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Imaggeo on Mondays: Earthquake Lake

Imaggeo on Mondays: Earthquake Lake

Despite its alluring turquoise waters and rugged mountain backdrop the story behind this beautiful lake is rather more troubling. In today’s Imaggeo on Mondays, the first post since our short break from the traditional format during the General Assembly, Alexander Osadchiev writes about the shaky origins of Sarez Lake.

Lake Sarez is situated in Tajikistan, deep in the Pamir Mountains. In 1911 a local earthquake caused a large landslide which blocked the valley of the relatively small Murgab River (which discharge is only 100-150 m^3/s). The valley is relatively young, on the geological scale at least, meaning it is deep and narrow and has steep sided slopes. This is the reason why the moderate volume of the landslide (about 2 km^3) was enough to form the tremendously high Usoi dam (about 550 m) – the tallest in the world either natural or man-made. The length of the Usoi dam is about 500 m which is almost equal to its height. However, lakes formed by landslide dams blocking river valleys are not uncommon in the Pamir Mountains or elsewhere around the world.

Most blocking dams are not high or solid enough to remain in place for extended periods of time. Initially, a river will seep through the dam eroding it, but usually the outflow discharge is less than the river inflow into the lake. Together with active sedimentation and silting, the water level in the lake steadily increases until it reaches the dam height. Eventually water starts flowing over the top of the dam and intensively destroys the dam. Yet due to a number of circumstances the behavior of the Sarez Lake was significantly different. On the one hand, the Usoi dam is solid enough not to have been significantly eroded in the more than one hundred years since it appeared. At the same time, it is porous: outflow and inflow volumes of water across the dam balance each other.  Crucially, this balance was obtained for a very high water level, close to the height of Usoi dam itself. Lake water levels oscillate near 500 m height, just 50m away from the top of the of 550 m dam. The height of the dam resulted in the large size of the Sarez Lake – its length is about 60 km and its volume exceeds 16 km^3.

This large volume of water (and potential energy!) situated high in the mountains (3263 m above the sea level) presents a hazard for millions of people in Tajikistan, Afghanistan, and Uzbekistan living below the Sarez Lake and along the banks of the Mugrab, Panj and Amu Darya rivers. The Usoi dam is solid enough to resist erosion and create such a big lake, but it is not known if it can withstand a big earthquake, which are not uncommon in the area. Not only can an earthquake directly destabilize Usoi dam, but an earthquake-induced landslide into the lake could cause a lake tsunami and result in the dam overflowing. Particularly, an area of friable soil forming a unstable slope, has been particularly identified as a risk. Following a large earthquake (8-9 on the Richter scale) it could presumably form a landslide.

The levels of monitoring and investigation of landslide hazards in the region and the risk presented by Lake Sarez itself are still largely understudied. Limited funding availability in Tajikistan and the remoteness of the lake – it can only be reached on foot, after several days of strenuous mountain trekking through an almost uninhabited, but unbelievably beautiful area – are amongst the main reasons this is so.

“The view of the Sarez Lake was the best prize for me and Zhamal Toktamysova at the final part of our 2-week trekking through the Pamir Mountains”, explains Alexander.

 

By Alexander Osadchiev, Shirshov Institute of Oceanology, Physical Oceanography, Moscow

Imaggeo on Mondays: Mirror Image

Imaggeo on Mondays: Mirror Image

This week’s Imaggeo on Mondays image is brought to you by Fabien Darrouzet, who visited the icy landscapes of Svalbard back in 2012. Whilst the aim of his trip was not to better understand the geology of the landscapes, his eyes were very much focused on goings on up, up in the sky, it didn’t stop him taking this still of the snow covered peaks.

This picture was taken in Svalbard (78° lat.) in June 2012. I was there for one week in order to observe the transit of the planet Venus in front of the Sun. I came here because at this time of the year, the Sun never sets, (midnight Sun), so it was possible to see Venus during most of the transit (for over six and a half hours!), and not only during its last minutes, as was the case for most parts of Europe.

During the day after the transit, I took a boat trip inside the fjords around Longyearbyen in order to discover the island and its local wildlife; I was interested in catching a glimpse of the elusive polar bears and hope to see seals too. We sailed in the Isfjorden, and in particular close to the southern border of a territory of Svalbard named Oscar II Land, where I took this picture from the deck of the boat. This area, and indeed all of Svalbard, is covered by snow most of the time, and just a few plants can germinate during July-August, when the average temperature is 5°C.

