Geology topics

Carl Thornber

Publications and source records attributed to Carl Thornber.

8 recordsLinked to original sources

USGS2023

Dynamics of magma mixing and magma mobilization beneath Mauna Loa – Insights from the 1950 AD Southwest Rift Zone eruption.

Eruptions from Mauna Loa’s Southwest Rift Zone (SWRZ) pose a significant threat to nearby communities due to high eruption rates and steep slopes resulting in little time for evacuation. Despite the large body of research done on Mauna Loa, knowledge of the timing and duration of magma residence and transfer through its internal plumbing system is still poorly constrained. This study presents a first quantitative look at thermochemical conditions and timescales of potentially deep storage and disaggregation of magmatic mush during the run-up to the voluminous 1950 AD SWRZ eruption. Details of heterogeneous compositions and textures of the macrocryst and glomerocryst cargo in 1950 AD lavas suggest magma mixing and crystal recycling along the entire plumbing system. Furthermore, the crystal cargo contains evidence for the direct interaction between primitive, deeply stored magma and pockets of more evolved magma stored at shallow to intermediate depths. An enigmatic attribute of 1950 near-vent lava is the near-ubiquitous presence of subhedral, unreacted Mg-rich orthopyroxene phenocrysts (Mg#>80). Phase relations of Mauna Loa olivine-tholeiite indicate that orthopyroxene joins olivine as a primary phase at pressures higher than 0.6 GPa. Coexisting Mg-rich olivine and orthopyroxene and the occurrence of harzburgitic (olivine-orthopyroxene) glomerocrysts provide evidence for cognate crystallisation at near-Moho (~ 18 km) depths (Thornber and Trusdell 2008 ). Petrogenetically diverse populations of glomerocrysts and macrocrysts alongside evidence of multilevel magma storage indicate a network of ephemeral and possibly interconnected magma pockets from near-Moho depths to the upper/mid-crust. Fe-Mg diffusion chronometry applied to 1950 AD olivine populations implies rapid mobilisation and transport of large volumes of magma (376×10 6 m 3 ) from near-Moho storage to the surface within less than 8 months, with little residence time (~ 2 weeks) in the shallow (3–5 km) plumbing system.

Hawaii↗

USGS2019

Dikes in the Koaʻe fault system, and the Koaʻe-east rift zone structural grain at Kīlauea Volcano, Hawaii

Two small scoria vents were discovered in the Koa‘e fault system, an extensional regime connecting the east and southwest rift zones of Kīlauea that was previously considered to be noneruptive. The chemical composition of the scoria suggests an early to middle nineteenth-century age. The vents prove that magma can intrude several kilometers into the central part of the Koa‘e fault system from the nearest rift zone, supporting previous seismic and geodetic inferences of intrusions into the Koa‘e fault system in the twentieth century. Geodetic studies for the past 50 yr document widening of the Koa‘e fault system at a time-averaged rate of ~4.5 cm/yr, involving mostly coseismic strains, but also creep and displacement related to dike intrusions. These rates are consistent with a longer-term widening rate for the past ~700 yr calculated from crack widths in a lava flow of about that age. The Koa‘e fault system blends into, and is a structural continuation of, the east rift zone. We interpret the locus of intrusion in the east rift zone to have migrated ~6.5 km SE during the past 100,000–125,000 yr, as estimated from linear extrapolation of measured displacement rates across the Koa‘e fault system and east rift zone. The inception of migration is consistent with the onset of the tholeiitic stage at Kīlauea as interpreted by previous studies. As the rift zone moved away from the summit, a marked curvature in the transport pathway developed in order for the rift zone to maintain its connection to the summit magma reservoir. The migration resulted in development of the SE-trending east rift connector, a term we prefer instead of the upper east rift zone. The connector supplies magma to the ENE-trending rift zone from the summit storage complex but is not itself the site of significant magma storage or eruption. The Koa‘e fault system merges into the southwest rift zone, which has been migrating southeastward for an uncertain period of time. Some magma that enters it passes from the summit reservoir complex through the southwest rift connector (seismic southwest rift zone), analogous to the east rift connector. Both connectors reflect the response of magma-transport pathways to asymmetric volcano spreading away from a relatively fixed summit magma reservoir. The ENE structural grain of the Koa‘e fault system and east rift zone pervades Kīlauea’s entire edifice. Most eruptions take place along this trend. The major exception is the southwest rift zone, which may reflect the stresses of Mauna Loa spreading and the Ka‘ōiki fault system. The dominant ENE grain emphasizes the importance of SSE-directed volcano spreading in controlling most of Kīlauea’s tectonic and eruptive behavior.

