Youngest Toba Tuff

The Youngest Toba Tuff (YTT)

The enormous (>2800 km3) Youngest Toba Tuff (YTT) erupted 74,000 years ago from Toba Caldera in Sumatra, Indonesia. The YTT is the most recent of four major eruptions from Toba over the last 1.2 million years. As one of the worlds largest and most recent supereruptions, the YTT is an ideal natural laboratory to understand the near-eruption dynamics and durations of magma storage occurring in large silicic systems.

My Research

Despite the global hazards posed by systems like Toba, the timescales and processes by which eruptible magma domains are generated, and the conditions of their storage prior to eruption remain poorly constrained and major questions remain, including:

  • When did system remobilization and generation of eruptible melt-rich magma domains occur?

  • Magma system zoning - how does it originate, and what are the conditions of storage in the different zones?

  • How much affinity do crystals have to each other and host glasses, and what processes is mineral-scale chemical zoning recording?

My work investigates these questions by utilizing the complex chemical growth record preserved in crystals as a window into magma system evolution. More specifically, I utilize a multifaceted and integrated approach is utilized that incorporates a variety of state-of-the-art analytical tools and methods on a variety of mineral phases derived from a suite of compositionally representative samples that cover YTT compositional spectrum. Mineral chemistry and zoning are characterized via micro-scale imaging and in-situ analyses and coupled with temporal constraints from both isotopic dating and the magnitude of chemical diffusional relaxation at crystal growth boundaries. Investigations largely center on the accessory minerals zircon and allanite, and the major phase quartz. Crystal rims are a key focus as they provide a record of the most recent stages of growth occurring within the magma before eruption, however investigation of crystal interiors is also included as they provide a more protracted magma system history and yield critical context to rim growth.

See below for some details on the previous and ongoing work.

Zircon and Allanite

To date, work on YTT zircon and allanite crystals focused on assessing what the last increment of their growth reveals about magma storage conditions in the melt-dominated portion of the system, and trying to reconcile timescales between accessory and major phases. It included analyzing the unpolished surfaces of zircon and allanite from multiple pumice samples for secondary ionization mass spectrometry (SIMS) determinations of composition and 238U-230Th crystallization ages. Depth sensitivity was increased from ~4 µm in a standard SIMS analysis to ~1 µm by coupling discrete measurement cycles with statistical treatments to detect zircon and allanite surface zoning domains. Moreover, in a novel approach, Rhyolite-MELTS modeling and accessory phase saturation modeling were integrated to constrain growth conditions of these phases in rhyolitic magmas, and resolve conflicting temporal records with major phases. Altogether, constraints from this modeling and assessment of the prevalence, magnitude, and compositional nature of eruption-aged growth on accessory phases are related back to magma system dynamics.

This work was published in Contributions to Mineralogy and Petrology in 2019.

From Tierney et al (2019)

From Tierney et al (2019)

From Tierney et al (2019)


A significant part of my prior and ongoing YTT research has on the record contained within YTT quartz crystals to investigate the crystallization history of the crystals and its relation to the evolution and storage of the YTT magma system. Quartz is an effective monitor of melt domain processes in the YTT because it is ubiquitous and its stability is sensitive to melt temperature, pressure, and composition. To date, only one aspect of this work has been published (see below), but more details will follow as additional aspects are published.

Tierney and Reid (2021)

Quartz-hosted melt inclusions are windows into the assembly and storage ofcompositionally zoned magmas systems like the giant magma body responsible for the 2,800 km3 Youngest Toba Tuff (YTT), Indonesia. Feldspar-sensitive element concentrations and interelement ratios in YTT quartz-hosted melt inclusions are found to cluster into three discrete melt inclusion populations. Each population represents a small fraction of the overall range in YTT melt compositions and no more than ∼12% to ∼21% fractional crystallization at mostly eutectoid conditions accounts for the trace elements variations within them. Kinships between host matrix glasses and co-hosted phases are interpreted as evidence of spatially discrete chemical domains of magma within a more broadly zoned magma system. Quartz growth in the dominant portion of the system apparently occurred after extraction of melt from mush, whereas chemically distinct crystal cores in the more crystal-rich, low-silica magmas may have been derived from mush during extraction or remobilized by mush rejuvenation. Collectively, the chemical zoning of the YTT appears to have developed prior to most quartz growth, likely “bottom-up” due to mush-derived melt heterogeneity.

From Tierney and Reid (2021)

From Tierney and Reid (2021)

From Tierney and Reid (2021)