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Origin and evolution of Panama Rock dike and dome, Banks Peninsula, New Zealand
Lorelei Curtin
Department of Geological Sciences, University of Canterbury, Christchurch 8041, New Zealand
Geology Department, Pomona College, Claremont, California 91711, USA
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ABSTRACT
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origin and stages of their formation. Understanding dome-forming eruptions are critical,
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Structural and geochemical studies of lava domes can provide insight into their
as the highly explosive eruptions from rhyolite domes have claimed many lives. In this
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study, an eroded dome on Banks Peninsula, New Zealand, was characterized through
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been formed endogenously in the crater of a scoria cone, fed by a radial dike from the
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areas of the dome can be loosely correlated to a higher obliquity of flow in this region;
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INTRODUCTION
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increases near dome-forming volcanoes, because the sudden collapse of lava domes has
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various geochemical and structural methods. The dome in question was found to have
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larger Akaroa volcanic complex. The curious concentric jointing exhibited in certain
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however, further studies are needed to better understand this relationship.
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The study of lava domes has become increasingly significant as population
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left many people vulnerable to pyroclastic flows and other volcanic hazards. Dangerous
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but an understanding of these systems is crucial in order to preserve human lives.
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world. There are four different types of lava domes, whose classifications are based on
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spiny/Pelean (i.e. Mt. Pelée, Santiaguito), lobate (i.e. Mt. St. Helens, Katmai, Pinatubo,
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Newberry). These distinctions are generally made based on morphology, surface texture,
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lava (Fink and Anderson 2000, Fink 1998).
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structures were analyzed to reconstruct the formation of the Panama Rock dome and dike
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lava domes generally form in more silicic environments than the one studied in this paper,
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Historically, active domes have been studied in various locations around the
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observations made during their formation as well as analog laboratory experiments:
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Redoubt), platy (i.e. Mt. Merapi, Soufrière), and axisymmetric (i.e. Medicine Lake,
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and eruptive mechanisms, including eruption rate, cooling rate, and yield strength of the
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In this study, whole-rock geochemistry via XRF and macro- and micro-scale
on Banks Peninsula, New Zealand (Figure 1). Panama Rock provides an excellent
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example for study as it has been partially eroded, allowing for the interior structures of
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Rock geochemically and structurally, and to determine the relationship between the dome
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structures provide insight into the origin of the dome, the type of dome, and the
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GEOLOGIC SETTING
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New Zealand (Figure 1). It is made up of two Miocene-aged volcanoes, Lyttelton and
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either an extensional tectonic regime or a mantle plume. Their formation has been
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subduction underneath Gondwana before its breakup “refertilized” the bottom of the
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between the lower lithosphere and the asthenosphere on which it sat. The negative
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allowing hot asthenosphere to rise and partially melt due to decompression. This melt
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today.
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events; the first created the Lyttelton volcano, active from 11.0 to 9.7 million years ago,
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years ago (Hampton 2009). Both of these volcanoes had two main phases of activity—a
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1980). Three main types of features can be seen on both volcanoes: constructional
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and sills; and erosional features, which form valley orientations and ridge patterns
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the dome to be easily accessed. The goal of this study is to fully characterize Panama
and the dike. Field relationships, jointing patterns, and micro-scale flow and shear
mechanics of dome growth.
Banks Peninsula is located to the southeast of Christchurch on the South Island of
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Akaroa. Although they are intraplate volcanoes in nature, they cannot be attributed to
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attributed to delamination of the lower crust (Timm et. al. 2009); melts related to
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lithosphere with high-density mantle material. This created a large density differential
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buoyancy of the lower lithosphere caused it to detach from the crustal material above,
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interacted with surrounding rocks, creating the complicated chemistry that is expressed
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Based on this theory, Banks Peninsula is believed to represent two delamination
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while the second, larger event created the Akaroa volcano, active from 9.3 to 8.0 million
large building phase followed by an interval of late-stage volcanism (Price and Taylor
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features, including lava flows, scoria cones, and domes; hypabyssal features such as dikes
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(Hampton 2009).
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Akaroa Volcano vary greatly. Each volcano exhibits its own suite of basalt-trachyte lavas
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Although they are similar in structure, the compositions of Lyttelton Volcano and
and dikes, which grew progressively more alkalic over time (Price and Taylor 2009).
