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Imaggeo on Mondays: Quartz lawns and crystal flowers

26 May

Petrologists spend a large part of their time peering down microscopes at wafer thin slices of rock to work out what they’re made of and how they were formed. What lies on the other side of the lens can be an incredibly beautiful pattern, a kaleidoscope of colour, or stark bands of black and white, all of which provide clues to the rock’s history, and the history of the landscape it came from. Bernardo Cesare, enthusiastic photographer and professor of petrology at the University of Padova, Italy, has captured some fantastic images of these slender rock sections, including the incredible image of ocean jasper, below.

Ocean jasper under the microscope – 30 micrometres thin and almost entirely quartz. (Credit: Bernardo Cesare via imaggeo.egu.eu)

Ocean jasper under the microscope – 30 micrometres thin and almost entirely quartz. (Credit: Bernardo Cesare via imaggeo.egu.eu)

Ocean jasper is a variety of jasper found only in Madagascar and is increasingly sought after as a precious stone, and with that, decent rock samples are hard to come by. “I have long been searching for affordable samples of ocean jasper, until I saw a necklace in a market stall. I cut all the beads, prepared thin sections from them, and put them under the microscope,” says Cesare.  “They turned out to contain a microscopic garden of quartz flowers in a fine-grained silica matrix. In places some coarser crystals form ‘rosettes’ in a lawn of fibrous quartz too.”

Jasper jewelery – it looks even more lovely under the microscope, don’t you think? (Credit: Tess Norberg/Nova Design)

Jasper jewellery – it looks even more lovely under the microscope, don’t you think? (Credit: Tess Norberg/Nova Design)

Ocean jasper is made up of many minute orbs, just a few millimetres in diameter, known as spherulites. They form when silica-rich volcanic rocks change from being glassy to crystalline, and are saturated with silica in the process. This crystallisation occurs in arrays of thousands of fibrous, needle-like crystals. “They can grow as perfect spheres, but where they’re too close to one another the growing spherulites impinge on each other,” Cesare explains. This close clustering causes the spherulites to have sharp boundaries where they meet, something clearly seen in Cesare’s snap of the crystal structure.

Even though almost everything in the image (the exception being the small black dots, which are opaque minerals) is made of quartz, a rainbow of colours can be seen. These rainbows are known as interference colours, and they appear when polarized light passes through a crystal. So what creates this rainbow?

When white light, first polarized by a filter, passes through the crystals of a rock, it is split in two components that travel at different speeds. These components interfere with the crystal structure in different ways depending on their wavelength and the crystal’s orientation. When light emerges from the crystal and is filtered for the second time, some wavelengths are suppressed, so the colour of the light is no longer white. Cesare explains why: “the interference colour depends on the type of mineral (more precisely on its birefringence), on its thickness, and on its orientation. This is why crystals of the same mineral (quartz) may display different colours in my image: because they have different optical orientations!”

There are even ways to test out this tool at home: “even without a slice of rock, readers can test how interference colours emerge using two “crossed” polarizing filters (for example two orthogonal lenses of some sunglasses) and placing a stretched piece of plastic bag between them,” says Cesare.

A shiny sample of ocean jasper (Credit: Druzy Macro)

A shiny sample of ocean jasper (Credit: Druzy Macro)

“Polarized light is one of the fundamental tools of a geologist: with a polarizing microscope and a thin section we can recognize different minerals without the aid of more sophisticated and expensive analyses. Looking at colours and at their changes, at the shapes and contours of mineral grains, at their sizes and mutual relationships, not only can we understand which minerals a rock contains, but also in which sequence they formed, if some deformation occurred during or after their crystallisation, if they were transformed into other minerals and, qualitatively, at which pressure and temperature conditions the rock originated or evolved.” All these observations form the basis for more detailed research, such as, working out how many millions (or billions) of years ago the rock formed.

You can find out more about ocean jasper and Cesare’s photographic style over at National Geographic and on Geology In Art. You can also find more of his photography over at www.microckscopica.org.

By Sara Mynott, EGU Communications Officer

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: Villarrica Volcano

14 Apr

This week’s Imaggeo on Mondays highlights the vulnerability of Villarrica’s slopes and zooms in on the volcano’s spectacular crater…

Villarrica, one of the largest stratovolcanoes in Chile, is also one of the country’s most active. The volcano is iced by glaciers that make the mountain a stunning scene, but also a dangerous one. The glaciers cover some 30 square-kilometres of the volcano and, during an eruption, the snow and ice melts to form lahars (rapid mud flows that move at 30-40 kilometres per hour). These flows present a formidable hazard to the towns that flank the volcano.

Villarrica from above – if you look closely, you can see the traces of lahars on the volcano’s northwest flanks. (Credit: NASA Earth Observatory/Jesse Allen/Robert Simmon)

Villarrica from above – if you look closely, you can see the traces of lahars on the volcano’s northwest flanks. (Credit: NASA Earth Observatory/Jesse Allen/Robert Simmon)

Villarica’s crater spans some 250 metres and takes the form of steeply sloping basalt. The cherry on the volcanic cake (and just out of shot here) is the lava lake at its summit. Villarrica is constantly degassing through this lava lake – a process that releases pressure below the surface. Without it, eruptions would be much more violent.

Peering into the crater of Villarrica Volcano, Chile. (Credit: Dávid Karátson via imaggeo.egu.eu)

Peering into the crater of Villarrica Volcano, Chile. (Credit: Dávid Karátson via imaggeo.egu.eu)

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: 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.

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