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Imaggeo on Mondays: Sunset on the Giant’s Causeway

Imaggeo on Mondays: Sunset on the Giant’s Causeway

Pictured here is the Giant’s Causeway – a region of basalt columns, created 50-60 million years ago during the Paleogene. The typical polygonal form of the bedrocks, a product of active volcanic processes from the past, is well underlined by the sunset’s light; that’s why I took the photo in the late evening. The separate cracks are extended by weathering over time and are filled eluvium, geological debris from the erosion.

After the lava cooled, approximately 40,000 columns have since been polished by sea wave action. I decided to show the slow action of the sea with a long exposure, because it’s a continuous process, not obvious at first to an untrained person, but nevertheless very important now. I think in one photo we can find a deep history of Earth’s development, which palaeogeographers are still trying to understand.

by Osip Kokin, Lomonosov Moscow State University, Russia

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 licenceSubmit your photos at http://imaggeo.egu.eu/upload/.

Imaggeo on Mondays: All seasons in one day

Imaggeo on Mondays: All seasons in one day

Storm fronts arrive on Britain’s west coast in waves throughout the winter (and very often the summer too!) It is not unusual to see rain falling on a beach with hail and snow lying, whilst a rainbow pierces the clouds.

Description by Mary Gagen, as it first appeared on imaggeo.egu.eu.

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: Robberg Peninsula – a home of seals

Imaggeo on Mondays: Robberg Peninsula – a home of seals

This picture is taken from the Robberg Peninsula, one of the most beautiful places, and definitely one of my favorite places in South Africa. The Peninsula forms the Robberg Nature Reserve and is situated close to the Plettenberg Bay on the picturesque Garden Route. “Rob” in Dutch means “seal”, so the name of the Peninsula is translated as “the seal mountain”. This name was given to the landmark by the early Dutch mariners, who observed large colonies of these noisy and restless animals on the rocky cliffs of the Peninsula. Seals still inhabit this area today.

The Peninsula is constituted of two rocky parts, rising above the sea level at about 150 m, and which are separated from each other by the “Gap” – a low and narrow sandy neck. The length of the Peninsula is about 4 km and the width is up to 750 m, which narrows down to 200 m at the “Gap”. The eastern rocky part forms an extension to the south – another tiny peninsula, “the Island” (on the picture). “The Island” is a small rocky area connected with the main Peninsula by a narrow strip of sand. This sand bar is essentially a beach washed by water at its both long sides. With “the Island” the entire Peninsula has a shape of a mitten, which thumb is looking down.

Robberg Island is entirely built up by the sedimentary rocks: quartzites of the Table Mountain Group (part of the Cape Supergroup), and Robberg’s Formation sandstones, conglomerates and breccias. Table Mountain Group is formed some 500-330 million years ago; and the Robberg’s formation, which is the part of the Uitenhage Group, is around 120 million years old and related to Gondwana break-down. Upper parts of the Peninsula are covered by the Quaternary dune deposits. At the moment the rocks of Robberg Peninsula are actively eroded by the Indian Ocean waters.

Robberg Peninsula is a beautiful home of many marine bird species, small reptiles, seals and indigenous plants. The Nelson Bay Cave, situated on the Robberg Peninsula, is one of the oldest caves, inhabited by human. It was occupied from 120 000 years ago, by various tribes, including San and Khoikhoi people. Nelson Bay Cave was formed due to the erosion caused by the ocean waves.

Description by Elizaveta Kovaleva, post-doctoral researcher at University of the Free State, in South Africa

If you pre-register for the 2018 General Assembly (Vienna, 08–13 April), you can take part in our annual photo competition! From 15 January until 15 February, every participant pre-registered for the General Assembly can submit up three original photos and one moving image related to the Earth, planetary, and space sciences in competition for free registration to next year’s General Assembly!  These can include fantastic field photos, a stunning shot of your favourite thin section, what you’ve captured out on holiday or under the electron microscope – if it’s geoscientific, it fits the bill. Find out more about how to take part at http://imaggeo.egu.eu/photo-contest/information/.

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/.

Geosciences Column: Do coastlines have memories?

do coastlines have memories

Did you know that the shape of coastlines is determined by the angle at which waves crash against the shoreline. It has long been thought that fluctuations in the wave incidence angle are rapidly felt by coastlines, which change the shapes of their shores quickly in response to shifting wave patterns.

Or do they?

Researchers at the British Geological Survey, Duke University (USA) and Woods Hole Oceanographic Institution in Massachusetts, have performed experiments which show that spits and capes hold ‘a memory’ of their former shapes and past wave climates, influencing their present geomorphology. The findings have recently been published in the EGU’s open access journal Earth Surface Dynamics.

Gradients in sediment distribution within wave-driven currents and shoreface depth play an important role in shaping coastlines. But the angle between an offshore wave crest and the shoreline is chief among the parameters which shape coasts worldwide.

