metamorphic geology that I take issue with on this page. It both teaches the basics and give an opportunity to challenge long-accepted assumptions.
|The slides from Prof. Jansma's lecture appear to the right. I have added my comments to the left.|
|Cleavage has traditionally been described as secondary to folding, as stated. This, in my opinion, is a concept that needs critical examination. Among the most common foliations are those parallel or approximately parallel to bedding planes. These occur in rocks that are sometimes completely unrecrystallized--for all intents and purposes unmetamorphosed. Sometimes specific beds will be foliated while others are not. This did not hinder workers from inferring regional isoclinal folding on a scale too big to be observed. It strikes me that this is speculation covering for speculation. As I discuss below, there are less drastic means to create these textures. The connection between folding and cleavage is in serious need of reconsideration.|
|Again, the matter of bedding parallel cleavage needs to be considered. Spaced cleavage does cut bedding in carbonate rocks, but in most phyllosilcate-bearing rocks, it is far more likely to be cutting bedding plane cleavage. This is especially true of crenulation cleavage.|
|In the case of bedding plane cleavage, compaction is an obvious source of shortening--though not the only one. Other forces are important too.|
|On a small scale, spaced cleavage may appear to be essentially parallel, but on a larger scale, it will bend to forces that created the foliation. As will be discussed below, those forces can be quite complex over relatively short distances or may be consistent over large areas..|
|In looking at these examples, one shouldn't forget the point made above--much if not most of the time, spaced cleavage in phyllosilicate bearing rocks will be cutting or superimposed on a primary foliation. None of the diagrams depicts an earlier foliation that is cross-cut.|
|Here a spaced cleavage
cuts a previously foliated rock. This would is a pattern
typical of unmetamorphosed rocks. The high angle to right
angle between the original cleavage and the spaced
cleavage is significant as well.
The styolitic type of spaced cleavage shown in this photo again shows the tenuous relationship to folding. The black in the photo is insoluble residue (probably carbonaceous) and this deformation is more akin to stylolites in carbonate rocks.
|Crenulation cleavage is usually interpreted to be the result of a second separate deformation event--D2--producing a second foliation. This is a concept that needs to be questioned. Interestingly, crenulation has some of the same features as spaced cleavage--it occurs at intervals and, most often, at a high angle to the first cleavage. In fact, many deformed rocks contain both spaced cleavage and crenulation with one grading into the other. The logical conclusion is that they are expressions of the same forces.|
|These photos show some of the relationship between spaced cleavage and crenulation.|
|The first photo confuses me. I would have said the crenulation was most prominent on the right.|
|Good examples relating spaced cleavage to crenulation. It seems likely that the physical processes that caused these two textures must operate under the same physical conditions. Or perhaps it is the same process.|
|This is an example drawn
from the equal volume school of thought on cleavages.
Conceivably it could be correct for some high grade
metamorphic rocks, but it seems inconsistent with much of
the rest of this presentation. I don't think it is safe
to interpret most rocks on the basis of this model.
Nevertheless, the take-away point of this drawing
remains. Rock can be more or less completely
reconstituted by a process that includes crenulation
A pressure solution origin for crenulation was only recognized in 1978. Full recognition of the importance of the process is far from complete.
|Slate is one kind of rock
that has been completely reconstituted by crenulation. It
is important to keep in mind, as noted above, that slaty
cleavage can the result of very lowest grade
metamorphism--or perhaps even non-metamorphic conditions.
There is something going on here--textures that some
people think of as pervasive metamorphic reconstitution
are thought of by others as not even being metamorphic.
Note the veins in the right photo. The breaks are classic indications of a pressure solution origin.
|The M domains are insoluble residue. They often contain carbonaceous material in addition to phyllosilicates.|
|Here we are getting to
the meat of the crisis in metamorphic geology. Despite
the recognition here and elsewhere, the pressure solution
revolution has not come. The forces of counter revolution
are fighting back. The following frames discuss some of
the evidence that pressure solution is the mechanism that
formed the cleavage structures discussed above, but that
is not enough. The 'how' and 'why' are not all here.
