Smithsonian Institution
Global Volcanism Network Bulletin v. 20, no. 4, April 1995

Fogo (Cape Verde)  Fire fountains continue but lava extrusion rate
      declines

Fogo
SW Cape Verde Islands, Atlantic Ocean
14.95N, 24.35W; summit elev. 2,829 m
All times are local (= GMT - 3 hours)

On 2-3 April a fissure eruption began on Fogo Island from the SW
flank of Pico cone (Fogo Peak) within the 8-km-diameter Cha Caldera
(Bulletin v. 20, no. 3). During the initial stage of the eruption
there was a burst or jetting of gas, followed by ejection of large
blocks and fire fountaining. A lava flow cut off the main road to
local villages by the morning of 3 April, and ash fell on the
island. Approximately 1,300 residents in the caldera were
evacuated.
Volcanologists from the United States, Portugal, and France were
requested by the Cape Verdean government to help monitor and
evaluate the activity. Jo~o Gaspar (Universidade dos A ores) and
colleagues observed the activity until 11 April. U.S. Geological
Survey (USGS) volcanologists, assisted by Cape Verdean geologists,
installed a seismic station and monitored the eruption during 10-25
April. Additional information about the vent activity during 14-19
April was provided by Henry Gaudru and members of the Societe
Volcanologique Europeenne who visited the volcano. Fran ois Le
Guern (CNRS France) monitored the volcano on 25-27 April.
Summary of activity, 3-16 April. Detailed activity reports through
16 April have already been published (Bulletin v. 20, no. 3). Seven
vents were active on the first day of the eruption, with
fire-fountains feeding pahoehoe lava flows, ejection of volcanic
bombs, and a gas-and-ash plume 2,000 m high. A scoria cone was soon
built, from which lava flows were directed SW before turning NW
towards the caldera wall. As the main aa flow approached the
caldera scarp it turned N, covering the settlement of Boca de Fonte
by 9 April and approaching Portela and Bangaeira (see map in
Bulletin v. 20, no. 3). Less vigorous fire fountaining continued on
12-16 April, and fed new lava flows on top of the previous aa flow.
There were occasional periods of Strombolian spatter ejections. By
late on 16 April the remobilized flow-front was ~4 km from the
source vent and only a little more than 500 m from the nearest
house in Portela.
Activity during 17-25 April. Except where noted otherwise, the
following observations are from the USGS team and their Cape
Verdean colleagues. Activity continued on 17 April with little
change at the vent. Spatter fountains rose 100-150 m, and the cone
was ~150 m high. Volcanic tremor amplitude remained moderate to
strong. The N end of the aa flow advanced ~150 m during 16-17
April, to ~420 m SW of the nearest house in Portela, and the E side
of the flow moved 20-50 m ENE. The W side of the flow advanced >100
m and by 1430 had crushed half of the winery at Boca de Fonte.
After these breakouts blocked the access road a new road was
created through agricultural fields, forcing residents rescuing
belongings to walk an additional 500 m. Flow movement was barely
perceptible after 1430 and largely restricted to short spiny
pahoehoe and aa oozes at flow margins, although lava output at the
vent was unchanged.
Between 1630 and 2030 on 17 April, Gaudru noted that Strombolian
explosions were less vigorous and that the main lava channel had
widened from 2-3 m to 5-6 m because of lava-block obstructions. The
W flank of the cone was also covered by cinders. Explosive activity
increased at 1900, sending incandescent ejecta 150-200 m above the
rim of the cone. A flame visible behind the E part of the cone was
apparently coming from a small vent on the upper E flank. At 2000
explosions began ejecting material >300 m W instead of vertically.
Tremor amplitude began to increase around 0650 on 18 April, and at
0740 became continuous at about twice the previous amplitude.
Eruptive style changed from fire fountaining to Strombolian
activity, with spatter discharged by loud gas bursts every 3-8
seconds. Lava production increased during the morning; by noon the
lava was largely pahoehoe in the upper 300 m of the channel.
Estimated channel dimensions and the speed of lava in it yielded
production rates of 4-8.5 x 10^6 m^3/day. Microearthquakes were
intermittent, with three larger events (all M <1) at 1314 and 1803
on 18 April, and at 0426 on 19 April.
Seismograph records showed that activity during 0110-0320 and
0426-0610 on 19 April was characterized by strong explosive bursts,
which were interpreted to be vent clearing episodes after pieces of
the cone and newly erupted spatter closed the conduit. After 0610
the seismicity indicated a return to fire-fountaining. A favorable
wind direction permitted a close approach to the vent and lava
channel to verify the volume estimate, but the lava appeared
somewhat more viscous/sluggish. There was no measureable movement
at the edges of the aa flow on 19 April after <3 m of movement the
day before, however, lava continued ponding in its channel near the
middle of the flow.
