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1 Complex plumbing of monogenetic scoria cones: New insights from the Lunar Crater Volcanic Field
(Nevada, USA)
*Amanda R. Hintz arl6@buffalo.edu
Greg A. Valentine gav4@buffalo.edu
SUNY University at Buffalo, 411 Cooke Hall, Buffalo, NY, 14260 (USA)
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*corresponding author
Tel. +1 (716) 645.4298
Fax +1 (716) 645.3999
9 Abstract
10 The complexity of monogenetic basaltic eruptions is largely governed by the shallow plumbing of the
11 volcanoes that develop in the upper 10s of meters of the crust, as well as within the growing edifice during the
12 eruption. We present data pertaining to geometry and dynamics of plumbing features associated with two
13 monogenetic volcanoes observed within in the Lunar Crater Volcanic Field (Nevada; USA) that have been
14 eroded to varying degrees, exposing numerous intrusive bodies. The majority of the intrusive bodies observed
15 within the eroded scoria cone and vent remnants are radial dikes and dike sets, some of which have irregular
16 shapes and patterns (such as arcuate and en echelon). We infer that as the dikes propagated away from the
17 conduit, the overall shape and geometry was influenced by a combination of the lithostatic load exerted by the
18 cones, the immediately surrounding topography, and the density contrast between the dike and the cone; with
19 the regional tectonic stresses playing a negligible role. Internally, the dikes display textural features such as
20 multiple chilled margins and/or distinct bandings of vesicle populations, suggesting multiple injections or pulses
21 of magma originating from the vent/conduit area was incrementally added to the dikes. The overall geometry of
22 the dikes in relation to the vents, orientation and distribution of internal textures of the dikes suggest that
23 increased magma pressure in the conduit caused failure of the conduit walls such that magma ‘leaked’ out of the
24 conduit and into the cones, propagating outward as radial dikes or dike sets. Pressure fluctuations within the
25 magmastatic column capable of producing such features can be associated with a variety of dynamic processes
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26 including; the ascent of slugs of gas through the magma column, variations in the source pressure of the
27 ascending magma, as well as the weight of relatively degassed magma and temporary blockages of the conduit.
28 We infer that the magma overpressures necessary to drive pulses of magma into individual dikes, once
29 established, easily overcomes the force exerted by the lithostatic load of the surrounding scoria cone. Our work
30 suggests that the dikes propagated laterally near the cone-substrate contact and in some cases might have fed
31 lava boccas around the base of the cone. Irregular shapes of some dikes is likely related to the interaction
32 between viscous intruding magma and weakly consolidated scoria cone deposits. The geometry of the dikes
33 and the degree of connection to a central magma column has important implications for degassing patterns, gas-
34 melt coupling, and eruption dynamics associated with monogenetic eruptions.
35 Keywords: monogenetic, scoria, eruptive style, eruption dynamics, Strombolian, shallow plumbing, dike
36 propagation
37
38 1. Introduction
39 Monogenetic volcanoes represent a major proportion of terrestrial volcanoes ( Wood, 1979 ), occurring in
40 groups or clusters (i.e. volcanic fields; Connor and Conway, 2000 ), or in association with polygenetic volcanoes
41 (e.g. Mauna Kea, Hawaii, and Llaima volcano, Chile). Scoria cone-forming eruptions are often the result of
42 complex combinations of effusive and explosive eruptive styles. Cones are built by the accumulation of
43 pyroclastic lapilli, spatter, bombs and blocks near an eruptive center and are typically the cumulative record of
44 several or more transitions between these eruptive styles. Historical observations of eruptions of this type of
45 volcanism have been limited (e.g. Parícutin; Foshag and Gonzalez-Reina, 1956; Mount Etna; Calvari and
46 Pinkerton, 2004 ), though the information from these first-hand accounts has greatly improved our
47 understanding of monogenetic eruptive dynamics. Detailed field observations of relatively young deposits that
48 have experienced little to no erosion display evidence for a range of eruptive behaviors ( Valentine et al., 2005,
49 2007; Pioli et al., 2008 ). These eruptive behaviors may include Hawaiian-style fire fountaining, Strombolian,
50 and violent Strombolian explosive activity; all of which are frequently accompanied by simultaneous lava
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51 effusion from vents on outer cone slopes or at the base of a cone or construct ( Luhr and Simkin, 1993; Di
52 Traglia et al., 2009; Pioli et al., 2008; Genareau et al., 2010 ). Monogenetic volcanoes that have been more
53 deeply eroded may expose portions of their shallow crustal plumbing systems as well as the sometimes complex
54 dike-to-conduit transitions ( Valentine and Krogh, 2006; Keating et al., 2008 ). The recent research in these areas
55 highlights the necessity for furthering our understanding of the driving forces controlling the nature of these
56 eruptions.
57 The primary mechanism by which mafic magmas ascend from their source regions (asthenospheric or
58 lithospheric mantle) through the lithosphere to the surface is through self-propagating fractures or dikes ( Lister
59 and Kerr, 1991; Wilson and Head, 1981 ). Dike ascent at shallow depths is susceptible to the effects of surface
60 topography and pre-existing structures such as faults; particularly in tectonically controlled volcanic fields
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62 where magma flux is relatively low ( Valentine and Perry, 2007 ). Here, in the upper hundreds of meters of crust, ascending dikes can be “captured” by steeply dipping pre-existing joints and faults (
Delaney et al., 1986 ;
63 Connor and Conway, 2000; Valentine and Krogh, 2006; Gaffney et al., 2007 ). When a dike ascends beneath
64 variable topography, eruption and subsequent magma flow will be focused toward the lowest elevation where
65 the dike is first intersects the Earth’s surface ( Gaffney and Damjanac, 2006 ). Once an ascending dike passes
66 through the boundary between the underlying rock and the developing volcanic cone, we expect that its
67 propagation will be determined mostly by the stress field induced by the topography of the cone and the
68 pressure in a feeding fissure (surface expression of a dike) or central conduit. However, little is known about
69 these shallow plumbing geometries and its relationship to eruptive behavior , especially in cases where
70 explosive and effusive activity occur simultaneously. Theoretical treatments of plumbing and dike propagation
71 within basaltic volcanic edifices often assume relatively simple shapes ( Menand and Phillips, 2007; Pioli et al.,
72 2009 ), and the need for field-based observations to link the laboratory models with the real-world interpretation
73 motivates the work presented here.
