GeoLog

Geomorphology

Imaggeo on Mondays: the rocks that look like Swiss cheese

Imaggeo on Mondays: the rocks that look like Swiss cheese

Over the course of centuries and millennia, the force of winds, seas, ice and rains, sculpt rock formations around the globe. From the world-famous glacier carved landscapes of Yosemite National Park, to the freeze-thawed hoodoos at Bryce National Park, through to the wind battered stone pillars of South China Karst, boundless geological formations have been transformed by the power of erosion and weathering.

When the force of winds and salty waters combine, their effect on the surface of rocks is quite unique. In some costal environments, a network of holes, of all shapes and sizes, puncture otherwise smooth and silky rocks. This form of weathering is aptly known as honeycomb weathering (though some of you might be more familiar with terms such as cavernous weathering, alveoli/alveolar weathering, stone lattice, stone lace or miniature tafoni weathering). Limestones, sandstones and granites are most affected.

Exactly how the interaction of the sea breeze and the salt in ocean waters results in the distinctive ‘Swiss cheese’ weathered pattern remains a bit of a mystery.  One of the front running theories proposes that it is the culmination of physical and chemical weathering.

Evaporation of leachate causes a deposition of the rocks minerals on its surface which leads to a decomposition of the rocks interior. Additionally salt weathering caused by oceanic brackish water as well as temperature changes support the formation of this feature,” explains Michael Grund, a researcher at Karlsruhe Institute of Technology.

In Corsican, Tafoni, means hole or perforated rock, so it is not surprising that this form of weathering sometimes takes its name after the Tafoni rock formation on the southern coast of Sardinia, where Michael snapped a superb example of the potholed intrusives which dominate the 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/.

Imaggeo on Mondays: The road to nowhere – natural hazards in the Peloponnese

Imaggeo on Mondays: The road to nowhere – natural hazards in the Peloponnese

The Gulf of Corinth, in southern Greece, separates the Peloponnese peninsula from the continental mainland. The structural geology of the region is complex, largely defined by the subduction of the African Plate below the Eurasian Plate (a little to the south).

The Gulf itself is an active extensional marine basin, i.e., one that is pulling open and where sediments accumulate. Sedimentary basins result from the thinning, and therefore sinking, of the underlying crust (though other factors can also come into play). The rifting in the region is relatively new, dating back some five million years, and results in rare but dangerous earthquakes.

The active tectonics result in a plethora of other natural hazards, not only earthquakes.  Minor and major faults crisscross the area and have the potential to trigger landslides, posing a threat to lives and infrastructure. A road, swept away in a landslide, in the northern Peloponnese (along the southern margin of the Corinth rift) is a clear example of the hazard.

“This photo was taken in the Valimi fault block [editor’s note: a section of bedrock bound on either side by faults], east of the Krathis valley. West of this valley, the landscape is characterised by  narrow and deep gorges as the present day rivers cut into the well-consolidated conglomerates deposited during the active extension of the basin,” explains Romain Hemelsdaël, author of this week’s imaggeo on Mondays photograph.

Characteristically, sediments deposited in actively extensional rifts where the Earth’s crust and lithosphere are being pulled apart, as at the Gulf of Corinth, change in size (both horizontally and vertically) and composition. To the east of the Krathis valley, the sediments are being uplifted and are dominated by less competent sandstones and siltstones, as opposed to the conglomerates found in the Valimi fault block.

“The present landscape along this part of the rift margin forms large valleys covered by active landslides,” clarifies Romain. “In this photograph, the road was initially constructed directly on silts which were deposited by lakes and rivers. Up the hill, a temporary track currently replaces the road but this track still remains within an active landslide.”

 

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: Living flows

Imaggeo on Mondays: Living flows

There are handful true wildernesses left on the planet. Only a few, far flung corners, of the globe remain truly remote and unspoilt. To explore and experience untouched landscapes you might find yourself making the journey to the dunes in Sossuvlei in Namibia, or to the salty plain of the Salar Uyuni in Bolivia. But it’s not necessary to travel so far to discover an area where humans have, so far, left little mark. One of the last wilds is right here in Europe, in the northern territories of Sweden. Today’s spectacular photograph of the Laitaure delta is brought to you by Marc Girons Lopez, one of the winners of the 2016 edition of the EGU’s Photo Contest!

The photograph shows a part of the Laitaure delta, at the entrance of Sarek National Park (Northern Sweden). Sarek is one of the oldest national parks in Europe and it is often considered to be one of the last wild areas in Europe. The Sami people, however, have traditionally used these lands.

This delta is formed by the Rapa River when it flows into Lake Laitaure. The Rapa River springs from the Sarektjåkkå glacier and is fed by over thirty glaciers. The specific flow of the Rapa River — the ratio between its flow and the area of its catchment — is the highest in Sweden. The magnitude of the flow has strong seasonal fluctuations which are reflected in the sediment transport, which can be as high as 10,000 tons per day during the summer. This heavy sediment load gives the river its characteristics greyish colour. The different colours in the backwater zones may be produced by dissolved organic matter from decomposing vegetation.

The delta in this area is flanked by  patches of montane forests along the river banks in an area otherwise covered by marshes. Regarding the fauna, according to Wikipedia the Eurasian teal, the Eurasian wigeon, the greater scaup, the red-breasted merganser, the sedge warbler and the common reed bunting are common in the Laitaure delta.

By Marc Girons Lopez, researcher at the Centre for Natural Disaster Science, Uppsala University

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: Coastal erosion

Imaggeo on Mondays: Coastal erosion

Coastlines take a battering from stormy seas, gales, windy conditions and every-day wave action. The combined effect of these processes shapes coastal landscapes across the globe.

In calm weather, constructive waves deposit materials eroded elsewhere and transported along the coast line via longshore-drift, onto beaches, thus building them up. Terrestrial material, brought to beaches by rivers and the wind, also contribute.  In stormy weather, waves become destructive, eroding material away from beaches and sea cliffs.

In some areas, the removal of material far exceeds the quantity of sediments being supplied to sandy stretches, leading to coastal erosion. It is a dynamic process, with the consequences depending largely on the geomorphology of the coast.

Striking images of receding coastlines, where households once far away from a cliff edge, tumble into the sea after a storm surge, are an all too familiar consequence of the power of coastal erosion.

In sandy beaches where dunes are common, coastal erosion can be managed by the addition of vegetation. In these settings, it is not only the force of the sea which drives erosion, but also wind, as the fine, loose sand grains are easily picked-up by the breeze, especially in blustery weather.

Grasses, such as the ones pictured in this week’s featured imaggeo image, work by slowing down wind speeds across the face of the dunes and trapping and stabilising wind-blown sands. The grasses don’t directly prevent erosion, but they do allow greater accumulation of sands over short periods of time, when compared to vegetation-free dunes.

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/.

Follow

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

Join other followers: