The 2009 eruption of Rinjani volcano (Lombok, Indonesia)


The majestic Segara Anak lake filling the caldera of Rinjani volcano. The cinder cone is Gunung Baru (new mountain). The lake covers an area of 11km2 and its volume was (before the eruption) estimated at 1.02km3. This is probably the largest hot volcanic lake in the world. The lake elevation is 2000 meters asl. Photo from July 2007 by Benjamin Barbier (ULB).


Segara Anak lake background:


The lake is neutral (pH: 7-8) and its chemistry dominated by chlorides and sulfates with a relatively high TDS (Total Dissolved Solids: 2640 mg/l). This unusual TDS as well as the lake surface temperatures (20-22°C) well above ambient temperatures (14-15°C) for this altitude, reflect a strong input of hydrothermal fluids. Numerous hot springs are located along the shore at the foot of Gunung Baru volcanic cone (see FLIR image below). Bathymetric profiles show also several areas with column of gas bubbles escaping from the lake’s floor and indicating a significant discharge of CO2 gas into the lake.


Studies of Rinjani volcanic lake are part of a cooperation agreement between Indonesia and Belgium. The funding agency is the CUD, the main Belgian development cooperation agency for Universities. Geochemical and physical studies of the Segara Anak lake started in the framework of this collaborative effort during the summer 2006. Recently, a monitoring station for continuous measurements of lake level and temperature and meteorological parameters was installed during the summer 2008.
Scientific teams involved in this study are:  

- for Indonesia: Center of Volcanology and Geological Hazards Mitigation (CVGHM), Bandung, Indonesia: Akhmad Solikhin, Devy Syabahna and Syegi Kunrat.
- for Belgium: Université Libre de Bruxelles (ULB): Alain Bernard, Benjamin Barbier, Robin Campion and Corentin Caudron. Facultés Universitaires Notre Dame de la Paix (FUNDP): Vincent Hallet and David Lemadec.





Topographic map of Rinjani caldera from Bakosurtanal-Indonesia. Square dimension is 1 kilometer. CTD and CTD-B are locations of conductivity temperature-depth profiles (see below on this page). Meteo is the site of the meteorological station monitoring air temperature and humidity, wind speed and direction and solar net flux. (#51-54) are the locations of hot springs from table below.



Bathymetric map of Segara Anak lake build from 65 kilometers of echo-sounder surveys conducted in 2007 and 2008. Maximum depth of the lake is 205m.




FLIR thermal camera of Segara Anak lake. Picture taken from the summit of Rinjani volcano (elev. 3726m). Thermal plumes on the left and right sides of Gunung Baru volcanic cone are hot springs discharging hydrothermal waters into the lake. A thermal anomaly is also visible at the top of the volcanic cone. Photo by Robin Campion (ULB) in July 2007.




Precursory signals of the May 2009 eruption :


- During a field work carried out in April 10-14, 2009, significant changes in the temperature and chemistry of some of the hot springs were observed. An increase in temperature and acidity of two hot springs (#53 et 54) was recorded (see table below). This increasing acidity was confirmed later in the lab as the consequence of an increase in the sulfates to values that were never observed before (since 2004). For the neutral hydrothermal system of Rinjani volcano, this increase in acidity was interpreted as the consequence of an increase in the magmatic (SO2) degassing at depth.
- The Fe concentrations in spring #54 usually below detection limits peaked at 120 mg/l. This change in chemistry produced a spectacular coloration of the lake waters that became yellowish-brown because of the precipitation of ferric hydroxide Fe(OH)3 (see photo below).
- A chemical plume of low pH and dissolved oxygen was clearly observed at the lake surface with an extension of several hundred meters away from the hot spring.
- pH profiles as a function of depth recorded at several locations showed also a clear acidification of Segara Anak lake especially at shallow depths (15-20meters).

- Lake surface temperatures increased slightly from 20°C in July 2008 to 22°C in early April 2009. Most if not all of this heating can be attributed to meteorological effects i.e reduced evaporation at the lake surface because of low-wind conditions compared to the dry season. Heat budget calculated for the period August 2008-April 2009 using meteo data (humidity, wind speed, air temperature and net solar flux) shows that heating of the lake occurred mainly during periods where heat lost by the lake to the atmosphere was reduced.


A report of these field observations made on April 17 to the CVGHM headquarter in Bandung prompted the Volcanological Survey of Indonesia to send another team to Rinjani volcano. The new team arrived at the summit of the volcano on May 2 2009 , the day the eruption started.





Geochemistry of Segara Anak lake (from CTD location, see above) and hot springs. NA: not analyzed.


April 2009 increase in the acidity and sulfate contents of hot springs 53 and 54.


Evolution of the pH of the lake waters with depth. The April 2009b data are from the location CTD-B on the NE of Gunung Baru (see map) and show a clear acidification of the lake waters. Other profiles are from CTD location. 2008 profile recorded in July 2008. Measurements made with a SBE Seacat 19plus profiler.


April 12, 2009. North East end of the lake. Chemical plume at the surface of Segara Anak lake due to the discharge of hot spring (#54) waters into the lake. Measurements made with a SBE Seacat 19plus at a depth of 0.8 m and with a sampling rate of 4Hz.


The shoreline close to the hot spring #54 on April 12, 2009. The brown color of the water and of the coating on the rocks is an amorphous ferric hydroxide that precipitates when hydrothermal fluids are oxidized by mixing with the lake waters. The human scale is Benjamin Barbier (ULB) who is 181cm tall. Changes in lake level are the consequence of the rainy season (see below). Photo by A. Bernard.




Evolution of surface lake waters and atmospheric temperatures during the period 23 July 2008-13 April 2009. Bottom graph shows for the same period the variation in lake level. Large increases in lake level observed in January and February 2009 are the consequence of heavy rainfalls during the rainy season.


Heat budget of Segara Anak lake. Most of the heating periods of the lake occurred when the heat released by the surface of the lake to the atmosphere was lower than the heat supplied from the hydrothermal system (Delta_E net <0). Peaks of heat losses correspond to period of strong winds. Thermal power supplied to the lake by the hydrothermal system is assumed to be close to 1700MW and constant for the modeling.


Eruptive activity May - August 2009


The eruptive activity started apparently on May 2, 2009. The activity is characterized by mild eruptions that produced a small lava flow and low altitude ash-poor gas plumes. Here follows a report by Robin Campion (ULB) who was on the spot last June.

Mild activity was observed from the SE rim of Rinjani caldera during 9-11 June. Pressurized incandescent gas was released at a 1-2 second intervals by a vent located in the 2004 crater, on the south flank of Gunung Barujari. At variable intervals (10 seconds to 10 minutes), stronger gas jets threw lava fragments at height <100m. A second vent in the same crater produced occasional ash jets.  A third lower vent was emitting a viscous lava flow that was reaching Segara Anak Lake.  Contact between the lake and the lava delta produced limited evaporation of the lake and warm current at the surface of the lake (see FLIR thermal picture below). Increased discharge of the hot springs located on the southern flank of gunung Barujari produced distinct plume with orange-red color. Weak changing winds carried the steam and gas plume (with low ash content) north and westwards at an altitude of 3000-4000m. Activity did not show variations over the 3 days observation period.

Results of the geopysical monitoring by CVGHM as well as other data about the current eruptive activity are published by the Smithsonian Institution - Global Volcanism Program. BGVN, vol.34 N°6 of June 2009.



Map of temperatures of Segara Anak lake recorded by ASTER satellite on 26 May 2009 at 14:45 UT. Thermal infrared bands 13 and 14 of ASTER processed with Split-Window algorithm.




Above: eruptive activity of Rinjani as of 10 June 2009. Below: FLIR thermal camera picture of Gunung Baru and Segara Anak lake. A thermal plume of hot waters is drifting from the entry point of the lava flow in the lake. Temperature scale is for lake waters. Photos by Robin Campion (ULB).


ASTER satellite image of the North part of Segara Anak lake, 29 July 2009 at 14:46 UT. Thermal infrared bands 13 and 14 of ASTER processed with Split-Window algorithm. Contours are in degrees C. Maximum temperature in the lake : 57°C.



ASTER false natural color image from August 21, 2009 at 02:35UT. The new lava covers an area of 650,000m2 and changed significantly the shoreline. The lake area is now reduced by 460,000m2. Inside box shows the previous shoreline and the new lava in red.



Same ASTER image (August 21, 2009) as above but processed to enhance the Fe(OH)3 precipitates in suspension in the lake waters. Scale of precipitates in arbitrary units (not calibrated). The source of this strong chemical plume is located close to hot springs 53 and 54 still discharging Fe-rich waters into the lake.



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