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Akaroa has overall lower SiO2 concentrations and trace element and isotope chemistry
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however, has rocks of higher SiO2 content, and geochemistry suggestive of mixing
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suggest the source was a carbonated eclogite in a peridotite matrix. Lyttelton volcano,
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between asthenospheric (Akaroa-type) melts with lithospheric pyroxenite melts (Timm et
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al 2009).
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is a dome approximately 250 m long by 150 m wide that is intersected by a 4 m-thick
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One feature on Akaroa volcano, Panama Rock, located 3 km west of Le Bons Bay,
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dike. The dome is separated into two parts; an inner dome and an outer amphitheater. The
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the inner dome (Figure 2). The eroded center allows for an excellent view of the jointing
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The rock it is made of is an apheric trachyte mostly composed of dark grey groundmass
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dike terminates inside the amphitheater, but is not exposed between the amphitheater and
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pattern around the amphitheater, which seems to trend with the overall shape of the dome.
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with few (<10%) feldspar phenocrysts.
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METHODS
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accessible parts of the dome (Figure 2). Four samples were taken; one from the inner core
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and one from the dike (Figure 2). The three dome samples were oriented. The whole-rock
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Strike and dip measurements were taken along various jointing planes around all
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of the dome, one from the outer amphitheater, one from the slope alongside the dome,
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geochemistry of each sample was analyzed via X-Ray Fluorescence, using the Philips
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University of Canterbury. Samples were prepared and analyzed via the fused disk method
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unoriented thin section taken from sample 4.
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Geochemistry and Texture
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into the trachyte range, based on their total alkali to total silica ratio (Figure 3). All
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groundmass, with the occasional felsdspar phenocryst. The phenocrysts show no
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the thin section-scale is defined by the alignment of the feldspar crystals, which make up
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PW2400 Sequential Wavelength Dispersive X-ray Fluorescence Spectrometer at the
outlined in Hartung 2011. Oriented thin sections were prepared from samples 1-3, and an
RESULTS
The three samples collected from the dome and the sample from the dike all fall
samples lack defined texture at the hand-sample scale; they are >97% dark grey apheric
preferred alignment. The trachytic texture observed in the groundmass of the samples at
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90% of the groundmass. The rest of the groundmass is made up of a few augite and
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the vertical plane, oriented north-south. The general trend of the crystal alignments were
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were observed in individual thin sections, and no thin sections in the horizontal plane
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olivine crystals, and fractures coated with oxidized iron. The thin sections were taken in
measured as degrees from horizontal, and are plotted on Figure 4. No textural domains
were taken. Phenocrysts were present in all samples most of which have multiple
fractures at 90-degree angles to the edges of the crystals (typical feldspar cleavage). No
vesicles were observed.
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Structure
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patterns, and three domains can be defined: platy jointing, columnar jointing, and
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the eastern side of the dome. Columnar jointing is observed on the western edge of the
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DISCUSSION
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The macro-scale structures observed at Panama Rock are defined by jointing
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irregular jointing. The platy jointing is present around the outer amphitheater and down
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dome and in the dike. The inner dome is irregularly jointed.
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Lava and Dome Type
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trachytes. The magma feeding these eruptions is the most mature seen on Banks
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material. The large amount of brittley deformed feldspar crystals that have been aligned
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aligned by internal flow of the lava. During cooling, the deformation changes for more
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crystallinity of the lava upon eruption causes it to be much more viscous than its
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The geochemical and textural data are typical of Akaroa volcano’s late activity
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Peninsula, implying a long fractionation process and/or increased interaction with host
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implies that most, if not all, of the crystals were already formed upon eruption, and were
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ductile processes (flow) to brittle processes, causing the crystals to fracture. The high
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chemistry implies.
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compared to Fink and Griffiths’ classifications. Previous work on this subject includes a
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or cryptodome, includes the obvious deformation and uplift of the sedimentary cover
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dipping away from the source. In the Italian case the contact zones between the dome and
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In order to typify the dome that Panama Rock was originally, results were
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study in Central Italy (Cimarelli and Rita 2006). The evidence for an endogenous dome,
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above, while an exogenous dome was formed via the superimposition of lava flows,
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the sedimentary cover were well exposed; however, the local geology of Panama Rock
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precludes these observations from being made.
