Just add water N37º 46.398 W25º 18.239 S. Miguel
(Azores)
Introduction
Active volcanic regions worldwide 
are
common for observation of mineral and thermal waters discharges,
which can contribute to social and economic well-being. For
example, they are developed as spas, bottled for drinking purposes,
and used for heat production for domestic and greenhouses supply.
Another useful application of the thermal water geochemistry is for
geothermal resources prospection and the monitoring of volcanic
activity.
The Azores
The archipelago of the Azores is made of nine islands of volcanic
origin in the North Atlantic Ocean, between 37 to 40ºN and 25 to
31ºW. It has an area of 2333 km2 and approximately 237500
inhabitants. The islands are scattered along a 500 km
WNW-ESE-trending strip, and emerge from the Azores plateau, a
triangular-shaped zone of anomalous and irregular shallow
topography.
The Azores archipelago lies close to the triple junction between
the American, the Eurasian and the African plates.
The Azores plateau is traversed by important tectonic
structures, like the Mid-Atlantic Ridge (MAR) which divides the
Azores islands in two main sectors: the western group of islands,
Flores and Corvo, are to the west of the MAR whereas the central
(Terceira, Pico, Faial, São Jorge and Graciosa) and the eastern
groups of islands (São Miguel and Santa Maria) emerge toward the
east of that structure. The geological setting of 
the
Azores archipelago enhances the multiplicity of surface
hydrothermal manifestations. Mineral and thermal waters springs as
well as fumarolic grounds exist over the archipelago, especially on
the islands of São Miguel, Terceira, Pico, Faial, Graciosa and
Flores.
Ground water
Groundwater at the Azores occurs in two major aquifer systems: (1)
the basal aquifer system, which corresponds to freshwater lenses
floating on underlying saltwater, and (2) perched-water bodies. The
basal aquifer system is in the coastal area, presenting generally a
very low hydraulic gradient, and groundwater extraction is from
drilled wells.
The perched-water bodies correspond to pervious units, with
impermeable to very low-permeability layers at the bottom and,
where topographic conditions are favourable, they are drained by a
large number of springs on the volcano slopes. Therefore, these
aquifers at altitude correspond to confined layers or to leaky
aquifers which can lose water through aquitards bounding them from
above, with a flow-like water over a succession of roof tiles.
H2O in S. Miguel Island
São Miguel, the major island of the archipelago and belonging to
the Azores eastern group, has an area of about 747 km2 and
approximately 125800 inhabitants.
Thermal waters in the Azores are first referenced in 1583 but the
study of mineral and thermal water discharges in São Miguel started
in 1873. The first reference to the therapeutic use of the
thermal waters dates from 1614. Owing to the known therapeutic
value of these waters several thermal hospitals were constructed,
especially in the 19th century. However, the importance of these
spas has decreased with time and at present several
‘‘banhos’’ are closed or in ruins.
Geology of S. Miguel
The geology of São Miguel is dominated by three volcanic centres
which correspond to the major active trachytic central volcanoes of
Fogo, Sete Cidades and Furnas, linked by rift zones. During the
last 5,000 years, the activity of the three active central
volcanoes is recorded by 57 volcanic eruptions, with an output of
4.6 km3 of rock. The island o S. Miguel is made up of a series of
accreted volcanoes, the oldest corresponding to the Nordeste shield
volcano, a deeply eroded structure in the easternmost sector of the
island, in which the oldest outcrop was dated at about 4 M.a.
and the more recent at 0.95 M.a. Volcanic activity has spread
from east to west, forming the remaining volcanic complexes from
São Miguel, of which the Picos complex is the most recent,
corresponding to an area dominated by basaltic volcanism features,
marked by scoria cones, many of them aligned along fractures
trending WNW–ESE and NW–SE, and where the last eruption
occurred in historical times.
Hydrothermal manifestations occur mainly in the three active
volcanic centres, especially at the Fogo and Furnas
volcanoes.
Furnas is the easternmost of the three active composite volcanoes
on the island São Miguel. This volcano is bounded in the west by
the Povoacão caldera, which corresponds to an older and presently
inactive volcano, and to the east by the Fogo volcano. The Furnas
volcanic centre summit has subsided in several stages of caldera
collapse. An older caldera, about 7·5 km wide with a 290-m-depth
depression, encloses a younger caldera with a diameter of
approximately 5 km. Two smaller subsidence events took place within
the younger caldera in the last 5,000 years and explain the origin
of the depressions where the Furnas village and the Furnas lake are
found today. The growth of the Furnas central volcano started about
100000 years ago, mainly as a result of eruptions of trachytic
pumice, and the majority of the outcrops in the western caldera
wall postdate a 48000±4,000-year-old lava flow. In the last 5,000
years, at least ten explosive eruptions took place in Furnas, two
of which were accompanied by extrusion of trachytic domes
corresponding with the major event of the Furnas C deposit. The
last eruption at Furnas, dated at 1630 A.D., occurred in the
southern part of the caldera and had an explosive phase which
produced a tuff/pumice ring complex. In historical accounts by the
first inhabitants of S. Miguel, two eruptions were recorded: one in
1439–1443 A.D. and the other in 1630 A.D. 
The most important thermal features at the Furnas volcano lie on
a E-W-trending tectonic lineament which cuts across the Furnas
caldera complex. Distributed along this tectonic lineament, two
fumarole grounds can be distinguished, one at the Furnas lake
margin, 290 m a.s.l., and the other toward the east at the Furnas
village, 200 m a.s.l. Another fumarole is in the Ribeira dos
Tambores valley. Associated with these fumaroles are boiling water
discharges. Near the Furnas village, numerous scattered thermal
springs can be observed as well as several CO2-rich mineral cold
waters.
This EarthCache takes you to investigate the ground water of one
of the Azores tourist hot spots: the Furnas volcano on the Island
of S. Miguel.
Ground zero takes you to a fountain that is made up of two spouts
each with their own proper name.
In order to qualify for a found,
- Please tell me what the names of the spouts are,
- Which spout yields HOT water and
- Whether the temperature of this water is below or above 36.3
ºC.
Your correct answers by mail will be answered with permission to log
this cache and tally another found.
Refs:
Cruz, JV, 2003. Groundwater and volcanoes: Examples from the Azores
archipelago. Environmental Geology 44: 343-355.
Guest JE, Gaspar JL, Cole PD, Queiroz G, Duncan AM, Wallenstein, B,
Ferreira T, Pacheco, JM (1999). Volcanic geology of Furnas Volcano,
São Miguel, Azores. Journal of Volcanology and Geothermal Research
92 (1-2), pp.1-29.
No Ponto zero desta EarthCache vão encontrar
duas bicas, cada uma delas com o seu nome próprio.
Para fazerem o "found" preciso que respondam Às seguintes
perguntas:
1 - Qual o nome das duas bicas?
2 - Qual delas é que deita água QUENTE?
3 - A água quente tem uma temperatura acima ou abaixo de 36.3
ºC?
Enviem-me as respostas por mail que depois de verificadas valerão mais um
“found”.
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