remote sensing Article Repeat Glacier Collapses and Surges in the Amney Machen Mountain Range, Tibet, Possibly Triggered by a Developing Rock-Slope Instability Frank Paul Department of Geography, University of Zurich, 8057 Zurich, Switzerland; [email protected]; Tel.: +41-44-6355175 Received: 19 February 2019; Accepted: 19 March 2019; Published: 24 March 2019 Abstract: Collapsing valley glaciers leaving their bed to rush down a flat hill slope at the speed of a racing car are so far rare events. They have only been reported for the Kolkaglacier (Caucasus) in 2002 and the two glaciers in the Aru mountain range (Tibet) that failed in 2016. Both events have been studied in detail using satellite data and modeling to learn more about the reasons for and processes related to such events. This study reports about a series of so far undocumented glacier collapses that occurred in the Amney Machen mountain range (eastern Tibet) in 2004, 2007, and 2016. All three collapses were associated with a glacier surge, but from 1987 to 1995, the glacier surged without collapsing. The later surges and collapses were likely triggered by a progressing slope instability that released large amounts of ice and rock to the lower glacier tongue, distorting its dynamic stability. The surges and collapses might continue in the future as more ice and rock is available to fall on the glacier. It has been speculated that the development is a direct response to regional temperature increase that destabilized the surrounding hanging glaciers. However, the specific properties of the steep rock slopes and the glacier bed might also have played a role. Keywords: glacier surge; glacier collapse; rock-slope instability; hazard; Landsat; Sentinel 2; Tibet 1. Introduction Glacier surges have recently been in the focus of several scientific studies [1], largely because remote sensing data with a sufficiently high spatial and temporal resolution allow accurate tracking of surface and morphological changes as well as creation of dense time-series of flow velocities [2–12]. During a surge, large amounts of ice are transported at comparably high velocities (several m/day) from an upper reservoir area downward to a receiving zone, possibly creating a strong advance of the terminus. After a surge, the ice becomes stagnant and melts away while in the reservoir zone new ice accumulates [8]. Whereas an improved understanding of possible surge mechanisms slowly emerges [13–15], collapses of glaciers (where the ice is removed from a flat bed) have so far only rarely been reported. The two most prominent and best-documented examples are the 2002 collapse of Kolkaglacier in the Caucasus [16–19] and the 2016 collapse of two small valley glaciers in the Aru mountain range of Tibet [20–23]. In both regions the collapses caused long-distance (several km) mass movements with very high velocities (200 to 290 km/h) [19,22] that have not been thought possible before they occurred. In contrast to the frequent ice break-offs at steep hanging glaciers [24], the Kolka and Aru glaciers rested on beds of a comparably low slope and should thus have been resistant to mechanical failure. The history of events leading to the failure was different in both cases and they are thus unique on their own. Kolkaglacier is a well-known surge-type glacier [17] that is mostly nourished by avalanche snow and has been removed from its bed during its 2002 collapse, possibly due to a massive Remote Sens. 2019, 11, 708; doi:10.3390/rs11060708 www.mdpi.com/journal/remotesensing Remote Sens. 2019, 11, 708 2 of 18 loading with debris from rock fall of its surrounding slopes [25]. Kolkaglacier had a regular surge in 1969/70 and might have also collapsed in 1835 and 1902 [17]. The Aru glaciers were not known as surgingRemote before Sens. 2017 [23, 9,], x FOR but PEER showed REVIEW a typical pattern of surge-related elevation changes2 before of 17 they collapsed [21]. The twin-glacier collapse removed the lower parts of both glaciers, likely due to a surge in 1969/70 and might have also collapsed in 1835 and 1902 [17]. The Aru glaciers were not change in thermal/hydrologic conditions at the glacier bed [22]. known as surging before [23], but showed a typical pattern of surge-related elevation changes before Thethey collapses collapsed reported [21]. The here twin-glacier for a glacier collapse (GLIMS removed ID the G099443E34824N, lower parts of both Inventory glaciers, likely ID CN5J352E0017 due to ) ◦ locateda on change the north-western in thermal/hydrologic slope conditions of the Amney at the Machenglacier bed (A’ny [22]. ê Maqên) mountain range (34.822 N, 99.44◦ E) tookThe already collapses place reported in 2004, here and for again a glacier in 2007 (GLIMS and 2016.ID G099443E34824N, Figure1a shows Inventory the location ID of the mountainCN5J352E0017) range and thelocated glacier on the and north-western Figure1b isslope an obliqueof the Amney perspective Machen view(A’nyê up-glacier Maqên) mountain showing the range (34.822° N, 99.44° E) took already place in 2004, and again in 2007 and 2016. Figure 1a shows study region after the second collapse in 2008. The surges and collapses of this glacier have so far not the location of the mountain range and the glacier and Figure 1b is an oblique perspective view been reportedup-glacier [26 showing] or analyzed the study in region the scientific after the literature,second collapse although in 2008. they The repeatedlysurges and collapses buried aof country road thatthis has glacier been have reconstructed so far not been after reported each event[26] or (Figureanalyzed A1 in a).the Hence,scientific localliterature, authorities although are they aware of the collapsesrepeatedly and buried an information a country road board that describing has been reconstructed the 2004 avalanche after each event had been(Figure installed. A1a). Hence, The board can be seenlocal authorities in a picture are fromaware a of tourist the collapses [27] and and showsan information the advancing board describing glacier the a few2004 weeksavalanche before its third collapsehad been in installed. 2016 (Figure The board A1b). can The be seen small in valleya picture glacier from a (sizetourist about[27] and 1.5 shows km2) the has advancing a homogenous, glacier a few weeks before its third collapse◦ in 2016 (Figure A1b). The small valley glacier (size about gently sloping2 tongue (slope about 13 ) with a length of about 2 km and an elevation range of 450 m 1.5 km ) has a homogenous, gently sloping tongue (slope about 13°) with a length of about 2◦ km and (from 4800an elevation to 5250 range m). Theof 450 tongue m (from is 4800 nourished to 5250 bym). iceThe from tongue a steepis nourished (mean by slope ice from 36 )a andsteep its once connected(mean upper slope part, 36°) and that its reaches once connected as high uppe as 5900r part, m. that reaches as high as 5900 m. Figure 1.Figure(a) Location1. (a) Location of the of the Amney Amney Machen Machen mountainmountain range range in innorth-eastern north-eastern Tibet, Tibet, China. China. The The collapsedcollapsed glacier glacier is marked is marked with with a red a red square. square. TheThe location of of the the stud studyy region region is marked is marked in the inset in the inset with a yellowwith a yellow arrow. arrow. Image Image sources: sources: Screenshots Screenshots from Google Google Earth. Earth. (b) (Obliqueb) Oblique perspective perspective view view of the collapsedof the collapsed glacier glacier as seen as seen on on a Quickbirda Quickbird imageimage acquired acquired on on 13.1.2008, 13.1.2008, a few a months few months after the after the second collapse. The steep and partly already glacier-free rock walls can be seen in the background. second collapse. The steep and partly already glacier-free rock walls can be seen in the background. The extent of the 2004 collapse is still visible in the foreground, the deposit from the 2007 collapse The extentcan ofbe theseen2004 on top collapse of it. Image is still source: visible Screenshot in the from foreground, Google Earth. the deposit from the 2007 collapse can be seen on top of it. Image source: Screenshot from Google Earth. The Amney Machen mountain range is covered by numerous glaciers (covering about 80 km2), Theof Amney which several Machen have mountain been classified range in is covereda previous by study numerous as surge glaciers type and (covering strongly aboutadvancing 80 km 2), of which severalbetween have 1966 beenand 1981 classified [28]. Time in series a previous of Landsat study images as surge (available type since and 1987) strongly reveal advancing that several between 1966 andof 1981the larger [28]. Timeglaciers series in this of Landsatmountain images range ha (availableve surged since again 1987) during reveal the past that three several decades. of the larger Without temperature measurements, any assumptions about thermal conditions of the ice and rock glaciers in this mountain range have surged again during the past three decades. Without temperature walls are speculative. However, thermokarst features that can be found in the valley floor to the measurements,southwest any of the assumptions mountain range about and thermal the permafrost conditions zonation of the map ice andby Gruber rock walls (2012) are [29] speculative. are However,providing thermokarst evidence features that the thatglaciers can higher be found up co inuld the be valleypoly-thermal floor or to cold-based. the southwest The purpose of the mountainof range andthis the study permafrost is providing zonation an overview map on by the Gruber glacier surges (2012) and [29 collapses] are providing between evidence1987 and 2017 that based the glaciers higher upon the could analysis be poly-thermal of freely available or cold-based.