Hawaii↗

USGS2017

238U–230Th–226Ra–210Pb–210Po disequilibria constraints on magma generation, ascent, and degassing during the ongoing eruption of Kīlauea

The timescales of magma genesis, ascent, storage and degassing at Kīlauea volcano, Hawai‘i are addressed by measuring 238 U-series radionuclide abundances in lava and tephra erupted between 1982 and 2008. Most analyzed samples represent lavas erupted by steady effusion from Pu‘u ‘Ō‘ō and Kūpahianaha from 1983 to 2008. Also included are samples erupted at the summit in April 1982 and March 2008, along the East Rift Zone at the onset of the ongoing eruption in January 1983, and during vent shifting episodes 54 and 56, at Nāpau crater in January 1997, and Kane Nui O Hamo in June 2007. In general, samples have small (∼4%) excesses of ( 230 Th) over ( 238 U) and ∼3 to ∼17% excesses of ( 226 Ra) over ( 230 Th), consistent with melting of a garnet peridotite source at melting rates between 1 × 10 –3 and 5 × 10 –3 kg m –3 a –1 , and melting region porosity between ∼2 and ∼10%, in agreement with previous studies of the ongoing eruption and historical eruptions. A small subset of samples has near-equilibrium ( 230 Th/ 238 U) values, and thus were generated at higher melting rates. Based on U–Th–Ra disequilibria and Th isotopic data from this and earlier studies, melting processes and sources have been relatively stable over at least the past two centuries or more, including during the ongoing unusually long (>30 years) and voluminous (4 km 3 ) eruption. Lavas recently erupted from the East Rift Zone have average initial ( 210 Pb/ 226 Ra) values of 0·80 ± 0·11 (1σ), which we interpret to be the result of partitioning of 222 Rn into a persistently generated CO 2 -rich gas phase over a minimum of 8 years. This ( 210 Pb) deficit implies an average magma ascent rate of ≤3·7 km a –1 from ∼30 km depth to the surface. Spatter and lava associated with vent-opening episodes erupt with variable ( 210 Pb) deficits ranging from 0·7 to near-equilibrium values in some samples. The samples with near-equilibrium ( 210 Pb/ 226 Ra) are typically more differentiated, suggesting decadal timescales of magma storage in shallow conduits or reservoirs that were not degassing. Lava and spatter samples erupted in the East Rift Zone and at the summit had ( 210 Po) ∼0 at the time of eruption, which results from efficient partitioning of Po into the CO 2 - and SO 2 -rich gas phases during and prior to eruption. Summit ash and Pele’s hair samples from 2008 differ from lava and lapilli samples in that they have elevated initial ( 210 Po), ( 210 Pb/ 226 Ra), and Pb concentrations because of Po condensation on tephra particles, and incorporation of fumarolic Po and Pb into erupted tephra fragments during quenching.