Svalbard is dominated by glaciers (60% of all the surface), which are important indicators of global warming and can reveal possible answers as to what the climate was like up to several hundred thousand years ago. The glaciers are studied and analysed by scientists in order to better observe and understand the consequences of the global warming on Earth.

 

By Fabien Darrouzet, Belgian Institute for Space Aeronomy

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: Drumlins Clew Bay

Imaggeo on Mondays: Drumlins Clew Bay

During ice ages landscapes are sculpted by the power of advancing glaciers. From rock scratches, to changing mountains and the formation of corries, cirques and aretes, through to the formation of valleys and fjords, the effects of past glaciations are evident across the northern hemisphere landscape.

Perhaps not so familiar, drumlin fields are also vestiges of the erosive power of ancient ice sheets. Glacial deposits tend to be angular and poorly sorted, meaning they come in lots of different sizes and shapes. The extreme of this are glacial erratics. Drumlins are are elongated hills made up of glacial deposits and they represent bedforms produced below rapidly moving ice. Our Imaggeo on Monday’s image this week is of Clew Bay in western Ireland and shows the streamlining of drumlins into an extensive drumlin field of glacial sediment. The drumlins here formed during the rapid thinning of the fast moving central parts of the western sector of the British-Irish Ice Sheet, in a process known as deglacial downdraw – probably between 18,000 and 16,000 years ago. The ice was streaming through bays in western Ireland both during and at the end of the Last Glacial Maximum (also known as LGM). This was the time in which the ice sheets covered most of northern America, Europe and Asia. In Clew Bay the ice was a minimum of 800m thick and flowing out into a series of tidewater glaciers situated along the length of Ireland’s western shelf.

By Prof. Peter Coxon, Head of Geography, School of Natural Sciences, Trinity College Dublin & Laura Roberts

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

Imaggeo on Mondays: Pyroclastic flow, Montserrat

Below the warm and tranquil waters of the Caribbean, some 480 km away from Puerto Rico, the North America Plate is being subducted under the Caribbean Plate. This has led to the formation of the Lesser Antilles volcanic arc; the result of the formation of reservoirs of magma as fluids from the down going North America Plate are mixed with the rocks of the overlying Caribbean Plate.

The continued magma generation is expressed violently at the surface on Monserrat Island, which has been the subject of extensive scientific scrutiny since the mid-1990s. This is all because of Soufrier Hills volcano, a Pele’ean type lava dome complex. This means that rather than explosive eruptions taking place, very viscous lava is slowly erupted from the volcano’s vent. The lava is so sticky and gooey that instead of flowing away, down the flanks of the volcano, it accumulates in the vent area and forms a large plug. Lava domes come in a range of shapes and sizes, in the case of Soufrier Hills, it tends to be circular and quite spiky.

Just because the eruptions on this Carbbien Island aren’t generally as spectacular, as for instance at Mt Etna in Italy, they are no less deadly! A common hazard associated with the building up of a dome by the continued accumulation of volcanic material means they can become dangerously unstable and collapse. The volcanic material careers down the flanks of the volcano in the form of pyroclastic density currents (PDCs). The largest such collapse ever observed took place in July 2003 and numerous smaller flows have occurred since. One rather large collapse happened in early 2010, when the dome atop Soufrier Hills had grown to be 1150 m asl (above sea level). After a period of unrest which started in late 2009 and was characterised by seismicity and extrusion of lava from the vent, there was a catastrophic dome collapse in February which reduce the summit height by almost 100m!

Pyroclastic flow, Montserrat. Credit: Alan Linde (distributed via imaggeo.egu.eu)

Pyroclastic flow, Montserrat. Credit: Alan Linde (distributed via imaggeo.egu.eu)

“The photo is taken from a spot at the water’s edge (just behind me) that was previously about 200 m out to sea. A PDC pushed the shoreline out by as much as ~600 m,”

says Alan Linde, who took this photograph of the smoking black landscape in April 2010.

Alan and the research team from the Department of Terrestrial Magnetism (DTM, Carnegie Institution for Science) have been involved with studying Soufrier Hills since 2003. By installing a network of very sensitive instruments in small shafts dug into the ground in and around the volcano, known as borehole strainmeters, they can measure changes in the size and volume of the ground as a result of dome collapses and explosive eruptions.

 “One of our borehole sites, very close to the coast, was almost destroyed by the hot ash. There is a clear change (from before to after the flow) in the tidal signals recorded by that site because an area of ocean loading has been removed as a result of the ash filling in and moving the coastline. The volcano is behind the small mountains, obscured by cloud.”

 

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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