Understanding of basaltic eruption dynamics and mechanisms : effusive and explosive eruptions in Hawaiʻi
Hawaiʻi is viewed as the “type example” for many fundamental basaltic volcanic phenomena (Garcia, 2017). The extensive geophysical and visual monitoring network maintained by the Hawaiian Volcano Observatory (HVO) (Guffanti et al., 2010) allows for numerous basaltic eruption styles and their product...
| Main Author: | |
|---|---|
| Format: | Thesis |
| Language: | English |
| Published: |
2019
|
| Subjects: | |
| Online Access: | https://eprints.utas.edu.au/33348/ https://eprints.utas.edu.au/33348/1/Holt_whole_thesis.pdf |
| Summary: | Hawaiʻi is viewed as the “type example” for many fundamental basaltic volcanic phenomena (Garcia, 2017). The extensive geophysical and visual monitoring network maintained by the Hawaiian Volcano Observatory (HVO) (Guffanti et al., 2010) allows for numerous basaltic eruption styles and their products to be described in great detail. Hawaiian volcanoes are famous for effusive eruptions and they are mainly built of pahoehoe and a’a lavas. However, two Hawaiian volcanoes, Kīlauea and Mauna Loa, have been the location of numerous examples, both historical and modern, of other low intensity basaltic eruptive styles. These include Hawaiian-style fountaining, persistent lava lake activity and rootless littoral cone eruptions (Heliker et al., 2003; Jaggar and Finch, 1924; Moore and Ault, 1965; Orr, 2008; Patrick et al., 2016). The products of such activity are rarely preserved in the rock record, and yet they are a common and typical part of these volcanoes’ long-term behaviour and provide valuable constraints on fundamental dynamic processes. Three case studies of such ephemeral eruptive activity, each focussed on a different phenomenon, are presented in this thesis: five episodes of Hawaiian-style fountaining in the 1983-present eruption of Puʻu ʻŌʻō on Kīlauea, the 2010-2018 stable Halemaʻumaʻu lava lake at the summit of Kīlauea, and the rootless littoral cones at Puʻu Kī and ʻAuʻau Point on Mauna Loa. These studies provide a better understanding of the full spectrum of activity typical of basaltic volcanoes. Tephra from the early Hawaiian fountaining episodes of the ongoing eruption of Puʻu ʻŌʻō in the East Rift Zone (ERZ) of Kīlauea provides an opportunity to study the vesicle microtextures of pyroclasts erupted from a single vent over a prolonged period of time. The results of microtextural analysis of pyroclasts from five of Puʻu ʻŌʻō’s high (>200 m) Hawaiian fountaining episodes (episodes 32, 37, 40, 44 and 45) erupted during 1985–1986 are reported. The aims of the research are to constrain the parameters that led to large variations in fountain height at Puʻu ʻŌʻo, and the extent to which pyroclast residence times in the fountain modified microtextures. The results confirm the finding of Stovall et al. (2011, 2012) that pyroclasts from a single Hawaiian fountain can vary greatly in texture (from bubbly to foamy) and have vesicle volume densities (N\(^m\)\(_v\)) and vesicle to melt ratios (V\(_G\)/V\(_L\)) that vary by an order of magnitude. Pyroclast microtextures can range from high density (typically >500 kg m\(^{-3}\)), high N\(^m\)\(_v\) (2.2×10\(^6\) to 4.4×10\(^6\)), low V\(_G\)/V\(_L\) (2.06 to 4.65) bubbly textures to low density (<450 kg m\(^{-3}\)), low N\(^m\)\(_v\)(2.5×10\(^5\) to 6.1×10\(^5\)), high V\(_G\)/V\(_L\) (5.49 to 11.05) foamy textures due to extensive growth and coalescence of vesicles within the fountain after fragmentation. Due to the extensive modification by post-fragmentation vesiculation processes pyroclasts that have primary textures remain elusive and, as a result, it is not possible based on N\(^m\)\(_v\) to quantitatively describe the fluid dynamics of the magma in the shallow conduit in such a way that allows us to make detailed comparisons between distinct fountaining episodes at a single vent. These data show good correlation between Δ(V\(_G\)/V\(_L\)) and peak fountain height, which implies that Δ(V\(_G\)/V\(_L\)) can be used as a proxy for peak fountain height in the absence of direct visual observations of the eruption. Two rootless littoral cones, ʻAuʻau Point and Puʻu Kī South Cone, located on the southwestern flank of Mauna Loa, Hawaiʻi have wholly unique cone geometries and cone-forming lithofacies when compared to other examples of historic rootless littoral cone activity elsewhere on Mauna Loa and on Kīlauea. Both cones have, or intitally had, circular geometries and are built from alternating clastogenic lavas and thermally oxidised, slightly welded lapilli and bombs, all of which are deposit features typically associated with ‘dry’ magmatic styles of volcanism. They differ greatly from other examples of rootless littoral activity in Hawaiʻi defined by half-cone deposits consisting predominantly of thinly planar-bedded to cross-bedded, bimodal deposits composed mainly of ash (70-90%) and minor lapilli and bombs. Both ʻAuʻau Point and Puʻu Kī South Cone were formed by prolonged periods of confined mixing (Mattox and Mangan, 1997) of lava and water-saturated substrate driving a cyclic eruption of littoral lava fountaining and bubble burst activity. Despite being formed by confined mixing, these two rootless littoral cones differ from examples in Iceland of rootless cones formed by confined mixing where lavas flowed over water-saturated substrates (Fagents and Thordarson, 2007; Hamilton et al., 2017), highlighting a critical difference in the nature of the molten fuel coolant interactions that drove the rootless eruptions in each case. The identification of rootless cones that have formed from phreatomagmatic activity but have deposits that appear to be of ‘dry’ magmatic in origin (thermally oxidised, variably welded lapilli and bombs), such as Puʻu Kī South Cone and ʻAuʻau Point, casts doubt on the diagnostic features that are used to distinguish between phreatomagmatic and ‘dry’ magmatic activity, and presents a specific challenge for volcanologists working in the ancient rock record. The recent summit eruption of Kīlauea began in 2008 and from 2010 to it's demise in May 2018 consisted of a stable lava lake within the Overlook Crater within Halemaʻumaʻu. The physical and chemical processes that occured at the surface of the lava lake were directly related to processes that occured throughout the entire magmatic system of Kīlauea Volcano. In this study, a combined dataset of high-speed thermal infrared videos and near-infra-red time-lapse sequences is used to define and map the spatial distribution of the passive outgassing processes that took place on the surface of the Halemaʻumaʻu lava lake from the 1st to the 6th of December 2013. Observed outgassing features include both boundary and intra-plate bubble bursts, spot vents and surface doming, all of which are different surface manifestations of buoyant gas slugs impacting on the base of the viscoelastic crust of the lava lake surface. At this time, gas slugs arriving at the lake surface were localised in diffuse ring geometry at a fixed point on the lake surface which was not affected by lateral movement of the surface crust. The lateral movement of the surface crust is commonly attributed to convection of magma at deeper levels in the lake. Therefore, this geometry is consistent with the formation of gas slugs at depths below where the lava is mechanically coupled to the surface flow regime. The gas slugs then likely rose through the lava lake decoupled from internal convection. These observations provide basic, qualitative constraints on the internal processes of the lava lake at Halemaʻumaʻu as well as the foundation for further quantitative studies of the internal convection regimes of stable lava lakes at Kīlauea and elsewhere around the globe. |
|---|