Hawaii↗

USGS2016

Timing and composition of continental volcanism at Harrat Hutaymah, western Saudi Arabia

Harrat Hutaymah is an alkali basalt volcanic field in north-central Saudi Arabia, at the eastern margin of a large Neogene continental, intraplate magmatic province. Lava flow, tephra and spatter cone compositions in the field include alkali olivine basalts and basanites. These compositions contrast with the predominantly tholeiitic, fissure-fed basalts found along the eastern margin of the Red Sea. The Hutaymah lava flows were erupted through Proterozoic arc-associated plutonic and meta-sedimentary rocks of the Arabian shield, and commonly contain a range of sub-continental lithospheric xenoliths, although the lavas themselves show little indication of crustal contamination. Previous radiometric dating of this volcanic field (a single published K–Ar age; 1.8 Ma) is suspiciously old given the field measurement of normal magnetic polarity only (i.e. Brunhes interval, ≤ 780 Ka). We report new age determinations on 14 lava flows by the 40Ar–39Ar laser step heating method, all younger than ~ 850 Ka, to better constrain the time frame of volcanism, and major, trace and rare earth element compositions to describe the chemical variation of volcanic activity at Harrat Hutaymah. Crystal fractionation was dominated by olivine ± clinopyroxene at a range of upper mantle and crustal pressures. Rapid ascent and eruption of magma is indicated by the array of lower crustal and lithospheric xenoliths observed in lava flows and tephra. Modeling suggests 1–7% melting of an enriched asthenospheric mantle source occurred beneath Harrat Hutaymah under a relatively thick lithospheric cap (60–80 km).

Harrat Hutaymah↗

USGS2016

Asthenosphere–lithosphere interactions in Western Saudi Arabia: Inferences from 3 He/ 4 He in xenoliths and lava flows from Harrat Hutaymah

Extensive volcanic fields on the western Arabian Plate have erupted intermittently over the last 30 Ma following emplacement of the Afar flood basalts in Ethiopia. In an effort to better understand the origin of this volcanism in western Saudi Arabia, we analyzed 3 He/ 4 He, and He, CO 2 and trace element concentrations in minerals separated from xenoliths and lava flows from Harrat Hutaymah, supplemented with reconnaissance He isotope data from several other volcanic fields (Harrat Al Birk, Harrat Al Kishb and Harrat Ithnayn). Harrat Hutaymah is young (< 850 ka) and the northeasternmost of the volcanic fields. There is a remarkable homogeneity of 3 He/ 4 He trapped within most xenoliths, with a weighted mean of 7.54 ± 0.03 R A (2σ, n = 20). This homogeneity occurs over at least eight different xenolith types (including spinel lherzolite, amphibole clinopyroxenite, olivine websterite, clinopyroxenite and garnet websterite), and encompasses ten different volcanic centers within an area of ~ 2500 km 2 . The homogeneity is caused by volatile equilibration between the xenoliths and fluids derived from their host magma, as fluid inclusions are annealed during the infiltration of vapor-saturated magmas along crystalline grain boundaries. The notable exceptions are the anhydrous spinel lherzolites, which have a lower weighted mean 3 He/ 4 He of 6.8 ± 0.3 R A (2σ, n = 2), contain lower concentrations of trapped He, and have a distinctly depleted light rare earth element signature. 3 He/ 4 He values of ~ 6.8 R A are also commonly found in spinel lherzolites from harrats Ithnayn, Al Birk, and from Zabargad Island in the Red Sea. Olivine from non-xenolith-bearing lava flows at Hutaymah spans the He isotope range of the xenoliths. The lower 3 He/ 4 He in the anhydrous spinel lherzolites appears to be tied to remnant Proterozoic lithosphere prior to metasomatic fluid overprinting. Elevated 3 He/ 4 He in the western harrats has been observed only at Rahat (up to 11.8 R A ; Murcia et al., 2013), a volcanic field situated above thinned lithosphere beneath the Makkah-Medinah-Nafud volcanic lineament. Previous work established that spinel lherzolites at Hutaymah are sourced near the lithosphere-asthenosphere boundary (LAB), while other xenolith types there are derived from shallower depths within the lithosphere itself (Thornber, 1992). Helium isotopes are consistent with melts originating near the LAB beneath many of the Arabian harrats, and any magma derived from the Afar mantle plume currently appears to be of minor importance.