74 Here we present field data on shallow plumbing geometry from two eroded scoria cone volcanoes,
75 informally referred to as the Kimana and Broken Cone volcanoes; located within the Lunar Crater Volcanic
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76 Field (Nevada, USA; Fig. 1). The overall geometry of the observed intrusive bodies (dikes and dike sets)
77 suggests that the size and shape of the scoria cones, the immediately adjacent topography, and conduit pressure
78 were the dominant influences on the major trends of the dikes. We infer based on the internal structures of the
79 dikes (e.g. vesicle and crystal flow banding), that propagation was dominantly radially away from central
80 conduits within the cones. Multiple bands of vesicles and chilled margins within several dikes, suggest that
81 they widened and lengthened incrementally, a result of pressure fluctuations in the magma column filling the
82 conduit. The irregular shapes of some dikes, as well as irregularities within otherwise ‘normal’ dikes, are likely
83 related to the interaction between the viscosity of the intruding magma and the weakly consolidated pyroclastic
84 deposits of the scoria cone. Additionally, irregular dikes occurring near the distal edge of the Kimana cone
85 might reflect a dike approaching the free surface or edge of a cone near its base or foot. We describe the
86 implications of these data for fluid dynamic studies and for eruptive processes.
87
88 2. Lunar Crater Volcanic Field
89 The Lunar Crater Volcanic Field (LCVF) is a north-northeast trending belt of Late Miocene to
90 Pleistocene mafic volcanoes in the south-central Basin and Range Province (Fig. 1) that was preceded by
91 Oligocene and Early Miocene caldera-forming volcanism. Alkali basalts and smaller volumes of trachytes and
92 basanites erupted beginning ~6 Ma ( Foland and Bergman, 1992; Yogodzinski et al., 1996 ) and continued to as
93 recently as ~38 Ka ( Shepard et al., 1995 ), indicating that LCVF is essentially an active field in the sense that it
94 is likely to produce more monogenetic volcanoes in the future. The older, Mio-Pliocene, part of the field lies to
95 the south, partially covering the Reveille Range, while the younger Plio-Pleistocene part extends northeastward
96 onto the southern part of the Pancake Range. Volcanism gradually migrated northward in the LCVF, although
97 during any given time window volcanoes formed over a large fraction of the total area ( Naumann et al., 1991;
98 Foland and Bergman, 1992 ). Yogodzinski et al. ( 1996 ) suggested that this apparent northward migration
99 largely reflects a temporal change in a mantle-melting anomaly. For the purposes of our work, the longevity of
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100 the field, the varying degrees of erosion, and the northward-younging trend , means that there is a range of
101 erosional exposure, including the interiors of vent areas and their shallow plumbing systems which are the focus
102 of this paper.
103 Volcanic landforms in the LCVF include abundant scoria cones that display a wide range of volatile-
104 driven explosive activity (Hawaiian, Strombolian, and violent Strombolian), and a variety of lava flow fields.
105 Over 200 vents, or vent remnants, have been identified, although a detailed count has not been completed. At
106 least three maar craters and tephra rings suggest localized explosive interaction between rising magma and
107 groundwater or surface waters ( Scott and Trask, 1971; Dickson, 2004; Valentine et al., 2011 ). Vents occur in
108 many arrangements, including; clusters where vent locations appear relatively random and scattered, aligned
109 vents, and as isolated vents ( Stickney, 2004 ). Most of the LCVF eruptions…
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111 3. Kimana Volcano
112 Kimana volcano ( Kimana - Shoshone Indian for ‘butterfly’; personal communication with Duckwater
113 Shoshone Tribe ), is located in the southern portion of the LCVF on the northwestern side of the Reveille Range
114 (Fig. 1). Previous investigators did not, to our knowledge, sample Kimana; however, a K-Ar age of 5.7 ± 0.2
115 Ma was obtained by Marvin et al. ( 1973 ) for a basalt flow immediately to the north (~4.8 km; Fig. 2a). The
116 similar morphologies of the immediately adjacent dated volcano and Kimana suggest that Kimana is of similar
117 age (Late Miocene to Early Pliocene). Kimana and other basalts of this area are thought to be some of the
118 earliest basaltic eruptions in this field.
119
120 3.1 Topographic and structural setting
121 Magmas feeding the Kimana eruption passed through at least 2 km of Oligocene to Miocene rhyolite
122 tuff, as well as dacite to andesite lavas ( Ekren et al., 1973; Martin and Naumann, 1995 ), covering an area of ~26
123 km
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with basaltic lava and pyroclastic deposits; the latter being the remnant of the source scoria cone. The
124 preexisting topography has been reconstructed based on present contact elevations between the basalt and older
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125 deposits. The lavas were emplaced on a sloping surface dipping ~4° north-northeast (Fig. X1 – xsection of
126 Kimana). Lava flows were diverted by a paleohill composed of Early Miocene rhyolite tuffs and plugs
127 associated with the northern Reveille Range caldera ( Martin and Naumann, 1995 ). The lavas have a generally
128 down-stepping morphology, implying that they cascaded down one or more preexisting escarpments (Fig. X1 –
129 xsection of Kimana). The inferred paleotopography also suggests that a small paleoridge may have diverted
130 portion of lava toward the northeast of the vent area (Fig. 2a), producing what is now an isolated mesa of
131 basaltic lava.
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133 3.2 Description of eruptive products
134 The Kimana deposits have an approximate volume 0.4 km
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, most of which is the lava flow fields. The
135 lavas consist of a several stacked ‘a‘ā flows (compound lava fields) that form two large lobes as lava flowed
136 down from the summit vent area. Individual ‘a‘ā flow units range from ~4 m to more than 20 m thick. Lavas
137 are dark grey to black, porphyritic with large phenocrysts of plagioclase, commonly >1 cm (long dimension).
