Scales and frequencies of Snake-River type super-eruptions of the Yellowstone hot-spot track, USA

Lead Research Organisation: University of Leicester
Department Name: Geology

Abstract

This project concerns the largest explosive eruptions on Earth. Super-eruptions are amongst the most catastrophic events that affect the Earth's surface: they have immediate and devastating regional environmental consequences and affect global climate. To assess their role with regard to crustal evolution, regional or global environmental change, we first need to need to know more about how often they occur, at what scales, and about the eruption styles. To do this we must consult the geologic record. But despite recent interest, relatively few examples are known and still fewer have been studied in detail. Most work has been on reconstructing single examples in far-flung locations (e.g. Toba volcano in Indonesia). We plan a different and complementary approach: to quantify the number, frequencies and volumes of successive large eruptions over an extended period at a single geotectonic setting. We know something about the frequencies of large eruptions during relatively short periods of time at continental arcs (e.g. Taupo Volcanic Zone; San Juan Mts) but there is virtually no quantitative work on super-eruption frequencies in the continental intraplate setting - despite a growing realisation of the importance of silicic volcanism within Large Igneous Provinces. It is well-known that Yellowstone, USA, has erupted catastrophically in recent times, but perhaps less widely appreciated is that this was just the last of a series of numerous very large explosive eruptions that burned a track along the Snake River eastwards from Oregon to Yellowstone from 16 Ma to present. Some of the earlier eruptions were probably as large as at Yellowstone, if not larger, but we know astonishingly little about them: we do not even know how many occurred, or their true extents. This is despite the region being one of the Earth's largest, most accessible, and best-preserved silicic volcanic provinces; yet large tracts of it remain unstudied and the overall stratigraphy has not been resolved. Magma erupted in a super-eruption is pulverised and enters vast ground-hugging density currents that deposit thick layers of ash, called 'ignimbrites'. Repetition of similar eruptions in the Snake River Plain has produced many broadly similar ignimbrites that are not simple to tell apart from one another. Therefore, we shall divide local successions into 'eruption-units' and then fingerprint each unit in detail using a combination of detailed physical and chemical analysis, dating, and palaeomagnetic tools; for example, recording any distinctive vertical variations in the chemistry of the crystals and glasses. Preliminary fieldwork, mineral chemistry and 40Ar-39Ar has already allowed us to correlate two units in adjacent ranges and gives us confidence to predict that the detailed approach planned will, for the first time, let us to correlate individual units across large distances, and so establish the number and volumes of the eruptions. The age data together with the palaeomagnetic data for each unit will provide a new regional time-event framework, to interpret the province's eruption history and assess whether the volumes and frequencies of large explosive eruptions have changed through time, including comparison with the known data from the Yellowstone area. The site of volcanism shifted eastwards with time, enabling us to consider spatial variations and possible links with crustal deformation. The work will add to future understanding about what controls eruptions on this large scale (e.g. heat flux at depth?), about rates of magma generation and storage, and the inter-relationships between magmatism and the crust. The multidisciplinary research team has much experience in the methods to be employed, and in the Columbia River - Yellowstone Volcanic Province. Developing, for the first time, a long-needed regional time-event framework, and drawing together the different research groups will act as a catalyst for extensive further research.

Publications


10 25 50
 
Description Main discoveries.

1. We have discovered a new super-eruption on the Yellowstone hotspot-track (Knott et al., 2016a). It was explosive, and comparable in size to the 3 well-known, recent super-eruptions from the Yellowstone volcanic field. We have dated the new super-eruption and estimated the eruption volume. It is the c. 8.1 Ma 'Castleford Crossing eruption', with a volume of 1900 km3 erupted from near Twin Falls in Idaho, USA. In fact, the size of the Castleford Crossing eruption is likely to be substantially larger than this, because the widely dispersed atmospheric ash of unknown volume was not included in the volume estimate. We have deep-drilled the source volcano, and proved that the (single) proximal ignimbrite exceeds 1.3 km thick (base not seen) where it fills the concealed volcanic caldera.

2. We have shown that the frequency of large explosive eruptions from the Yellowstone hotspot track during mid-Miocene times was twice that of Yellowstone (Knott et al., 2016a,b). The frequency of giant explosive eruptions from this province seems to have decreased over time.

