Geological mapping of the central area of Terceira Island (Azores

Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185
Comunicações Geológicas (2014) 101, Especial I, 283-288
IX CNG/2º CoGePLiP, Porto 2014
ISSN: 0873-948X; e-ISSN: 1647-581X
Geological mapping of the central area of Terceira Island
(Azores, Portugal): associated volcanostratigraphy, ages
and genetic implications on the Malha-Balcões-Chamusca
lava caves system
Cartografia geológica da região central da ilha Terceira
(Açores, Portugal): vulcanoestratigrafia, idades associadas
e implicações genéticas no Sistema Cavernícola MalhaBalcões-Chamusca
J. C. Nunes1,2*, A. Calvert3, S. Medeiros1, E. A. Lima1, F. Pereira2,4, M. P. Costa2,4,
P. Barcelos2,4, M. R. Carvalho5
Artigo Curto
Short Article
© 2014 LNEG – Laboratório Nacional de Geologia e Energia IP
Abstract: The detailed geological mapping of the Basaltic Fissural
Zone in the central area of Terceira Island, the field stratigraphical
relationships observed and published and new isotopic ages now
provided allowed to correlate the 26 volcanic caves existing in the
mapped area (between Pico da Bagacina area and the north coast)
with the correlated volcanic episodes/eruptions.
The main conclusions obtained include: i) the association of a 10
volcanic caves system (including the lava-tube caves of Malha,
Balcões and Chamusca) with Holocene (potentially younger than
6,600 years BP) pahoehoe lava flows extruded from Pico Galiarte
scoria cone; ii) the association of 8 volcanic caves (namely Natal
cave) with the eruptive episode associated with the “Pico do Gaspar
fissural system”, with an age younger than 12,100 years BP; iii) the
mapping of the lava flows associated with the Pico Gordo scoria cone
(that formed 4 volcanic caves), dated of about 4,480 years BP and
that preceded the 1761 A.D. historical eruption, which corresponds to
the late eruption In the mapped area.
Keywords: Geological mapping, Volcanic caves, Azores islands,
Ages, Eruptive history.
Resumo: A cartografia geológica de detalhe da região central da ilha
Terceira integrada na Zona Basáltica Fissural, as relações
estratigráficas observadas in situ, idades isotópicas publicadas e
novas datações agora apresentadas, permitiram correlacionar as 26
cavidades vulcânicas existentes nesta região (e.g. entre a zona do
Pico da Bagacina e o litoral norte) com os episódios/erupções
vulcânicas que estiveram na sua génese.
De entre os principais resultados merece especial destaque: i) a
associação de um sistema de 10 cavidades vulcânicas (onde se
incluem as grutas da Malha, dos Balcões e da Chamusca) a escoadas
lávicas pahoehoe emitidas pelo Pico Galiarte, de idade holocénica
(potencialmente de idade inferior a 6.600 anos BP); ii) a associação
de 8 cavidades (de que faz parte a Gruta do Natal) ao episódio
eruptivo associado ao “sistema fissural do Pico Gaspar”, de idade
inferior a 12.100 anos BP; iii) a cartografia das escoadas lávicas
associadas à erupção do cone de escórias do Pico Gordo (que
originou 4 cavidades vulcânicas), datada de há cerca de 4.480 anos
BP, a qual antecede a erupção histórica do ano de 1761 A.D., que
corresponde à erupção mais recente ocorrida na zona cartografada.
Palavras-chave: Cartografia geológica, Cavidades vulcânicas, Ilhas
dos Açores, Idades, História eruptiva.
1
Universidade dos Açores, Departamento de Geociências, Apartado 1422,
9501-801 Ponta Delgada, Açores, Portugal.
2
GESPEA – “Grupo de Estudo do Património Espeleológico dos Açores”,
Edifício Matos Souto, Piedade, 9930-210 Lajes do Pico, Açores, Portugal.
3
United States Geological Survey, 345 Middlefield Road MS 977, Menlo Park,
California 94025, USA.
4
Associação “Os Montanheiros”, Rua da Rocha, nº 8, 9700-169 Angra do
Heroísmo, Açores, Portugal.
5
Departamento de Geologia e Centro de Geologia, FCUL, Universidade de
Lisboa, Edifício C6, Campo Grande, 1749-016 Lisboa, Portugal.
*
Corresponding author / Autor correspondente: jcnunes@uac.pt
1. Introduction
The Azores Archipelago is located at the triple junction
between the North American, Eurasian and African plates.
The first two are separated by the Mid-Atlantic Ridge
located between Flores and Faial islands, while Eurasian
and African plates are separated by a complex tectonic
structure, the Azores-Gibraltar Plate Boundary, that in the
area of the archipelago comprises a wide shear zone (e.g. the
Azores block, Luis et al., 1994), with a rough elongated
triangular shape. The northern edge of this shear zone is
established by the Terceira Rift, a WNW-ESE hyper-slow
spreading centre (Vogt & Jung, 2004) incorporating
Graciosa, Terceira and S. Miguel islands.
Terceira Island, together with Graciosa, São Jorge, Pico
and Faial islands form the Central Group of the
Archipelago. The island presents active volcanism (with a
wide variety of rocks compositions and structures) and an
important seismic activity. Terceira Island includes four
central polygenetic volcanoes with caldera (Cinco PicosSerra do Cume, Guilherme Moniz, Pico Alto and Santa
Bárbara, from east to west) and a Basaltic Fissural Zone
284
(Nunes, 2000), mainly in the central and SE parts of the
island (Fig. 1).
Volcanic products extruded from these central
volcanoes, mostly in Quaternary times include ignimbrites,
pumice fall deposits, strongly evolved silicic lava flows (as
domes and coulées of comendite to pantellerite
J. C. Nunes et al. / Comunicações Geológicas (2014) 101, Especial I, 283-288
composition), scoria and spatter deposits, surtseyan tuffs
and basaltic lava flows. Unlike other islands of the
archipelago, Terceira volcanism main feature is the
presence of extensive effusive episodes of silicic nature
(domes and coulées, 40-90 m thick) which represent about
80% of the erupted material (Self, 1976).
Fig. 1. Volcano-tectonic sketch of Terceira Island: 1- Cinco Picos-Serra do Cume Volcano; 2- Guilherme Moniz Volcano; 3- Santa Bárbara Volcano; 4Pico Alto Volcano; 5- Basaltic Fissural Zone, including the 1761 A.D. and the Algar do Carvão lava flow, in yellow. Dotted areas include trachyte s.l.
domes and coulées. a- volcanic and tectonic lineament; b- fault scarp; c- crater rim; d- caldera rim; e- basaltic s.l. eruptive centre (scoria cone/tuff cone); fsilicic eruptive centre (dome/coulée); g- eruptive fissure; h- fumaroles; i- thermal waters spring. Stars point out locations of 14C ages of the Lajes and
Angra ignimbrites. UTM coordinates in metres (Nunes, 2000). Rectangle indicates mapped area of figures 2 and 3.
Fig. 1. Esboço vulcano-tectónico da Ilha Terceira.
After the settlement of the island (in the fifteenth
century) three volcanic eruptions took place: the first one in
1761, the second in 1867 and the last one on the period
1998-2001. The 1761 eruption occurred on-land, east of
Santa Bárbara Volcano (with two distinct episodes and
locations), while the later two were located at sea, NW of
the island (about 2.5 km and 8.5 km offshore, respectively).
The Basaltic Fissural Zone is developed on two main
sectors of the Terceira Island: i) in the central part of the
island, namely along the Pico da Bagacina-Pico do Gaspar
volcano-tectonic lineament, and, ii) in the SE part of the
island including the S. Sebastião-Porto Martins and the
Cinco Picos areas (Fig. 1). The associated volcanism is
strongly controlled by NW-SE to W-E fractures, is almost
exclusively of the Hawaiian to Strombolian style, and was
responsible for the building up of about 75 monogenetic
volcanoes, mostly scoria cones and eruptive fissures, and
emission of aa and pahoehoe lava flows.
Given its volcanic origin and the dominant basaltic
nature of its eruptions, Azores archipelago presents a wide
diversity of volcanic caves in all but Corvo Island.
Presently, 272 natural caves are known in the Azores
Islands, corresponding to about 32 kilometres of
underground paths, with peculiar geologic features and live
forms (Costa et al., 2008). These caves are of different
types: lava-tube caves, volcanic pits, fractures and erosion
caves, including the combination of these structures. Most
caves are located in the islands of Pico with 129, Terceira
and S. Miguel islands, with 69 and 28 caves respectively.
About 70% of the Terceira island caves are located in
the Basaltic Fissural Zone, thus the importance to map the
geological formations with which the caves are associated.
Since the area of São Sebastião-Porto Martins was already
mapped (cf. Montesinos et al., 2003) – where caves such as
Gruta da Madre de Deus, Gruta de Santo António and
Galeria do Sequeira were associated to the 2,000-3,000
Geological mapping of the central area of Terceira Island (Azores)
years old Fonte Bastardo eruption – the main goals of the
present work were to obtain a detailed geological map of the
central sector of the Basaltic Fissural Zone and establish the
local volcanostratigraphy, allowing correlations with the
volcanic caves emplaced in that area.
2. Geological mapping and volcanostratigraphy
Detailed geological mapping was carried out on the north
part of central sector of the Basaltic Fissural Zone (Fig. 1)
since the known volcanic caves are located in this area.
The field work was done in the period 2009-2011 and
included
the
establishment
of
the
detailed
volcanostratigraphy of the area, the nature of its
volcanism, the eruptive styles and the mapping of the
major tectonic lineaments and fractures. Geochemical
analyses of collected samples were performed at
Activation Laboratories, Ltd., in Canada.
The obtained geologic map is presented in figure 2,
which includes the location of the main entrance of the 26
volcanic caves and pits existing in the area. The mapped
area is limited to the W by the volcanism associated with
the Santa Bárbara Volcano, while to the NE and SE,
respectively, the Pico Alto and the Guilherme Moniz
polygenetic silicic volcanoes formations crop out.
Geological mapping and local volcanostratigraphy was
also supported on isotope age determinations available for
the area, namely 14C and Ar/Ar ages reported in Calvert et
al. (2006). Additionally, the present work includes
unpublished Ar/Ar ages of samples collected in 2006 by A.
Calvert at Gruta dos Balcões and Gruta do Natal lava
caves. Compiled age information relevant for the present
work and its relations with the geological map and
volcanostratigraphy of the area are presented in table 1 and
in figure 3. Table 2 presents the geochemical composition
of the samples analyzed and the correspondent rock
designation in accordance to the Le Bas et al. (1986)
diagrams presented in figure 4.
All the gathered information allows new insights on the
volcanism of Terceira Island and especially on the eruptive
history of this sector of the Basaltic Fissural Zone (BFZ),
as presented below.
285
 the eastern sector includes the scoria cones of Pico da
Bagacina (hawaiite), Criação do Filipe/Achadas
(mugearite) and “536”cone/Biscoito da Atalhada (basalt),
whose aa-type lava flows where extruded to the south.