Low-angle waves – those with approach the coast at an angle of 45° or less – have a smoothing effect on the coastline and keep its shape relatively steady. On the other hand, high-angle waves – those with slam against the shore at an angle of 45° or more – introduce instability and perturbations which shape the coast.

The figure shows the experimental set-up used in the study. It also nicely illustrates how coastlines are shaped by the angle of the incoming wave. The arrows indicatenet flux direction under waves incoming from the left; arrow lengths qualitatively indicate the flux. Sand is not transported through cells which are in shadow for a particular wave. From C. W. Thomas et al., 2016.

Alterations to the patterns of shorelines are caused by enhanced erosion and/or deposition, driven by changes in wave climate. Ultimately, coastline geomorphology evolves depending on the relative degree of high and low-angle waves in the wave climate, as well as the degree of irregularity in the wave angle distribution.

Climate change will alter the wave climate, particularly during storm events, so we can expect shorelines to shift globally. Predicting how coastlines will adapt to changing climatic conditions is hard, but more so if coastlines retain a memory of their past shapes when responding to changing wave regimes.

Flying spits (finger-like landforms which project out towards sea from relatively straight shoreline) and cuspate capes (a triangular shaped accumulation of sand and shingle which grows out towards sea) are particularly susceptible to climate change. They form when high angle waves approach the shore at a slant. Animal communities living within fragile marine and estuarine ecosystems largely depend on the protection they offer. They are also of socio-economic importance as many shelter coastal infrastructures. Understanding how they will be affected by a changing climate is vital to develop well-informed coastal management policies.

To understand how changing wave climates affect the evolution of flying spits and cuspate capes (from now on referred to as spits and capes), the team of researchers devised experiments which ran on a computer simulation.

They generated an initially straight shoreline and set the wave conditions for the next 250 years (which is the length of time it takes in nature) to allow the formation of spits and capes.

To test whether pre-existing coastal morphologies played a role in shaping coastlines under changing wave climates, over a period of 100 years (which is loosely the rate at which climate change is thought to be occurring under anthropogenic influences), the scientists gradually changed the angle at which waves approached the coast.  After the 100 year period the simulation was left to run a further 650 years under the new wave conditions.

The investigation revealed that when subjected to gradual changes in the angle at which waves approach the shoreline, capes take about 100 years to start displaying a new morphology. The tips of the capes are eroded away and so they slowly start to shrink.

Spits adjust to change much more slowly. Even after 750 years the experimental coastlines retain significant undulations, suggesting that sandy spits retain a long-term memory of their former shape.

Snapshots of simulated coastline morphologies evolved under changing wave climate. U is the fraction of waves which are approaching the shoreline at 45 degress or higher. Coastlines evolved for 250 years under initial conditions. (aii, bii)> The U values of the changed wave climate show the coastline morphologies evolved 200 and 500 years after the wave climate is changed at 250 years, and the morphologies evolved over 1000 years under static wave climates with the same U. From C. W. Thomas et al., 2016. See paper for full image caption. Click to enlarge.

The implications of the results are far reaching.

Be it implicitly or explicitly, many studies of coastal geomorphology assume that present coastal shape is exclusively a result of present wave climate. The new study shows that even with steady wave climate conditions at present, coastline shapes could still be responding to a past change in wave climate.

Reconstructions of ancient coastal geographies and paleo-wave climates might also be approached differently from now on. The researchers found that as spits adjust to changing wave climates they can leave behind a complex array of lagoons linked by beach bridges. Though there are a number of process which can lead to the formation of these coastal features, researchers must also consider alterations of coastlines as a response to changing wave climate from now on.

The findings of the study can also be applied to the management of sandy coastlines.

Currently, forecasts of future shoreline erosion and sediment deposition are made based on observations of how coasts have changed in recent decades. The new study highlights these short observation timescales may not be enough to fully appreciate how our beaches and coasts might be reshaped in the future.

This is especially true when it comes to climate change mitigation. Decisions on how to best protect the world’s shores based on their environmental and socio-economic importance will greatly benefit from long-term monitoring of coastal geomorphology.

But more work is needed too. The experiments performed by the team only consider two types of coastline morphology  (spits and capes) and only two types of wave climate. While the experiments provide a time-scale over which spits and capes might be expected to change, other factors not considered in the study (wave height, shoreface depth, etc…) will alter the predicted timescales. The time-scales given by the study should be used only as a guideline and highlight the need for more research in this area.

 

By Laura Roberts Artal, EGU Communications Officer

 

References

Thomas, C. W., Murray, A. B., Ashton, A. D., Hurst, M. D., Barkwith, A. K. A. P., and Ellis, M. A.: Complex coastlines responding to climate change: do shoreline shapes reflect present forcing or “remember” the distant past?, Earth Surf. Dynam., 4, 871-884, doi:10.5194/esurf-4-871-2016, 2016.