203 Old and the New, Long and the Short: Perspectives on Integration of Timescales of Magmatic Processes: Special Session Related to MGPV Awards to Madison Myers and Anita Grunder

We encourage papers that challenge and give insights into the interplay between timescales, methodologies, and processes to build a cohesive understanding of the lifespan, evolution, storage, and eruption of magma, from plutonic to volcanic systems.

Tuesday, 21 October 2025
1:30 PM - 5:30 PM
HBGCC, 216AB

Diffusion Timescales in Olivine Suggest Weeks of Unrest Precede Explosive Flank Eruptions at Nyiragongo and Nyamulagira Volcanoes, DR Congo

Paper number: 203-4
Program: Technical Sessions
Time: 2:30 PM – 2:45 PM

Authors

Abstract

Nyiragongo and Nyamulagira (DR Congo) are two of the most active volcanoes in the East African Rift. Nyiragongo is well known for its active lava lake and dangerous, fast-moving lava flows. Explosive eruptions from >200 scoria cones and tuff rings along the distal flanks of the volcanoes also pose significant hazards to the rapidly growing population in the city of Goma, but the magmatic processes and timescales leading to these flank eruptions remain poorly understood. Here, we analyze the major and trace element geochemistry of olivine from six prehistoric tephra-forming flank eruptions. Zoning patterns in olivine were characterized qualitatively using Backscatter Electron Imaging and quantitatively with electron microprobe analysis. Compositional transects were fit and diffusion timescales were estimated using a numerical model employing a Monte-Carlo technique to assess boundary conditions. In most cases, models assumed an initial step function concentration profile, where zoning results from a combination of diffusion and growth processes.

Primitive (Fo>85) olivine from Mg-rich tephra have 10-50 micron-thick zoned rims, most of which are normally zoned. Median timescales modeled using these zoned rims are 14-32 days, indicating a few weeks of potential unrest before a flank eruption begins. Alternatively, if using the faster cation diffusivities from recent experimental work (Shea et al., 2023), the median unrest-to-eruption timescale for high-Mg olivine decreases from 24 days to 2.4 days. For the lower-Mg olivine (Fo72-86) in more Mg-poor tephra, normal zoning is present over longer length scales. Modeled timescales are much longer – 9 months to 4.5 years. If the faster diffusivities are again used, the median timescale decreases from 1.9 years to 2.4 months. Two distinct processes can be inferred for flank eruptions based on our results: (1) many flank eruptions involve intrusion of primitive melts (Fo85-91) into crustal magma storage zones that contain more evolved melts (Fo77-86), triggering eruption within a few weeks or less; (2) crustal storage zones with the most evolved magmas (Fo72-76) are sometimes intruded by more primitive melt, but the mixing-to-eruption timescales are longer (a few years or less), making it uncertain whether the magma mixing event is the trigger that destabilized the magma body. Our results can be used in seismic and geodetic monitoring studies to interpret precursory signals of future flank eruptions at Nyiragongo and Nyamulagira.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413655

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