Field work

4.3 Sample preparation and analysis ... 31

4.1 Introduction

The reconstruction of the eruptive processes at Maungataketake, Motukorea, and North Head volcanoes was based on studying the characteristics of the pyroclastic deposits. Conventional methodology was followed, including field work together with dry sieving, binocular, petrographic, scanning electron microscope (SEM) work, as well as energy-dispersive spectrometry (EDS). In addition, selected juvenile pyroclasts from Maungataketake and North Head were analyzed via electron microprobe and Maungataketake fine-ash was analyzed by X-ray diffraction. Data obtained via these methods was combined with information on AVF paleo-environment, geology, and hydrogeology to reconstruct the eruption dynamics and history of each studied volcano, and to gain insight into the role of the substrate and environment on phreatomagmatic processes.

4.2 Field work

The general guidelines for field work carried out in this study were mainly taken from Sohn and Chough (1989; 1992; 1993) and Chough and Sohn (1990). These authors developed a system for describing and grouping the stratigraphical and sedimentary characteristics of ejecta ring deposits of phreatomagmatic volcanoes in Jeju Island, South Korea. There, deposits are well exposed from proximal to distal locations along axes perpendicular to the crater rim. Lateral and vertical changes in sedimentary structures, bedding and lamination characteristics, thicknesses of deposits, grading, sorting, vertical unconformities, grain size and componentry are exhibited throughout. The authors note that any identified variation in these characteristics may provide clues into the changing mode of transport and the sedimentation and deposition rates of pyroclasts within specific pyroclastic units from proximal to distal locations. Through detailed characterization, each unit can be compared with the overlying and underlying units to recognize vertical and lateral variations. In this way, the dynamics in the construction of the volcano can be interpreted, as well as the changes in eruptive styles. Unfortunately, such extensive outcrops are not found in the AVF. However, to a certain extent lateral and vertical variations are shown in Maungataketake in a cliff that is roughly perpendicular to the crater rim (chapter 5). Except for one outcrop exhibited over a short distance (tens of meters), lateral variations from proximal to distal locations are not exposed at Motukorea, rather deposits were examined along a cliff that runs sub-perpendicular to the direct radial

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travel path from the crater. At North Head, the exposures are on a line roughly parallel to the cone rim. Therefore in the last two cases only vertical variations were recorded.

The morphological features of the phreatomagmatic edifices were identified in the field and with the aid of aerial photographs, Google Earth satellite images, and a shaded relief model of the LiDAR derived Digital Surface Model.

Layers of <1 cm-thickness are termed laminae, while beds comprise units >1 cm-thick (following Ingram, 1954). Particles were grouped using the pyroclast size terms of Sohn and Chough (1989), which is a subdivided version of the widely accepted classification of pyroclast fragments by Schmid (1981). Unconsolidated volcaniclastic particles are termed ash or lapill, whereas consolidated equivalents are

termed tuff if composed of ash-size-pyroclasts, or lapilli-tuff if composed dominantly of lapilli-sized

pyroclasts (after Schmid, 1981). Lapilli-tuff also indicates a consolidated, poorly-sorted mixture of both lapilli and ash-sized grains. Field observations at a macroscopic scale, the significant vertical changes in juvenile grain size and distribution, and the dominant sedimentary structures, allowed the grouping of beds into “units”, which are composed of a single or multiple beds (with internal laminae or not). At Maungataketake (chapter 5) and Motukorea (chapter 6), distribution of juvenile clasts played an important role in definition of units, since lithic grain size and distribution was relatively uniform throughout the entire sequences. In these cases most lithics were in the ash grade, apart from the relatively scarce presence of coarse lithics (lapilli to blocks). Therefore the horizons (either laminae or beds) containing juvenile fragments (usually dark, coarse ash to lapilli) were used as indicators of changes in pyroclast distribution and many sedimentary characteristics were able to be characterized on this basis. North Head is mostly composed of juvenile lapilli whose characteristics (size and distribution) allowed the definition of units. Once these units were identified, key representative locations were selected for detailed description. At each key site the following tasks were carried out: recognition and description of sedimentary structures and bedforms; identification of vertical changes, as well as lateral ones, and distinguishing of boundary types and grading; measurements of units, sedimentary structures, and bedforms; visual estimations of grain size and componentry; identification of juvenile and lithic morphology and their proportions. Sampling of pyroclastic deposits was done where possible. Usually, the samples were collected from spots that appeared to represent the grain size distribution of an entire unit at a specific location. In very poorly sorted tephra ring deposits, where changes in grain size and sedimentary characteristics occur over short vertical and lateral distances (even on a decimeter scale), representative sampling of phreatomagmatic sequences can be difficult to achieve. Since segments of units or the entire unit are usually entirely characterized by either tuff or lapilli-tuff, sampling was done specifically on one of these two types of deposit independently from the presence of minor associations of “anomalous” grain size distributions (e.g. juvenile-lapilli trains or isolated pockets composed of coarser particles). Single lithic or juvenile fragments were also sampled for further observations. The total number of samples (including tuff or lapilli

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tuff, lithics, and juvenile fragments) for each volcano is 46 from Maungataketake, 26 from Motukorea and 39 from North Head. Except for Motukorea, which is an island in the Harbour (Fig. 3.1), the access to the other two volcanoes allowed several visits to check and cross-check the information gathered in previous visits.

The term lithofacies was adapted (from Sonh and Chough, 1989) and lithofacies and their associations were recognized when possible. Each ltihofacies and its nomenclature was described on a case by case basis, generally being characterized by a sedimentary bedform(s) or structure(s) that is relatively widespread within a single unit or a number of units. Conversely, the sporadic presence of certain sedimentary structures were not considered as separate lithofacies or taken into account for interpretative aspects. Lithofacies are typically labelled in the figures; their characteristics, descriptions and interpretations are summarised in tables.

The criteria used to define the different key sites, units, subunits and sequences for each volcano is described in each corresponding chapter: Maungataketake (chapter 5), Motukorea (chapter 6), North Head (chapter 7).