Contextualizing the View Hill Scoria Cone, Akaroa Volcanic Complex

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1 Contextualizing the View Hill Scoria Cone, Akaroa Volcanic Complex Spencer Irvine 1, 2, Darren Gravely 2, Sam Hampton 2 1 Williams College, MA, USA, 2 University of Canterbury, NZ Abstract A recently discovered scoria cone located at View Hill, Banks Peninsula, New Zealand is well exposed on the side of a ridge. Analyses of these scoria deposits reveal eight different facies types based on grain size, clast percentage, average clast size, and degree of welding. These lithofacies have been mapped along with strike and dip measurements to locate the probable eruptive center. A schematic crosssection of the scoria cone was also drawn to demonstrate the cone s internal morphology. These diagrams indicate changing eruptive styles over the course of the cone s evolution and highlight complex relationships to the surrounding features. One of the features contacting the scoria cone includes overlying lava flows, which form a steep contact relationship. This indicates that the cone had already been completely formed by the time the lavas were deposited. The scoria cone is also contacted by dike intrusions, which likewise postdate the cone, though the origin of these deposits is unclear. As such, the View Hill scoria cone demonstrates that even in localized areas, multiple eruptive events took place in the Akaroa Volcanic Complex. Introduction A well-preserved scoria cone located at View Hill, Banks Peninsula, New Zealand records the eruptive history of Akaroa volcano. Last active 8 Ma, Akaroa volcano is primarily composed of basaltic lava flows and shallow intrusives, though it also consists of pyroclastic deposits such as the recently mapped scoria cone at View Hill. Though scoria cones have been identified and described on Akaroa volcano before (Johnston et al. 1997, Gorbey, D., Markey, E.), detailed analyses of Akaroa s volcanism and formation are limited. Previous studies have demonstrated that scoria cones are formed as a result of strombolian style eruptions, which are characterized by bursts of gas and coursegrained pyroclasts (Valentine et al. 2005). Scoria cones develop from strombolian eruptions over a series of four stages. In the first stage, a low-rimmed mantlebedded pyroclastic ring forms. During stage two, an exterior talus develops. Stage three begins with the destruction of the original rim by inward migration of the talus pile. In the final stage, the talus grows beyond the ballistic limit of ejecta

2 (McGetchin et al. 1974). Scoria cones are useful tools in analyzing eruptive history, as they record flank explosive activity away from the central conduit of the volcano. This study investigates the relationship between the View Hill scoria cone and the construction of Akaroa Volcanic Complex by examining the cone s development and morphology. By reconstructing the scoria cone, one can pinpoint the location of the vent and therefore the eruptive center. It also helps in understanding the relationship between the scoria cone and nearby lava flows. This will help explore the eruptive history of Akaroa and its formation. Geologic Setting Banks Peninsula, located on the east coast of New Zealand s South Island, consists of two late Miocene aged composite shield volcanoes. These volcanoes, Lyttelton and Akaroa, were last active Ma and Ma, respectively. The deposits from these volcanoes can be split into four volcanic groups: Lyttelton, Mt. Herbert, Akaroa, and Diamond Harbor. This study focuses on the deposits from the Akaroa volcanic group, which include the rocks erupted from the central and flank vents in eastern Banks Peninsula. Collectively, these comprise Akaroa volcano, a 1200 km 3 composite volcano of dominantly alkali lavas and shallow intrusives (Sewell 1988). The development of Akaroa volcano can be divided into an early phase and a main phase, corresponding to their timing in regards to the evolution of the volcano. The early phase consisted of early basaltic lava flows, the development of trachytic material in domes, and pyroclastic and epiclastic deposits. Comprising these deposits are scoria cones containing basaltic scoria and lava bombs, including the focus of this study. The main phase is characterized by more basalt and hawaiite lava flows with trachyte and basalt dikes intruding flows and pyroclastics (Dorsey 1988). View Hill, the field site for this study, consists of two ridges in northeastern Banks Peninsula and is located several kilometers inland of Little Akaroa. The ridges are composed of stacked lava flow sequences, pyroclastic deposits, basaltic dikes,

