Cape Verde volcanic archipelago - Fogo, the 1995 fissure eruption.

The 1995 fissure eruption in Cha das Caldeiras :

After a seismic swarm, discussed below , the initial strombolian activity   localizes from April 2 on a crack on the southwest flank of Pico do Fogo . A cinder cone called Pico Pequeno, was built there .
 
Fogo - Cha das Caldeiras -  in the foreground, the lava flows, then the cinder cone Pico Pequeno (1995) and in the background, the Pico do Fogo stratovolcano - photo © 2013 Carole and Frederic Hardy

                Fogo - Cha das Caldeiras - Pico Pequeno - photo © 2013 Carole and Frederic Hardy

On April 3 , the crack opened to two kilometers, and the eruption evolves into Hawaiian type , with lava fountains rising to over 400 meters in height, which feed a lava flow that crosses the road to the village of Portela .

               The eruption of Fogo in 1995 - the formation of Pico Pequeno - via Geowiki

Photo of Henry Gaudru / SVG / at a United Nations mission in Cape Verde during the eruption of Fogo in 1995 - via Geowiki

           Fogo - Cha das Caldeiras - the road cut by lava in 1995 was reopened - photo Cayambe

a : Location and overview map of the Cape Verde Islands (inset) and map of the Fogo-Brava

platform with 500 m bathymetric contours and 200m topographic contours on Fogo. Dotted areas mark

seismically active regions in the channel between Fogo and Brava (seismic crisis 1994) and at Cadamosto

seamount (Heleno et al 1999; Grevemeyer et al. 2010). The rectangle shows the position of Fig. 1b.

b :Map of eruption centers and main fissures of the 1995 eruption showing the the syn-eruptive scoria cone of Pico Pequeno and sample localities with numbers (modified after Brum da Silveira et al. 1997). Grey shading: 1995 lava flows, dotted: scoria cones.  // in   Elliot Hildner / University of Bremen 

The main crack houses seven active vents , and consists of an ENE- WSW radial fracture at Pico do Fogo and a segment oriented NNE-SSW at Mount Beco ( figure above / b ).
It gradually closes by the aa lava flows , which add pahoehoe lobes during the first two days of eruption.

The fountaining, hawwaiian type, of Pico Pequeno continues until April 17 to give way to the strombolian activity until the end of the eruption.
On April 23, a new vent opens to the northwest base of the Pico Pequeno and ejects spatter and lava pahoehoe, covering the aa flows previously issued .

The activity then decreases gradually , with short explosive phases .

Eruption of 1995 - cards Joao Gaspar / GVP
left , cast of 03.04 to 11.04.95
right flows from 02.04 to 18.05.95

Fogo - Cha das Caldeiras - Crater of the cinder cone Pico Pequeno - © photo Carole and Frederic Hardy 2013

 
Cha das Caldeiras - the crater of Pico Pequeno cinder cone and lava flows in the background - © photo Carole and Frederic Hardy 2013

At the end of the eruption, on May 26, the flows have covered 6.3 km². The volume of volcanic material emitted is estimated at 54-68 Mm³ . (Torres & al.1997 ). More than 13,000 people have been evacuated and the town of Boca Fonte completely destroyed.

The breached Pequeno crater is littered with blocks and lined with slag of various colors. Near the cone , there is a sample of various bombs : In spindle, in balls, in crust of bread, and rare turtle bombs, with a smooth and concave shell, a flat belly and an excrescence like a nose ( plug LAVE )

Pre- and syn-eruptive seismicity (see diagram above / a )

In January-February 1994 , a small seismic crisis is located in the channel between Fogo and Brava, then under and west of Brava. It is interpreted as reflecting the movement of magma. On Fogo, a swarm of microearthquakes marks the SW of the island and the tremor was observed in Cha das Caldeiras .
From March 25, 1995 , small earthquakes are felt intermittently by residents of the caldera ; they are becoming more common on April 2 , and a strong shock mark the opening of the eruptive fissure .

A volcanic tremor is seen, moderate to strong, during the eruption in the caldera , with episodes of significant increase in amplitude, on 18 and 19.04 , in the passage of the dominant Hawaiian to Strombolian activity .
Just after the end of the eruption, a seismic swarm marks the interaction between groundwater and fluid magma that withdraws.

                  Cha das Caldeiras - lava 1995 -Sylvie De Weze / EllysEyeland Geo series

Analysis of the plumbing system of the eruption 1995

Eruptive products were mineralogically and chemically zoned , with phonotephrites issued at the beginning of eruption, cogenetic to the  basanites then issued, and this succession is interpreted as a change in a pocket, with limited connection with the main magma chamber. In climbing, the magma briefly stationed in the lower crust.
               Fogo - interferogram between 06.1993 and 11.1995 - Amelung & Day

Based on interferograms provided by satellites (1993-1998 ERS SAR) , scientists estimate that the ground deformation during the eruption of 1995 is a result of a feeder dyke , rather than the presence of a magma chamber surface, and suggest that the magma comes from a reservoir at more than 16.5 km. depth ( Amelung & Day ) .

The ascent of magma is produced in 1995 by the plumbing system of Fogo, without interaction with the near Brava system, and cause and effect link with the concurrent seismic crisis around Brava in 1994.
The feeder dyke was oriented plan NO, and plunging at least 4 km. depth. Seismic crisis between the two islands in 94 was likely caused by the spread of one or two dykes that have not reached the surface.

 

Schematic model of the magma plumbing system of the 1995 Fogo eruption (thicknesses of

crustal layers after Pim et al. (2008)). Fractional crystallization prior to eruption took place between 16

and 24 km depth in the uppermost lithospheric mantle. Early erupted phonotephrites are cogenetic with

the subsequently erupted basanites and are interpreted to have evolved in a small pocket with limited

connection to the main magma chamber. The ascending magma stalled briefly in the lower crust. Dotted

areas mark the positions of hypocenters during the small seismic crisis of 1994 (Heleno da Silva et al.

1997; Heleno and Fonseca 1999), which may have been caused by propagating dikes preceding the 1995

eruption (dikes A and B) and/or by dikes related to a plumbing system beneath Brava (C).

Magma ascent of the 1995 eruption occurred along the established magma plumbing system of Fogo without interaction with the plumbing system near Brava. A seismic crisis in the channel between Brava and Fogo does not necessarily herald a new eruption on Fogo. // in   Elliot Hildner / University of Bremen.

Sources :

- Global Volcanism Program - Fogo

- Magma storage and ascent of historic and prehistoric eruptions of Fogo,
Cape Verde Islands: A barometric, petrologic and geochemical approach - by Elliot Hildner / University of Bremen 

- GRL - InSAR observations of the 1995 Fogo eruption - Implications for the effects of collapse events upon island volcanoes - by F.Amelung & S.Day.

- LAVE - fiche Pico do Fogo - link