Investigating Volcanic Reconstruction using Akaroa Lava Benches

Transcription

1 Investigating Volcanic Reconstruction using Akaroa Lava Benches Jessie Bersson 1,2, Sam Hampton 1 1 University of Canterbury, Christchurch, New Zealand, Department of Geological Sciences 2 Whitman College, Walla Walla WA, Department of Geology Key Words: lava bench, planeze, volcanic reconstruction, Akaroa Abstract Volcanic reconstruction is highly valuable for interpreting volcanic and erosive processes of eroded edifices, but reconstruction of the Akaroa Volcanic Complex (AVC) on Banks Peninsula is lacking. Lava benches and planezes may be used as markers of paleo-topography in the reconstruction of the AVC by projecting these remnant surfaces to produce linear and exponential profiles, similar to methods used by Karátson et al. (2016) in their reconstruction of the Fataga and Roque Nublo volcanoes. A methodology was developed using lava benches and planezes to produce a reconstruction of the AVC assuming conical symmetry. Variable summit estimations from these projections may support the multiple eruptive center theory (Hampton, 2009; Hobbs, 2012) rather than a singular summit. Introduction Volcanic geopmorphic signatures and erosional landscapes features are key parameters in the reconstruction of volcanic edifices. Reconstruction of the Akaroa Volcanic complex is lacking, but some geoscientists have looked to topographic surfaces, called lava benches, as a potential tool for volcanic reconstruction. Lava benches are planar surfaces that can be correlated throughout and between the dramatic ridge and valley topography of Banks Peninsula. They represent periods of inactivity and erosion between distinct effusive volcanic phases of the Akaroa Volcanic complex (AVC). These bench surfaces have been recognized by geoscientists studying Bank Peninsula (Hampton 2009, Barefoot 2015) and preliminarily mapped by Worthington (2016). Lava benches have been researched in conjunction with planezes,

2 triangular features preserving a portion of the volcanic flank bound by drainage paths, and both serve as paleo-topographic markers. Lava benches are a new concept in volcanic geomorphology with growing relevance and primarily studied on Banks Peninsula. Karátson et al. (2016) has successfully identified, extracted and analyzed planezes and quasi-planar surfaces, features comparable to lava benches, to reconstruct phases of volcanic growth. Lava benches are a new volcanic geomorphological concept with growing potential, especially in Banks Peninsula. Lava benches and planezes have great potential as primary volcanic features for marking paleo-topography in volcanic reconstruction. Volcanic reconstruction is greatly valuable, especially with highly eroded edifices such as Banks Peninsula, as it provides insight into the structural, volcanic, and erosional evolution of the volcano. While lava benches are primarily recognized in Banks Peninsula at this time, the presence of similar features in the Canary Islands (Karátson et al., 2016) suggests these features may be found on eroded volcanic edifices elsewhere, making these features ever more relevant. This project will use elevation profiles from DEM images to investigate the significance of lava bench surfaces within the AVC, providing insight into the volcanic and geomorphic processes on Banks Peninsula. Building off of previous research (Hampton 2009, Barefoot 2015, Worthington 2016) this project will produce a preliminary volcanic reconstruction using bench surfaces and planezes as markers of paleo-topography. Geologic Setting Banks Peninsula on the eastern coast of the South Island of New Zealand is primarily composed of deposits from two prominent Miocene volcanoes, Lyttelton Volcano and Akaroa Volcano. Lyttelton and Akaroa volcanism was prompted by intraplate volcanism occurring throughout the Cenozoic. (Timm et al., 2009) Eastern Banks Peninsula is primarily composed of deposits from the Akaroa Volcanic Complex (AVC), active between 9.4 and 8.6 Ma. (Timm et al., 2009) Banks Peninsula is a highly eroded volcanic edifice, researchers have used primary erosive features to try and understand how the landscape has evolved. Hampton and Cole (2009) used the orientations of ridges, valleys, and dikes to locate eruptive centers and support a multiple vent model for the Lyttelton Volcanic Complex. Hobbs (2012) used similar methods to suggest Akaroa also has multiple eruptive centers. Hampton and Cole (2009) further suggest wide

