Talk:Ignimbrite
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[edit] Unsourced and possibly original research
I removed the following dense tract of text not because it isn't interesting, but because it is, well, dense, very scientific, related to only one ignimbrite event, and is probably hacked from someone's paper or thesis. It is also overly technical for a public access encyclopedia. Please edit and discuss in general terms. Oh, and citations, pls.Rolinator (talk) 13:00, 26 April 2008 (UTC)
[edit] Welding and Rheomorphic flow
A model was proposed by based on observations on the Wall Mountain Tuff, suggests that the Rheomorphic structures such as a pervasive foliation and a preferred stretching direction of pyroclasts were formed during laminar viscous flow as the ignimbrite comes to a halt. It is also suggested that the reason that some tuffs display more structures than others is a result of temperature of transport and deposition. If the flow is sufficiently hot the particles will agglutinate and weld at the surface of sedimentation to form a viscous fluid, this is primary welding. If during transport and deposition the temperature is lower then the particles will not agglutinate and weld, although welding may occur later if compaction reduces the minimum welding temperature to below the temperature of the glassy particles, this is secondary welding. This secondary welding is most common and suggests that the temperature of most pyroclastic flows is below the softening point of the particles. Schmincke et al suggested that there was a change from particulate flow to a viscous fluid involving the entire cooling unit in the last few metres en masse. This disagrees with Chapin et al who suggest transformation at a boundary layer at the base of the flow and that all the materials pass through this layer during deposition.
The reason for some ignimbrites to have primary welding and some to have secondary welding is primarily because of differences in the chemical compositions. They suggest that in alkaline and peralkaline tuffs the high iron, sodium and total alkali contents were responsible for lowering the viscosity and enabling primary welding. However it has been suggested that there is not enough chemical variation between primary and secondary welded ignimbrites, within ignimbrite suites and between suites, for this to be the primary reason, although they agree that it is a factor. Chapin et al also disagree with this because the Wall Mountain Tuff is calc-alkaline in composition but still has primary welding. They suggest that emplacement temperature is the only variable that can meet all the criteria for primary welding, or lack of, in ignimbrites.
There is also a suggestion, based on predictions from modelling pyroclastic flows as turbulent suspension currents fed by fountaining eruptions, that emplacement temperature is controlled largely by eruption temperature, because of the way in which mass flux, gas content and eruption temperature compensate each other during eruption and transport. This suggests that cooling during transport and eruption is limited so high grade ignimbrites can be formed across the entire runout length providing the eruption temperature is greater than 1.3X the minimum welding temperature.
So if the eruption temperature is sufficiently high for primary welding to occur, there must be higher minimum welding temperatures locally in some ignimbrites for there to be lateral variations in welding.
Another model proposed is that the density current became stationary prior to the formation of the Rheomorphic structures. Structures such as pervasive foliation are a result of load compaction. The other structures are the result of remobilization by load and deposition on inclined topography. It is argued that a number of the structures cited by Schmincke and Swanson as evidence for late stage primary viscous flow are compatible with compaction structures. A further suggestion is that any viscous flow of the deposited must occur post compaction as the Wagontire Mountain tuff shows evidence of late stage viscous flow but has a foliation almost identical the Bishops Tuff. These tuffs have a similar chemistry and so must have undergone the same compaction process to have the same foliation.
The model of post depositional re-mobilization being responsible for rheomorphic structures was supported by reinterpretation a tuff that had been cited as support for a different model, the Green Tuff on Pantalleria, as an ash fall deposit, which meant that the rheomorphic structures must be post depositional as there is no lateral transport of fall deposits. They also held that strong similarities between the structural features of the Green Tuff and other ignimbrites, particularly on Gran Canaria, suggest that the rheomorphic structures in these ignimbrites were also formed post depositionally.
This interpretation is disagreed with on two counts. The Green Tuff has been reinterpreted as an ignimbrite since it was worked on . The Green Tuff contains imbricate fiamme which has been interpreted on Gran Canaria as a result of primary i.e. syn-depositional flow. They also suggest that secondary flow is not the only explanation for many of the structures observed in the Green Tuff; some of these structures have been well documented as forming during initial flow.
Ignimbrite D on Gran Canaria is a major example of a new model. They interpret imbrication of fiamme and phenocrysts to have developed during deposition, just below the boundary between the boundary between the particulate suspension and the sedimented parts of the flow. They interpret the imbrication as a result of shear exerted on the sedimented part by the moving particulate suspension. Ignimbrites re-examined conclude, after structural analysis, that sheathfolds and other rheomorphic structures are common in many ignimbrites and are indicative of a single stage of shear deformation. The Grey’s Landing ignimbrite has structure consistent with a single stage of shear deformation but varying orientations of sheathfolds with height in one geographic location. They hold the variations of orientations as evidence that welding and rheomorphic folding can begin during deposition, providing the pyroclasts are hot and viscous enough. It is suggested that the variation in sheathfold orientation is a result of varying flow direction with time, similar to the variations in sedimentary fabrics with time. This also suggests that the shear is a result of the drag exerted by the moving flow on the sedimented part, or how else could the orientation of rheomorphic folds be determined by the flow direction?