Observations made by Gaudru from 1230 on 18 April until 1230 on 19
April indicated that activity remained strong with incandescent
fragments rising >200 m and loud detonations. Explosions every 1-2
seconds, accompanied by earthquakes, ejected particles ranging in
size up to >1 m^3. Gas outbursts were more intense, and black
plumes hovered over the active cone. Partial obstruction of the
crater caused a larger explosion at 1745 on 18 April that sent gas
and cinders 500-600 m high. After several seconds of quiet,
stronger explosive activity began again with sounds that shook the
ground. The upper E flank crater sent an intermittent orange-red
flame 10-15 m high for several hours during this period, higher
than previous days. Eruptive activity observed by the Gaudru group
became more regular at 0100 on 19 April, when an intense episode
began that sent lava fountains >300 m high for several hours.
Explosive activity began again at dawn that lasted throughout the
morning of 19 April.
Tremor amplitude on 19 April changed from moderate-strong to
moderate around 1500, when Strombolian activity reverted back to
fire fountains. Fire fountain heights diminished somewhat on 20
April, rising generally 20-50 m above the vent. Intermittent
Strombolian activity continued with more energetic bursts that sent
viscous lava clots >160 m high. A full lava channel 200 m W of the
vent appeared much like it did the day before. A new aa lobe was
moving sluggishly on top of the earlier flow, and by 1700 its
distal end was ~600 m from the N end of the flow, nearest to
Portela.
Strong Strombolian activity on 21 April produced loud bursts of
viscous spatter 50-150 m high. A levee formed on top of the
spillway adjacent to the vent behind which fountains rose 10-20 m,
often interrupted by explosions. Lava exited through a hole in the
bottom of the levee into a W-flank channel roofed over in two
places. At the bottom of the spillway the lava entered a sinuous
channel, moving W and NW on top of the previously emplaced flow;
this channel remained full all day. The volume of lava erupted was
similar to values for the past several days, 4-8 x 10^6 m^3/day.
The 160-m-high cinder cone was no longer increasing significantly
in height, but impact craters as large as 5 m wide and 1 m deep,
created by fall of spatter bombs 0.5-2 m across, littered its
flanks and parts of the cinder-mantled caldera floor up to 200 from
the vent. As is common during eruptions of viscous mafic lava, the
inner walls of the cone collapsed into the conduit, resulting in
explosive vent-clearing episodes. The overriding aa flow on the E
side of the N flow moved another 6 m N during 21 April.
Volcanic tremor on 21-22 April continued at moderate to strong
levels, punctuated by frequent sonic bursts. Noisy Strombolian
bursts sent clots of spatter over the top of the cone and onto its
flanks. The volume of lava flowing into the channel was similar to
that of 21 April. At noon, lava from a new crack on the N flank of
the cone flowed 150 m N and soon stagnated. The aa flow advanced 2
m W near the new end of the road (150 m S of Boca de Fonte), and ~3
m NE on the E side of the N flow. Most of the volume of lava was
concentrated in an aa lobe that was very slowly overriding the
earlier flow. This lobe locally was at least 15 m thick and covered
an estimated 75% of the existing flow field.
Activity on 23 April was spectacular. Deafening explosions from
four discrete vents rocked the caldera all day; at times the ground
was in continuous motion from concussion waves. The overriding aa
lobe only moved ~4 m N on the E side of the main aa flow. However,
early in the afternoon a new vent opened at the NW base of the
cone. By 1700 lava was flowing W from this vent, and by 1807
spatter ejected to heights of 10-15 m was visible. Pahoehoe lava
flowed on top of older aa and soon joined the large stagnating aa
channel 500-700 m from the main cone. For the preceding 4 days the
seismograph had recorded sonic bursts and microseisms. It was
believed that shock waves associated with the bursts caused several
fractures on the cone. One of these cracks provided a new pathway
for lava to exit the cone, thus robbing the main channel of most of
its lava. Strong volcanic tremor was interrupted by frequent sonic
bursts.
Moderate to strong tremor continued on 24 April. At the main cone
in the morning, Strombolian bursts every few seconds sent spatter
fragments onto the cone's flanks. In the afternoon, the intense
sonic bursts and Strombolian activity that had characterized the
past few days were absent. A gray-black plume, laden with
fine-grained (<1 mm) juvenile particles and volcanic gases, rose to
heights approaching 1.5 km above the caldera floor. Lava in
relatively low volumes continued to erupt from the NW base of the
cone, moving horizontally from the cone into a tear-shaped cavity.
Once the lava reached the surface, degassing occurred, at times
intensely enough to drive low-level Strombolian activity. The
amount of visible degassing rivaled the plume from the main vent.
The depression and lava chute were 25-35 m long and 1-2 m wide.