To the north it is worth mentioning three important
episodes/eruptions:
a) the extensive pahoehoe basaltic lava field emitted from
Pico Galiarte (also known as “Pico dos Pedreiros”),
that flooded the flat areas near the axial part of the
BFZ, but, mostly, moved towards the north up to
Biscoitos village (Fig. 3) and probably reached the sea
(at the time the coast line was about 0.5 km further
south in the area of Biscoitos); this lava field includes
many lava caves (Fig. 3 and Table 1), such as the
Malha and the Chamusca lava caves, the most southern
and northern caves in this lava field. Since this unit
also includes the Balcões lava cave, the most important
one in Terceira island (4,421 m total length – Costa et
al., 2008), the volcanic unit composed by the Pico
Galiarte scoria cone and the extensive pahoehoe
basaltic lava field associated was named as “MalhaBalcões-Chamusca lava caves system”;
3. Discussion
The axial part of the mapped area is defined by the
volcano-tectonic lineament of Pico do Gaspar –
Arrochinha (“573” cone) – Pico da Bagacina – Criação
do Filipe/Achadas (see also Fig. 1). This WNW-ESE
alignment of spatter and scoria cones defines a narrow
and small ridge, from which altitudes decrease both to
north and south coasts. This alignment includes two
different sectors:
 the western sector includes the “Pico do Gaspar fissural
system”, with the Pico do Gaspar and Lombinhas spatter
cones and the Arrochinha scoria cone; the former fed a
pahoehoe-type basaltic lava flow that moved both to the
north and south; it includes the Gruta do Natal lava cave
among others (Fig. 3 and Table 1);
Fig. 2. Geological map of the Basaltic Fissural Zone (BFZ) in the central
area of Terceira Island (north sector) and location of the main entrances
(black circles) of the 26 associated caves. See figure 3 for legend.
Fig. 2. Carta geológica da Zona Basáltica Fissural, na região central da
ilha Terceira (sector norte) e localização da abertura principal (círculo a
negro) das 26 cavidades vulcânicas associadas Legenda: ver figura3.
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Table 1. Location of the entrances of the volcanic caves and associated
volcanic episode/eruption at the mapped area.
Tabela 1. Localização da entrada das cavidades vulcânicas existentes na
área cartografada e episódio/erupção vulcânica associada.
coulées of “Mistérios Negros” (“Va” in Fig. 3). It
should be mentioned that historical records and
descriptions of the 1761 A.D. eruption clear identify
this two phases, either in time and geographical terms,
either with detailed descriptions of the eruption
characteristics (see for example Drummond, 1981).
14
C age of Pico Gordo by Calvert et al. (2006) and new Ar/Ar ages (this
study)
Table 2. Geochemical composition of the samples, rock designation and
associated volcanic episode/eruption. Analysis by Activation Lab., Ltd.
(Canada).
Tabela 2. Composição geoquímica das amostras, classificação da rocha e
episódio/erupção vulcânica associada. Análises realizadas por Activation
Lab., Ltd. (Canadá).
b) the basaltic lava flows extruded from Pico Gordo
scoria cone, that flowed towards the north for about 7
km, reaching the sea on the Biscoitos village area; this
is the eruption that preceded the 1761 A.D. event in
this sector of the Basaltic Fissural Zone; charcoal
collected beneath this flow, near Biscoitos, yielded a
14
C age of 4,480±40 y BP (Calvert et al., 2006),
consistent with the local volcanostratigraphy and
mapped outcrops;
c) the hawaiite lava flow erupted in 1761 A.D. – the
“Mistério Velho dos Biscoitos” – as an aa-type lava
flow with an average thickness of about 3 meters,
strongly controlled by the paleomorphology of the
area; the first phase of this eruption, of silicic nature,
was also mapped, as the trachyte s.l. domes and
Fig. 3. Geological map and volcanostratigraphy of the mapped area (see
also table 1). 1-gravel and pebble beach deposits; 2- undifferentiated
lapilli mantling; 3- old BFZ scoria cone (dotted); 4- old BFZ lava flows;
5- Achadas eruption (scoria deposits dotted); 6- Biscoito da Atalhada
eruption (scoria deposits dotted); 7- caldera rim; I, II, III and IVintermediate to young BFZ volcanic episodes; V- 1761 A.D. eruption:
Va- Mistérios Negros trachyte domes and coulées; Vb- Mistério Velho
dos Biscoitos hawaiite eruption; PA pp- Pico Alto Volcano pumice
sequences, including ignimbrites; PA T- Pico Alto Volcano trachyte s.l.
domes and coulées; PA b&a- Pico Alto Volcano block and ash flow
deposit; SB pp- Santa Bárbara Volcano pumice sequences; SB B- Santa
Barbara Volcano basaltic s.l. lava flows; SB T- Santa Barbara Volcano
trachyte s.l. domes and coulées; GM- Guilherme Moniz Volcano pumice
sequences. Main entrances of volcanic caves marked as black circles and
other entrances marked as red circles.
