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Imaggeo on Mondays: Great sand dunes and beyond

Imaggeo on Mondays: Great sand dunes and beyond

Driving eastwards through the San Luis Valley in south central Colorado, United States, the Great Sand Dunes emerge at the foot of the Sangre de Cristo Range in the northeast of the region’s upland plain.

The origin story of these great dunes begins during a time of glacial melt, five to three million years ago, when the rivers of the surrounding mountains filled the basin with water and sediments, forming a lake covering large parts of the valley. This lake kept growing until the water pressure broke through volcanic deposits situated at the southern end of the valley. The water from the lake subsequently receded, draining into the Rio Grande River.

The predominant winds from the southwest then blew the exposed sand towards the Sangre de Cristo Range, eventually forming the dune field. Opposing winds from the mountains allowed the dunes to grow vertically. Many questions about the exact formation of the dunes remain today, and scientists are still studying how these dunes took shape and evolve.

Today the highest sand dunes of North America reach up to 229 m (as high as the Golden Gate bridge reaches from the sea surface), covering an area of 78 km2. The dunes and the surrounding mountains are now protected by the Great Sand Dunes National Park and Preserve.

Visiting this breathtaking landscape at the end of my vacation through Utah and Colorado was really fascinating. As a geoscientist even on holidays I am always wondering about the processes that shaped these beautiful landscapes.

Meriel J. Bittner, University of Copenhagen


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

Imaggeo on Mondays: Civita di Bagnoregio – the dying town

Imaggeo on Mondays: Civita di Bagnoregio – the dying town

On top of a steep cliff standing out from the surrounding countryside, lies the small town of Civita di Bagnoregio, one of the most famous villages of Italy. It is often called the dying town, although more recently people have started to refer to it as fighting to live. What this little town is fighting against is the threat of erosion, as its walls are slowly crumbling down.

Located in central Italy, about a 100 km north of Rome, the town of Civita dates back to the Etruscan civilization, about 2500 years ago. It was most likely built on top of a hill for military reasons, since the 200 m of difference in height would provide perfect panoramic views. The city’s major development took place during the Middle Ages, and its well-preserved medieval character is one of the features that makes this city so magnificent nowadays. However, in 1695, a terrible earthquake demolished most of Civita by triggering a major landslide below, and forced people to move to the neighbouring village of Bagnoregio. This was not the only landslide that threatened the city. For centuries, Civita has been fighting against the natural degradation of the cliff, with recurring landslides slowly taking down the edges of the plateau, causing some of the medieval buildings to collapse and plummet into the ravine (Figure 1).

Figure 1. Evolution of the upper urbanised area of Civita di Bagnoregio from historical maps, showing many buildings destroyed by landslides during the past centuries. Credit: Margottini, C. & Di Buduo, G. Landslides (2017).

The geology of the plateau explains why this town is so susceptible to landslides (Figure 2, Delmonaco et al., 2004). The top of the plateau consists of a 20 m thick layer of consolidated rock formed from volcanic ash (ignimbrite), also known as tuff. The tuff was deposited by pyroclastic flows (rapid currents of volcanic debris and hot gas) related to the neighbouring Vulsini volcanic complex. This massive tuff layer overlies a more stratified section of pyroclastic deposits, roughly 70 m in thickness. These quaternary volcanic deposits lie above a bedrock of Plio-Pleistocene clay, which can be found all over the valley. This succession forms a classic setting for landslides. In the fragile clay deposits, slope instability is represented by mud flows and debris flows, while the upper, volcanic part of the plateau suffers from rock-falls, toppling and block-slides as it becomes unstable. Landslides can be dated back to 1373 AD, with 150 landslides documented by scientists who investigated the local geomorphology (Margottini and Di Buduo, 2016).

Figure 2. Geological profile of the study area. Credit: Giuseppe Delmonaco.

