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Imaggeo on Mondays: A fold belt within a grain

Imaggeo on Mondays: A fold belt within a grain

Tiny crinkly folds form the main basis of today’s Imaggeo on Mondays. Folding can occur on a number of scales; studying folds at all scales can reveal critical information about how rocks behave when they are squeeze and pinched, as described by Sina Martin, from the University of Basel.

Although many geoscientists have seen such fold structures many times before, if you noticed the scale bar in the lower left of the image, you might be surprised of the small scale of these folds!

The presented image is a high-magnification image taken on an electron microscope, showing sub-micrometer scale folds developed within a deformed pyroxene grain – a chain silicate mineral, for example common in the oceanic crust of the earth. The folded layers are primary exsolution lamellae of more calcium rich and calcium poor chemical composition. These lamellae formed during the early, magmatic history of the pyroxene grain, where it crystallized and cooled down in a shallow intrusion. The folding subsequently took place during deformation and the following text will try to give a short overview on why and how these folds have formed.

The presented image was made using a back-scattered electron (BSE) detector, where different grey values indicate different chemical compositions. This effect originates from the fact, that some of the electrons, which “bombard” the sample in the electron microscope, are back scattered by the atoms near the sample surface and then detected by the BSE. Heavier atoms (with a greater atomic number, Z) have a higher probability to generate a backscattered electron. Consequently, where heavy atoms occur, more backscattered electrons reach the detector and the area appears bright, compared to dark- appearing areas, where light atoms prevail. Because of this sensitivity of the BSE image on chemical composition, we can see the exsolution lamellae in the pyroxene with different grey values.

Although the folds in this image occur on the nanometer- to micrometer scale, their geometry and mode of formation is the same as is observed in large-scale fold belts (e.g. the Helvetic nappes in the Swiss Alps). There, this fold type develops mainly in layered sediments, which have contrasting properties: alternating series of competent and incompetent layers leads to boundary instabilities and thus to folding. In the present case, the contrasting properties of the layers – also known as anisotropy – is a result of the formation of the exsolution lamellae and enables folding even at the very small scales seen within this single grain. One can even see the difference between the layers in the image: The darker lamellae change their layer thickness more readily (best seen in fold hinges – the place of strongest bend in the fold) than the brighter layers, indicating that the darker layers deform more easily..

This folded pyroxene is an astonishing example that certain processes, which generate geological structures, operate over multiple orders of magnitude in scale. Without a scale bar provided, it would not be possible to determine the scale of these structures and tell them apart from folds formed in outcrop or even on larger scales. Now, it should not be confused: such a pyroxene grain will not be encountered in the same tectonic regime as large-scale fold belts. But exactly for this reason, it is a beautiful example displaying the overall controlling importance of anisotropy over most other material properties, independent of scale. For the deformation of rocks, anisotropy almost always plays a key role in the deformability, and in general controls the development of structures such as folds like in the present case.

By Sina Martin, Department of Environmental Science, Geological Institute, Basel

Sina would like to thank  the Center of Microscopy (ZMB) at the University of Basel, where the image was taken and also thank the ZMB for providing the infrastructure.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

General Assembly 2015 – Highlights

It’s been just over a month since the EGU General Assembly 2015 in Vienna. The conference this year was a great success with 4,870 oral, 8,489 poster, and 705 PICO presentations. There were 577 unique scientific sessions, complimented by an impressive 310 side events, making for an interesting and diverse programme.

The conference brought together 11,837 scientists from 108 countries, 23% of which were students. Keeping abreast of everything that was going on throughout the week was made easier due to the distribution of 15,000 copies of EGU Today, and as a result of a keen media presence and their reporting of the scientific sessions. Thousands of visits to the webstreams, as well as GeoLog, meant  those at the conference and those who couldn’t make it stayed tuned to the best of the conference! We thank all of you very much for your attendance and active contribution to the conference.

Why not watch this video of the best bits of the conference and highlights of a productive week?

The conference this year, as showcased in the highlights video, celebrate a theme: A voyage through scales. The theme was an invitation to contemplate the Earth’s extraordinary variability extending from milliseconds to its age, from microns to the size of the planet. The range of scales in space, in time – in space-time – is truly mindboggling. Their complexity challenges our ability to measure, to model, to comprehend. The range of scales were explored across four exhibition spots throughout the conference centre.

One of the exhibitions, ‘The scales in art‘, invited conference participants to participate in the dialogue between science and art. At the space, attendees watched the artistic interpretation of the theme developing over the week, with artist Eva Petrič.

We hope to see many of you at next year’s EGU General Assembly 2016 which takes place on: 17 – 22 April 2016, in Vienna, Austria.

Imaggeo on Mondays: A thermal inversion

Imaggeo on Mondays: A thermal inversion

This week’s Imaggeo on Mondays image is brought to you by Cyril Mayaud, from the University of Graz (Austria), who writes about an impressive hike and layers of cold and warm air.

