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Imaggeo on Mondays: Volcanic rope

7 Apr

On Hawaii, lava fields fall into two camps – pahoehoe and a’a. This week’s Imageo on Mondays puts the two into perspective…

Pahoehoe fields are created when the lava is well insulated at the surface. The cooled rock on top prevents a lot of heat escaping and lets the lava flow beneath a tough skin of basalt. This skin is pulled and distorted by the moving lava, creating ripples and wrinkles that resemble rope.

Pahoehoe lava. (Credit: Martin Mergili via imaggeo.egu.eu)

Here’s a close up! (Credit: Martin Mergili via imaggeo.egu.eu)

Pahoehoe flow. (Credit: Tari Noelani Mattox, USGS)

And here’s the bigger picture! (Credit: Tari Noelani Mattox, USGS)

The other type, a’a, sounds a lot like it feels. Cooled a’a fields are notoriously difficult to walk over and the jagged rocks can quite happily shred your walking boots to pieces. A’a lava is much more viscous than pahoehoe and flows uninsulated over the surface. They form tall fronts of rough basalt blocks that are pushed forward and swallowed by the lava as it moves downslope.

A’a lava (and a little pahoehoe in the foreground). (Credit: USGS)

A’a lava – and a little pahoehoe in the foreground fro good measure. (Credit: USGS)

Want to see some lava flow on the go? The BBC has a collection of excellent clips.

Imaggeo is the EGU’s open access geosciences image repository. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.

Imaggeo on Mondays: A rolling stone gathers no moss

31 Mar

Philippe Leloup brings us this week’s Imaggeo on Mondays, with tales from a mountain trail that show a geologist can never resist a good rock!

In reality, this shiny slab of rock is about 20 centimetres across. Polished to perfection, the layers of marble and amphibole are beautiful to behold. (Credit: Philippe Leloup via imaggeo.egu.eu)

In reality, this shiny slab of rock is about 20 centimetres across. Polished to perfection, the layers of marble and amphibole are beautiful to behold. (Credit: Philippe Leloup via imaggeo.egu.eu)

This image is that of a polished slab of a rock composed of interlayered marbles and amphibolites. The sample was once part of a small dry-stone wall bordering an outdoor kitchen along a trail along the Ailao Mountain Range in China (or Ailao Shan in Chinese).

As I passed by, a small black eye looked at me, and I couldn’t resist asking the owner to give me that stone – one that could easily be replaced by any other rock nearby, and he kindly agreed. The rock was special for me because I felt that its structure would be spectacular.

The Ailao Range is part of the Ailao Shan – Red River shear zone, a region that stretches for more than 1000 kilometres – from southeast Tibet to the Tonkin Gulf. During the Oligo-Miocene, the Indochina bock (encompassing Vietnam, Cambodia, Laos and Thailand) was pushed away from the collision between the Indian and Asian continents and moved several hundreds of kilometres towards the southeast along that ~10 kilometre-wide shear zone. Today, evidence that intense ductile deformation occurred are found in gneiss and marbles showing steep foliation, horizontal lineation, and numerous left-lateral shear features – a type of deformation that leaves rocks looking like this:

 A thin section microphotograph (total width ~0.5 mm) showing several feldspar crystals with bended tails. These tails show that they have slowly rotated counter-clockwise. These rolling structures are characteristic of left-lateral ductile deformation. (Credit: Philippe Leloup via imaggeo.egu.eu).

A thin section microphotograph (the total width is about 0.5 mm) showing several feldspar crystals with bended tails. These tails show that they have slowly rotated counter-clockwise. These rolling structures are characteristic of left-lateral ductile deformation. (Credit: Philippe Leloup via imaggeo.egu.eu).

When I cut the rock it turned out that the amphibole layer had a very special shape, like a Swiss roll, resulting from simple shear – something that revealed spectacular colours and a stunning shape when seen in section.

By Philippe Leloup, University of Lyon

Imaggeo is the EGU’s open access geosciences image repository. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.