3 and trachytic lava domes. This study focuses on a previously unmapped scoria cone found on the western-most ridge of View Hill (figure 1). Methods Fieldwork involving mapping, data collection, and sample collection was conducted in February This research focused on scoria deposits, so features such as location of the deposits, color, grain size, structural measurements (strike and dip), clast measurements, and clast percentage were recorded. Photographs of the deposits were also taken and used to correlate different lithofacies across the field area. Using grain size, clast size, and clast percentage as parameters, the deposits were categorized into one of eight facies types based on those described by Johnston et al and refined with analyses of field observations. Using ArcMAP 10.2, the lithofacies were then mapped according to their observed locations and inferred extent. This geologic map of the scoria cone, while accounting for the lithofacies presumed proximity to the vent, was utilized along with strike and dip measurements to locate the potential eruptive center. A schematic cross-section of the scoria cone was constructed with CorelDRAW V6 to depict the cone s internal structure and highlight the shifting eruption styles that were involved in building it. The relationship between the scoria cone and the overlying lavas was examined using structural measurements (strike and dip) of contacts and Google Earth to infer their respective source locations. Results Fieldwork uncovered an extensive scoria cone on the northwestern ridge of View Hill. Much of the scoria cone has been eroded away, though compared to other scoria cones on Banks Peninsula it is quite well preserved. The scoria cone sharply contacts overlying basaltic lava flows on its eastern and southern flanks, and farther down the ridge to the northwest, the scoria is intruded by basaltic and trachyadesitic dikes.

4 Scoria Lithofacies Classification Analysis of field data revealed eight different facies types. These classifications were based on Johnston et al and modified using field observations to better describe the View Hill scoria cone. The defined lithofacies include ash bed, ash-rich scoria, intermediate scoria, non-flattened scoria, mixed scoria, clast-dominated scoria, loosely welded scoria, and densely welded scoria. To determine these deposit types, grain size, clast percentage, average clast size, and degree of welding were all considered. Each deposit type was then matched with the scoria outcrops observed in the field, and their field locations noted. These categories were then given a characteristic description based on recorded field observations of the deposits. The results of this classification are presented in table 1. Representative photographs of each deposit type are shown in figure 2. The deposits are listed in order from finer grain size, less clast-dominated, and smaller clast size to coarse grain size, more clast-dominated, and larger clast size. Scoria Cone Reconstruction Using the relative location of the different facies types, a geologic map of the scoria cone was constructed (figure 3). The map depicts the eight different scoria deposit types, the overlying lavas, and the dike intrusions. The field locations are marked in red and match those in table 1. Strike and dip measurements are mapped in locations where they were recorded. In areas where there is no field location, the extent of the deposits is inferred, assuming consistent thicknesses and factoring in a facies typical proximity to the vent. This understanding of scoria cone morphology was considered along with the strike and dip measurements to pinpoint the eruptive center of the scoria cone. For instance, welded scoria is typically more proximal to the vent, while ash-rich scoria is more distal, and beds tend to dip away from the vent source. The hypothesized vent location in the facies map represents the most likely area for eruptive activity. Based on the scoria facies map, a schematic cross-section of the scoria cone was constructed (figure 4). The units below the topography (represented by the solid black line) are drawn according to the contacts and thicknesses from the

5 geologic map of the scoria cone (figure 3). Those above the line are inferred, as they were not observed in the field. The units are mirrored across the proposed vent location as it is assumed that the eruptions produced symmetrical deposits. The cross section highlights the change in eruption styles from more strombolian to more hawiian over the course of the scoria cone s development. Google Earth image analyses indicate that the scoria cone rests on a large flat bench area, which likely represents the paleotopography that was present at the time the scoria cone formed. Measurements of the scoria cone from its base where it lies on the paleotopography to its top where it contacts the lava suggest that the cone is roughly 250 m tall. Meanwhile, the lateral extent of the scoria cone is estimated to be upwards of 1000 m based on the dip of the units and measurements of the scoria cone in Google Earth. These dimensions fit closely with those described by the equation, Hco = 0.18Wco where Hco is the height of the scoria cone and Wco is the width (Porter, 1972). Discussion The scoria facies at View Hill represent hawaiian and strombolian style eruptions, as indicated by the presence of welded and non-welded scoria facies, respectively. Hawaiian eruptions typically produce welded scoria, as they are less explosive and cause less magma fracturing. Conversely, strombolian eruptions will favor non-welded scoria. More violent eruptions will cause more fracturing and therefore create more ash-rich scoria, whereas less violent eruptions will produce blocks and bombs with lapilli matrices. These eruption styles and deposit types are characteristic of scoria cone development. The morphology of the View Hill scoria cone, including its dimensions and internal structure, are similarly very characteristic of scoria cones not only on Banks Peninsula, but also across the world. In the context of View Hill, the scoria cone has a clear relationship with the surrounding features, which is depicted in figure 5. Their relationship suggests that the hawaiian and strombolian eruptions that built the scoria cone occurred before those that created the overlying lava flows, as

