From: Benjamin Van Wyk De Vries 
Date:         Fri, 1 Dec 1995 09:48:51 MST
Subject:      Cerro Negro activity report

Cerro Negro
Saturday -Sunday 25/26th November

I apologise for the delay in sending this information, which was caused by
computer failure. The data has already been reported in Nicaragua.
B. van Wyk de Vries.
  1. Observations from Managua, La Paz Centro, Leon and Rota 25th November
  2. Observations of La Mula Lava Flow
  3. Observations from Cerro La Mula and Northern Lavas 25/11
  4. Lava flow observations 26th November
  5. Seismic Activity
  1. Observations from Managua, La Paz Centro, Leon and Rota 25th November.

    The eruption plume of Cerro Negro was clearly visible, as on earlier days, from Managua. It was a diffuse grey column, turning horizontal at about 2000m, where white clouds formed and drifted westwards over the Pacific coast. From La Paz Centro at 09:00 hrs distinct pulses of ash could be seen rising from the crater. These were dark grey, on exit from the crater, forming mushroom shapes, before drifting westwards and being incorporated into the plume. When passing below the plume near Leon, at 09:30 hrs no ashfall was encountered.

    Ash fall has been reported in Leon and Corinto, however, and subsequent observations showed that there is great variation in the amount of fine material ejected. A times only massive bombs are thrown out, while at others strong explosions sent up dense ash clouds. Ash and scoria are falling continuously on the west base of the cone, with occasional periods of heavy fall, with <5 cm-size highly vesicular scoria. Heavy ash emission lasted from single pulses to several hours.

    From Rota (the most popular tourist entrance to the volcano) the new cone was clearly visible at 10:00 hrs and an unusual spinelike protrusion was jutting out on the northern edge of the crater. (This was later found to be a lava dome).

    Pulses of ash and bombs had a frequency of 20 per minute, a characteristic periodicity in this eruption. Pulses were strong enough to maintain a constant fountain of bombs, some of which were large and visibly red at 5km. Bombs rose to a maximum of 600 m. Explosions were audible at this distance.

  2. Observations of La Mula Lava Flow

    The lava flow which was observed forming between Cerro La Mula and Cerro Negro on 23rd November had advanced westwards about 500m down a small valley and was moving at about 1m per hour between 10:30 and 20:00 hrs 25/11. We have called this La Mula flow. The flow was about 30m wide. By 06:00 26/11 the flow had stopped advancing, red glow was still visible in cracks. A tree at the front was not burnt and remained intact at the last observation at 12:00 26/11. Occasional creaks and falling rocks still occurred.

  3. Observations from Cerro La Mula and Northern Lavas 25/11.

    At 11:00 hrs little ash was emitted. Pulses continued to throw bombs in a near continuous fountain. Some bombs were metres across and deformed and broke up in flight. Most were still ejected vertically, but a significant number were now exiting at low angles and falling low on the flanks. Sharp whooshing sounds occurred with some pulses accompanied with burning gas.

    The new cone had grown considerably, and was about 30-50 m below the old crater summit. The crater was about 40m across. Bombs fell mostly on the cone and rolled down to the base. The small breach where the lava flow of 23/11 exited was partly covered by a new blocky flow, which appeared to come straight from the new cone, though no exit vent was visible. (I believe it was produced by accumulated, still liquid ejecta beginning to flow outwards, as seen on 22/11). The flow had advanced to half way down the flank and was covering another blocky lava, which had reached the base.

    To the east of the lava exit a dome about 100m wide and 40m high had grown. This initially had a serrated, spined, pyramidal shape, but changed rapidly throughout the day. By 18:00 hrs it had become table shaped. Blocks were continually spalling off the dome, which also sustained a continuous rain of bombs from the new cone. Three small but thick lava tongues extended to the base of the cone from the dome, and were advancing slowly. A small, thinner lava tongue was following the east side of the thick lavas.

    The two flows seen moving northwards on the 23rd had reached about 1 to 1.5 km from the volcano. The western lobe was the largest, being about 400m wide and 3-5 m thick at the front. It had a small lobe extending down the gully below Cerro La Mula and another extending eastwards into a depression in the old northern lava field. Forward movement was visible in red plastic lava exposed at the front and over a three hour period the flow advanced about 12m.

    The eastern lobe has extended into forest at the east side of the old northern lava field and was moving at about the same rate at the other lobe. A low ash covered area with a small cinder cone separated the two lobes. The sides of each flow were slowly (about 1m per hour) encroaching on this and thickening. >From the cinder cone all the thick lava lobes below the dome could be seen advancing. Many areas of the dome were glowing red.

  4. Lava flow observations 26th November.

    By 06:45 lava fronts had advanced 20-50m since nightfall 25/11. The main west lobe had spread out eastwards and a block 15m wide had moved about 100m in the middle of the flow. The dome was less pronounced that the previous day and was blocky rather than spiny. A thick (20m) lava had formed to the north east and was still active.

    The cone had changed shape and grown about 10m overnight.

  5. Seismic Activity

    Throughout this time seismic tremor recorded on the local stations or Cerro Negro, Miramar, Momotombo, Leon and San Cristobal remained at high levels. Tremor is continuous and distinctly felt up to 1.5 km from the cone. As of this writing the energy was slowly increasing.

More information and diagrams/ photographs will be made available on the WWW
soon and a comprehensive report is being prepared for the Global Volcanism

Any enquiries about this report should be addressed to B. van Wyk de Vries at,
The Open University, Milton Keynes, UK.
Tel +44 1908 652558,
Fax +44 1908 655151,

Or to Wilfried Strauch at INETER, Managua
Tel +505 2 492761
Fax +505 2 496987
email   wil@ineter.nic