Without those, the supporting evidence to sustain the
revolution will not come.
This reminds me of a revolution in ore deposits. When I went to school in the early 1970s, many deposits were recognized as syngenetic. Previously they had been ascribed to replacement. But details apparently supported the replacement theory. No one figured out how the syngenetic theory could explain these details and in recent years there has been a swing back to the old theory--much to the detriment of ore deposit science.
|Note that pressure
solution works in both sedimentary and metamorphic rocks.
It is not an exclusively metamorphic process. In fact, it
may be a mistake to call it a metamorphic process at all.
Legions of workers still do not even consider the possibility that constant volume deformation does not work. That is the paradigm in which they work.
|Pressure solution presents problems for geochemists as well as structural and metamorphic geologists.|
|Note that some clasts can
take on the shape of ventifacts.
A provocative discussion of how quartz pebble conglomerates could be the result of diagenetic pressure solution is found in Cox, R., Gutmann, E., & Hines, G.; Diagenetic origin for quartz pebble conglomerates; Geology V. 30, No. 4, p 323-326, April 2002. This issue is important for a number of reasons. First, the world's largest gold deposits (those in the Witwatersrad basin)--and some significant uranium deposits (those in the Elliot Lake-Blind River area of Ontario, for example)--are found in these rocks. Second, the occurrence of "detrital" pyrite in Archean quartz pebble conglomerates is an important--perhaps the most important--piece of evidence used by those advocating an oxygen free atmosphere in the early evolution of the Earth. If indeed these rocks are the result of pressure solution, then some serious rethinking is necessary.
|Note the temperature conditions of pressure solution.|
|This is a very misleading cartoon. Recall the styolitic spaced cleavage that apparently cross-cuts pre-existing unfolded foliation above. I don't think that spaced cleavage progressed through a folding stage anymore than the styolitic spaced cleave shown for carbonate rocks. Folding is not a necessary step in the process. Again, this cartoon is a relict of constant volume metamorphic theories.|
|Metamorphic conditions need not reach greenschist facies for formation of sericite and chlorite. In some situations, these minerals form before or during diagenesis. This is especially true in volcanic rocks.|
|Once we get to gneiss,
the constant volume model might well apply. That is not
the focus of this page, so I don't consider it.
And so we reach the end of Professor Jansma's presentation.
At this point I am hoping readers will appreciate that pressure solution is important in formation of metamorphic-appearing rock fabrics but that it can occur at pressures and temperatures that are not typically thought of as metamorphic. In particular, I am hoping that readers will appreciate that fabrics can be developed in near-surface and diagenetic situations. Certainly, these are conditions where fluids and pathways (porosity and permeability) are available to facilitate pressure solution.
Pressure Solution--an Hypothesis on the How and Why--Shear Stress
A lingering problem with the explanation of fabrics presented above is its incompleteness. While the mechanism of pressure solution suggests that fabrics need not be metamorphic and can be near-surface or diagenetic, only metamorphic situations are offered to explain the fabrics. For example, the origin of slaty cleavage in essentially unmetamorphosed rocks shown above is described as a result of progressive isoclinal folding. Also, the connection between bedding plane cleavage and high angle spaced cleavage or slatey cleavage is still attributed to two distinct "metamorphic" D1 and D2 events. The constant volume forces may still win the war.
For the how and why, I look to research on faulting. Recent workers have been interested in explaining how ductile deformation can occur in faults at lower crustal levels. To do this, they have concocted rock analogue experiments using mixtures of clay or mica and halite. Halite was chosen because it is known to be subject to pressure solution at room temperature. Below I refer to the work of Andre Niemeijer, http://igitur-archive.library.uu.nl/dissertations/2006-0315-200011/ .