6 evidenced by the nature of the contact between the two units (highlighted in red in figure 5). The dip of the contact between the ash bed and lava at location 10 is 21 to the east and near location 2 the dip of another contact between the scoria and lava is 35 to the east. Away from the scoria cone the lavas have only a gentle dip to the east. These values indicate that the scoria cone precluded the deposition of the lavas, as the scoria cone must have been present to affect the dip of the lava. Additionally, there is evidence of baking of the ash at the scoria-lava boundary at location 10 (figure 2A). The ash is thermally altered and appears reddish in color, and it is highly fractured and brittle. This phenomenon occurs when ash comes into contact with hot fluids, including lava, signifying that the ash and top layers of the scoria cone predated the lava flows. Intrusions discovered in the field area were found at location 6 (represented by bright yellow in figures 3, 4, and 5). Whereas the scoria deposits are of hawaiite composition, these intrusions consist of a basaltic dike and a trachy-andesitic dike occurring next to each and intruding through the scoria. As such, they likely represent a different eruptive event, distinct from the activity that formed the scoria cone. There is evidence that intrusions are indicative of later-stage activity, as they do not fall within the proposed vent zone and do not appear to follow the same conduit. Alternatively, it is possible that the vent source migrated during the construction of the cone, raising the potential for the intrusions to be directly related to the scoria cone. However, no current data collected in the field indicates this. Conclusion The View Hill scoria cone records shifting eruption styles on the flanks of Akaroa volcano, away from the central conduit. It also signifies eruptive activity that is distinct from the more characteristic lava-forming eruptions that created the ridges at View Hill. Even in localized areas such as View Hill, multiple eruptive events are represented, indicating that the Akaroa volcanic system had variable eruptive styles and was quite complex. Future work can be done to analyze the dike intrusions near the vent of the

7 scoria cone. This research focused primarily on the scoria cone at View Hill and did not have adequate time to decipher these features. Work can try to determine the timing of the dikes formation relative to each other, how far along the scoria cone s development they formed, and how they are related the scoria cone in terms of following the same conduit or representing later-stage intrusive events. Acknowledgements I would like to thank Patrick Thieringer and Eric Barefoot for their assistance and keenness while conducting fieldwork. I would also like to thank Frontiers Abroad for the amazing opportunity to study in New Zealand and my parents for supporting me in my adventure.