138 The flow fields appear to be only partially eroded around their outer edges and preserver most of their original
139 lobate structure. Erosion and soil formation have obscured the flow-top textures on the uppermost flows,
140 though top and bottom clinker layers (up to 1 m thick) are observed where stacked flow interiors have been
141 exposed by ephemeral stream erosion. The lavas extend for ~7-9 km to the west and northwest from the
142 summit area and drop in elevation from ~2200 m to ~1650 m.
143 The pyroclastic deposits associated with Kimana consist of variably-welded and non-welded lapilli to
144 blocks and bombs; mostly confined to the summit area, where they sometimes form massively bedded moderate
145 to densely welded mounds (Figs. 2b and 3). Lapilli are mostly 2-5 cm; reddish-brown and less commonly gray
146 in color. Unwelded scoria clasts generally have sub-rounded shapes with abraded surfaces, with vesicularities
147 that often increases near the center of the clast. Moderate to densely welded lapilli deposits are most prominent
148 in and around the highest part of the summit area, exposed in a roughly circular 2-km
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area, and to a lesser
149 degree as smaller patches on the surface of the lava flows. A smooth, oxidized skin on lapilli clasts is often
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150 observed on broken clasts that have partial fluidal or flattened shapes, likely being pieces of larger ribbon or
151 spindle clasts. In many areas where deposits are unconsolidated, the lapilli and smaller bombs have begun to
152 form a weak desert pavement; clasts are underlain by as much as 20 cm of fine eolian sediment in undisturbed
153 areas.
154 Blocks and bombs are scattered on the surface throughout the summit, but are most abundant on the
155 east-southeast flank. Bombs range to >0.5 m (long dimension), with most preserving aerodynamic shapes such
156 as ribbons or spindles. The largest observed bomb (60 x 40 x 30 cm) is a well-preserved spindle with a smooth,
157 glassy, and a red skin with a moderately vesicular inner layer (~10 cm thick); it was cored with a smaller
158 oxidized scoria bomb that itself had an elongated, flattened shape. This was the case for many of the larger
159 broken bombs we examined. This is indicative of ‘recycled’ clast textures often associated with Strombolian
160 eruptions ( McGetchin et al., 1974; Valentine and Gregg, 2008 ).
161 We infer, based on the characteristics of the eruptive products described above, that Kimana erupted
162 explosively as well as effusively. The majority of pyroclastic deposits around the Kimana vent area (Figs. 2b
163 and 3) reflect a Strombolian eruptive style based on grain sizes and textures ( Valentine and Gregg, 2008 ).
164 However, the moderately to densely welded, near-vent deposits could also be interpreted as the result of
165 Hawaiian-style fire fountaining. The highest part of the summit area (Fig. 2b) is interpreted to be the vent area
166 for the pyroclastic products. Curiously, no intrusive body such as the remnant of a magma-filled conduit is
167 observed. We suggest that either the magma column did not reach this altitude in the original cone, or that the
168 magma level dropped at the end of eruptive activity and was replaced by pyroclastic debris.
169
170 3.3 Intrusive bodies
171 The summit area preserves at least eight major intrusive features that crop out for several 10s of meters
172 to over 100 m from the inferred eruptive center, predominantly in a radial pattern (Fig. 2b). These features are
173 more resistant to erosion than the surrounding pyroclastic deposits and protrude several decimeters to meters
174 above the surrounding area (Fig. 3). The features are inferred to be dikes and dike sets, as evidenced by their
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175 discordant nature and linear trends. Vesicular interiors and chilled margins (where preserved) support their
176 being emplaced as tabular intrusions into cooler “country rock” (in this case, pyroclastic deposits of the scoria
177 cone). Of these eight dikes and dike sets, five were mapped, sampled, and described in detail; summaries of
178 their physical characteristics are given in Table 1. Dikes A.1, D and G were not mapped to the same level of
179 detail as the other dikes because they are poorly exposed.
180 Many of the contacts, internal textures and overall shapes described in detail below for Dike set A and
181 Dike C, are representative of the other dikes and dike sets on Kimana, so we describe these two in some detail,
182 followed by a summary of the characteristics of the other dikes. The terms segment and set are used in the
183 descriptions. Dike sets, as observed in Dikes A, E and F (Fig. 2b), refer to groups of dikes whose planes are
184 laterally offset, yet have parallel trends. This differs from the segments observed in Dikes A.1, D, G and
185 possibly within the individual dikes of E and F. We use segments to imply that though a dike appears
186 discontinuous at the present level of exposure, the pieces are aligned and closely spaced, suggesting they were
187 connected at the time of emplacement.
188 None of the dikes were traceable to the vent (see Fig. 2b). We infer that the dikes are connected to the
189 vent/conduit below the current level of exposure for four reasons: (1) the dike geometry suggests a generally
190 radial or nearly radial pattern away from the inferred vent area (aka Kimana summit); (2) the topography
191 sharply increases around the summit area, thus the same level of erosional exposure limits our observations; (3)
192 of the dikes measured, most continually widen towards the summit, suggesting that the source of the magma to
193 feed/propagate the dikes was from the conduit; and (4) the internal vesicle banding and phenocryst orientations
194 suggest magma flowed away from the vent/conduit. We illustrate the geometric relationship between the dike
195 exposure and the original cone below in Section 3.3.4.
196
197 3.3.1. Dike A
198 Dike A (Figs. 2b and 4a) is a ~90°E-trending dike that extends from a large amorphous mound of lava for ~ 80
199 m before thinning significantly, becoming very discontinuous and disappearing into the surrounding colluvium.
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200 The width of the dike averages ~1 m. It is tabular and often stands for half a meter or higher above the
201 surrounding colluvium. Halfway along the medial section of the dike a small ~7 m long segment or possibly
202 smaller dike appears for ~7 m. Due to the level of erosion in this area, we were unable to determine if this
203 outcrop is a segment of Dike A, or another dike all together.
