Geology topics

J. McPhie

Publications and source records attributed to J. McPhie.

3 recordsLinked to original sources

USGS2019

SUSTAIN drilling at Surtsey volcano, Iceland, tracks hydrothermal and microbiological interactions in basalt 50 years after eruption

The 2017 Surtsey Underwater volcanic System for Thermophiles, Alteration processes and INnovative concretes (SUSTAIN) drilling project at Surtsey volcano, sponsored in part by the International Continental Scientific Drilling Program (ICDP), provides precise observations of the hydrothermal, geochemical, geomagnetic, and microbiological changes that have occurred in basaltic tephra and minor intrusions since explosive and effusive eruptions produced the oceanic island in 1963–1967. Two vertically cored boreholes, to 152 and 192 m below the surface, were drilled using filtered, UV-sterilized seawater circulating fluid to minimize microbial contamination. These cores parallel a 181 m core drilled in 1979. Introductory investigations indicate changes in material properties and whole-rock compositions over the past 38 years. A Surtsey subsurface observatory installed to 181 m in one vertical borehole holds incubation experiments that monitor in situ mineralogical and microbial alteration processes at 25–124 ∘C. A third cored borehole, inclined 55∘ in a 264∘ azimuthal direction to 354 m measured depth, provides further insights into eruption processes, including the presence of a diatreme that extends at least 100 m into the seafloor beneath the Surtur crater. The SUSTAIN project provides the first time-lapse drilling record into a very young oceanic basaltic volcano over a range of temperatures, 25–141 ∘C from 1979 to 2017, and subaerial and submarine hydrothermal fluid compositions. Rigorous procedures undertaken during the drilling operation protected the sensitive environment of the Surtsey Natural Preserve.

Scientific Drilling↗

USGS2019

Eruption and fountaining dynamics of selected 1985–1986 high fountaining episodes at Kīlauea volcano, Hawai'i, from quantitative vesicle microtexture analysis

Tephra from the early Hawaiian fountaining episodes of the ongoing eruption of Pu'u 'Ō'ō 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. We report the results of microtextural analysis of pyroclasts from five of Pu'u 'Ō'ō's high (>200 m) Hawaiian fountaining episodes (episodes 32, 37, 40, 44 and 45) erupted during 1985–1986. This analysis was carried out to constrain the parameters that led to large variations in fountain height at Pu'u 'Ō'o, and the extent to which pyroclast residence times in the fountain modified microtextures. Our results confirm the finding of Stovall et al., 2011 , Stovall et al., 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. This range in vesicle texture and population is due to extensive growth and coalescence of vesicles within the fountain after fragmentation. Only one pyroclast from four of five episodes was found to have textures interpreted as indicative of the vesicle population near the moment of fragmentation: bubbly texture, high density (typically >500 kg m −3 ), high N m v (2.2 × 10 6 to 4.4 × 10 6 ), and low V G /V L of 2.06 to 4.65. We demonstrate a linear correlation between Δ(V G /V L ) and peak fountain height across a range of Hawaiian fountains from Kilauea. This correlation could be used to infer peak heights of unobserved Hawaiian fountaining eruptions after further testing using well-recorded events.

Hawai'i↗

USGS1990

Phreatomagmatic and phreatic fall and surge deposits from explosions at Kilauea volcano, Hawaii, 1790 a.d.: Keanakakoi Ash Member

In or around 1790 a.d. an explosive eruption took place in the summit caldera of Kilauea shield volcano. A group of Hawaiian warriors close to the caldera at the time were killed by the effects of the explosions. The stratigraphy of pyroclastic deposits surrounding Kilauea (i.e., the Keanakakoi Ash Member) suggests that the explosions referred to in the historic record were the culmination of a prolonged hydrovolcanic eruption consisting of three main phases. The first phase was phreatomagmatic and generated well-bedded, fine fallout ash rich in glassy, variably vesiculated, juvenile magmatic and dense, lithic pyroclasts. The ash was mainly dispersed to the southwest of the caldera by the northeasterly trade winds. The second phase produced a Strombolian-style scoria fall deposit followed by phreatomagmatic ash similar to that of the first phase, though richer in accretionary lapilli and lithics. The third and culminating phase was phreatic and deposited lithic-rich lapilli and block fall layers, interbedded with cross-bedded surge deposits, and accretionary lapilli-rich, fine ash beds. These final explosions may have been responsible for the deaths of the warriors. The three phases were separated by quiescent spells during which the primary deposits were eroded and transported downwind in dunes migrating southwestward and locally excavated by fluvial runoff close to the rim. The entire hydrovolcanic eruption may have lasted for weeks or perhaps months. At around the same time, lava erupted from Kilauea's East Rift Zone and probably drained magma from the summit storage. The earliest descriptions of Kilauea (30 years after the Keanakakoi eruption) emphasize the great depth of the floor (300-500 m below the rim) and the presence of stepped ledges. It is therefore likely that the Keanakakoi explosions were deepseated within Kilauea, and that the vent rim was substantially lower than the caldera rim. The change from phreatomagmatic to phreatic phases may reflect the progressive degassing and cooling of the magma during deep withdrawal: throughout the phreatomagmatic phases magma vesiculation contributed to the explosive interaction with water by initiating the fragmentation process: thereafter, the principal role of the subsiding magma column was to supply heat for steam production that drove the phreatic explosions of the final phase. ?? 1990 Springer-Verlag.

Bulletin of Volcanology↗