Dec. 1, 2018 – Over the last 15 years one of the biggest advances in the study of igneous petrology – the study of rocks that are formed from magmas and lavas – has been the realization that magma bodies are often composite features, i.e., they tend to be built progressively by many injections of magma and subsequent cooling.
If magma doesn’t erupt after an injection event, it will sit in the crust and slowly cool, forming crystals. And it can sit in that state – mostly crystals with a tiny amount of liquid magma – for a very long time.
Yellowstone has had at least 23 smaller eruptions since its last super-eruption ~631,000 years ago, making this type of eruption more likely than a super-eruption. Although smaller than super-eruptions, these more frequent eruptions are still the size of the largest eruptions of the 20th Century, such as the 1991 Pinatubo eruption (~5 km3), and could pose a considerable volcanic hazard.
Rejuvenation of previously intruded silicic magma is an important process leading to effusive rhyolite, which is the most common product of volcanism at calderas with protracted histories of eruption and unrest such as Yellowstone, Long Valley, and Valles, USA.
Although orders of magnitude smaller in volume than rare caldera-forming super-eruptions, these relatively frequent effusions of rhyolite are comparable to the largest eruptions of the 20th century and pose a considerable volcanic hazard. However, the physical pathway from rejuvenation to eruption of silicic magma is unclear particularly because the time between reheating of a subvolcanic intrusion and eruption is poorly quantified.
This study uses geospeedometry of trace element profiles with nanometer resolution in sanidine crystals to reveal that Yellowstone’s most recent volcanic cycle began when remobilization of a near- or sub-solidus silicic magma occurred less than 10 months prior to eruption, following a 220,000-year period of volcanic repose.
Our results reveal a geologically rapid timescale for rejuvenation and effusion of ~3 km3 or 3 billion cubic meters, of high-silica rhyolite lava even after protracted cooling of the subvolcanic system, which is consistent with recent physical modeling that predict a timescale of several years or less.
Future renewal of rhyolitic volcanism at Yellowstone is likely to require an energetic intrusion of mafic or silicic magma into the shallow subvolcanic reservoir and could rapidly generate an eruptible rhyolite on timescales like those documented here.
The high-spatial-resolution analyses of sanidine phenocrysts in this study indicate a time scale of only several months to dozens of months for rejuvenating silicic intrusive material at Yellowstone to produce its most frequent type of eruption after a protracted period of volcanic repose, during which the subvolcanic system existed as near-solid or subsolid state waiting for remobilization.
These brief time scales are remarkably like others calculated for eruption triggering, but in those cases due to magma mixing in established, liquid-dominated silicic magma chambers (10–60 yr.)
The recurring eruptions of rhyolite lava at Yellowstone caldera are individually comparable in size (~1–10 km3, or 1-10 billion cubic meters) to recent silicic eruptions at Chaiten (Chile) or Pinatubo (Philippines), and would likely lead to considerable social and economic disruption and possible interference with North American air travel.
If the ~3 km3 volume, or 3 billion cubic meters, for the SCL (Scaup Lake flow) and South Biscuit Basin rhyolites is derived from a subvolcanic area outlined by their vent zones, then a ≥40–60-m-thick region of near-solidus or subsolid rhyolite may have been remobilized.
Recent physical modeling indicates that a near-solidus body of this size could be rejuvenated by an intrusion of hot, near-liquidus rhyolite within several years or less consistent with our results.
Today, Yellowstone’s vigorous hydrothermal system circulates through a several-kilometers-thick cap of subsolid, fractured rhyolite above a shallow crustal reservoir (~10,000 km3 = or 10,000,000,000,000, ten trillion cubic-meter) of near-solidus magma mush containing 5%–32% or ~200–600 km3 (200 billion- 600 billion cubic meters) of rhyolitic melt and a near-solidus lower crustal reservoir (~46,000 km3, or 46 billion cubic meters) containing ~2% or ~900 km3, or 900 billion cubic meters of basaltic partial melt,; both are ultimately sustained by intrusion of maﬁc magma at a rate similar to the active Hawaiian hotspot.
The results of this study reveal that a sufﬁciently energetic rejuvenation of Yellow-stone’s shallow crystal-melt mush and/or hydro-thermally altered wall rock could lead to an effusive eruption within months.
Fortunately, any signiﬁcant rejuvenation of the reservoir is likely to be associated with deformation or seismicity and identiﬁable by geophysical monitoring (e.g., Fialko et al., 2001; Wicks et al., 2006)
Because ground deformation can change permeable pathways for heat, gas and hydrothermal fluids, patterns of documented ground deformation along the Yellowstone caldera’s northern rim are relevant when discussing the NGB hydrothermal system.
From 2004–2008, researchers documented rapid uplift of the resurgent Sour Creek Dome and northern caldera rim subsidence documented a subsidence rate of 3 cm/year in the Norris area between 2004 and 2006. Rates of subsidence slowed from 2006 to 2007 with 8 mm subsidence from September 2007 to September 2008 (Dzurisin et al., 2012). Dzurisin et al. (2012) state that “minor subsidence in the Norris area also seems to have stopped by the third quarter of 2009, as indicated by the CGPS station NRWY”
Other factors in a Supervolcano eruption
Scientists had thought that these huge volcanoes gradually built up more and more molten rock until the pressure got to be too much. But they are now realizing that much of the period between eruptions — as much as a million years — is probably quiet. To help understand how to forecast supervolcano eruptions, a team of geologists quantified the effects of tectonic stress on the rocks that house these sleeping giants.
Geologist Patricia Gregg of University of Illinois is a co-author on the study. She explained in a statement:
“Supervolcanos tend to occur in areas of significant tectonic stress, where plates are moving toward, past or away from each other.”
Haley Cabaniss, a PhD student at University of Illinois, is the study’s first author. Her work focuses on computer modeling of 3-D magma reservoirs of volcanos, in order to determine the how systems fail and ultimately. She explained how the models used in this study showed that tectonic stress does have a profound effect on the stability of supervolcanoes, but that these stresses aren’t the only factor to cause an eruption. She said:
“Any tectonic stress will help destabilize rock and trigger eruptions, just on slightly different timescales. The remarkable thing we found is that the timing seems to depend not only on tectonic stress, but also on whether magma is being actively supplied to the volcano.”
The researchers found that, in any given tectonic setting, the magma reservoirs inside supervolcanoes appear to remain stable for hundreds to thousands of years while new magma is being actively suppled to the system. Gregg said:
“We were initially surprised by this very short timeframe of hundreds to thousands of years. But it is important to realize that supervolcanoes can lay dormant for a very long time, sometimes a million years or more. In other words, they may remain stable, doing almost nothing for 999,000 years, then start a period of rejuvenation leading to a large-scale eruption.”