LITHOS↗

USGS2011

New episodes of volcanism at Kilauea Volcano, Hawaii

Mid‐2007 was a time of intense activity at Kilauea Volcano, Hawaii (see Figure 1). In June, the long‐lived Pu'u 'Ō'ō—Kupaianaha eruption, a dual‐vent system along the east rift zone (ERZ) that has been erupting since 1983 [Heliker et al., 2003], paused due to the outbreak of a new vent farther up the rift (see Figure 2). The Pu'u 'Ō'ō vent collapsed following that activity, and the resulting reorganization of the magma plumbing system led to the formation of a second new eruptive vent 2 kilometers downrift of Pu'u 'Ō'ō. These events were well documented by geological, geophysical, and geochemical monitoring. This article summarizes results from these monitoring efforts and interprets the changes that have occurred at Kilauea since June 2007.

Hawaii↗

USGS2007

Vapor transfer prior to the October 2004 eruption of Mount St. Helens, Washington

Dome lavas from the 2004 eruption of Mount St. Helens show elevated Li contents in plagioclase phenocrysts at the onset of dome growth in October 2004. These cannot be explained by variations in plagioclase-melt partitioning, but require elevated Li contents in coexisting melt, a fact confirmed by measurements of Li contents as high as 207 µg/g in coexisting melt inclusions. Similar Li enrichment has been observed in material erupted prior to and during the climactic May 1980 eruption, and is likewise best explained via pre-eruptive transfer of an exsolved alkali-rich vapor phase derived from deeper within the magma transport system. Unlike 1980, however, high Li samples from 2004 show no evidence of excess ( 210 Pb)/( 226 Ra), implying that measurable Li enrichments may occur despite significant differences in the timing and/or extent of magmatic degassing. Diffusion modeling shows that Li enrichment occurred within ∼1 yr before eruption, and that magma remained Li enriched until immediately before eruption and cooling. This short flux time and the very high Li contents in ash produced by phreatomagmatic activity prior to the onset of dome extrusion suggest that vapor transfer and accumulation were associated with initiation of the current eruption. Overall, observation of a high Li signature in both 1980 and 2004 dacites indicates that Li enrichment may be a relatively common phenomenon, and may prove useful for petrologic monitoring of Mount St. Helens and other silicic volcanoes. Lithium diffusion is also sufficiently rapid to constrain vapor transfer on similar time scales to short-lived radionuclides.

Geology↗

USGS2004

What makes hydromagmatic eruptions violent? Some insights from the Keanakāko'i Ash, Kı̄lauea Volcano, Hawai'i

Volcanic eruptions at the summit of Kilauea Volcano, Hawai'i, are of two dramatically contrasting types: (1) benign lava flows and lava fountains; and (2) violent, mostly prehistoric eruptions that dispersed tephra over hundreds of square kilometers. The violence of the latter eruptions has been attributed to mixing of water and magma within a wet summit caldera; however, magma injection into water at other volcanoes does not consistently produce widespread tephras. To identify other factors that may have contributed to the violence of these eruptions, we sampled tephra from the Keanakako'i Ash, the most recent large hydromagmatic deposit, and measured vesicularity, bubble-number density and dissolved volatile content of juvenile matrix glass to constrain magma ascent rate and degree of degassing at the time of quenching. Bubble-number densities (9X10 (super 4) -1X10 (super 7) cm (super -3) ) of tephra fragments exceed those of most historically erupted Kilauean tephras (3X10 (super 3) -1.8X10 (super 5) cm (super -3) ), and suggest exceptionally high magma effusion rates. Dissolved sulfur (average=330 ppm) and water (0.15-0.45 wt.%) concentrations exceed equilibrium-saturation values at 1 atm pressure (100-150 ppm and approximately 0.09%, respectively), suggesting that clasts quenched before equilibrating to atmospheric pressure. We interpret these results to suggest rapid magma injection into a wet crater, perhaps similar to continuous-uprush jets at Surtsey. Estimates of Reynolds number suggest that the erupting magma was turbulent and would have mixed with surrounding water in vortices ranging downward in size to centimeters. Such fine-scale mixing would have ensured rapid heat exchange and extensive magma fragmentation, maximizing the violence of these eruptions.

Hawaii↗