From: Wilfried Strauch 
Date:         Fri, 24 Nov 1995 21:21:41 MST
Subject:      Cerro Negro Eruption

Instituto Nicaraguense de Estudios Territoriales, Managua, Nicaragua Eruption of Cerro Negro, Nicaragua

Initial Observations
Wilfried Strauch, Virginia Tenorio, Rolf Schick

During a visit to the crater on Monday, 13 of November no precursors such as surface effects, increased degassing, or high fumarole temperatures were detected.

The eruption start was detected by the seismic service of INETER from data of CNGN seismic station (distance 500 m to the crater) at 11:45 AM 19/11/95. It began with 30 minutes of mildly increasing seismic activity, a pause and then continually increasing strength. This coincides with the reports of local residents who noticed the first explosions at this hour.

Local people and radio stations reported in the afternoon about increasing activity. Especially from the town of Malpaisillo (about 10 km north of Cerro Negro) from where there is a good sight to the interior of the crater, people informed about fire and flames coming from the crater.

On the day before the eruption only 2 small seismic events were recorded by the local seismic station CNGN, but these were not above the usual background intensity or number.

The Civil Defence of Nicaragua was alerted and since the afternoon of 19/11 has been working amongst the local population to inform and prevent undue alarm. Problems from ash fall have been very slight, and the major preoccupation is lava, which is expected to flow northwards from the cone towards the town of Malpaisillo. Since much of the area has been covered by previous lavas the material damage is not expected to be high.

First observations on the midnight of 19/11 to 20/11 indicated mild strombolian activity with vertically directed ejecta and gradually increasing strength. The characteristics of the eruption have remained similar in the following days, but with increasing intensity.

Observations of Activity 20:00 21/11/95 to 10:30 22/11/95

B. van Wyk de Vries and Pedro Perez

At 20:00 hrs on 21/11 incandescent bombs were being thrown up to 300-400m above the lip of the 1992 crater edge. Ash content is very low compared with the 1992 and May-August 1995 activity, and bombs were often very large (meters across), which deformed and broke up in flight. Pulses of material averaged 20 per minute, maintaining an almost constant fountain of material. Trajectories are mostly near-vertical, usually not less than 80 degrees. Because of this few bombs have fallen outside the crater, which is being filled up. Pedro Perez climbed to the crater edge to take photographs and observe. A new cone is growing within the old one, which at 07:00 hrs 22/11 had almost reached the lip of the 1992 crater. The new cone's crater is slightly to the NW of the May-August activity. A second construction 50m west of the new crater appears to be a lava dome, from which at 07:00 hrs a small lava flow 2-5m wide and 50m long, was flowing. The flow was following the edge of the new cone, towards the lowest part of the 1992 crater. The new cone has a steep (>45 degrees) basal scarp, 2-5m high, followed by a level part and then a steeper slope (25 degrees) to the crater. The scarp resembles a lava flow front, and behind it small faults cut into the cone. From these observations we conclude that the cone is built of still plastic material and is spreading outwards. Activity over the period of observation was not constant. Ejecta pulses maintained a frequency of 20 per minute, however the size of each pulse and its duration varied. At times the pulses were short discrete events, while at others they merged into an almost continuous fountain. Short periods (5-10 sec) of non-ejection occurred, usually terminated with a loud bang. Longer periods of low activity took place from 12:10 to 12:30 22/11, and 02:00 to 02:15, terminated by gradually increasing activity. From 02.55 to 03:10 ejecta heights were <150m but ash content and degassing were much higher, as seen by dark clouds escaping with each explosion and a thick, white lower plume, which appeared to be escaping from the location of the dome. By 05:00 hrs the eruption had regained previous levels and was increasing to near constant fire fountain like activity, bomb size was increasing and pulse frequency increasing to 22 per minute, but counting was difficult since pulses were merging. The eruption continued at this level as (from 06:00 hrs to 10:30 hrs) we walked around the cone setting up deformation survey points with Leica differential GPS equipment. From 09:30 to 10:00 a series of strong explosions ejected material to the lower slopes of the cone Sand to gravel size ash was falling to the west of the cone, but no large ejecta. In comparison to the massive 1992 ejecta the present material is highly vesicular with millimetre-size vesicles, olivine, pyroxene and plagioclase are present and some plagioclase crystals are 1cm in length. Large bombs inspected on the N side were similar in vesicle content to the small particles, but with dense interiors.

Continuing Observations

Helman Taleno, Leonel Urbina, Cristian Lugo

During the evening of 22/11 the new cone overgrew the side of the 1992 crater to the north and material began spilling down the north slope towards Cerro La Mula. From 19:00 to 23:00 a tongue of lava had appeared over the edge of the old crater, following from the path of the small lava seen at 07:00 22/11. The front moved at less than 1 m per hour, but spilled rocks of the front which constantly fell down to the base of the main cone.

Seismic Activity

Wilfried Strauch

Tremor from the eruption began at 11:45 19/11, the amplitude increased continously and saturated 02:00 21/11 the nearest station CNGN (Cerro Negro, 60 dB gain). Tremor is detected on short period seismic stations within a 30km radius (seismic stations at volcanoes San Cristobal and Momotombo and near the city of Leon). Energy release has increased continuously and continues to do so at present 12.00, see RSAM record of seismic station MIRN (Miramar) in a distance of 7 km. Tremor is felt over 1km away when sitting down as a smooth rocking motion, and makes levelling of surveying equipment difficult, as the levelling bubble dances around.

As of 12:00, 23/11 the eruption continues unabated, as detected by seismometers at Cerro Negro, and other seismic stations.

For more informations contact Dr.Wilfried Strauch, INETER, Managua
Fax (505)-2-496087, Tel (505)-2-492761,  email