Between a Rock and a Hard Place

Rocky secrets – fifty shades darker?

Charly Stamper October 14, 2013

Mel Auker is an Earth Sciences PhD student in the School of Earth Sciences at the University of Bristol. A mathematician by trade, Mel’s PhD uses numerical approaches to better understand past, present, and future global volcanic hazard and risk.

You might remember Charly’s “Fifity Shades of Grey” post, highlighting the colourful spectrum of rocks; fifty shades of grey they are not. On a purely aesthetic level, rocks are incredibly varied and interesting.

But the thing is, a lot of the samples us volcanology and petrology PhD students spend our time looking at are some shade of grey. The trick to staying interested in all things grey is to learn to read between the lines. Grey volcanic ash, for example, has a whole host of dark secrets to share if you know what to look for… [Read More]

Science snap (7): Thrusting under our noses

Charly Stamper October 10, 2013

As Earth Science researchers, we are extremely fortunate that fieldwork often necessitates trips to exotic and far-flung places. But sometimes we are guilty of ignoring the riches right on our doorstep.

In Bristol (UK), perhaps our greatest geological asset is the Avon Gorge. At the end of the Last Glacial Maximum, torrents of icy meltwater scoured out a 2.5km long gouge through a series of Devonian and Carboniferous limestones and sandstones. The bottom of the 90m deep gorge is now filled with the River Avon and the sheer cliffs of the north side are home to fossil corals, rare plants and challenging climbing routes; they also expose an excellent thrust fault.

This particular example lies at the intersection between Bridge Valley Road and the Portway, just underneath the Clifton Suspension Bridge (see here for map). Compressional forces associated with the formation of the supercontinent Pangea (~290 Ma) caused the the older Clifton Down Limestone to be thrust over the younger Upper Cromhill Sandstone. Friction along the overhanging fault plane deformed the younger sediments, and the resulting instability of the rock face has caused major issues for the adjacent roads.

Thrust fault in the north side of the Avon Gorge where the older grey Clifton Down Limestone (right) has been thrust over the younger red Upper Cromhall Sandstone (left); the intensity and friction of the thrusting is manifest in the deformation of the younger sediments. The fault outcrops at the intersection between Bridge Valley Road the Portway (A4) and is conveniently located adjacent to set of traffic lights and a cycle path – look out for it next time you’re stuck on a red light or peddling past.

Conference diaries: Goldschmidt 2013

Charly Stamper October 7, 2013

Following on from Mel Auker’s report on her visit to Japan for the The IAVCEI Scientific Assembly, Bristol PhD students Kate Hibbert and Sorcha McMahon tell us about their recent trip to Florence for Goldschmidt 2013.

What? The annual Goldschmidt conference is a major geochemistry conference, alternating between Europe and North America each year. With over 4,000 delegates from all over the world attending, this year’s gathering consisted of 22 themes, and many sessions within. Topics varied from the geochemistry of bottled water, to a Martian origin for life on Earth, with a new Volcanoes and Hazards session added this year. Talks were given all day every day, with poster sessions and refreshments each evening. Many social events were offered too, including a cheese and wine evening, film showings and a conference banquet.

When? The conference was from the 25^th – 30^th August, 2013, and started with an icebreaker on Sunday 24^th.

Where? Florence, Italy. The conference was held in the Firenze Fiera Congress and Exhibition Centre, just a few minutes walk from the city centre. [Read More]

Science Snap (6): SEM images of a high-pressure experiment

Charly Stamper October 3, 2013

Sorcha McMahon is a third year PhD student in the School of Earth Sciences at the University of Bristol. Sorcha is investigating how strange igneous rocks called carbonatites may have formed, using both natural samples and high-pressure experiments.

Sorcha's SEM SS

These back-scattered electron (BSE) images are a typical view of one of the high-pressure experiments that I run on the piston-cylinder apparatus, here in the BEEST labs at the University of Bristol. Such photographs are taken using the Scanning Electron Microscope (SEM), and are an essential stage in the analysis of run products as the different shades, textures and compositions are used to identify different mineral and melt phases.

The image on the right shows an entire capsule (a metal container that holds the powdered sample) and its contents after it has experienced conditions of 1375^oC and 30 kbar (equivalent to ~100 km depth) for 24 hours. The AuPd capsule (an alloy that can withstand up to ~1400^oC before melting at this pressure) appears brighter than the phases produced because this material has a higher atomic mass than the minerals (more information in Charly’s post).

The two images on the left show closer shots of the same experiment, labelled with the different minerals. In varying shades of grey; garnet, olivine, clinopyroxene (cpx) and orthopyroxene (opx) are typical minerals found in lherzolite (‘normal’ mantle) assemblages. As I am working in a synthetic carbonate-bearing system (CMAS-K₂O-CO₂), my run products contain an abundance of carbonate minerals, such as dolomite. At higher temperatures, melt may be observed, and is identified by its ‘streaky’ quenched texture.