The Furnas Fumarole Field
A
cache
by mtrevas
Hidden
:
3/9/2009
Difficulty:
Terrain:
Size: 
(Not chosen)
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The Furnas Fumarole
Field - S. Miguel (Azores)
Introduction:
A fumarole (Latin
fumus, smoke) is an opening in Earth’s crust, often in the
neighbourhood of volcanoes, 
which emits steam and
gases such as carbon dioxide, sulphur dioxide, hydrochloric acid,
and hydrogen sulphide. Fumaroles are
commonly called by the word
solfataras, from the Italian solfo, sulfur. The general
public tends to use fumarole interchangeably with solfatara,
unrelated to chemistry. Less commonly, the
term mofette is used. Technically speaking, solfatara describes a
steam vent rich in sulphuric compounds, such as sulphur dioxide or
hydrogen sulphide, while moffetes are rich in carbon
dioxide. 
Fumaroles
are not quiet geothermal features. They have
been described as
hissing, roaring, and even thundering. The sounds are dependent on
the force of the steam being released into
the atmosphere. Slowly diffusing emissions will have lessened
sounds. Forceful, blasting emissions
produce greater sounds. Fumaroles can occur on the surface of lava
flows and thicker pyroclastic flows. The flows require a release of
heat to cool off. Small cracks and crevices in the crust also allow
steam from deeper within the Earth to escape. Fumaroles can be
present singularly or in large fields full of them. Fumaroles are
common globally wherever volcanism is occurring.
Depending
on the heat source, fumaroles can be short or long term feature,
ranging anywhere from weeks to many
hundreds of years. The hotter the heat source is, whether it be an
internal rock mass or a volcanic flow, the longer it will take to
cool off, so the longer the fumarole will exist. A fumarole field
is an area of thermal springs and gas vents where magma or hot
igneous rocks at shallow depth are releasing gases or interacting
with groundwater. From a simplistic point of view and the
perspective of groundwater, fumaroles could be described as a hot
spring that boils off all its water before the water reaches the
surface.
Geological setting of the Furnas volcano:
Furnas is the easternmost of the three active central volcanoes on
the island of São Miguel in the Azores. Unlike the other two
central volcanoes, Sete Cidades and Fogo, Furnas does not have a
well-developed edifice, but consists of a steep-sided caldera
complex 8×5 km across. It is built on the outer flanks of the
Povoação/Nordeste lava complex that forms the eastern end of São
Miguel.
Constructive flanks to the volcano exist on the southern side
where they form the coastal cliffs, and to the west. The caldera
margins tend to reflect the regional/local tectonic pattern which
has also controlled the distribution of vents within the caldera
and areas of thermal springs.
Activity at Furnas has been essentially explosive, erupting
materials of trachytic composition. Products associated with the
volcano include plinian and sub-plinian pumice deposits,
ignimbrites and surge deposits, phreatomagmatic ashes, block and
ash deposits and dome materials.
Most of
the activity has occurred from 
vents
within the
caldera, or on the caldera margin, although strombolian eruptions
with aa flows of ankaramite and hawaiite have occurred outside the
caldera. The eruptive history consists of at least
two major caldera collapses, followed by caldera infilling. Based
on 14C dates, it appears that the youngest major collapse occurred
about 12,000–10,000 years BP.
New 14C dates for a densely welded ignimbrite suggest that a
potential caldera-forming eruption occurred at
about 30,000 years BP. Recent eruptions (<5000 years old) were
mainly characterised by alternating episodes of magmatic and
phreatomagmatic activity of plinian and sub-plinian magnitude,
forming deposits of interbedded ash and lapilli. An historical
eruption is documented in 1630 AD; new evidence suggests that
another occurred during the early occupation of the area at about
1440 AD.
The cache:
To claim this EarthCache you need to
perform two tasks.
Go to the given coordinates and tell me how many fumaroles are
present here, if they emit steam and sound and what shape are their
openings.
However, you must take a
picture of yourself near the tile panel at N37º 46.377' W025º
18.266 (78 m to the east of GZ), and incluede this photo in your
log.
E-mail me the answers and I will get back to you with permission to
log the cache.
EarthCaching: Observe,
learn, enjoy and be safe!
Refs:
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.
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