204 Discreet bands with high vesicle concentrations are present along the length of the dike. The bands are
205 paired symmetrically along the central plane and range from sub-cm to ~30 cm in width Individual bands
206 within each pair are parallel to the dike walls for most of their lateral extent, but curve inward and meet at their
207 distal ends (Fig. 4b), which are in-board of the distal end of the dike. Thicknesses of the bands are greatest
208 where they meet. Often, multiple bands are observed nested within larger vesicle bands; as many as five vesicle
209 bands were observed to be nested. Vesicle shapes range from spherical to elongate and irregular; generally,
210 spherical and irregular vesicle populations were observed along the centers of the dikes, while elongate vesicles
211 were observed closer to the margins, indicative of shear stress along the boundary. The latter vesicle shapes
212 allowed us to identify magma flow direction, here interpreted to be westward, away from the vent area. We
213 also note that the crystallinity observed in hand sample along the length of Dike A showed an increase of
214 feldspars, both in size and population (2% distal - 50 % proximal; Table 1), with decreasing distance from the
215 central conduit area.
216 Dike A also preserves several examples of multiple fine-grained, relatively glassy bands, which we
217 interpret as chilled margins within the main dike, at several localities along its length (Fig. 4c,d). Several
218 observations of the margins showed similar convergence and termination patters as the vesicle bands previously
219 described. In some instances the margins converge toward the center of the dike and terminate, similar to the
220 distal terminations of paired vesicle bands.
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222
223
3.3.2. Dike C
Dike C (Figs. 2b and 5a), extends from a semi-arcuate mound of variably welded pyroclastic deposits
224 near the eastern flank of the summit. Proximally, the dike trends continuously S55°E for 40 m, then bends
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225 slightly to a new trend of S70°E for the last 50 m until it gradually tapers into the colluvium. The bend nearly
226 halfway along the length of the dike corresponds to an intersection with a semi-arcuate mound of moderately
227 welded spatter. Dike C is the most coherent and best preserved of all the observed dikes. In many places, one
228 or both of the chilled margins are preserved, allowing for measurements of the dike’s true width. The dike
229 tapers in width from 1.1 m near its proximal end to 0.27 m at its most distal exposure and though the dike drops
230 in elevation by ~10 m, it generally stands >1 m above its immediate surroundings.
231 The interior of the dike has one consistent vesicle-rich band that extends parallel along the interior with
232 vesicle sizes ranging from several mm to more than 5 cm. The largest vesicles show indications of coalescence
233 such as varying degrees of connectivity between bubbles and irregular shapes (Fig. 5b). Though vesicles do not
234 occur in multiple or nested bands as with Dike set A and others, the vesicles within Dike C are elongated
235 parallel to the main trend of the dike, indicating flow direction (Fig. 5b). Dike C also has a rather low
236 phenocryst content compared to the other dikes with plagioclase crystals averaging between 1 - 3 vol.%.
237
238
239
3.3.3 Other dikes and intrusive bodies
Dike Set E (Figs. 2b and 6) is the only dike observed that is not associated with a lava body or mound of
240 welded pyroclastic material at its proximal end. The dike set consistently trends N45°W for nearly 180 m
241 before its last dike tapers into the colluvium. Dike Set E is discontinuous and is composed of at least eight
242 small dikes, each with individual lengths ranging from 3 -10 m, some of which are segmented. The dike set has
243 a maximum width of ~17 m, though individual dikes are typically ~1 m wide. As with Dike set A, we infer that
244 the well-aligned, smaller dikes, especially those to the northeast, are individual dikes (where laterally offset) but
245 there are also segments of a single dike that are/were connected to a primary dike. This is based on the lateral
246 offsets and adherence of agglutinate spatter on both margins of most of the small dikes, rather than one
247 continuous dike that has preferentially eroded into this configuration over time. Dikes within Dike set E have
248 paired bands of vesicles , similar to Dike set A.
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249 Dike B, though discontinuous, is linear and in many ways similar to the main dike in Dike set A in terms
250 of its vesicularity and crystallinity, though its simplicity of shape and coherence is similar to Dike C (Fig. 2b
251 and 7a). The dike’s proximal end extends away from the inferred conduit area in a north-northeast direction
252 from a ~900 m
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mound of lava and variably welded pyroclastic deposits (Fig. 7b). The contrast between
253 morphology of the pyroclastic mound and of the dike allows for it to be traced for ~10 m into the mound before
254 it is lost.
255 Dike set F is encased in moderate to densely welded spatter on either side along much of its length, thus
256 preserving its original margins and interior. The dike set extends from the summit area with a trend of N60ºW
257 for 167 m and has an elevation change of 16 m. Similar to Dike Set E, Dike Set F is composed of at least eight
258 smaller dikes that range in length from several meters to several 10s of meters. Typically the individual
259 segments range from 1-2 m wide and in most cases one or both margins are preserved along with adjacent
260 moderately welded pyroclastic deposits. While some of the segments of Dike Set F were observed to have
261 complex internal vesicle banding, on whole, the segments are relatively dense with vesicularities ranging from
262 1-5%. Vesicles were generally irregularly shaped and ranged from several mm to 0.8 cm.
263 Several linear to semi-arcuate intrusive basaltic features share some characteristics with the dikes
264 described above in terms of the overall outcrop shape, high aspect ratio and internal features. A feature might
265 show internal banding of crystals or vesicles, but the chilled margins were missing, or there might be an outcrop
266 that clearly displays chilled margins, but it grades into a massive mound of coherent lava (a lava body) over a
267 small lateral distance (1-2 m). In some cases it was unclear whether the outcrop is an irregularly shaped dike,
268 ponded lava, or simply an artifact of erosion that resulted in an dike-like shape. These ambiguous features are
269 mapped as ‘lava’ in Fig. 2b. In map view, the shape of some features, particularly the two features just south
270 and west of Dikes C and D, might be interpreted as circumferential or tangential dikes.
