GeoLog

Tectonics and Structural Geology

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/.

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/.

Imaggeo on Mondays: The largest fresh water lake in world

Lake shore in Siberia. Credit: Jean-Daniel Paris (distributed via imaggeo.egu.eu)

Lake shore in Siberia. Credit: Jean-Daniel Paris (distributed via imaggeo.egu.eu)

Most lakes in the Northern hemisphere are formed through the erosive power of glaciers during the last Ice Age; but not all. Lake Baikal is pretty unique. For starters, it is the deepest fresh water lake in the world. This means it is the largest by volume too, holding a whopping 23,615.39 cubic kilometres of water. Its surface area isn’t quite so impressive, as it ranks as the 7th largest in the world. However, it makes up for that by also being the world’s oldest lake, with its formation dating back 25 million years – a time during which mammals such as horses, deer, elephants, cats and dogs began to dominate life on Earth.

Located in a remote area in Siberia, perhaps, most impressive of all is how Lake Baikal came to be. It is one of the few lakes formed through rifting. The lake is in fact, one of only two continental rifted valleys on our planet. Typically, “continental rift zones are long, narrow tectonic depressions in the Earth’s surface”, writes Hans Thybo, lead author of a paper on the subject. The Baikal rift zone developed in the last 35 million years, as the Amurian and Eurasian Plate pull away from one another. Eventually, the stretching of the Earth’s surface, at continental rifted margins, can lead to continental lithosphere splitting and the formation of new oceanic lithosphere. Alternatively, as is the case in Siberia, extensive sedimentary basins can be formed; bound by faults, they are known as grabens. It is by this process that Lake Baikal was formed and now houses around 20% of the world’s fresh water!

But this is not where the amazing facts about today’s Imaggeo on Monday’s picture end. The lake is the origin of the Angara River, along which you’ll find the manmade Bratsk Dam, the world’s second largest dam! The shoreline pictured in this photo by Jean- Daniel Paris, is from this impressive dam. Completed in 1964, this artificial reservoir is home to almost 170 billion cubic meters of water (equivalent to the volume held by 68 million Olympic sized swimming pools!).

However, it’s not the impressive water bodies in this inaccessible location in Siberia that are of interest to Jean-Daniel. In fact, this photograph was taken from a research aircraft, which flew over the region for an investigation that spanned a period of several years. Its aim was to measure how concentrations of CO2 and CO varied across the region. Acquiring this data would allow the team of scientist to better understand the sources of the gases, in this remote area of Russian, due to anthropogenic activities and biomass burning.

Reference

Thybo, H., Nielsen, C.A.: Magma-compensated crustal thinning in continental rift zones, Nature, 457, 873-876, doi: 10.1038/nature07688, 2009

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: An explosive cloud

Imaggeo on Mondays: An explosive cloud

One of the world’s most volcanically active regions is the Kamchatka Peninsula in eastern Russia. It is the subduction of the Pacific Plate under the Okhotsk microplate (belonging to the large North America Plate) which drives the volcanic and seismic hazard in this remote area. The surface expression of the subduction zone is the 2100 km long Kuril-Kamchatka volcanic arc: a chain of volcanic islands and mountains which form as a result of the sinking of a tectonic plate beneath another.  The arc extends from Hokkaido in Japan, across the Kamchatka Peninsula, through to the Commander Islands (Russia) to the Northwest. It is estimated that the Pacific Plate is moving towards the Okhotsk microplate at a rate of approximately 79mm per year, with variations in speed along the arc.

There are over 100 active volcanoes along the arc. Eruptions began during the late Pleistocene, some 126,000 years ago at a time when mammoths still roamed the vast northern frozen landscapes and the first modern humans walked the Earth.

Many of the volcanoes in the region continue to be active today. Amongst them is Karymsky volcano, the focus of this week’s Imaggeo on Mondays image. Towering in excess of 1500 m above sea level (a.s.l), the volcano is composed of layers of hardened lava and the deposits of scorching and fast moving clouds of volcanic debris knows as pyroclastic flows. You can see some careering down the flanks of the volcano in this image of the July 2004 eruption. The eruptive column is the result of a

“strong Vulcanian-type explosion, with the cloud quickly rising more than 1 km above the vent. The final height of the eruption cloud was approximately 3 km and in the image you can clearly see massive ballistic fallout from multiple hot avalanches on the volcanoes slopes,”

explains Alexander Belousov, a Senior Researcher at the Institute of Volcanology and Seismology in Russia and author of this week’s photograph.

 

USGS map of the Kuril-Kamchatka trench, showing earthquake locations and depth contours on downgoing slab. Credit: USGS, USGS summary of the 2013 Sea of Okhotsk earthquake, via Wikimedia Commons.

USGS map of the Kuril-Kamchatka trench, showing earthquake locations and depth contours on downgoing slab. Credit: USGS, USGS summary of the 2013 Sea of Okhotsk earthquake, via Wikimedia Commons.

If you pre-register for the 2015 General Assembly (Vienna, 12 – 17 April), you can take part in our annual photo competition! From 1 February up until 1 March, every participant pre-registered for the General Assembly can submit up three original photos and one moving image related to the Earth, planetary, and space sciences in competition for free registration to next year’s General Assembly!  These can include fantastic field photos, a stunning shot of your favourite thin section, what you’ve captured out on holiday or under the electron microscope – if it’s geoscientific, it fits the bill. Find out more about how to take part at http://imaggeo.egu.eu/photo-contest/information/.

Follow

Get every new post on this blog delivered to your Inbox.

Join other followers: