GeoLog

Spain

Imaggeo on Mondays: Nummulites, the living lentils

Imaggeo on Mondays: Nummulites, the living lentils

This photograph depicts a close-up of Eocene limestones from the Sardinero Formation in Cantabria (Northern Spain). The limestone is rich with foraminifera shells, most of them from the Nummulitidae family. These organisms once lived in a very shallow sea that separated Europe from Iberia in the late Mesozoic and early Cenozoic era. Later the sea basin’s  closure led to the formation of the Pyrenees.

The remarkable abundance of these Paleogene and Eocene fossils around the globe led Eugène Renevier, a Swiss geologist from the 19th century, to propose merging the two Epochs into the Nummulitic Period, a suggestion that did not succeed.

Nummulitidae were also some of the first microfossils recognized in literature. Their features were described in Book XVII of Geography, written by the Greek geographer and historian Strabo around the beginning of the 1st century. In this passage Strabo finds the Nummulitidae fossils embedded in the rocks used to build Egyptian pyramids, however he mistakes the shells for lentils dropped by rushed workers. It reads below:

One of the marvellous things I saw at the pyramids should not be omitted: there are heaps of stone-chips lying in front of the pyramids; and among these are found chips that are like lentils both in form and size; and under some of the heaps lie winnowings, as it were, as of half-peeled grains. They say that what was left of the food of the workmen has petrified; and this is not improbable.

Indeed, in my home-country, in a plain, there is a long hill which is full of lentil-shaped pebbles of porous stone; and the pebbles both of the seas and of the rivers present about the same puzzling question; but while these latter find an explanation in the motion caused by the current of water, the speculation in that other case is more puzzling.

It has been stated elsewhere that in the neighbourhood of the quarry of the stones from which the pyramids are built, which is called “Trojan”, and that there are caves at the foot of it, and a village near both these and the river which is called Troy, being an ancient settlement of the captive Trojans who accompanied Meneläus but stayed there.

Despite this initial description referring to lentil-sized Nummulitidae, the size of these fossils varies considerably among different species, and can reach up to 12 cm, some of which have been found in Cenozoic rocks in the same area where the photograph was taken. Such a size is remarkable, considering the fact that Nummulitidae are unicellular organisms.

There are lots of genera within the Nummulitidae family, the most common of which are Assilina and Nummulites. The fossils in the photograph are about 1 cm in length and pertain to the Nummulites genus. Nonetheless, more elongated and white fossils from the Discocyclinidae family are also present in the limestone; one of them can be found right below the crawling snail.

While taking some pictures of the outcrop, I noticed the snail in the lower part of the frame, slowly crawling towards the top of it. To my surprise, it chose to follow the path drawn by two large Discocyclinidae shells, and I rushed to take this photograph. Two living beings together but separated by 50 million years, embodying a sort of spacetime paradox.

By Manuel de Paz Álvarez, the University of Oviedo

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submittheir 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: Landslide on the Cantabrian coastline

Shimmering blue seas, rocky outcrops and lush green hills sides; this idyllic landscape is punctuated by a stark reminder that geohazards are all around us. Irene Pérez Cáceres, a PhD student at the University of Granada (Spain) explains the geomorphology behind this small scale landslide on the Asturian coastline.

Landslide on the Cantabrian Sea. Credit: Irene Pérez Cáceres (distributed via imaggeo.egu.eu)

Landslide on the Cantabrian Sea. Credit: Irene Pérez Cáceres (distributed via imaggeo.egu.eu)

This picture was taken in May 2011 in the coast of Llanes (Asturias, Spain). I was living in Oviedo (Asturias, Spain) doing my Master in the structural geology of the Axial Zone of the Pyrenees. Thus, geomorphology and geohazards are not my specialty or area of expertise. However, the landslides are well known and studied in this region, and people from Asturias call them Argayos.

This argayo is situated in Niembru Mountain, over the San Antolín beach, constantly affected by waves and swell of tides of the Cantabrian Sea, and continuous rain typical in the region. It was defined as a rotational landslide with two fracture surfaces, possibly conjugated in wedge shape. It is approximately 50 meters high and 60 meters width at its base. The slide volume is calculated at 45000 m3. It is carved in quartzite altered by the water rain infiltration through crevices in the surface. The initial displacement was between 10 and 15 meters in the scar. Experts say this landslide is still active, moving and evolving continuously. It is an imminent risk for the swimmers, but it is very difficult to control it, due to the size and the slope, and the technical requirements to stabilize the rock. On the other side of this mountain, further landslides are evident, as a result of the building of a road.

These natural geomorphological processes are very common in the north of Spain, mainly in riverbeds, as well in other nearby beaches. The main causes are the abundant (and sometimes heavy) rainfall, the typically clay rich soils, steep slopes, building works that destabilize the slopes, and the absence of vegetation in some areas. They vary in in size and volume, and can sometimes have important material consequences and can pose a significant risk for the local inhabitants. The annual economic cost for repairing the damage caused by these processes is estimated to be 66 million of euros in this region.

Studies carried out in the Department of Geology of the University of Oviedo (Mª José Domínguez and her group), indicate that 70% of the landslides in Asturias happen when it rains over 200 mm during over a period of a minimum of three days. Research has also been carried out to try and predict when landslides might happen, examining numerous landslides over the last 20 years approximately. It seems that one conditioning factor is the exact location of new buildings, being that ancient constructions used to be in secure zones, probably because people observed more minutely to the nature, but the new ones are more vulnerable.

To conclude, detailed geological and geomorphological studies are always recommended to carry out before constructions. Thereby it is possible to minimise this common geohazard in Asturias.

By Irene Pérez Cáceres, PhD Student, Granada University.

 

If you pre-register for the 2015 General Assembly (Vienna, 12 – 17 April), you can take part in our annual photo competition! From 1 February up until 1 March, 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/.