3. We have regionally correlated 2 large ignimbrite sheets and estimated the area covered them. The Brown's View Ignimbrite sheet covers 3,300 km2 (volume =100 km3) and the Wooden Shoe ignimbrite sheet covers 4,400 km2 (volume =130 km3 and extends from Idaho into northern Nevada.

4. We have documented evidence for substantial subsidence of the US continental crust in response to the intrusion of large volumes of hot basaltic magma related to the Yellowstone hotspot. A 100 km-wide strip of continental crust has subsided by over 5 km, and a significant component of the subsidence occurred during major explosive volcanism as recorded by thickness variations and kinematic indicators in successive, radiometrically dated ignimbrite sheets. The subsidence produced the Snake River Basin, the development of which was coeval with Basin-and-Range extension (Knott et al., 2016a,b).

5. We have demonstrated that a method that combines whole-rock trace-element geochemistry, mineral chemistry, palaeomagnetism, geochronology and volcanological fieldwork, is a viable means to correlate monotonous rhyolitic Snake-River-type ignimbrites regionally, as a way to resolving the regional stratigraphy and the volumes of successive explosive super-eruptions.

6. We have shown that the rock magnetism (TRM) of glassy flow-banded rheomorphic ignimbrites is significantly affected by shear-induced anisotropy, leading to magnetic orientations that do not faithfully reflect the Earth magnetic field at the time of cooling (Finn et al. 2015). We have developed a method to mitigate this effect, that should work for glassy ignimbrites and lavas elsewhere in the world.

7. We have produced a new stratigraphic scheme for the southern part of the central Snake River Plain, Idaho, USA, with the record of numerous large explosive eruptions. This stratigraphy is underpinned by new type locations, field, geochemical, mineralogical, and magnetic characteristics, and Ar-Ar geochronology (Knott et al. 2016a,b)
Exploitation Route More results in preparation for publication.

The stratigraphy, geochemical, palaeomagnetic, mineralogy and geochronological data published will be valuable for further research into super-eruptions of the USA, tephrostratigraphy, and assessments of the impact of major volcanic events on regional and global environments.

The correlation and rock magnetism methods developed should have widespread applications to other volcanic provinces, and the volcanic stratigraphies and correlations achived should provide the basis for future volcanological studies
Sectors Education,Environment
URL http://leicester-geology.beardyscientist.com/volcanology/snake-river-plain.html
 
Description Invited participant and presenter at scientific conference 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Co-presentation (Lecture), Chaired a lively discussion. Conference field trip which I co-led to look at the Snake River Plain deposits generated a great deal of questions and discussion.

Lots of discussion in workshops and field after presentations, followed by scientific publications on various topics by several authors present.
Year(s) Of Engagement Activity 2009
URL http://www.geosociety.org/penrose/reports/09pcrpt1.htm
 
Description Participation in several scientific conferences 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact 5 presentations in successive years at AGU sparked questions and interest from US and international volcanologists and palaeomagnetists
3 presentations at VMSG AGM, generated discussion and further invites for talks at international meetings
includes other presentations in Europe and USA.

Invitations to submit papers, give talks.
Year(s) Of Engagement Activity 2009,2011,2012,2013,2014
URL http://fallmeeting.agu.org/2013/
 
Description Schools visit, talks and discussion on supervolcanoes 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Visits to primary and high schools in the Leicester to present talks and lead discussion about super-eruptions, including the Yellowstone Snake River Volcanic Province. Increasing awareness and education.
Year(s) Of Engagement Activity 2015,2016
 
Description Super-eruptions of the Snake River Plain - Yellowstone Hotpot website. http://leicester-geology.beardyscientist.com/volcanology/snake-river-plain.html 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact The results are ongoing, and the website will be added to as new scientific results arrive and are published

People reported that they had learned new things from the website.
Year(s) Of Engagement Activity 2014
URL http://leicester-geology.beardyscientist.com/volcanology/snake-river-plain.html
 
Description invited contributor, International Volcanology School 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact My 2 talks parked numerous questions afterwards, and the fieldcourse I led sparked many questions and the discussion continued after the meeting.

I have had invitations to collaborate with workers in Italy and Canada and will take an Italian student on a volcanology course
Year(s) Of Engagement Activity 2014
URL http://www.aivulc.it/index.php?page=shop.product_details&product_id=23&flypage=flypage.tpl&pop=0&opt...