It seemed that the fate of Civita de Bagnoregio was to slowly disappear, but the city experienced a major turning point in 2013, when mayor Francesco Bigiotti decided to charge an entrance fee for people who wanted to visit the town. Tourists now pay a few euros to cross to the sloping footbridge towards the town. This proved to be a smart move, since people became more attentive and treated the site with more respect. The money raised by the entrance fee partly goes to preserving Civita’s fragile beauty and since 2015, the dying city received the UNESCO World Heritage status. This recognition of cultural heritage now leads to more investments from the regional government in order to preserve the historical site.

If you have the opportunity to visit the Civita, you will first enjoy a magnificent view on the town and the surrounding valley, before descending into the valley to cross the footbridge that provides the only gateway to the town. After a short climb towards the entrance, you’ll pass through an old arc, immediately bringing you back to medieval times. Then, all there is left to do is wander through the charming, quiet streets, observing the beauty of the classical quiet Italian village. Visit the Geology and Landslides museum, have lunch at one of the many authentic restaurants, or walk all the way to the end of the village, away from the other tourists. From there, a small trail leads into the countryside, where you can enjoy the magnificent views on the sharply eroded, clayey ridges in the surrounding badlands valley.

Previously referred to as the dying town, it now seems that there is some hope left after all for Civita di Bagnoregio. Something that will never change, however, is the interplay between mankind trying to survive in a hostile, but strategic environment of immense beauty, and nature that follows its own course of dismantling and eroding the existing relief.

By Elenora van Rijsingen, Ecole Normale Supérieure, Department of Geosciences, France

How extreme events impact Earth’s surface: reports from the 6th EGU Galileo conference

How extreme events impact Earth’s surface: reports from the 6th EGU Galileo conference

Throughout the year, EGU hosts a number of meetings, workshops, and conferences for the geoscience community. While the EGU’s annual General Assembly brings more than 15,000 scientists together under one roof, the EGU Galileo Conferences allows a smaller number of scientists to discuss and debate issues at the forefront of their discipline. In this blog post, the organisers of the 6th Galileo Conference “Perturbations of earth surface dynamics caused by extreme events” reflect on a week of insightful presentations and discussions on rare and catastrophic events.

“How do extreme events perturb Earth surface dynamics?” This question kept us busy during the entire week of the 6th EGU Galileo Conference “Perturbations of earth surface dynamics caused by extreme events”, which took place in Nepal from 13-19 October 2019. As organisers, we had aimed for a slightly unusual conference venue. We kept the nice hotels to a minimum of two nights and took the participants out to the Bhote Kosi for some camping for the remainder of the week to foster discussions and idea exchange.

The Bhote Kosi valley, about four hours’ drive north east of Nepal’s capital city Kathmandu, was heavily impacted by the April 2015 Gorkha earthquake and a subsequent glacier lake outburst flood event in 2016. This valley still today carries the signs of these earlier events in the form of large landslides, unstable slopes, and reworked river beds. As such, the valley serves as an ideal natural laboratory to better understand and quantify how the Earth’s surface responds to such perturbations. The Bhote Kosi had been a basecamp for a number of us studying natural hazards during the multiple field campaigns organised after the Gorkha earthquake, and this conference was a great opportunity to share what we have learned over the past years while directly illustrating the conference topics.

This conference brought together scientists studying a range of rare/extreme events and their broader impacts on Earth surface processes, biogeochemical cycles and human systems. Credit: Monique Fort

What seemed easy in the early days of planning did not come without inevitable doubts as the conference came closer. How do we make sure we have enough tents for everyone, how do we deal with the frequent power cuts, how do we make sure to cater enough local beer to thirsty geoscientists, and what if everyone contracted food poisoning? Fortunately, 60 participants, including ten Nepali colleagues and many early career scientists, blindly followed us without much afterthought and we were off for a busy and promising week.

The talks and posters covered most extreme event triggers: from earthquakes to volcanic eruptions and from wildfires to storms and tsunamis. These presentations provided food for thought for the geomorphologist, the geochemist, and the seismologist alike. Nepal, with the aftermath of the Gorkha earthquake, was well represented in these presentations, but many other parts of the world were covered as well.