271
272 3.3.4 Relationship between dikes and the inferred Kimana cone
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273 Based on the presence pyroclastic deposits in the summit area of Kimana, mapped contacts with pre-
274 Kimana rocks, and assumed slope angles of 30° (approximate angle of repose for scoria), it is possible to infer
275 the original size and shape of the cone. We estimate that the original cone height was up to 288 m above the
276 pre-Kimana ground surface, with a basal diameter of ~650 m (Fig. 8). There is some uncertainty in this estimate
277 due to the current level of erosion and assumed simple cone shape. Nevertheless, these values of cone height
278 and diameter provide an approximate basis for considering the relationships between the intrusions and the host
279 cone discussed in Section 5. The preserved dikes occur in the lower half of the inferred original cone (Fig. 8).
280 We do not know how deeply each dike extended below the current outcrops.
281
282 4. Broken Cone Volcano
283 4.1 Topographic and structural setting
284 The second site described in this paper, Broken Cone, is located in the northern part the LCVF where it
285 overlaps the Pancake Range (see Fig. 1). Broken Cone is part of a series of three small north-northeast
286 trending, ~1.5 km long, line of small basaltic bodies at the foot of a range high mesas and hills of Miocene
287 ignimbrite and rhyolite lavas (Fig. 9a and 9b). To the west of Broken Cone is an alluvial basin that is partly
288 occupied by the Lunar Lake playa. Although the existing geologic map ( Scott, 1969 ) does not show a fault at
289 Broken Cone, it is likely that it coincides with a major basin-bounding normal fault or is on a fault that is part of
290 a basin-bounding fault zone. Such faults are mapped to the northeast and southwest of Broken Cone along the
291 same general trend, and ~3 km to the southwest one such fault extends into a northeast-southwest trending
292 fissure vent system associated with another volcanic center. While Broken Cone itself is composed primarily of
293 coarse pyroclastic material, as described below, the two basalt bodies immediately to the northeast are remnants
294 of an associated lava field. The three bodies are no longer connected but it seems likely that the lavas vented
295 from Broken Cone and filled in a topographic low between ignimbrite cuestas representing the tops of
296 southeast-tilted fault blocks. Due to subsequent erosion, the lava field remnants now form low mesas (inverted
297 topography). Although the youngest activity in the LCVF occurs only ~10 km away, we infer that Broken Cone
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298 is probably of (late?) Pliocene age, due to the level of erosion and surface modification of the nearby lava fields
299 (see Wells et al., 1985 ).
300
301 4.2 Description of eruptive products
302 Broken Cone consists of a wedge of basaltic pyroclastic deposits on the northwest-facing steep cuesta
303 slope, and is cross cut by two basaltic dikes (Dikes 1 and 2; Figs. 10 and 11). The pyroclastic deposits are
304 composed of crudely bedded (on dm to m scale), flattened bombs and less abundant, coarse scoria lapilli (Fig.
305 12). Bombs are typically between 20-80 cm (long dimension), but larger ones reach ~300 cm with up to ~80
306 cm along short dimensions. Most bombs are reddish grey to reddish brown in color; they are lenticular in
307 shape, resembling flattened spindle bombs, with coarsely vesicular, frothy interiors and quenched, less vesicular
308 rinds that range from intact to fractured and torn due to expansion of the clasts just after deposition. A smaller
309 fraction of these reddish clasts are ribbon-like or flattened rags, decimeters long and centimeters thick, that are
310 draped over underlying clasts, indicating that they were highly fluid upon impact with the ground. Other bombs
311 are relatively dense and dark grey, with smaller vesicles and imbedded, reddish scoria lapilli, suggesting mixing
312 of previously erupted clasts into magma just prior to explosive ejection, perhaps by grain avalanches down
313 crater walls. The deposits are mostly only slightly to moderately welded, such that individual clasts are distinct
314 and only partly bonded to neighboring clasts. However, some localized horizons, which are laterally
315 discontinuous over length scales of meters, are more densely welded such that relict clast shapes and variations
316 in texture can be distinguished, but the porosity between clasts is largely closed and they are well bonded to
317 each other. We did not observe any xenoliths from upper crustal rocks. Finally, a poorly sorted lapilli tuff
318 consisting of ignimbrite and vesicular basalt lapilli, poorly exposed near the western margin of the basaltic
319 pyroclastic deposits, probably records mixing of colluvium and scoria cone material as basaltic material
320 accumulated on the steep slope of the cuesta.
321 We interpret the deposits described above to have accumulated in a relatively near-vent environment;
322 the coarse clast sizes and the evidence for high deposition temperatures are consistent with a vent location
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323 within a few hundred meters of the preserved deposits, and probably as close as 100-200 m. The facies
324 characteristics are consistent with Strombolian activity if the vent was relatively close or Hawaiian if it was
325 farther away ( Valentine and Gregg, 2008 ). At most locations the poorly defined bedding of the deposits dips
326
~20º to the north-northeast, roughly parallel to the slope of the cuesta on which they lie (an east-southeast-tilted
327 fault block). Thus, the bedding orientation by itself provides no constraint on vent location, which might in
328 other cases be suggested by a radial pattern of inward dipping beds, formed in the crater of a scoria cone. To
329 the south and east of the deposits, there is no evidence of a basaltic vent, but only the gradually tapering Dike 1
330 (see below) and the surface of the cuesta. To the west and north is the Lunar Lake basin with its alluvial fill. It
331 appears that the deposits are a remnant of proximal pyroclastic accumulation on a hillside, and that the vent was
332 within ~200 m to the north or northwest. We infer a scoria cone existed in that area along the basin-bounding
333 fault mentioned above, but has since eroded, subsided, and been buried by sediments, as has happened in the
334 nearby Southwest Nevada Volcanic Field ( Valentine and Perry, 2009 ). Only the partly welded, resistant
335 deposits that accumulated on the footwall side of the normal fault are preserved on the surface (Fig. 9).
