GeoLog

Imaggeo on Mondays

Imaggeo on Mondays: A spectacular rainbow

Imaggeo on Mondays: A spectacular rainbow

Back in February 2005, François Dulac and Rémi Losno worked in the field in the very remote Kerguelen Islands (also known as the Desolation Islands). Located in the southern Indian Ocean they are one, of the two, only exposed parts of the mostly submerged Kerguelen Plateau.

Our work consisted in sampling atmospheric aerosols and their deposition by rain on the island, which is a meeting point for the roaring fourties (strong westerly winds found in the Southern Hemisphere between 40 and 50 degrees latitude) and the equally turbulent furious fifties (which occur at more southerly latitudes still).

The aim of the study was to evaluate the input of chemical elements (in very low concentrations) derived from continental soil dust, to the remote surface waters of the Southern Ocean. Given the scarcity of land areas at this latitude, the particles were expected to have travelled long distances before arriving at Kerguelen.

For example, iron – one of the major elements in the Earth crust and soils – is of particular interest in this oceanic area because it is a micro-nutrient that limits the productivity (and related CO2 sink) of the Southern Ocean.

The island’s air was often very clear and the horizontal visibility unusually high, as can be seen in the photo. It highlights that atmospheric aerosol concentrations (the mixture of solid and liquid particles from natural and anthropogenic sources) are very low in this environment. Field sampling and subsequent chemical analyses require constraining protocols adapted to ultra-traces in order to minimize contamination of samples and blank levels.

The unique atmospheric conditions also meant we had problems estimating distances: we often found ourselves underestimating the stretch between two points during our long walks between the base and our remote sampling stations. In addition, the combination of very clean air, low sun and fast running atmospheric low-pressure systems carrying water clouds at low-level over the cold ocean make rainbows relatively frequent.

Walking back to the base after changing samples, we were caught in a rain shower. Raindrops were almost falling horizontally due to the high wind speed, leaving the soil dry downwind of the stones and rocks lying on the ground. A few minutes later clouds had passed and sunlight reflecting and diffracting in the cloud droplets offered us a spectacular semi-circular rainbow.

It was particularly special because it displayed an infrequent combination of (i) the main, classic, bright rainbow that shows up at 138-140 degrees from the direction of the sunlight, (ii) a secondary rainbow due to double reflection of sunlight in droplets that appears higher on the horizon at an angle of about 127-130 degrees and with an inversion of colours compared to the main bow (red inside), and (iii) one supernumerary rainbow with pastel green, pink and purple fringes on the inner side of the primary bow.

This stacked rainbow is caused by interferences and was first explained in 1804 by Thomas Young. It indicates the presence of small, uniformly sized droplets.  The dark area visible here on the right-hand side between the primary and secondary rainbows is called the Alexander’s band, after the ancient Greek philosopher Alexander of Aphrodisias comments on Aristotle’s Meteorology treatise, published in the early 3rd century. It is due to a lack of light resulting from the fact that diffracted rays are either reflected back inside the primary rainbow (causing this area to be brighter) or outside the secondary rainbow.

By François Dulac, Laboratoire des Sciences du Climat et de l’EnvironnementCEA/LSCE, Gif-sur-Yvette, France

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: Autumnal Larch

Imaggeo on Mondays: Autumnal Larch

For a fantastically picturesque train ride, consider travelling by rail between Lanquart and Davos (in Switzerland). You’ll be rewarded with stunning Alpine views, especially in autumn when the Larches, surrounded by Spruces, turn yellow and cast pretty reflections in the waters of the mountain lakes. Seen here is Schwartzsee, located only a few meters from ‘Davos Laret’ train station.

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: Angular unconformity

Imaggeo on Mondays: Angular unconformity

It is not unusual to observe abrupt contacts between two, seemingly, contiguous rock layers, such as the one featured in today’s featured image. This type of contact is called an unconformity and marks two very distinct times periods, where the rocks formed under very different conditions.