Overall, this conference demonstrated the role of extreme events as geomorphic actors, able to shape landscapes and affect biogeochemical cycles. This conference also highlighted the large range of possible geomorphic responses, both in terms of magnitude and spatial extent, suggesting that the question of how these extreme events should be defined (are they large or are they rare events?) should ultimately be left to the investigators. It is however clear that in terms of geomorphic impact, an extreme event should lead to an observable perturbation above a, to-be defined, background variability, and be followed by a recovery period that leads to an old or new steady-state. As such, extreme events are not created equal and future research is needed to understand why such a range of responses are encountered.

Conference attendees had the opportunity to discuss questions and topics at the forefront of their field, from ethics in science to international cooperation. (Credit: Monique Fort) 

Time for discussion also allowed us to debate on the morality of post-disaster scientific work. We concluded that basic research questions related to these events need to be pursued and frequently require immediate mobilisation of scientific equipment and personal. However, this discussion also highlighted the need for clear and transparent international coordination so as to not interfere with relief efforts and avoid being perceived as greedy ambulance-chasing scientists. This important discussion was backed by input from a large Nepali delegation, providing an insight into how they had perceived these questions directly after the recent earthquake. Further discussions focused on the commonalities of different extreme events and the possibility to define a common framework that would allow us to compare the geomorphic impact of an earthquake to that of a storm or a wildfire.

Finally, this conference allowed us to lay the foundation blocks for future international coordination efforts. While the exact contours remain to be defined, all participants emphasised the need to prioritise research questions and resources in the case of rapid response efforts. These efforts require clear coordination with affected countries and funding bodies, but for instance also encourage scientific actors to agree on common publication strategies upfront.

Conference participants tour the Bhote Kosi valley to learn more about how extreme events can shape landscapes. (Credit: Monique Fort) 

In the middle of this busy schedule, a day of field excursion provided a welcome change. From small to large, the Bhote Kosi has it all: boulders, landslides, debris flows etc… Driving up the valley all the way to the Nepal-China border provides a humbling experience of how these idyllic landscapes can be turned into deadly traps in the blink of an eye. With closer scrutiny it becomes obvious that the whole landscape has been shaped by a myriad of these catastrophic events, directly questioning the notion of extremes.

After six days of presentations, posters, and late night discussions, it was time to close this intense, yet educational week. In the end there weren’t too many power cuts, no one got sick, most of us managed to shower with hot water and only a few reported spiders in their tents. In line with the local Nepali customs, the end of the conference was celebrated by inspired dancing until late at night when the first shuttles back to the airport started to take people back to Kathmandu.

By Maarten Lupker, ETH Zürich, Switzerland

60 scientists from all over the world came together for the opportunity to debate and discuss issues related to rare/extreme events and how they impact Earth system dynamics. Credit: Monique Fort


This conference was jointly organised with the Nepal Geological Society (NGS), without which this week would have never existed. While many people were involved, we would like to extend special thanks to Basanta Raj Adhikari and Ananta Prasad Gajurel from Tribhuvan University as well as the former president of NGS, Kabi Raj Paudyal and the present one Ram Prasad Ghimire. Bhairab Sitaula also provided invaluable help in all logistical aspects of this conference.

The conference was also co-sponsored by the US National Science Foundation, which provided overseas travel grants. Support from DiGOS & GFZ Potsdam were also greatly appreciated.

The organiser team: Christoff Andermann, Kristen Cook, Sean Gallen, Maarten Lupker, Christian Mohr, Ananta P. Gajurel, Katherine Schide, Lena Märki

Imaggeo on Mondays: Rocks weather, soil takes form

Imaggeo on Mondays: Rocks weather, soil takes form

This image depicts the soil formation, that is weathering of rock. Soil is actually formed by weathering of rock by physical, chemical and biological methods. This image depicts physical and biological weathering. Physical weathering by the action of temperature and biological weathering by the growth of some species of grass through the cracks. Image clicked from Trivandrum district of Kerala, India.

Description by Alwyn Biju, as it first appeared on

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