November 2011 eruption

ERUPTION ALERT – Nov.-Dec. 2011 eruption of Nyamulagira
Information report n°6 – 7th December, 2011

Eruption summary

Nyamulagira volcano has started to erupt on 6th November 2011, 5:55 PM (UTC+2). This eruption is located ± 12 km east-northeast of the Nyamulagira crater, close to one of the 1989 eruptive sites. The previous eruption of this volcano occurred 22 months earlier, in January 2010. The eruptive fracture is oriented approximately East-West, perpendicularly to the rift direction. After a week of eruption, the lava flows reached their maximum length of ~11.5 km.

Information from optical satellite imagery

Optical images acquired during the second half of November 2011 do not allow a good observation of the Nyamulagira eruption due to important cloud cover. The Landsat 7 ETM+ image acquired on 21st November 2011 shows that the lava flow did not grow in length, but widened and developed new branches (Fig. 1).

Figure 1 – The Landsat 7 ETM+ image acquired on 21st November 2011 reveals a new branch that developed from the central part of the initial flow (pink arrow). Another branch may have developed towards the west (pink question mark) but cloud cover and dense plume prevent from confirming it. (c) B. Smets, RMCA 2011

Information from SAR satellite imagery

A new ENVISAT-ASAR radar image was acquired on 2nd December 2011. Comparison between pre- and post-eruptive radar images allowed us detecting and mapping part of the new lava flow, where it overlap the former 1989 lava flow (Fig. 2). A second branch is detected in the northern half part of the flow and a third smaller one has developed on the western side of the main flow (pink arrows on Fig. 3).

Figure 2 – Detection of the 2011 lava flow using a comparison between SAR coherence images. The frame c shows the area of the 1989 eruption that is now covered by lavas and pyroclasts of the 2011 eruption (in white). (c) B. Smets, RMCA 2011

The surface change which is linked to the presence of the lava flow represents an area of about 14.7 x 106 m². Assuming a mean thickness of 3 m, a common value for Nyamulagira, the estimated corresponding volume of lava is ~44 x 106 m³. This estimation based on coherence images only refers to part of the new lava flow that overlaps the former 1989 lava flow and is therefore underestimating the actual volume of the ongoing eruption.

Figure 3 – The partial mapping of the 2011 lava flow (black lined area) using the comparison of two SAR coherence images (i.e. one before the 2011 eruption minus one that covers the 2011 eruption) reveals two new flow branches (pink arrows), which still follow probable paths suggested by the lava flow simulation. (c) B. Smets, RMCA 2011