Telheiro Beach is located at the western tip of the Algarve; Portugal’s southernmost mainland region and the most touristic too.

The area, famous for its famous rocky beaches and great seefood, shows a spectacular Variscan unconformity between the highly-folded greywackes and shales of the Brejeira Formation (Moscovian-Carboniferous) and the horizontally placed red sandstones and mudstones of the Group Grés de Silves (of Late Triassic age: 237 and 201.3 million years old). There is a hiatus of about 100 million years between the two formations.

The Variscan period ranges from 370 million to 290 million year ago and is named after the formation of a mountain belt which extends across western Europe, as a result of the collision between Africa and the North American–North European continents.

The imposing sea cliffs produce a privileged place to observe the end of the Variscan Cycle and the beginning of the Alpine Cycle.

It is possible to visit the outcrop on foot, from the top of the cliffs to the beach, although the path is of high degree of difficulty. When going down to the beach one can begin to visualise the typical lithologies of the Grés de Silves. Toward its top you can see red to green Mudstones (dominant) intercalated with rare dolomites and immediately above the unconformity plane it is possible to observe the red sandstone with cross stratification. The highly-folded turbidites (a type of sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean) of the Brejeira Formation are located below the unconformity.

The folds feature chevron geometries (where the rocks have well behaved layers, with straight limbs and sharp hinges, so that they look like sharp Vs). The folding is the result of the final deformation phase of the Variscan compression.

The beds of sedimentary rocks show sedimentary structures attributed to sedimentation in a turbidic environment (turbititic currents), namely the Bouma sequence and sole marks like flute, groove and load casts.

                                                                                                     By André Cortesão, Environmental Engineer and Geoscientist collaborator of the University of Coimbra Geosciences Centre

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: A spectacular view of moss-covered rocks

Imaggeo on Mondays: A spectacular view of moss-covered rocks

Geology has shaped the rugged landscape of the Isle of Skye – the largest island of Scotland’s Inner Hebrides archipelago. From the very old Precambrian rocks (approximately 2.8 billion years old) in the south of the island, through to the mighty glaciers which covered much of Scotland as recently as 14,700 years ago, the modestly-sized island provides a snap-shot through Earth’s dynamic history.

A far cry from its modern cold, foggy and drizzly weather, back in the Jurassic age (250 million years ago, or so), the island was part of hot and dry desert. Over time, the sea encroached the low-lying plain, depositing sands and muds, and later sandstones, as well as thin limestones and shales across the island. The best examples of these rocks are found on the western side of the island, on the Strathaird Peninsula, but they can also be found on northern and eastern coastal stretches too.

Fast-forward to the Tertiary period (approximately 60 million years ago) and the landscape changed dramatically. The calm tropical waters had made way for explosive eruptions, which vented lavas from crack’s in the Earth’s crust. The lavas blanketed large areas of the north of the island, covering the sediments deposited back in the Jurassic.

Long after the surface explosive activity ended, the cracks in the Earth’s crust continued to serves as pathways for molten magma to move below the surface. In the norther part of the island, the lava travelled sideways, pushing its way between the layers of Jurassic sedimentary rocks. The black lavas, layered between the lighter coloured limestones and sandstones (as pictured above), are in stark contrast with the present-day moss-covered cliffs.

The most spectacular examples of this layering of volcanic units atop sedimentary rocks can be seen not far from where this photograph was taken, at Kilt Rocks, in south Staffin. Visitors to the area can also enjoy Mealt waterfall, where water from Mealt Loch (the Scottish word for lake) tumbles spectacularly into the Sound of Raasay.

Marius Ulm, who captured today’s featured image, is a civil/coastal engineer meaning a totally different aspect of the geology captured his attention:

“From a coastal engineering point of view, what is interesting is the missing moss-cover at the cliff’s toe. There is a line which marks the transition where the rocks stop being covered by moss also indicates how high water regularly rises due to tides. It tells us the tidal range (difference between low and high water) reaches up to 5 m in this area.”

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/.