Ignimbrite D on Gran Canaria was examined by different workers. They identified four zones within the ignimbrite. A basal vitrophyre with pure uniaxial flattening, perpendicular to the foliation. An overlying shear zone with asymmetric fabrics and a strain ellipse similar to stretched oblate bodies. The next zone is similar to the shear zone but without evidence of a rotational strain component. The top zone is slightly deformed to non deformed, it has randomly orientated sub spherical pyroclasts which may be preserved syn depositional clast shapes. There are pressure shadows in all zones, which formed around rigid clasts. In the basal and central zones the pressure shadows are symmetrically orientated, in the shear zone their inclined planes are parallel to each other and always dip opposite to the dip of the basal substrate. There are folds within all zones. Kobberger et al identified two types of folds, primary folds with there axes dipping parallel to the dip of the slope and younger folds that are vergent with their axes dipping opposite to the dip of the slope. The second type of fold is only seen in the shear zone. They suggest that flow related fabrics were developed after deposition and significantly below the surface of sedimentation. They believe that the shear exerted by the particulate suspension is not significant enough to form the structures observed. They also suggest that load compaction on an inclined slope caused remobilization and the rheomorphic structures. They hold the evidence for this is that rheomorphic fabrics occur where compaction exceeds 5m.
This supports and contradicts both models, in some factors. They agree that post depositional movement is possible and does occur i.e. where a fold affects two separate ignimbrites, but suggest that non-particulate flow during deposition is far more significant than has been proposed.
The west TL ignimbrite on Gran Canaria was examined and evidence found for rheomorphic flow. They found preserved welding fabrics with imbrication. In one model proposed these fabrics were formed by pure flattening, and so should be horizontal or slope parallel. It is suggested that these inclined fabrics are the result the initial deformation being rotational, and infer from this that the deformation occurred in a rheomorphic shear zone around the base of the flow and was later preserved in the vitrophyre.
[edit] Chemical and compositional zoning
Vertical and lateral zoning is common in ignimbrites. Compositional zoning is marked by variations in the composition or abundance of juvenile or lithic clasts. Variations in the composition of lithic clasts can provide information about changes in the depths of explosive fragmentation or substrate erosion during an eruption. Variations in the juvenile clasts can provide information on changes of the composition of the magma being erupted. Vertical zoning of juvenile clast composition has been used to infer the presence of a compositionally stratified magma chamber beneath the volcano. Two explanations are recognized to explain the most common form of vertical juvenile zoning; trending upwards from more evolved silicic pumice at the base to less evolved mafic pumice at the top. The first explanation is progressive extraction from a density stratified magma chamber, so the lighter more evolved magma is extracted first. An alternative is that a new batch of magma is intruding an older more evolved magma. Both of these explanations require the ignimbrite to have been deposited by progressive aggradation in order for the vertical compositional zoning to be recording temporal changes. Gradual, abrupt or varied changes in the compositional zoning reflect gradual, abrupt or varied temporal changes in the composition of the magma being erupted. After examination of the Zaragoza Ignimbrite, a “double” zoning was discovered. They observed that the composition of the pumice at the base of the flow was rhyodacite but vertically became andesitic, there was then a concentration of altered lithic blocks above which the proportion of andesitic pumice decreased and the composition of the pumice returned to rhyodacite. From these observations the presence of a compositionally stratified magma chamber prior to the eruption of the Zaragoza Ignimbrite, with the draw-up depth of the eruption increasing and decreasing with waxing and waning mass flux is favoured.
The TL ignimbrite, Gran Canaria is a compositionally zoned ignimbrite. From observations a history of the magma chamber that was responsible for the eruption was constructed. Prior to the eruption the magma chamber was stably zoned with comendite magma overlying trachyte magma. It is believed that the compositions of the magmas are consistent with the comendite being derived by fractional crystallisation of the trachyte. They also suggest that there was downward movement of alkali feldspars within the trachyte layer, to enrich the lower, less evolved trachytes in Ba. Prior to the eruption there was an injection of intermediate lava which dispersed through the trachyte layer as globules. Before, during and after the eruption there was partial mixing or mingling of all compositions. They also suggest that as TL is a high grade ignimbrite there may have been post depositional mingling after deposition. The TL ignimbrite has been split into three zones; the comendite zone, the mixed zone and the trachyte zone. Topography has an effect on the zoning where it is deposited on relief the mixed zone may often overlap the valley-filling comendite zone to rest directly on the palaeo-topography.
[edit] Sorry this was me
I put this in before i knew better. I am working through the information and hopefully adding the relevant information. I am also including the relevant citation. P.S. I wish that this was original research, sadly just a lowly literature review. Russjass (talk) 16:01, 15 May 2008 (UTC)
[edit] Rheomorphic flow-where should it go?
I have put rheomorphic flow in the deposition section, not because that is where i think it belongs (which i dont) but because i do not know where else to put it. —Preceding unsigned comment added by Russjass (talk • contribs) 16:37, 15 May 2008 (UTC)