Imaggeo on Mondays: Rockscape

24 Mar

A geologist out in the field is often the one doing the mapping, but sometimes you might just find a map while you’re out there. Martin Reiser shares how he stumbled one such stunning feature…

Oxidised and eroded limestone layers, resembling patterns of a geological map. (Credit: Martin Reiser, distributed via imaggeo.egu.eu)

Oxidised and eroded limestone layers, resembling patterns of a geological map. (Credit: Martin Reiser, distributed via imaggeo.egu.eu)

The picture shows colourful marly layers in a Triassic limestone of northwest Albania (Lezhe region). The marly layers have developed intense reddish and greenish colours due to exposure to reduction and oxidation processes (redox-conditions). Subsequent partial erosion has then exposed different levels of these coloured layers and created a pattern resembling a geological map with horizontal layering. Although it might look artificial or even painted, the colours of the rock were really that intense!

The map-like pattern caught my eye when we visited that outcrop during a fieldtrip to Albania in 2011. The fieldtrip was organised by the University of Berlin to study the geology of Albania and specifically focused on the regional tectonic evolution from the Jurassic until the present.

By Martin Reiser, University of Innsbruck

Imaggeo is the EGU’s open access geosciences image repository. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.

Imaggeo on Mondays: Winter waterfalls reveal their secrets

17 Mar

Cyril Mayaud is kicking of this week’s Imaggeo on Mondays with an insight into what waterfalls in winter can tell us about their local hydrology… 

The picture below shows the lower Peričnik waterfall during winter season. This cascade system is composed of two successive waterfalls that stretch some 16 metres (upper fall) and 52 metres (lower fall) high and is one of the most beautiful natural sights in the Triglav National Park. The cliff is located at the western rim of a U shaped valley and is composed of a very permeable conglomerate rock, which is made up of glacier material that accumulated at the rims of the valley back when the glacier retreated.

Peričnik waterfall from behind the scenes. (Credit: Cyril Mayaud

Peričnik waterfall from behind the scenes. (Credit: Cyril Mayaud

The high permeability of the rock provides an important path for water transfer, letting it infiltrate between the level of the upper and the lower fall. This transfer is particularly visible if you walk in the passage under the fall, where the infiltrated water falls at an intensity comparable to a strong shower. Winter is also a fascinating time to visit the falls and see how the water flows from the upper level to the lower level. The low temperatures freeze the dripping water, creating a picturesque landscape with beautiful ice stalactites and draperies.

Peričnik waterfall, an amazing sight in Slovenia’s Triglav National Park. (Credit: Cyril Mayaud, distributed by imaggeo.egu.eu)

Peričnik waterfall, an amazing sight in Slovenia’s Triglav National Park. (Credit: Cyril Mayaud, distributed by imaggeo.egu.eu)

As hydrogeologist, I see two key scientific points of interest in this picture: the first relates to the water transfer between the two levels, which is delayed during winter (due to the low temperatures) as it shows a spatial snapshot of the infiltration processes through the outcrop. The second underlines the importance of accurately quantifying all the different hydrological processes in a given catchment in order to better understand its hydrological behaviour. As an example, the storage of water as snow is really important for mountainous catchments (like the catchment of the Fraser River in British Columbia) and plays a prominent role in retaining water during the cold season and releasing it during spring/summer.

The waterfall in summer, a wonderful view. (Credit: Cyril Mayaud)

The waterfall in summer, a wonderful view. (Credit: Cyril Mayaud)

A parallel could be also made with the hydrological behaviour of karst aquifers, which depend on a variety of processes, each with different time scales. Because these aquifers contain fractures with a huge size range (from cracks less than 1 mm wide to conduits bigger than 10 metres), these aquifers allow water to infiltrate in two very different ways, and are said to have a double infiltration capacity: rapid and localised infiltration through sinkholes and ponors, and slow, diffuse infiltration of rainwater in the unsaturated zone. The origin and path of the water can normally be differentiated during chemical sampling in the spring.

By Cyril Mayaud, University of Graz  Austria

Imaggeo is the EGU’s open access geosciences image repository. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.

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