3 valleys and prominent bays, such as Okains bay, represent areas of heavy drainage between eruptive centers. Methods The complex nature of Akaroa s volcanic and erosive processes provides an excellent basis for the development of volcanic reconstruction methodology. Past studies have looked to geomorphic features on Banks Peninsula to gain insight into the processes that have shaped this dramatic landscape (Sumner, 2014; Worthington, 2016). This study aims to refine previous methodologies, building off of existing understandings of lava benches to gain further insight into the significance of these features. Using Digital Elevation Model (DEM) data, this study provides a new perspective on lava benches as markers of paleo-topography of the AVC. The methods of projection and reconstruction used in this research are largely modeled after the methodology used by Karátson et al. (2016) in their reconstruction of paleovolcanoes on Gran Canaria, Canary Islands using quasi-planar surfaces (QPS) and planezes as topographic markers. Karátson et al. define QPS features as scattered paleosurfaces, comparable to lava benches present in Akaroa. The highly eroded edifices of the Fataga volcano and Roque Nublo volcanoes present similar challenges as the AVC for volcanic reconstruction. Research was conducted in three phases: Extraction and Analysis, Classification and Interpretation of topographic markers, and reconstruction of the paleo topography using lava bench projections. Extraction and Analysis Transects were drawn from the central high point of Onawe Peninsula in Akaroa Harbour, radiating out through the Northeastern section of Banks Peninusla (Figure 2). This central point was chosen based on its central location to the now eroded construct of the Akaroa Volcanic Complex. Thirty-one transects were drawn, separated into sections A, B, C, and D (Figure 1). Each section is separated by a large bay, as previous research (Hampton, 2009; Hobbs, 2012) suggest prominent bays may indicate the conjunction of two drainage systems from two separate

4 vents. By dividing transects into sections, the elevation profiles could be used to provide further insight into the multiple vent theory posed by Hampton (2009) and Hobbs (2012). Elevation data was extracted from ArcGIS and imported into Excel, were elevation profiles were constructed using a 1:1 vertical to horizontal ratio to produce a proportionate representation of the topography. Classification and Interpretation Elevation profiles were imported into CorelDraw version and examined individually (Figure 4) and by section (Figure 5), with overlying profiles, to provide isolated and contextualized views of the topography. Individual elevation profiles were interpreted first. Topographic markers, lava benches and planezes, were identified by slope angle, direction, and continuity, and then correlated with visual analysis. Slopes between 0 and 20 dipping away from Akaroa Harbour were isolated, based on values Karátson (2016) posed for the classification of quasi-planar surfaces, features comparable to lava benches. Higher slopes, up to 25, may be considered near the summit. Slope direction must also be considered, as it should reflect a central summit, which has been defined as the location of Onawe Peninsula. Surfaces must be planar, showing some continuity of slope, as they must reflect the generally flat surfaces of lava flows. These surfaces were then correlated with visual analysis of the field sites. GoogleEarth and photos from Banks Peninsula were used to ensure the surfaces isolated within the profile elevations truly reflect topographic markers. Projection and Reconstruction Topographic markers identified in the individual elevation profiles were correlated within the profile and connected with a linear projection and a separate exponential projection. These projections represent two interpretations of the original edifice, assuming conical symmetry. Lines produced reflect slope and slope direction of identified topographic markers. Projections were then incorporated into the section elevation profiles to correlate projected lines within each sector of northeastern Banks Peninsula. Projections of similar elevation and slope were grouped into lava bench formations, each representing different growth stages of the volcano.

5 Lava bench formations were then projected compared between the different sections. The Lava bench formations were then projected upslope to Onawe Peninsula, the estimated summit. This projection, collaborated with the projections of each section produced an estimation of the height of the summit of the Akaroa Volcanic Complex. Volcanic reconstructions of each sector will provide a platform to better understand the significance and potential of lava benches and planezes. Results Bench projections Projected lava benches create a cross-sectional view illustration of the effusive growth stages of the Akaroa Volcanic Complex (Figure 6). Each lava bench projection represents the surface of a volcanic growth formation. These volcanic growth formations represent at least one distinct period of effusive activity, with the top volcanic growth formation being the youngest stage of growth. The top volcanic bench projection is the most valuable for volcanic reconstruction, as it signifies the youngest, highest paleo-topography. Summit Estimations The minimum summit elevation may be estimated from the top lava bench projection (Figure 7). The summit location, arbitrarily defined as central Onawe Peninsula, is the left-most primary data point on each topographic profile. Linear projections are largely extensions of topographic markers. Exponential projections reflect increasing slope approaching the eruptive center. Discussion Volcanic reconstruction of the AVC is complicated by the contested eruptive history and shape of Akaroa. Recent studies (Hampton, 2009; Hobbs 2012) suggest the AVC had multiple eruptive centers, thus multiple eruptive peaks rather than a singular summit. The projected summit elevations (Table 1) in this research assume a conical shape, using Onawe Peninsula as an arbitrary center. The variability of the data raises the question why do we have varying topographic highs with different sectors? With a conical volcano we would expect more of a correlation between the projected summit elevations. This variability may support the multiple