Fig. 3. Carta geológica e vulcano-estratigrafia da zona cartografada (ver
também Tabela 1). Indicada a abertura principal (círculo a negro) e outras
aberturas (círculo a vermelho) das cavidades vulcânicas.
Geological mapping of the central area of Terceira Island (Azores)
Besides these broadly speaking Holocene age
episodes/eruptions, other basaltic s.l. eruptive centers were
identified and mapped as “old BFZ” scoria deposits/lava
flows (Fig. 3 and Table 1) – like the Pico Viana, Pico das
Caldeirinhas, “592/Pico Gordo 2” and the “565” scoria
cones – whose lava flows were buried by the younger
pyroclasts and lava flows mentioned above.
As a final remark, it should be mentioned that at Alto da
Caparica/Cancela area an old trachyte flow outcrop (“PA T”
in Fig. 3) surrounded by the pahoehoe lava flows of the
Malha-Balcões-Chamusca lava caves system, extruded from
the Pico do Galiarte scoria cone. This kipuka, previously
mapped by Lloyd & Collis (1981) as the “Biscoitos
Formation”, belongs to the Pico Alto polygenetic silicic
volcano and represents the older formation cropping out in
the area, presently buried by the BFZ products.
Fig. 4. Classification of the samples on the TAS diagram, Na2O + K2O
versus SiO2, expressed in %, after Le Bas et al. (1986).
Fig. 4. Classificação das amostras com base no diagrama de TAS, Na2O
+ K2O versus SiO2, expressa em percentagem, segundo Le Bas et al.
(1986).
Unlike the silicic formations related with Pico Alto
Volcano, to the West of the Pico Gordo lava flows there
is an extensive lava field of a hawaiitic compound
pahoehoe lava flow, which is very distinctive, since it is
highly porphyritic and containing abundant acicular
phenocrystals of plagioclase (often 5-10 mm long). This
287
lava field extends westwards up to Altares village and
Matias Simão spatter cone, forming the sea-cliffs up to
Biscoitos harbor. This lava flow is attributed to Santa
Barbara Volcano and is locally covered by the Pico
Gordo lava flows or mantled by a widespread lapilli fall
deposit (see Fig. 3). Up-hill, closer to the alignment of
domes and coulées from the northeastern flank of Santa
Bárbara Volcano, this flow is also buried by Santa
Bárbara pumice deposits.
As an attempt to better constrain the ages of the mapped
volcanic episodes/eruptions, two samples from the areas
close to Gruta dos Buracos (sample 06AZT15) and Gruta
do Natal caves (sample 06AZT16) were analyzed for
Ar/Ar dating, following the methodology and
specifications indicated in Calvert et al., 2006. Although well
constrained age determinations were not obtained for these
samples, it seems reasonable to point out that (Fig. 5):
 Step heating of basalt sample 06AZT15 yielded a
plateau age of -2.4 ± 9.0 ka and an isochron age of -1.1
± 18.9 ka (2σ); the sample is constrained by the limit of
the plateau age as younger than 6,600 years and the limit
of the isochron age as younger than 17,800 years;
 Step heating of basalt sample 06AZT16 yielded a
plateau age of -16.5 ± 15.4 ka and an isochron age of 11.8 ± 23.9 ka (2σ); the sample is constrained by the
limit of the isochron age as younger than 12,100 years.
 In spite the 2σ errors associated with the obtained
ages and the limited isochron/plateau ages of the
samples – in close relations with the youth of this
lava flows – it seems reasonable to use these ages
as indicative time intervals for the occurrence of
the associated volcanic eruptions/episodes (see
also Table 1).
 Thus, an “Holocene age” (potentially younger
than 6,600 y BP) can be assigned for the MalhaBalcões-Chamusca lava caves system (Pico
Galiarte eruption) and an age of “younger than
12,100 y BP” can be assigned for the “Pico do
Gaspar fissural system”, the later responsible for
the formations of Gruta do Natal lava cave. Those
assigned Ar/Ar ages are consistent and in good
agreement with field data gathered from
stratigraphical relationships among the geological
formations and the detailed mapping of the
outcrops.
