Geology topics

Nathan Lee Andersen

Publications and source records attributed to Nathan Lee Andersen.

4 recordsLinked to original sources

USGS2026

The magmatic-hydrothermal system of the Three Sisters volcanic cluster, Oregon, imaged from field gravity measurements

From 2019 to 2024, gravity surveys were conducted at the Three Sisters volcanic cluster (TSVC), measuring 246 gravity sites using a spring relative gravimeter. We calculated the residual Bouguer anomaly and identified three main zones with negative anomalies, ranging from −4 to −8 mGal, located southwest and west of South Sister, within an area that has been uplifting for the past two decades. After inversion, we obtain a 3D density model of the subsurface and identify low-density bodies extending from the surface down to 3 km. We estimate a total of 15 km 3 of crustal bodies with density close to 2 g/cm 3 that could store up to ~5 km 3 of water, forming an extensive hydrothermal system beneath the TSVC. We explore the possible combinations of melt compositions and temperatures that could create a bulk density close to our reference crustal density (2.5 g/cm 3 ) using MELTS thermodynamic simulations. Our results indicate that a magmatic mush with as little as 15% partial melt of bulk rhyolitic composition or as much as 52%–57% partial melt of a bulk dacitic composition could be stored in a magmatic system under TSVC without generating a detectable gravity anomaly. Episodic magma injections at the base of the magmatic system, such as the 1998–2000 intrusion at ~6 km depth, would bring heat and gas to the hydrothermal system while maintaining a low melt fraction in the magmatic mush, as imaged at other Cascade volcanoes.

Oregon↗

USGS2025

Utilizing crystal records to determine the underlying architecture and timescales of magmatic processes during the 2022 Mauna Loa eruption

Details of Mauna Loa’s plumbing system and magma storage dynamics are relatively poorly constrained, relative to nearby Kīlauea volcano. Recent studies have begun to address this gap in knowledge using geophysical imaging or olivine-focused petrology. Here, we use the crystal cargo from the 2022 summit and Northeast Rift Zone eruption to advance our understanding of Mauna Loa’s storage regions and pre˗eruptive processes. We identify textural and geochemical populations of olivine, orthopyroxene, clinopyroxene, and plagioclase macrocrysts as well as evolved glomerocrysts consisting of orthopyroxene, clinopyroxene, and plagioclase. Summit versus rift zone trends can be distinguished between specific crystal populations and are used to infer magma transfer pathways. Melt inclusion barometry from three orthopyroxene populations indicate crystallization depths of ~2–3 km which coincide with previous olivine˗hosted fluid inclusion barometry across Mauna Loa’s recent 200˗yr eruptive history. We combine orthopyroxene˗hosted melt inclusion barometry with diffusion chronometry on the same crystals using Fe˗Mg interdiffusion to estimate timescales of mixing and recharge prior to the eruption. Summit orthopyroxene yield timescales of weeks to months, aligning with geophysically recognized pre˗eruptive intrusion events and previously determined olivine timescales. Northeast Rift Zone orthopyroxene crystals have resorbed cores and yield short pre˗eruptive mixing timescales on the order of hours to days. Chemistry and timescales from the orthopyroxene populations suggest some of the 2022 magmas may have bypassed the shallowest portions of the summit reservoir system and were transferred directly into the rift zone syn˗eruptively.

Hawaii↗

USGS2025

The transition from melt accumulation to eruption initiation recorded by orthopyroxene Fe-Mg diffusion timescales in late Holocene rhyolites, South Sister volcano, Oregon Cascade Range

South Sister volcano, Oregon Cascade Range, USA, has repeatedly erupted rhyolite since ca. 40 ka. The youngest such eruptions are the ca. 2 ka Rock Mesa and Devils Chain rhyolites, erupted several hundred years apart from two multi-vent complexes separated by 3–6 km. Fe-Mg interdiffusion models of orthopyroxene rims from both rhyolites produce timescales up to several-thousand years, but dominantly decades-to-centuries. Notably, the timescales of step-normal zoned orthopyroxene rims (i.e., normally zoned with a steep chemical gradient) from the Rock Mesa rhyolite are longer than those of reversely zoned crystals, whereas the Devils Chain produced mostly decadal timescales for both zoning types. Despite the proximity and broadly similar products of these episodes, their respective timescales indicate distinct sequences of events leading up to each eruption. The Rock Mesa timescales record centuries of magma chamber growth followed by decades of predominantly magma rejuvenation, reorganization, and destabilization. In contrast, the Devils Chain episode was preceded by a single episode of coupled rhyolite extraction, rejuvenation, and hybridization. Rare, high-An plagioclase cores and evidence of reheating implicate cryptic emplacement of mafic magma at the base of the rhyolite reservoirs. However, the diffusion timescales do not unequivocally support a single magma recharge event that affected both. Fluid fluxing and the reorganization of melt into buoyant magma chambers likely provided the source of increasing pressurization that initiated each eruption after several decades. Geodetic models of ongoing deformation west of South Sister could consider these processes in addition to magma emplacement.

Oregon↗

USGS2024

Zircon constraints on the eruptive sequence and magma evolution of rhyolites at South Sister volcano, Oregon

We present 230 Th- 238 U crystallization ages and trace element compositions for zircons spanning the late Pleistocene to Holocene rhyolite eruptive record at South Sister volcano in the central Oregon Cascade Range. Most zircon ages are between 100 and 20 ka, with very few in secular equilibrium (>350 ka). The weighted mean of zircon ages for the two oldest South Sister rhyolites, 31.5 ± 2.1 and 39.1 ± 2.4 ka, are significantly younger than the associated 40 Ar/ 39 Ar ages, 47.4 ± 9.7 and 51.4 ± 9.7 ka. We propose that these 40 Ar/ 39 Ar dates, performed on plagioclase separates, are compromised by a subtle amount of excess Ar and therefore the younger weighted mean zircon ages yield more reliable eruption ages. These results imply that the interval of rhyolite eruption at South Sister during the late Pleistocene was both shorter and more productive than previously thought and that eruption at South Sister initiated after Middle Sister. Compositionally, zircons from the Pleistocene rhyolites are broadly similar and show down-temperature zircon and plagioclase crystallization trends. However, we argue that destabilized amphibole and titanite in a common mush also exert leverage on the Pleistocene zircon trace element compositions. Divergence in the Eu/Eu* ratio between the Pleistocene and Holocene lavas implies chemically distinct magma reservoirs originating from the Pleistocene rhyolite eruptive sequence and the Holocene eruptive sequence. This work suggests a higher flux of rhyolite volcanism than previously thought and characterizes magmatic storage distinctions between the Pleistocene and Holocene rhyolites, aiding in the assessment of future eruptive hazards at South Sister volcano.

Oregon↗