Niemeijer subjected his mixture of mica and salt in a brine to shearing and observed the results. At low velocities, pressure solution was the limiting factor. Movement was accommodated by sliding and ductile flow of the phyllosilicate grains and limited by pressure solution. A strong foliation developed. At higher velocities, porosity developed and the flow became granular as in a gauge--or to add my interpretation, a pseudoconglomerate. Interestingly the rock was weaker--that is there was less resistance to the shearing force--at lower velocities than at slightly higher velocities. When granular flow and porosity developed, the rock again weakened.
|The photo at the right is a thin section of one of the textures produced. To me this appears to be a classic crenulation cleavage. Crenulation was not mentioned in the work, however. The photo on the far right shows a closer view of the textures developed.|
|The analysis shown in the
figure to the right shows how the shear stress is
translated to a normal stress that could account for
crenulation or stylolite formation. http://igitur-archive.library.uu.nl/dissertations/2006-0315-200011/
As pressure solution progresses, the microlithons of the horizontal foliation would be shortened to the point where the vertical fabric would be dominant. In not-so-extreme cases, verticle structures would be enhanced by this process forming spaced cleavage. Escape of pore fluids on the spaced cleavage would result in greater compaction, thus greater shear stress, and greater normal stress on the spaced cleavage and enhancement of the fabric.
Neimeijer's and his professors' work seems to suggest that a shear stress will tend to create fabrics in two directions--one parallel to the shear stress and one at a high angle. Compaction and volume loss that are enhanced by pressure solution should also accentuate high angle crenulation and spaced cleavage.
Inherent in the stress model suggested by Niejmeir is the formation of adjacent high and low pressure zones. For pressure solution, this gives the opportunity for a recrystallization of the rock as minerals are dissolved from the high pressure zones and redeposited in the lower pressure zones. This could be a continuous process.
The catalyzing action of phyllosilicates may well be important too. Boles, et al., recently described low temperature pressure solution of quartz and feldspar in contact with muscovite. The rates began in the range of 1mm per year, but slowed substantially. If, as appears to be the case for a shear stress situation, adjacent high and low pressure zones can persist, then high rates of dissolution and precipitation may continue for some time. The result could be very intense foliations in larger bodies of rock.
(The results at higher shear velocities are significant as well. The development of higher porosities gives the opportunity for development of other textures including pseudoconglomerates. I hope to develop this more later.)
The source of a diffuse aerially extensive shear force is not hard to imagine. Sediments deposited on sloping shelves or in subsiding basins would be subject to a shear force. While the force may be weak, as Niemeijer found, the strength of the rock is low and a pressure solution mechanism may accommodate fabric development. Shear forces may be much greater in situations such as volcanic collapses and deformation could be much greater. Again, it might be expected to have two (or more) apparent fabrics at high angles.
Another recent PhD dissertation, http://www.geo.uni-potsdam.de/mitarbeiter/Safaricz/Safaricz_PhD_thesis/Safaricz_PhD.html,suggests that two high angle pressure solution fabrics is to be expected from a single stress regime--albeit one somewhat different than I have suggested here. Certainly, it has not escaped the field geologist (that endangered species) that high angle bedding plane and crenulation or spaced cleavage is very common--perhaps too common for the common constant volume metamorphic explanation.
A Thought Experiment--How and Where Pressure Solution Works
|Pressure solution, by its
nature requires porosity and permeability. It is by those
reservoirs and pathways that the solutions move material
around. Also, in high porosity and permeability
situations grain to grain pressures will be
higher--pressures are higher if the force is directed at
a few discreet points. Microcracks and other processes
that enhance pressure solution will be greater. But as
pressure solution progresses, it eliminates or limits the
conditions that allowed it to take place in the first
instance. The porosity and permeability disappear. The
diagram (shown above twice--but once more here)
Where this leads is to the idea that pressure solution may begin in a uniform or relatively uniform manner in rock that is homogeneously porous and permeable. As that porosity and permeability disappears, it takes place at more discreet sites. One would expect those to be places where fluids can flow--in fractures for example. Blocks or pieces of rock that are no longer porous or permeable, would only be subject to pressure solution on their margins. These are of course microlithons--or macrolithons, in some cases. In the extreme case where impervious rock pieces exist in a porous permeable matrix, they may be eroded on the outside by pressure solution and take on the appearance of conglomerates.
