Abstract
The AD 1761 eruption on Terceira was the only
historical subaerial event on the island and one of the last
recorded in the Azores. The eruption occurred along the fissure
zone that crosses the island and produced a trachybasalt
lava flow and scoria cones. Small comenditic trachyte lava
domes (known as Mistérios Negros) were also thought by
some to have formed simultaneously on the eastern flank of
Santa Bárbara Volcano. Following a multidisciplinary approach,
we combined geological mapping, paleomagnetic,
petrographic, mineral and whole-rock geochemical and structural
analyses to study this eruption. The paleomagnetic dating
method compared geomagnetic vectors (directions and intensities)
recorded by both the AD 1761 lava flow and Mistérios
Negros domes and revealed that the two events were indeed
coeval. Based on new data and interpretation of historical
records, we have accordingly reconstructed the AD 1761
eruptive dynamics and distinguished three phases: (1) a precursory
phase characterized by decreased degassing in the
fumarolic field of Pico Alto Volcano and a gradual increase
of seismic activity, which marked the intrusion of trachybasalt
magma; (2) a first eruptive phase that started with phreatic
explosions on the eastern flank of Santa Bárbara Volcano,
followed by the inconspicuous effusion of comenditic trachyte
(66 wt% SiO2), forming a WNW-ESE-oriented chain of lava
domes; and (3) a second eruptive phase on the central part of
the fissure zone, where a Hawaiian to Strombolian-style eruption
formed small scoria cones (E-W to ENE-WSW-oriented)
and a trachybasalt lava flow (50 wt% SiO2) which buried 27
houses in Biscoitos village. Petrological analyses show that
the two batches of magma were emitted independently without
evidence of interaction. We envisage that the domeforming
event was triggered by local stress changes induced
by intrusion of the trachybasalt dyke along the fissure zone,
which created tensile stress conditions that promoted ascent of
comenditic trachyte magma stored beneath Santa Bárbara
Volcano.
historical subaerial event on the island and one of the last
recorded in the Azores. The eruption occurred along the fissure
zone that crosses the island and produced a trachybasalt
lava flow and scoria cones. Small comenditic trachyte lava
domes (known as Mistérios Negros) were also thought by
some to have formed simultaneously on the eastern flank of
Santa Bárbara Volcano. Following a multidisciplinary approach,
we combined geological mapping, paleomagnetic,
petrographic, mineral and whole-rock geochemical and structural
analyses to study this eruption. The paleomagnetic dating
method compared geomagnetic vectors (directions and intensities)
recorded by both the AD 1761 lava flow and Mistérios
Negros domes and revealed that the two events were indeed
coeval. Based on new data and interpretation of historical
records, we have accordingly reconstructed the AD 1761
eruptive dynamics and distinguished three phases: (1) a precursory
phase characterized by decreased degassing in the
fumarolic field of Pico Alto Volcano and a gradual increase
of seismic activity, which marked the intrusion of trachybasalt
magma; (2) a first eruptive phase that started with phreatic
explosions on the eastern flank of Santa Bárbara Volcano,
followed by the inconspicuous effusion of comenditic trachyte
(66 wt% SiO2), forming a WNW-ESE-oriented chain of lava
domes; and (3) a second eruptive phase on the central part of
the fissure zone, where a Hawaiian to Strombolian-style eruption
formed small scoria cones (E-W to ENE-WSW-oriented)
and a trachybasalt lava flow (50 wt% SiO2) which buried 27
houses in Biscoitos village. Petrological analyses show that
the two batches of magma were emitted independently without
evidence of interaction. We envisage that the domeforming
event was triggered by local stress changes induced
by intrusion of the trachybasalt dyke along the fissure zone,
which created tensile stress conditions that promoted ascent of
comenditic trachyte magma stored beneath Santa Bárbara
Volcano.
| Original language | English |
|---|---|
| Article number | 22 |
| Number of pages | 21 |
| Journal | Bulletin of Volcanology |
| Volume | 78 |
| Issue number | 22 |
| DOIs | |
| Publication status | Published - 1 Mar 2016 |