Overview

Giant meteorite impacts are the largest catastrophic events in Earth history. They are a fundamental process in planetary evolution and have profound effects on the global environment, atmosphere and biota. They melt and vaporise the crust, forming craters 10–100+ km across with regional ejecta blankets and global fallout layers. Our understanding of what happens is informed by forensic-style investigation of the end-products.

Melt-bearing impact ejecta deposits (known as ‘suevites’)1,2 are important because they contain shocked target rock, minerals, and fragments of chilled melt that provide valuable information about the unusual conditions and processes during impact events. However, there is no consensus on how these deposits actually form, and so a fundamental aspect of impact cratering remains poorly understood. Some experts think the melt-bearing deposits form early, as part of the initial ejecta curtain, but others think they form after the crater has formed, when cold surface water interacts with a giant sheet of hot melt2 in what is known as a ‘fuel-coolant explosion’ – explosions of this type are known to happen at some volcanoes.

You will carefully interpret the fragmentation processes at giant impact-events using state-of the-art techniques normally used to study explosive volcanic eruptions4. This fresh perspective will shed new light on processes that occur at impact sites, bearing in mind, of course, that several factors differ significantly with those at volcanoes.

where fuel-coolant explosivity has been proposed, and compare these to impact deposits where fuel-coolant explosivity is not thought to have been important. A strength of the research is that it will for the first time compare quantitatively the shapes of glass shards from different impact sites, where the chemistry of the target rocks, and the size of the meteorite impactor differed. By using the same techniques at different impact sites, the project will draw-out similarities and differences between the different impact events.

The research will provide novel results for the Planetary Geology, and Earth Science communities, including new conceptual models and much-needed constraints for existing impact models.

Giant impact events are the most catastrophic events to affect the Earth’s surface, yet some key processes remain poorly understood

Methodology

The shape and vesicularity of former melt particles in suevite deposits will be imaged by optical and scanning electron microscopy. Image-analysis software will be used to process and quantify a range of shapes and vesicle characteristics. The rheology of the melt at different impact sites will be estimated using glass compositions analysed on Leicester’s LA-ICPMS, along with estimated temperatures and volatile contents.

The new shard-morphology data from different impact-craters will be compared, including Ries1,3 (Germany), Chixulub5 (Mexico) and Sudbury (Canada), and there will be the opportunity to characterise and compare these with other suevites, including Stac Fada3 (Assynt, Scotland), Gardnos (Norway), and from the extensive worldwide collection of suevites at Berlin Natural History Museum.

Results will be compared with published data from well-studied explosive volcanic eruptions (including magmatic and phreatomagmatic eruptions) and the degree of vesicularity of the melt at the time it was fragmented will be inferred, shedding new light on the fragmentation processes.

Training and Skills

CENTA students benefit from 45 days training throughout their PhD study including a 10 day placement. In the first year, you will be trained as a single cohort on environmental science, research methods and core skills. Throughout the PhD, training will progress from core skills sets to master classes specific to the student's projects and themes.  

You will benefit from hands-on training in SEM, EMP, LA-ICPMS and analysis and in CT-scanning. You will become proficient in image-analysis, and by the end of the project you will have developed expertise spanning both physical volcanology and impact-cratering. Field training on how to interpret pyroclastic and impact deposits will be provided. You will join a lively PhD student community at Leicester, and be part of a volcanology research group. Presentation at national and international conferneces will be encouraged.

Timeline

Year 1: Initial reading on impact processes, and fragmentation at volcanoes. FIeld training at a volcano. Optical and SEM analysis of glass shards from Ries impact crater, Germany, and the Stac Fada, in Scotland (field visits). Image-processeing and analysis to quantify and compare particle shapes and vesicularities. Study visit to Bristol university. Participation at a national conferences (e.g. VMSG).

Year 2: Chemical characterisation of glass shards. Study visit to Berlin to use electron microprobe and examine suevites from other major impact sites. Field visit to a large impact site at Sudbury (field advice from local geologists), Canada. Write-up initial findings. Begin CT-scanning and 3-d image-processing.

Year 3: Final analyses, and plot shard shape data. Synthesise results, with a comparison between impact sites, and with published data on volcanic eruptions. Present at international conferences (International Meteoritics Society). Write-up papers for international journals.

 

Partners and collaboration (including CASE)

Prof Mike Branney is an expert on explosive volcanism and the deposits of pyroclastic density currents. He is spearheading a research initiative to re-evaluate fragmental deposits at large impact craters.

Prof Jan Zalasiewicz is a geological stratigrapher.

Dr Tiffany Barry is an igneous geochemist at the University of Leicester, and oversees Leicester’s analytical facilities.

Dr Lutz Hecht is a impact-specialist at the meteoritics group, Berlin, with experience at several impact sites.

Prof Kathy Cashman (Bristol) is a physical volcanologist at Bristol University with world-leading expertise on explosive fragmentation.

Further Details

Please contact Prof Mike Branney, University of Leicester, mjb26@le.ac.uk.