336
337 3.3 Intrusive bodies
338
339
The two dikes that are exposed at Broken Cone provide additional insight into plumbing at very shallow levels, in this case partly inside a cone and partly in the “bedrock” of the cuesta that the cone abutted on its
340
341 southeast side. Dike 1 is ~135 m long and along most of its length trends north-northeast and dips steeply (66-
88°) to the east (Figs. 10 and 11). In most places, it is ~1.5-2 m thick, but swells to 4.9 m where it intrudes the
342 basaltic pyroclastic deposits. The dike thins to the south where it intrudes Miocene ignimbrite, gradually
343 tapering to a few dm and then terminating with a blunt tip; one or two separate segments occur along strike
344 several meters to the south of the termination, but it is not known if they are truly separate from the main dike
345 or if the separation is a result of partial cover of a single, thinning dike by colluvium. Near the southern
346 termination, a small, ~60 cm thick dike extends nearly orthogonally from the main Dike 1; this dike extends
347 continuously in ignimbrite country rock for 6 m and then breaks into a group of very small en echelon segments
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348
349 before completely ending (see Fig. 11 inset ). Toward the north, Dike 1 ranges from 2-4 m thick and its dip shallows until it is nearly horizontal (9° E) at its blunt northern termination. Unfortunately, there is very limited
350 exposure of the basaltic pyroclastic host of this part of the dike due to loose surface rubble, but limited exposure
351 suggests that the dike may be nearly concordant (sill-like) at that end. In many places along the southern 2/3 of
352 the dike, it has platy joints, with joint spacing of about one decimeter, which are approximately parallel to the
353 dike margins; this causes the dike to crop out as basalt slabs. Locally, however, the contact-parallel jointing is
354 not so closely spaced and the dike crops out as a single intact body. Joints that are perpendicular to the dike
355 margins are present all along the length of the dike, but these are typically several decimeters apart.
356 Dike 1 preserves complex internal textures in a few places where fresh basalt is exposed on its surface
357 (as opposed to the weathered surface where textures are obscured by desert varnish). Bands of relatively highly
358 vesicular basalt, ranging in width up to ~10 cm and with vesicularities as high as ~30%, alternate with thinner
359 bands that are typically 1-2 cm wide and that have very low vesicularity (Fig. 13), in many ways similar to
360 vesicle banding observed at Kimana. Viewed in horizontal (plan view) exposure, these alternating bands of
361 alternating vesicle rich and poor bands across the width of the dike, and the variations in rock texture often
362 correspond with the presence of contact-parallel joints.
363 Dike 2 is relatively simple, with a straight north-northwest trend up the hillside where remnants of
364 Broken Cone are exposed, and a dip of ~70°E along its entire length (Fig. 11). Outcrop is continuous except for
365 a gap of several meters where it is covered by talus. Near its lower (northern) end the dike is ~1 m wide and its
366 termination has a blunt shape. In contrast, the upper (southern) part of the dike has a consistent thickness of
367 ~50 cm and it gradually tapers to zero over a distance of 4 m at its top. Jointing perpendicular to the dike
368 margins has typical spacing of 50-100 cm, and dike-parallel jointing is locally platy as described for Dike 1.
369 Dike 2 extends about 2 m below level attained by the sub horizontal, northern end of Dike 1 (Fig. 10). The two
370 dikes are connected by a ~30 cm wide extension from the blunt end of Dike 1. Dike 2 is relatively dense (low
371 vesicularity) and massive in texture.
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372 Most of Broken Cone is lost to erosion and subsidence into the Lunar Lake basin, limiting the level of
373 detail that can be deduced about its internal plumbing. The two exposed dikes radiate from their intersection,
374 similar to radial dikes from the center of the Kimana cone described above. If the dikes had radiated from a
375 central conduit, one might expect a volcanic neck to be exposed at the point where they intersect (such as
376 described at Paiute Ridge and Grants Ridge; Keating et al., 2008 ), but this is not the case. We note that both
377 dikes appear to end with blunt terminations. It is possible that Dike 2, which is sub-vertical at its northernmost
378 and lowest exposure, is close enough to the basin-bounding fault that its outcrop has been truncated by fault
379 motion, and the outcrop does not actually correspond to the original termination of the dike. If this is the case,
380 Dike 2 might have been emplaced by radial propagation from the conduit and subsequently disconnected as the
381 main part of the volcano subsided along the basin-bounding fault (Fig. 14). Dike 1 may have similarly been
382 connected to a conduit just to the north, although this might have been complicated as reflected in the sub-
383 horizontal orientation of the dike at its northern end. If the dikes were connected to a conduit, it must have been
384 located very close (within a few 10s of meters to the north) to the exposed dike ends. This is also consistent
385 with the pyroclastic facies described above and with the feeder system of the cone having occupied a normal
386 fault.
387
388 5. Discussion
389 Here we discuss probable processes and mechanisms of dike and dike set emplacement within Kimana
390 and Broken Cone, the influence of the regional and local structures on the internal plumbing, and implications
391 of the observed features for eruption processes at monogenetic volcanoes.
392
394
393 5.1 Dike emplacement
Multiple sets of paired vesicle bands and internal chilled margins within the radial dikes of the
395 volcanoes provide evidence for incremental lateral growth or propagation away from a central conduit (see Fig.
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396 4.b-d). We propose that repeated batches of magma, or pulses of magma, were injected into the host cone
397 during the eruption and record pressure fluctuations in the shallow plumbing (Fig. 15). Processes capable of
398 producing these fluctuations are: (1) an increased magma column height associated with lava ponding, lava
399 lakes or ascending gas slugs (during Strombolian activity); (2) increasing density of magma due to stagnation
400 and degassing; or (3) blockages at the vent or within the upper conduit. The pressure fluctuations associated
401 with these processes, expected to be on the order of 10s of kPa and can result in failure of conduit walls and
402 propagation of radial dikes, with magma flowing into the dikes in pulses rather than with a steady flux. The
403 evidence for these multiple injection episodes within the intrusive bodies are consistent with effusive and
404 explosive eruption styles, that are inferred from the preserved pyroclastic and lavas of both volcanoes.
405 Examples of dike dilation and propagation during effusive activity might include the situation where a
406 column of magma in a conduit has become stagnant due to insufficient driving pressures required to overflow
407 the cone. Here, the column of magma might efficiently degas and increase in density, as is observed in lava
408 ponds and lakes (refs). Additionally, a change in the ascent rate of rising magma might increase the magma
409 column height, temporarily increasing the “magmastatic” pressure at a given level in the conduit. In these
410 cases, a dike would grow incrementally as more magma is added to the column from below (the source).