8 References Dorsey, C. J. (1988). The geology and geochemistry of Akaroa volcano, Banks Peninsula, New Zealand. Gorbey, D. (2014). Scoria Cone Analysis in Le Bons Bay, Banks Peninsula, New Zealand. Frontiers Abroad (University of Canterbury). Hampton, SJ, & Cole, JW. (2009). Lyttelton Volcano, Banks Peninsula, New Zealand: primary volcanic landforms and eruptive centre identification. Geomorphology, 104(3), Johnston, DM, Cole, JW, & Houghton, BF. (1997). Physical volcanology of Miocene basaltic pyroclastic deposits at Pigeon Bay: remnants of flank scoria cones of Akaroa volcano, Banks Peninsula, New Zealand. New Zealand journal of geology and geophysics, 40(1), Kereszturi, Gábor, & Németh, Károly. (2013). Monogenetic basaltic volcanoes: genetic classification, growth, geomorphology and degradation. Updates in volcanology new advances in understanding volcanic systems. InTech, Croatia, Markey, E. (2014). Uncovering Akaroa s Eruptive History: Reconstructing a flank scoria cone in Le Bons Bay, Banks Peninsula. Frontiers Abroad (University of Canterbury). McGetchin, Thomas R, Settle, Mark, & Chouet, Bernard A. (1974). Cinder cone growth modeled after northeast crater, Mount Etna, Sicily. Journal of Geophysical Research, 79(23), Patrick, Matthew R, Harris, Andrew JL, Ripepe, Maurizio, Dehn, Jonathan, Rothery, David A, & Calvari, Sonia. (2007). Strombolian explosive styles and source conditions: insights from thermal (FLIR) video. Bulletin of Volcanology, 69(7), Porter, Stephen C. (1972). Distribution, morphology, and size frequency of cinder cones on Mauna Kea volcano, Hawaii. Geological Society of America Bulletin, 83(12), Sewell, R. J. (1988). Late Miocene volcanic stratigraphy of central Banks Peninsula, Canterbury, New Zealand. New Zealand journal of geology and geophysics,

9 (1), Timm, Christian, Hoernle, Kaj, Van Den Bogaard, Paul, Bindeman, Ilya, & Weaver, Steve. (2009). Geochemical evolution of intraplate volcanism at Banks Peninsula, New Zealand: interaction between asthenospheric and lithospheric melts. Journal of Petrology, 50(6), Valentine, G. A., & Gregg, T. K. P. (2008). Continental basaltic volcanoes Processes and problems. Journal of Volcanology and Geothermal Research, 177(4), Valentine, G. A., Krier, D., Perry, F. V., & Heiken, G. (2005). Scoria cone construction mechanisms, Lathrop Wells volcano, southern Nevada, USA. Geology, 33(8),

10 Appendix Figure 1: Location map of the scoria cone relative to View Hill. Inset: The location of View Hill on Banks Peninsula, NZ.

11 262 Table 1: Table of scoria lithofacies classification. Name Description Code Field Location Ash Bed Unconsolidated, very fine grained red A.B. 10, 14 ash 5-12cm thick; occurs in sharp contact with overlying lava flows and grades into scoria below. Ash-rich scoria Brown to red ash matrix containing cm-scale dark gray to red vesicular A.R.S. 1, 13 Intermediate scoria Nonflattened scoria Mixed scoria Clastdominated scoria Loosely welded scoria Densely welded scoria clasts occuring at <20% Mixture of non-flattened clasts and bombs in a predominate orange to red ash matrix. Ash to clast ratio falled between that of A.R.S. and N.S. Clast supported deposits (>20%) of dark gray to red vesicular clasts and non-flattened bombs. The matrix is typically dark red ash to lapilli. Welding is not present. Both non-flattened and flatten clasts and bombs present in a red-orange ash to lapilli matrix. Clast dominated deposits containing non-flattened bombs occuring at >50% in a red-orange ash to lapilli matrix. Gray to black loosely welded, highly vesicular scoria with a knobly surface. Mm-scale clasts are visible. Black, welded scoria containing cmscale vesicular lenses and flattened cowpat bombs. I.S. 8, 11, 12 N.S. 1, 2, 5, 7 M.S. 4, C.D.S. 11, L.W.S. 15, D.W.S. 3, 4

12 Figure 2: Representative photographs of each scoria facies type and their corresponding field location. A) Ash bed (location 10); B) Ash-rich scoria (location 1); C) Intermediate scoria (location 12); D) Non-flattened scoria (location 1); E) Mixed scoria (location 4); F) Clast-dominated scoria (location 11); G) Loosely welded scoria (location 15); H) Densely welded scoria (location 3).

13 Figure 3: Scoria facies map based on the relative position of the deposit types observed in the field with field locations and structural measurements. The hypothesized vent location is circled in white A A Figure 4: Schematic cross-section of the scoria cone. The units and cross-section line correspond to those in figure 3. Units above the elevation profile (solid black line) are inferred and therefore faded.

14 Figure 5: Overview image of the scoria cone and overlying lavas to highlight their relationship. The contact between the scoria and lava is traced in red, and the other units are mapped according to figure 3.