Lava moving at 1 m/s then spilled out of the chute and entered a
channel, which was 3-5 m wide, with a speed of 6 m/minute. The flow
in the chute and lava channel was initially pahoehoe, changing to
aa with increasing distance. The new lava channel joined the former
channel, now stagnant in its upper part, 500-700 m below the cone.
This new channel caused the hydraulic head within the main cone to
be lowered, resulting in decreased Strombolian activity.
By 25 April the lava extrusion rate slowed to ~250,000 m^3/day, and
tremor amplitude was somewhat diminished. Spatter generally was not
visible within the cone and only rarely did isolated fragments
clear its top. However, lava that had ponded in the aa channel
advanced on the S side of the earlier large flow. This advance,
which probably began late on 24 April, moved as much as 0.5
m/minute during the afternoon. Most of the new lobe was aa, with
minor pahoehoe. The thermocouple temperature was 1,065 deg C
(steady for several minutes) in the pahoehoe. At about 1500-1700
loud explosions at vents within the main cone increased in
frequency, although spatter output did not change.
Activity in late April-early May. At the request of the Cape Verde
government, the French Embassy in Praia and the Ministere de l'
Environnement in Paris arranged for Fran ois Le Guern (CNRS) to
observe the activity during 25-27 April. Incandescent scoria
fountains rose 50 m over the crater 5-10 times/day followed by
quiet periods. Sometimes explosions with black ash or transparent
brown or blue haze lasted a few tens of minutes. Lava output was
estimated to be 1 x 10^6 m^3/day on 26 April with a lava front 300
m long, decreasing by 10-15% on the following days. On 27 April
lava advanced <0.5 m/hour.
>From late April through 2 May a team from the International
Federation of Red Cross and Red Crescent Societies reported that
lava continued to flow from the crater, though at a much reduced
rate, and had already covered 5 km^2 of cultivated land including
five houses and a winery that was a major source of income for the
displaced. At that time the flow was contained inside the existing
banks of lava. News reports indicated that after a period of
non-explosive emissions and weak lava flow production, the eruption
strengthened slightly on 7 May with greater lava output. On 8 May
the United Nations coordinator in Praia reported decreased activity
with some explosions and moderate to strong tremor. The lava
emission rate was relatively low, coming from vents at the NW base
of the cone.
Displaced persons and future plans. Apart from the destruction to
outlying buildings, the villages themselves remained intact but
largely deserted in early May. During the day there was regular
foot traffic as people removed items of use to the camps, including
livestock. The Red Cross of Cape Verde has volunteers in four camps
containing 157 families. The camps are: Sao Filipe, population 534
(including 313 children); Patim, population 88 (53 children);
Achada Furna, population 156 (90 children); and Mosteiros,
population 90 (55 children). Adding the ~150 people living with
friends and relatives, the total number of displaced person comes
to 1,014. These numbers fluctuate as people return to the area and
re-evacuate following felt earthquakes.
With emergency needs met, government officials believe that the
focus should be on the resettlement of displaced persons. The
United Nations DHA-Disaster Mitigation Branch was focusing on civil
protection preparedness planning for future volcanic eruptions and
other natural disasters.
On 10 May, at the request of the Cape Verde government, a team of
four geologists and two students from the Universidade dos A ores
went to Fogo to study the eruption. Their objectives are to monitor
the progress of the eruption and to begin research related to gas
release and the risks of contamination of public water supplies.
Information Contacts: Richard Moore, U.S. Geological Survey, Mail
Stop 903, Federal Center Box 25046, Denver, CO 80225 USA (Email:
rbmoore@gccmail.cr.usgs.gov); Frank Trusdell, U.S. Geological
Survey, Hawaiian Volcano Observatory, Hawaii National Park, HI
96718 USA; Veronica Carvalho Martins, U.S. Embassy, Rua Hoji Ya
Henda 81, C.P. 201, Praia, Cape Verde; Arrigo Querido and Helena
Tatiana Osorio, INGRH Servicos Estudos Hidrologicos, C.P. 367,
Praia, Cape Verde; Fran ois LeGuern, CNRS Centre des Faibles
Radioactivites, 91190 Gif-sur-Yvette, France; Jo~o Gaspar and
Nicolau Wallenstein, Departamento Geoci ncias, Universidad dos
A ores, rue da Mae de Deus 58, 9500 Ponta Delgada, A ores, Portugal
(Email: nw@uac.pt); Henry Gaudru, Christine Pittet, Patrick Barois,
and Marc Sagot, Societe Volcanologique Europeenne (SVE), C.P. 1,
1211 Geneva 17, Switzerland; United Nations Department of
Humanitarian Affairs, Palais des Nations, 1211 Geneva 10,
Switzerland; International Federation of Red Cross and Red Crescent
Societies, C.P. 372, 1211 Geneva 19, Switzerland.