The process is not restricted to homogenous rocks, of course. Rocks are what they are. If a rock had impervious areas and pressure solution removed and rearranged the surrounding rock, then the rock could be changed in its nature. The rock might be folded. In extreme cases a bed with quartz veins might look like a quartz pebble conglomerate at the end of the process.
|This rock shows a quartz vein that has resisted "deflation" of the surrounding rock by pressure solution. Cube is 1cm. Chlorite sericite phyllite, Greens Creek Mine, Alaska.|
|Apparent rounded "cobbles" of calcareous argillite and rounded fine grained pyrite "clasts" and "pebbles"form by pressure solution and movement. Movement and shifting is evident from fractures in the cobbles as in the right center of the photo. Mine Argillite, Greens Creek Mine Alaska. Core is about 10cm diameter.|
A Possible Faux-Metamorphic Sequence
The crude chart below uses the principles discussed above to illustrate a suggestion of how pressure solution deformation might progress to form something that looks like a metamorphic sequence. The progression would not be one of increasing pressure or temperature as in conventional constant volume metamorphic explanations, but one of increasing strain and velocity of shear movement. It is a process that can take place in sediments, partly lithified rocks, or fully lithified rocks with sufficient porosity and permeability.
|Example:||Sedimentary or volcanic rock with high permeability and porosity||More or less horizontal bands of low permeability formed by pressure solution in clastic sediments, sedimentary rocks or stylolites are formed in carbonate rocks||Slippage and plastic flow on phyllosilicate cleavage planes with stress such that the strain is limited by pressure solution||Pressure solution begins to remove rock volume in irregular patterns which causes higher strain zones at high angles to the shear stress. That is, once a flaw forms on the sliding planes, the resulting high stress--as shown in Niemeijer's diagrams above--cause enhanced pressure solution in a way that creates a second fabric.||When strain exceeds the limit where pressure solution can accommodate movement, structure collapses and sliding or even flow is enabled by zones where hydrostatic pressure exceeds critical strengths. In some cases, movement--probably sudden movement--can form rock or sediment that mimics a conglomerate (a pseudoconglomerate).|
One more idea . . . Pressure Solution Folds
|The photo at the right--and observation of similar features in other rocks, leads me to the idea of folding by pressure solution. One of the knotty geometric problems of folding is a room problem. If one bends rocks, then rock from the core of the fold must move or disappear. Plastic deformation has been advanced as a reason, but many have found it unsatisfactory. Pressure solution is easier. The dissolved rock flows away with the water. Higher stress and enhanced fracturing on the axial plane make pressure solution a likely cause--at least on a small scale . . . perhaps large too. I will develop this more.|
The conclusion for now (which I expect to update as I develop this idea) is that low shear stresses can form bedding parallel or near bedding parallel foliation and a high angle spaced cleavage or crenulation cleavage. This can form under diagenetic conditions and need not be related to any regional metamorphic event. Instead, it can be the product of consolidation of sediments on a slope or in a subsiding basin. For submarine volcanic rocks, these textures would be expected in a collapse at the end of a volcanic cycle.
Send your comments to Eric Twelker, firstname.lastname@example.org
I would be appreciative of any pertinent references on this topic too.
Your comments here:
If you are interested in developing the ideas expessed here further, please do. I would appreciate an email letting me know what you come up with.
First posted: December 28, 2006
Last update: February 28, 2007 (Added to shear stress section)