411 Resulting in the same type of dike formation. Likewise, external events may have an effect on magmatic
412 pressures within the conduit. For example, an obstruction either within the conduit or at the mouth of the vent
413 might choke or completely block magma and volatiles from escaping. This scenario, though possibly
414 temporary, could increase the pressure within the conduit long enough to either initially breach the walls, or
415 provide for additional ‘pulses’ of magma to further propagate incipient dikes. There are additional mechanisms
416 for variations in pressure during a monogenetic eruption that are preserved in the dikes. A change in the ascent
417 rate of rising magma, or increase in density due to exsolution, instead of pressure changes due to the passing of
418 a gas slug, could allow the conduit walls to experience an increase in magmatic pressure. In this case, the dike
419 would grow incrementally as more magma is added to the column from the source resulting in the same type of
420 dike formation. Likewise, external events may have an effect on magmatic pressures within the conduit. For
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421
422 example, an obstruction such as a plug of solidified magma or an avalanche of pyroclastic material from the cone’s crater walls, might choke or completely block magma flow until sufficient pressure builds up to blow
423 through the obstruction. The temporarily increased pressure leading up to failure of the obstruction would be
424 transmitted to radial dikes that are connected to the main conduit, if they contain fluid magma.
425 During Strombolian activity, variations in magma pressure in a conduit can result from slugs of gas
426 rising up through a magma column (Fig. 16). As a slug rises from depth, the top of the magma column is
427 slightly elevated as the expanding (by decompression and coalescence) slug approaches the surface. At a given
428 level in the conduit, the “magmastatic” pressure (the product of the depth below the magma surface, the average
429 density of magma above the level, and gravitational acceleration) increases until the slug ascends above that
430 level. As the bubble passes this reference level, the average density of the overlying magma column decreases
431 (due to the presence of the slug) until the slug intersects the surface and erupts. Dynamic effects such as the
432 pressure gradient due to flow of liquid around a gas slug would add to this magmastatic variability. Passage of
433 successive slugs during periods of Strombolian activity (such as is recorded in the pyroclastic deposits at the
434 two volcanoes) can cause repeated fluctuations in magma pressure, pushing small increments of magma into
435 dikes.
436 In some cases, individual pulses of magma might be preserved along a dike in the form of ‘chilled’
437 bands (where the flow-front of the fresh, hotter magma is chilled against the older, colder magma) and vesicle
438 bands. Paired internal chilled margins and vesicle bands suggest that successive increments of magma were
439 pushed into the center of the dike, dilating and displacing the products of previous pulses outward; therefore
440 lengthening the dike. The banding of vesicles and internally chilled margins in many places are consistent with
441 lateral, outward propagation directions, though we note that in some outcrops where a vertical section was
442 exposed, possible vertical-up and vertical-down bands and textures (crystal and vesicle) are visible. At one of
443 the best exposed examples of this banded texture within the Broken Cone volcano (Dike 1; Fig. 13) both
444 horizontal and vertical exposure), the inner bands are observed to converge downward. This geometry is not
445 consistent with the final magma pulses coming from below, but could be consistent with lateral injection (such
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446 as from a conduit to the north) or downward injection that stopped at the level that is now exposed, though
447 lateral injection seems more likely. Similar features observed within the dikes throughout the Kimana volcano
448 are also consistent with this outward propagation direction that may have a slight upward (Fig. 5b) and even
449 downward (Figs. 4d and 13) component.
450
451 5.2 Dike geometry
452 Orientations, shapes and types of dikes (and other intrusive bodies) that form within volcanic constructs
453 can play important roles in the overall structure or architecture of a growing or established cone. The dikes
454 described here are predominately radial dikes that extended away from the central conduits within the former
455 cones, though the present topography and level of erosion does not directly expose the dike-conduit transition.
456 We infer that that this transition is presently buried by 10s of meters of less-erodible, densely-welded, vent-
457 filling deposits (see Fig. 3). Factors influencing dike orientation are discussed below but it should be noted that
458 individual dike (including segments and sets) propagation paths are affected locally by the presence of
459 heterogeneities of intruded material (e.g. variable degrees of welding or lack thereof in scoria beds), variations
460 in the underlying topography or topographical confinement of scarps, collapsed sectors or buttressing of the
461 cone.
462 Radial dikes within volcanic edifices result from near-field or local stresses produced by the
463 combination of the outward-decreasing load of the edifice and pressurized magma from the conduit ( Ode, 1957;
464 Nakamura, 1977 ). Note that because the dikes described here occur within the volcanic cone, we consider the
465 influence of regional stresses to be negligible. The distribution of gravitational stresses due to a volcanic load
466 acts to control the direction of the maximum and minimum compressive stresses, which in a cone is sub parallel
467 or parallel to the slope and tangential to the slope, respectively ( Acocella and Tibaldi, 2005 ). This is reflected
468 in the radial dike patterns observed the Kimana and Broken Cones, though several of the dikes display
469 indications of transtensional shear in the form of en echelon segments (e .g. Dike D, Figs. 2b and 5d).
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470 As is the case with many radial dike patterns on volcanoes, the dikes associated with Kimana have an
471 anisotropic pattern (see Fig. 2b) with most of the dikes extending in a south-southeast to eastern direction. This
472 apparent clustering is possibly just an artifact of the current erosion level, however, suggests that dikes were
473 most likely to form along a southeast ridge. This might be an indication that the original cone was elongate
474 along a southeast axis and never grew to a significant height above the surrounding area, as has been observed
475 in larger, stratovolcanoes with an isotropic radial dike pattern ( McGuire and Pullen, 1989 ). It is unclear if
476 Broken Cone’s dikes are isotropic or anisotropic as the dataset is too limited, though based on the hypothesized
477 original size of the cone, the dikes are likely anisotropic. The second mechanism influencing radial of dike
478 propagation is the mentioned above-mentioned pressurized magma coming from the conduit. This mechanism
479 is likely more relevant to the actual propagation of dikes rather than the orientation, though the addition of
480 magma is somewhat responsible for an outward compressive force as evidenced by the flow directions observed
481 in dikes of both volcanoes studied (see Section 5.1).
