GeoLog

Geomorphology

Imaggeo on Mondays: Sedimentary record of catastrophic floods in the Atacama desert

Imaggeo on Mondays: Sedimentary record of catastrophic floods in the Atacama desert

Despite being one of the driest regions on Earth, the Atacama desert is no stranger to catastrophic flood events. Today’s post highlights how the sands, clays and muds left behind once the flood waters recede can hold the key to understanding this natural hazard.

During the severe rains that occurred between May 12 and 13, 2017 in the Atacama Region (Northern Chile) the usually dry Copiapó River experienced a fast increase in its runoff. It caused the historic center of the city of Copiapó to flood and resulted in thousands of affected buildings including the University of Atacama.

The city of Copiapó (~160,000 inhabitants) is the administrative capital of this Chilean Region and is built on the Copiapó River alluvial plain. As a result, and despite being located in one of the driest deserts of the world, it has been flooded several times during the 19th and 20th century. Floods back in 2015 were among the worst recorded.

The effects of the most recent events are, luckily, significantly milder than those of 2015 as no casualties occurred. However, more than 2,000 houses are affected and hundreds have been completely lost.

During this last event, the water height reached 75 cm over the river margins. Nearby streets where filled with torrents of mud- and sand-laden waters, with plant debris caught up in the mix too. Once the waters receded, a thick bed of randomly assorted grains of sand  was deposited over the river banks and urbanized areas.

Frozen in the body of the bed, the sand grains developed different forms and structures. A layer of only the finest grained sediments, silts and clays, bears the hallmark of the final stages of the flooding. As water speeds decrease, the finest particles are able to drop out of the water and settle over the coarser particles. Finally, a water saturated layer of mud, only a few centimeters thick, blanketed the sands, preserving the sand structures in 3D.

The presence of these unusual and enigmatic muddy bedforms has been scarcely described in the scientific literature. A new study and detailed analysis of the structures will help better understand the sedimentary record of catastrophic flooding and the occurrence of high-energy out-of-channel deposits in the geological record.

By Manuel Abad and Tatiana Izquierdo, Universidad de Atacama (Chile)

 

Imaggeo on Mondays: Sneaking up from above

Imaggeo on Mondays: Sneaking up from above

Take some ice, mix in some rock, snow and maybe a little mud and the result is a rock glacier. Unlike ice glaciers (the ones we are most familiar with), rock glaciers have very little ice at the surface. Looking at today’s featured image, you’d be forgiven for thinking the Morenas Coloradas rock glacier wasn’t a glacier at all. But appearances can be misleading; as Jan Blöthe (a researcher at the University of Bonn) explains in today’s post.

The picture shows the Morenas Coloradas rock glacier, a pivotal example of actively creeping permafrost (ground that remains frozen for periods longer than two consecutive years) in the dry central Andes of Argentina. The rock glacier is located in the “Cordon del Plata” range, some 50 km east of the city of Mendoza.

The rock glacier fills the entire valley and slowly creeps downslope creating impressive lobes and tongues with steep fronts. With more than 4 km length, the Morenas Coloradas is one of the largest rock glaciers of the central Andes.

Taken from a drone, the picture looks straight up the rock glacier into the main amphitheatre-like valley formed by glacial erosion located at ~4500 m.a.s.l. From there, large amounts of loose debris are moved down the valley at speeds on the order of a few meters per year. The creeping process forms tongues of material that override each other, producing the characteristic surface with steps, ridges and furrows.

The central Andes of Argentina are semi-arid, receiving less than 500 mm of precipitation per year, mainly falling as snow during the winter. The region is famous for its wines, which are grow in the dry Andean foreland that is heavily dependent on meltwater from the mountains. How much of this meltwater is actually stored in ice-rich permafrost landforms is unknown.

As opposed to ice glaciers, rock glaciers show a delayed reaction to a changing climate, as large amounts of debris cover the ground ice, isolating it from rising air temperatures. With large areas located above the lower altitudinal limit of mountain permafrost of ~3600 m.a.s.l., the central Andes of Argentina might store significant amounts of water in the subsurface.

Using mainly near-surface geophysics, our research tries to quantify the water storage capacities in the very abundant and impressive rock glaciers of the region. The Morenas Coloradas rock glacier is of special importance in this regard, as first geophysical measurements date back to the 1980s. Since then, active layer thickness has dramatically increased in the lower parts of the rock glacier, indicating that also the ground ice of the permafrost domain of the central Andes is suffering under the currently warming climate.

