this
study
describes
only
the
upper
member
of
the
cycle

ABSTRACT
 


The
 aim
 of
 this
 work
 is
 to
 identify,
 to
 describe
 and
 to
 explain,
 by
 analytical
 and
 stratigraphic
 studies
 (that
 implied
 field
 and
 laboratory
 activities),
 the
 products
 from
 two
 eruptive
 units
 of
 La
 Fossa
 di
 Vulcano
 cone
 (Aeolian
 Islands).
 In
 the
 literature
 (Frazzetta
et
al.,
1983;
Keller,1980),
one
cycle
is
called
“Pietre
Cotte”,
and
the
other
one
 is
described
as
a
period
of
eruptive
activity
between
Pietre
Cotte
and
the
last
eruptive
 cycle
(1888‐1890),
but
without
a
name;
in
this
study,
it
is
called
“Eruzione
Intermedia”.


Based
 on
 lithological
 differences,
 the
 Pietre
 Cotte
 deposits
 were
 divided
 into
 two
 members;
 the
 lower
 member
 is
 called
 “Lower
 Pietre
 Cotte”,
 the
 upper
 one,
 “Upper
 Pietre
Cotte”;
this
study
describes
only
the
upper
member
of
the
cycle.



There
are
many
studies
regarding
Vulcano
Island
and
in
particular
concerning
La
Fossa
 cone,
but
there
are
no
monographic
researches
about
these
two
eruptive
units.


Field
activity
was
carried
out
from
December
2007
up
to
March
2009,
for
a
period
of
 one
 month,
 and
 covered
 an
 area
 of
 1
 Km²wide,
 including
 La
 Fossa
 Crater,
 erosional
 channels
in
the
SSE
side
of
the
cone,
La
Fossa
Caldera
(Grotta
di
Palizzi)
and
the
Pietre
 Cotte
lava
flow.
Seven
stratigraphic
sections
were
taken
into
account
in
order
to
study
 Upper
 Pietre
 Cotte
 sequence;
 three
 of
 them
 were
 natural
 outcrops,
 the
 others
 were
 obtained
 by
 hand‐dug
 trenches;
 they
 were
 described
 and
 correlated
 on
 the
 basis
 of
 field
 observations.
 Correlation
 was
 useful
 to
 reconstruct
 a
 primary
 deposits
 only
 eruptive
sequence,
to
which
the
analytical
study
was
referred.


The
study
of
Eruzione
Intermedia
was
carried
out
by
analyzing
only
one
stratigraphic
 sequence,
obtained
with
a
trench,
digged
in
a
lateral
channel
of
the
cone
(SSE
area).


Laboratory
 analysis
 comprised
 grain‐size,
 componentry,
 bulk
 density
 and
 DRE,
 vescicularity,
XRF
chemical
analyses,
petrographic
observations,
SEM
observations
and
 EDS
chemical
analyses.




The
 “type‐sequence”
 of
 Upper
 Pietre
 Cotte
 event
 can
 be
 divided
 into
 two
 sub‐

sequences,
 corresponding
 to
 two
 different
 phases
 of
 the
 activity.
 The
 first
 one
 was
 characterized
by
two
pumice
fallouts.
Laboratory
analyses
pointed
out

the
abundance
 of
 vescicular
 juvenile
 material
 (that
 shows
 two
 different
 kinds
 of
 pumice),
 characterized,
from
the
base
to
the
top,
by
a
decrease
of
average
density
from
1
g/cm³
 to
0,5
g/cm³(vescicularity
ranging
from
60%
to
80%)
and
a
low
crystallinity
(<5‐10%)
 values.


The
latter
phase
is
defined
by
an
alternation
of
fine
and
coarse
ash
deposits;
from
the
 base
to
the
top
of
the
sub‐sequence,
it
records
a
decrease
of
juvenile
(both
dense
and
 vescicular)
and
the
progressive
increase
of
lithics
(>50%
wt).


The
high
lithic
content
can
be
interpreted
as
the
result
of
vulcanian‐type
activity
(Self
et
 al.,
1978).
Laboratory
analyses
pointed
out
a
small
decrease
of
average
density
during
 the
time
(from
1,4
g/cm³

to
1,3
g/cm³
),
and
a
vescicularity
ranging
from
42%
to
47%.


Eruzione
 Intermedia
 deposits
 are
 defined
 by
 an
 alternation
 of
 fine
 and
 coarse
 fallout
 ash
deposits;in
this
work,
flow
deposits
described
by
literature
(Frazzetta
et
al.,
1984)
 were
not
found
in
the
field.
Laboratory
analyses
shows
uniform
trends
in
the
sequence
 both
for
dense
and
vescicular
juvenile
material
and
for
lithic
material
(40%
wt);
also
 these
deposits
can
be
interpreted
as
related
to
vulcanian‐type
activity
(Self
et
al.,
1978).



Moreover,
 from
 the
 base
 
 to
 the
 top
 of
 the
 stratigraphic
 sequence,
 average
 density
 increases
(from
1,51
g/cm³to
1,57
g/cm³)
and
vescicularity
values
decrease
(from
38%


to
41%).


Optic
 and
 electronic
 microscope
 observations
 highlighted
 the
 presence
 of
 enclaves,
 defined
by
compositional
and
textural
features
that
differ
from
juvenile
in
which
they
 are
enclosed;
furthermore,
this
study
pointed
out
textural
differences
and
crystallinity
 inhomogeneities
 among
 different
 juvenile
 products,
 in
 particular
 in
 the
 material
 belonging
to
the
two
first
fallout
deposits
of
Upper
Pietre
Cotte
sequence.


Chemical
analysis
highlighted
a
trachy‐rhyolitic
composition
for
Upper
Pietre
Cotte
and
 rhyolitic
for
Eruzione
Intermedia.