Thermal inversion is a meteorological phenomenon which occurs when a layer of cold air is trapped near the Earth’s surface by an overlying layer of warmer air. This can happen frequently at the boundary between mountainous and lowland regions such as in Slovenia and last for weeks, obscuring the sun from view to the people living below. When this phenomenon occurs over a large city, the consequence is that it can cause important pollution problems, as the lack of air circulation, prevents the rising and scattering of pollutants in the atmosphere.

The picture was taken at dusk from the top of the Porezen, a 1630 m high mountain located near the town of Cerkno and belonging to the Slovenian Prealps. This mountain is very popular among local hikers because its summit offers an impressive panorama of large parts of Slovenia, comprising the highest peaks of the Julian Alps, the Ljubljana Basin and even Snežnik Mountain located close to the Croatian border. A thick, low altitude, layer of clouds was covering the whole country during the preceding week, but clear, sunny skies prevailed above the clouds, a not too uncommon phenomena.. We made our way to the summit over several hours and spent some time enjoying the panorama and the sunset. As a result of the thermal inversion, the air temperature was warmer at the summit than in the surrounding lowlands. As soon as the sun began to set, the fog slowly started to move forward and cover the narrow valleys below the mountain. By the end of the day, the valley in the foreground was also totally engulfed by fog.

By Cyril Mayaud, Researcher at the University of Graz, Austria

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/

Imaggeo on Mondays: Foehn clouds

This week’s post is brought to you by Stefan Winkler, a Senior Lecturer in Quaternary Geology & Palaeoclimatology, who explains how the mountain tops of the Southern Alps become decorated by beautiful blanket-like cloud formations.

The Sothern Alps of New Zealand are a geoscientifically dynamic environment in all aspects. They are arguably one of the youngest high mountain ranges in the world formed at the plate tectonic boundary between the Australian and the Pacific Plate. Their dominating tectonic structure, the Alpine Fault running some 600 km mainly parallel to the mountain ranges of New Zealand’s South Island, caused not only an impressive horizontal displacement of rock formations, but also an overall vertical uplift of estimated c. 20 km during the past 10 – 15 Million years. Aoraki/Mt.Cook visible in the left background on the image with its height of ‘only’ 3724 m a.s.l. is the highest peak of the mountain range that is currently uplifted by 4 – 5 mm per year. Together with reconstructed uplift rates of up to 10 mm per year for the centre of the Southern Alps this indication how efficient and important weathering and erosion processes are in this region.

Foehn clouds over Aoraki/Mt.Cook. Credit: Stefan Winkler (distributed via imaggeo.egu.eu)

Foehn clouds over Aoraki/Mt.Cook. Credit: Stefan Winkler (distributed via imaggeo.egu.eu)

The ranges of the Southern Alps rise just 10 – 15 km inland the West Coast of the South Island as a wall parallel to the coast line up to 3,000 metres and more. They are a major topographic obstacle for the predominantly westerly airflow and provide a classic example of how ‘föhn’ winds are generated along mountain ranges perpendicular to an air flow. Föhn winds are dry and warm, forming on the downside of a mountain range. On the western slopes of the Southern Alps, orographic precipitation amounts to impressive 5,000 mm at the base and 10,000 mm + on in the high-lying accumulation areas of the mountain glaciers concentrating around the Main Divide. At and east of the Main Divide this locally named ‘Nor’wester’ creates impressive foehn clouds (altocumulus lenticularis, hogback clouds, seen in this week’s Imaggeo on Mondays image) that form in waves parallel to the Main Divide and are often streamlined by the high wind speed. The frequent occurrence of strong and warm Nor’westers contributes to the sharp decline of precipitation immediately east of the Main Divide.

The foreground of the image displays another aspect of this dynamic environment: the current wastage and retreat of glaciers in New Zealand. The section of the proglacial lake with its sediment-laden greyish water colour on the image would still have been covered by the debris-covered lower glacier tongue of Mueller Glacier only 15 years ago. Now, the terminus has retread to a position to the left outside the image. The lake is bounded by the glacier’s lateral moraine – unconsolidated accumulations of rock and soil debris resulting from weathering of the rock walks surrounding a glacire – that are more than 120 m high from base to top (or crest, to give it its technical name) and were last overtopped during the so-called ‘Little Ice Age’ when the glacier surface reached higher than its crest. At this glacier, the maximum of this Little Ice Age has been dated to 1720/30, but as late as during the late 20th century it remained close to its frontal maximum position and had only shrunk vertically. Today the lateral moraines are heavily reworked and eroded by paraglacial processes following the latest vertical and horizontal ice retreat. In some places on Mueller Glacier’s foreland the crest of lateral moraines retreat up to 1 m per year back and give again evidence of a very dynamic geo-ecosystem.

By Stefan Winkler, Senior Lecturer in Quaternary Geology and Palaeoclimatology at the Univeristy of Canterbury.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

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