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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

Communicate your Science Video Competition finalists: time to get voting!

For the second year in a row we’re running the EGU Communicate Your Science Video Competition – the aim being for young scientists to communicate their research in a short, sweet and public-friendly video. Our judges have now selected 3 fantastic finalists from the excellent entries we received this year and it’s time to find the best geoscience communication clip!

The shortlisted videos will be open to a public vote from now until midnight on 16 Apri; – just ‘like’ the video on YouTube to give it your seal of approval. The video with the most likes when voting closes will be awarded a free registration to the EGU General Assembly 2016.

The finalists are shown below, but you can also catch them in this finalist playlist and even take a seat in GeoCinema – the home of geoscience films at the General Assembly – to see the shortlist and select your favourite.

Please note that only positive votes will be taken into account.

The finalists:

Inside Himalayan Lakes by Zakaria Ghazoui. Like this video to vote for it!

 

Glacial Mystery by Guillaume Jouvet. Like this video to vote for it!

 

Floods by Chiara Arrighi. Like this video to vote for it!

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The winning entry will be announced during the lunch break on the last day of the General Assembly (Friday 17 April).

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/.

 

Imaggeo on Mondays: An explosive cloud

Imaggeo on Mondays: An explosive cloud

One of the world’s most volcanically active regions is the Kamchatka Peninsula in eastern Russia. It is the subduction of the Pacific Plate under the Okhotsk microplate (belonging to the large North America Plate) which drives the volcanic and seismic hazard in this remote area. The surface expression of the subduction zone is the 2100 km long Kuril-Kamchatka volcanic arc: a chain of volcanic islands and mountains which form as a result of the sinking of a tectonic plate beneath another.  The arc extends from Hokkaido in Japan, across the Kamchatka Peninsula, through to the Commander Islands (Russia) to the Northwest. It is estimated that the Pacific Plate is moving towards the Okhotsk microplate at a rate of approximately 79mm per year, with variations in speed along the arc.

There are over 100 active volcanoes along the arc. Eruptions began during the late Pleistocene, some 126,000 years ago at a time when mammoths still roamed the vast northern frozen landscapes and the first modern humans walked the Earth.

Many of the volcanoes in the region continue to be active today. Amongst them is Karymsky volcano, the focus of this week’s Imaggeo on Mondays image. Towering in excess of 1500 m above sea level (a.s.l), the volcano is composed of layers of hardened lava and the deposits of scorching and fast moving clouds of volcanic debris knows as pyroclastic flows. You can see some careering down the flanks of the volcano in this image of the July 2004 eruption. The eruptive column is the result of a

“strong Vulcanian-type explosion, with the cloud quickly rising more than 1 km above the vent. The final height of the eruption cloud was approximately 3 km and in the image you can clearly see massive ballistic fallout from multiple hot avalanches on the volcanoes slopes,”

explains Alexander Belousov, a Senior Researcher at the Institute of Volcanology and Seismology in Russia and author of this week’s photograph.

 

USGS map of the Kuril-Kamchatka trench, showing earthquake locations and depth contours on downgoing slab. Credit: USGS, USGS summary of the 2013 Sea of Okhotsk earthquake, via Wikimedia Commons.

USGS map of the Kuril-Kamchatka trench, showing earthquake locations and depth contours on downgoing slab. Credit: USGS, USGS summary of the 2013 Sea of Okhotsk earthquake, via Wikimedia Commons.

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/.

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