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J. C. Nunes et al. / Comunicações Geológicas (2014) 101, Especial I, 283-288
Fig. 5. Age spectra for samples 06AZT15 collected at Gruta dos Buracos (A) and 06AZT16 collected at Gruta do Natal (B) lava caves.
Fig. 5. Espectro da idade das amostras 06AZT15, colhida na Gruta dos Buracos (A) e 06AZT16, colhida na Gruta do Natal (B).
4. Final notes
Geological mapping in the central northern area of Terceira
Island revealed that:
1) Most volcanic caves in the area are associated with
the Malha-Balcões-Chamusca lava caves system, related with
the lava flows of Pico Galiarte eruption;
2) The 1761 A.D. hawaiite lava flow does not
incorporate any volcanic cave; lava-tube caves mapped in its
vicinity are related to underlying lavas and thus, occasionally
have some mapped branches underneath the 1761 A.D.
hawaiite lava flow.
The Basaltic Fissural Zone in the central area of Terceira
is a relatively thin volcanic unit (probably just of a few tens of
meters thick), whose lava flows occupied a topographically
depressed area with a general N-S trend, at the intersection of
Santa Bárbara Volcano, to the West, and Pico Alto and
Guilherme Moniz polygenetic volcanoes, to the East. This can
be inferred from the field relations with widely dispersed
hawaiite lava flows from Santa Barbara Volcano, cropping
out to the west of Biscoitos harbor, and with the Alto da
Caparica trachyte kipuka, presently surrounded by the Pico
Galiarte pahoehoe lava flows.
References
Calvert, T.A., Moore, R.B., Mcgeehin, J.P., da Silva, A.M.R., 2006.
Volcanic history and 40Ar/39Ar and 14C geochronology of Terceira
Island, Azores, Portugal. Journal of Volcanology and Geothermal
Research, 156, 103-115.
Costa, M.P., Nunes, J.C., Constância, J.P., Borges, P.A.V.,
Barcelos, P., Pereira, F., Farinha, N., Góis, J., 2008. Cavidades
Vulcânicas
dos
Açores.
Amigos
dos
Açores/Os
Montanheiros/GESPEA (Ed.), 48 p. ISBN: 978-989-95432-25.
Drummond, F.F., 1981. Anais da Ilha Terceira. Secretaria
Regional da Educação e Cultura (Ed.), Angra do Heroísmo,
vol. II, 288-290.
Le Bas, M., Le Maitre, R., Streckeisen, A., Zanettin, B., 1986. A
chemical classification of volcanic rocks based on the total
alkali-silica diagram. Journal of Petrology, 27, 745-750.
Lloyd, E.F., Collis, S.K., 1981. Geothermal prospection – Ilha
Terceira, Açores. Geological report, SRCI-LGT Report, Ponta
Delgada, 96 p.
Luis, J.F., Miranda, J.M., Galdeano, A., Patriat, P., Rossignol,
J.C., Victor, L.A.M., 1994. The Azores triple junction since 10
Ma from an aeromagnetic survey of the Mid-Atlantic Ridge,
Earth and Planetary Science Letters, 125, 439-459.
Montesinos, F.G., Camacho, A.G., Nunes, J.C., Oliveira, C.S.,
Vieira, R., 2003. A 3-D gravity model for a volcanic crater in
Terceira Island (Azores). Geophysical Journal International,
154(2), 393-406.
Nunes, J.C., 2000. Notas sobre a geologia da Ilha Terceira
(Açores). Açoreana, 9(2), 205-215.
Self, S., 1976. The recent volcanology of Terceira, Azores.
Journal of the Geological Society of London, 132, 645-666.
Vogt, P.R., Jung, W.Y., 2004. The Terceira Rift as hyper-slow,
hotspot-dominated oblique spreading axis: A comparison with
other slow spreading plate boundaries. Earth and Planetary
Science Letters, 218, 77-90.