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dome, as dome-forming lavas are felsic and highly viscous in nature. However, it is
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Usually it would be unexpected for lava of Panama Rock’s composition to form a
possible that the high crystallinity caused the lava to be viscous enough to behave like a
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rhyolite lava. It is unlikely that the dome was spiny, platy, or lobate, as it lacks radial
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lobes/spines are emplaced laterally. Panama Rock, on the other hand, has a distinctly
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features; these domes all have a central core, from which between a few and a dozen
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concentric morphology, with a relatively regular elliptical shape. Panama Rock’s tall
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any of these classifications, this study concludes that the dome was either a cryptodome,
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Griffith’s study.
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profile makes it also unlikely that it was an axisymmetric dome. Because it does not fit in
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or its morphology was constrained by some other means not accounted for in Fink and
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Inner Dome, Outer Dome, and Dike Relationship
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due to the intersection of radial dikes with the dip-slope of the flank of the volcano
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towards the center of Akaroa volcano, and is exposed at the current surface, underneath
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Previous studies have suggested that the formation of domes on Akaroa volcano is
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(Weaver and Sewell 1986). This seems likely, as the dike trends approximately 240°,
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Panama Rock, where it terminates. It had been previously thought that Panama Rock was
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formed by the extrusion of two overlapping domes fed by the dike (Hobden 1990).
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However, observations made during this study suggest otherwise. The separation between
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outer amphitheater is either platy jointing or columnar, while the central core is irregular.
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representing the top of the conduit that fed the dome. The difference between the
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margin, whether that is the country rock or a previously cooled part of the dome. This
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the outer amphitheater and the inner dome seem to be related to the jointing patterns—the
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It seems more likely that this was one continuous dome, with the central portion
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columnar jointing and platy jointing probably has to do with the proximity to the cooling
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implies multiple endogenous filling phases.
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presented. The presence of scoria deposits to the west and northeast of Panama Rock
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Based on these interpretations, a new model for the formation of Panama Rock is
suggest that perhaps a scoria cone provided the constraints on the shape of the dome
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previously mentioned. If, in the late stage of the scoria cone’s life, its eruption style
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of the crater would provide a buttress against which the steep sides of the dome could
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Dome Formation Mechanics
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structurally, similar to studies performed on metamorphic rocks. It is assumed that
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alignment can be used as a proxy for flow. However, the high crystal concentration
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became more effusive, it is probable that a lava dome could form in the crater. The walls
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form, and cause a possibly lobate dome to appear more elliptical (Figure 5).
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Flow indicators in lavas can be preserved in shallow intrusions and analyzed
the crystals rotate into an alignment that decreases resistance to flow, and thus the
could also impede rotation, causing inconsistent results (Smith 2002).
Flow was observed in three dome samples. The sample that showed the most
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obliquity was collected from the portion of the dome with the most pronounced
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results in shear planes nearly parallel to dike walls. Assuming this can be applied to
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dome contained pronounced shear planes parallel to the roof, resulting in the platy,
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platy jointing. This is consistent with Shelley (1985)—extreme obliquity of flow
the dome situation, the areas with a flow direction more oblique to the roof of the
“onion-skin” jointing.
FUTURE RESEARCH
Possibilities for further research would mainly focus on a more detailed micro-
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structural study of Panama Rock and its associated dike. Shelley (1985) provides detailed
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would be interesting to see if these methods could be applied to both the dike and dome.
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unavailable for this study. If more samples were available, as well, it is possible that
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implications for flow and shear in the dome (Smith 2002). With continued research, the
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could be tested.
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instructions on determining flow directions in the dikes of Lyttleton volcano, and it
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This requires thin sections cut parallel to the wall of the dike or dome, which were
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domains of crystal alignment and concentration could be identified, which have further
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model for shear and strain in a growing dome presented by Buisson and Merle (2002)
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ACKNOWLEDGMENTS
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Many thanks to Darren Gravely, Samuel Hampton, Paul Ashwell, Shane Cronin,
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Joshua Johnson, Daniel Hobbs, and Gabe Lewis (field assistant) for their contributions to
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REFERENCES
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