A final remark: Thanks goes to the entire team of this research project, namely Christian Halla, Estefania Bottegal, Joachim Götz, Lothar Schrott, Dario Trombotto, Floreana Miesen, Lorenz Banzer, Julius Isigkeit, Henning Clemens, and Thorsten Höser.

By Jan Blöthe, University of Bonn, Germany

Imaggeo on Mondays: Erosion

Imaggeo on Mondays: Erosion

In mountainous regions precipitation – be that in the form of rain, hail or snow, for example – drives erosion, which means it plays an important part in shaping the way the landscape looks. Precipitation can directly wear away at hillsides and creates streams and rivers, which leave their mark on the scenery by cutting and calving their way through it.

Take for instance the hills in the arid coastal region of Pisco Valley, in Peru (pictured above). Contrary to what you might think having first looked at the photograph, very little erosion of rock happens here. The solid rock which makes up the undulating hills is a hard-wearing grantic rock (not dissimilar to the stone you might covet for your kitchen countertops).

Over time, wind-blown sediments have blanketed the granites. Loesses, as the deposits are known, are very soft and range between 20 and 60 cm in thickness. The channels which slice the hillside are carved into the loesses, not the granites which lie below.

Rain is such a rare thing in these parts that soil barely forms (Norton et al., 2015) and it’s impossible for plants to grow on the soft substrate, leaving the slopes exposed to the elements. When the infrequent rains do come, small scale gullies, only a few centimetres deep cut their way into the sediments, taking away material loosened by torrential rainfalls at high speeds.

References

Kevin P. Norton, Peter Molnar, Fritz Schlunegger, The role of climate-driven chemical weathering on soil production, Geomorphology, Volume 204, 1 January 2014, Pages 510-517, ISSN 0169-555X, http://dx.doi.org/10.1016/j.geomorph.2013.08.030.

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: Concord at midnight

Imaggeo on Mondays: Concord at midnight

The high peaks of the Alps are always awe inspiring, but this midnight shot, captured by Alessandro Lechmann, a PhD student at the Institute of Geological Sciences at the University of Bern, further enhance their fragile beauty. With a warming climate threatening snow availability to even the highest peaks, it has never been more important to appreciate the importance of the glaciers which drape the mountain slopes.

This photograph shows a view from the Jungfraujoch (a saddle in the Bernese Alps, connecting the two four-thousander peaks Jungfrau and Mönch, at an elevation of 3,466 metres above sea level) towards the south-east down the Jungfraufirn (an arm of the Great Altesch Glacier).

Originating amidst three of the most famous mountains of the Swiss Alps (Eiger, Mönch and Jungfrau), this glacier flows southwards towards the Concordiaplatz, where it merges with the Ewigschneefäld and the Great Aletschfirn into the Great Aletsch Glacier. Even today, despite reports of receding glaciers in the Alps, it forms the largest and longest Alpine glacier.

In the countries surrounding the Alps, glacial landforms dominate the landscape. From drumlins, moraines (accumulations of glacial debris) and overdeepenings in the foreland to U-shaped valleys (Lauterbrunnen is a marvellous example) and cirques in mountainous regions. Although retreating at rates not seen previously, these glaciers carved the face of central Europe during the last glacial-interglacial cycles.

The building of the railway to the Jungfraujoch research station started in 1896 and was completed in 1912; an impressive feat considering the limited technology before the First World War. Perched precariously 3500 m above sea level, the research station (known for its prominent sphinx observatory), has contributed significantly   to the understanding of the atmospheric sciences, glaciology and cosmic ray physics.

The ridge which the Jungfraujoch is built on, marks the northern margin of the exposed crystalline core of the Alpine orogeny. Interestingly, this mountain ridge, in addition to being a geological boundary, is also a major watershed. Rain that falls north, flows via the Aare into the Rhine, which eventually discharges into the North Sea. Precipitation on the southern flank and melt water from the Jungfraufirn, on the other hand, joins the Rhone in the Valais valley, that ends up in the Mediterranean Sea. This highlights the importance of Alpine glaciers as a water stores which continue to provide water throughout the year.

By Alessandro Lechmann PhD student at the Institute of Geological Sciences at the University of Bern

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/.

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