6 eruptive center theory, as the inconsistent summit projections may represent the multiple eruptive peaks. Furthermore, do these height differences represent stages of greater volcanic growth, given new eruptive centers may be building off of existing topography, thus younger eruptive centers will have higher elevations? The projected summit elevation of sector D is significantly higher than sectors A, B, or C. Does this inconsistency reflect the heightened topography of a younger eruptive center? Conclusion This research aims to develop a methodology for the reconstruction of the Akaroa Volcanic Complex using topographic signatures, lava benches and planezes, showing the potential lava benches have for providing insight into volcanic and erosive processes. Topographic markers provide bases for volcanic reconstruction and summit estimations, which can be interpreted to better understand the volcanic shape. Variable summit estimations of the AVC may support the multiple eruptive center model, suggested by Hampton (2009) and Hobbs (2012). Further research may geochemically analyze lava bench surfaces or investigate the relationship between topography and age of eruptive center. This research provides a simplistic model for volcanic reconstruction that may be used as a basis for further interpretation and research.

7 Figures Figure 1. The Northeastern section of the Akaroa Volcanic Complex was studied, with data analyzed by sector. Sectors are separated by prominent bays Little Akaloa, Okains Bay, and Le Bons Bay.

8 Figure 2. Transects were drawn from central Akaroa Volcanic Complex, Onawe Penninsula, radiating out across the Northeastern section of Banks Peninsula.

9 Figure 3. Flow chart detailing the progression of methods used in this research.

10 Figure 4. Individual elevation profiles from section A. All transects originate from Onawe Peninsula.

11 Figure 5. Overlapping elevation profiles grouped by sector A, B, C, and D. All transects originate from central Akaroa Volcanic Compex, Onawe Peninsula and extend out to the ocean. Figure 6. Each colored section represent a Volcanic Growth Formation. Volcanic growth formations, separated by lava bench projections, represent effusive periods of volcanic growth. Each volcanic growth formation may be composed of multiple periods of effusive activity. The top volcanic growth formation represents the youngest growth of Akaroa.

12 Figure 7. Elevation profiles displaying the highest linear and exponential projections from each sector are shown. Projections of each elevation profile are comparable to at least one other elevation profile within that sector to ensure validity of summit elevation estimation.

13 Table 1. Akaroa central summit projected elevations. References Chiba, T., Kaneta, S., Suzuki, Y. (2008). Red Relief Image Map: New Visualization Method for Three Dimensional Data. The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, 37. Cotton, C.A. (1944). Volcanoes As Landscape Forms. Christchurch, New Zealand: Whitcombe & Tombs limited. Irvine, S. (2015). Contextualizng the View Hill Scoria Cone, Akaroa Volcanic Complex. Karátson, D., Yepes, J., Rodríguez-Peces, M., & Fornaciai, A. (2016). Reconstructing eroded paleovolcanoes on Gran Canaria, Canary Islands, using advanced geomorphometry. Geomorphology, 253.

14 Hampton, S.J. (2010). Growth, Structure and Evolution of the Lyttelton Volcanic Complex, Banks Peninsula, New Zealand. University of Canterbury. Hampton, S.J., Cole, J.W. (2009). Lyttelton Volcano, Banks Peninsula, New Zealand: Primary volcanic landforms and eruptive centre identification. Geomorphology, 104 (3-4), Hobbs, D.J. (2012). Remote Sensing Geomorphology of Akaroa Volcano and Detailed Mapping of Okains Bay, Banks Peninsula, NZ. Sumner, M. (2014). Correlation of Volcanic Benches and Implication on the Development of the Akaroa Volcanic Complex. Timm, C., Hoernle, K., Van Den Bogaard, P., Bindeman, I., Weaver, S. (2009). Geochemical Evolution of Intraplate Volcanism at Banks Peninsula, New Zealand: Interaction Between Asthenospheric and Lithospheric Melts. Journal of Petrology, 50(6), Worthington, B. (2016). Investigating Controls on Erosional Patterns within a Volcanic Complex.