482
483 5.3 Implications for theoretical models of basaltic systems
484 The geometry of dikes observed on Kimana and Broken Cone is similar to the proposed dike-to-conduit
485 geometric relationships between a main conduit and hypothesized lateral (radial) dikes that fed lava flows from
486 the flanks of Paricutin volcano (Mexico ; Pioli et al., 2008; Krauskopf, 1948 ). Pioli et al ( 2008 ) proposed that
487 the dikes connected to the conduit within Paricutin strongly influenced spatial segregation between gas and
488 magma, resulting in the simultaneous eruption of effusive (from flank vents or bocas) and explosive
489 products(from the summit crater). They hypothesized that to achieve this behavior; gas would need to segregate
490 at very shallow levels (i.e. within the cone or just below it), allowing for an accumulation of denser material that
491 could be mobilized into a laterally propagating dike and subsequently opening of new lava bocas; this type of
492 eruptive activity has also been suggested by Valentine et al. ( 2007 ) in their work on Lathrop Wells volcano
493 (Nevada). For the Paricutin case, we find that our observations at Kimana and Broken Cone are consistent with
494 the interpretations of Pioli et al. ( 2008 ) in several ways;
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503
504
505
506
507
508
509
510
511
495
496
497
498
499
500
501
502
1.
Gas segregation. Though not directly observable in our volcanoes, the preserved dikes intruded deposits of explosive activity that probably originated at the summits of the cones. The mapped lava flows are evidence of both explosive and effusive behavior, though we realize that we currently cannot constrain the two behaviors as being contemporaneous or not.
2.
The observed segregations at Paricutin are reported to have only occurred after the eruption had persisted for more than a month and once the cone was constructed. This is consistent with our observations that dikes propagated throughout the Kimana and Broken Cone edifices in that the cones had to exist in order to be intruded.
We infer that the dike-conduit geometries observed or deduced from the Kimana and Broken Cone systems are a welcome complexity that accurately reflects the time-integrated composite view of the internal plumbing systems within these volcanoes. Whereas the experimental dike-conduit junction work by Pioli et al. ( 2009 ) reflects the optimal flow geometry established by steady-state flow during an individual eruptive phase. While this kind of work has several excellent applications, the limitations of experimental work to capture the ‘big picture’ of the ever-increasing complexity within natural systems
(which continue to increase in complexity as an eruption progresses) are its inherent simplicity. As with most things, the devil is in the details when comparing experimental and real-world systems.
512
513
5.4 Cooling and degassing
Extensive dike formation and propagation may lead to substantial increases in near-surface degassing;
514 which coupled with other factors like changes in mass flux and microtextures, may allow for a changes in
515 eruption styles. The more complex the geometric network of active or actively degassing dikes, the area over
516 which gas can diffuse becomes larger. For example, the cross-sectional surface area of the Kimana vent at the
517 present level of exposure is ~700 m
2
, and the combined surface area of the vent plus the dikes (assuming an
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518 average width of ~1 m) is ~2000 m
2
. Thus, the addition of these radial dikes to the increases the surface area
519 over which degassing can take place is nearly 3-fold that of just the vent area alone.
520 Similarly, degree to which a dike network extends throughout a cone increases the rate of conductive heat
521 loss throughout the cone during emplacement. This implies that once the process of intracone dike formation
522 has progressed enough to significantly affect the degassing and therefore cooling of the volcanic system, it is
523 likely self-perpetuating.
524
525
526
5.5 Lava boca development
The dikes described here and possible processes governing their inception and growth are thought to be
527 directly related to lava boca formation. Lava bocas have been observed at numerous monogenetic volcanoes
528 (refs) and represent effusive magma that breaches the base or lower portion of a cone. Bocas can result from a
529 direct intersection of a feeder dike with the surface, or from the lateral migration of magma from the conduit.
530
The latter type of lava boca, called ‘breakout bocas’, are consistent with our observations and interpretations of
531 laterally propagating dikes that given a sufficient magma supply would ultimately intersect the free-surface of
532 the cone and possibly feed a lava flow. Given a new magmatic pathway, significant amounts of magma might
533 now be channeled through this portion of the cone, wiping out original flow structures in the dike, and
534 increasing local heat flow thus potentially removing or altering features like multiple chilled margins.
535
536 Conclusions
537 The remnants of the Kimana and Broken Cone scoria cones provide field-based evidence for complex
538 geometries of shallow plumbing and potential links with the eruptive dynamics of explosive and effusive
539 monogenetic volcanism. The exposed dikes in both volcanoes display complicated intrusive geometries along
540 with distinct vesicle bands that are interpreted to reflect multiple lateral and vertical pulsation/injections of
541 magma from a centralized feeder source that is consistent with Strombolian behavior ( Pioli et al., 2009 ). The
542 orientation and internal structures preserved within the dikes are a good first-order approach for determining the
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543 processes acting within the cones during the 'life' of the eruption. During cone building and post-cone building
544 activity, radial dike and dike sets extend from a central feeder source once the volcanic load is significant
545 enough for the ancillary dikes to intrude the edifice. The internal vesicle banding within the dikes are the result
546 of multiple pulses of magma into the dike, with each successive pulse injecting into its center, pushing the
547 previously injected magma and dike walls apart. Each thin, dense band represents a chilled margin of a magma
548 pulse, while the vesicular bands cooled more slowly, allowing more time for bubble nucleation and growth.
549 Magma segregation of gas and liquid (+ crystals) allows gases to build up in the central conduit area; however,
550 increased overburden can cause the internal pressure of the system to push magma laterally, causing the dikes to
551 extend. Individual vesicle orientations/fabrics as well as distinct bands of vesicles within the dikes are shown to
552 be a useful means of interpreting flow and propagation directions. Propagation directions, coupled with
553 processes acting throughout the dikes while still active are deterministic of eruptive style, while the overall
554 geometries of the dikes provide information for the regional and local structural control acting on and within the
555 cone.
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