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| Titel |
Viscosity of Campi Flregrei (Italy) magmas |
| VerfasserIn |
Valeria Misiti, Francesco Vetere, Piergiorgio Scarlato, Harald Behrens, Annarita Mangiacapra, Carmela Freda |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250041352
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| Zusammenfassung |
| Viscosity is an important factor governing both intrusive and volcanic processes. The most
important parameters governing silicate melts viscosity are bulk composition of melt and
temperature. Pressure has only minor effect at crustal depths, whereas crystals and bubbles
have significant influence. Among compositional parameters, the water content is critical
above all in terms of rheological behaviour of melts and explosive style of an eruption.
Consequently, without an appropriate knowledge of magma viscosity depending on the
amount of dissolved volatiles, it is not possible to model the processes (i.e., magma ascent,
fragmentation, and dispersion) required to predict realistic volcanic scenarios and thus
forecast volcanic hazards.
The Campi Flegrei are a large volcanic complex (~150 km2) located west of the city of
Naples, Italy, that has been the site of volcanic activity for more than 60 ka and represents a
potential volcanic hazard owing to the large local population.
In the frame of a INGV-DPC (Department of Civil Protection) project devoted to design a
multidisciplinary system for short-term volcano hazard evaluation, we performed viscosity
measurements, under dry and hydrous conditions, of primitive melt compositions
representative of two Campi Flegrei eruptions (Minopoli-shoshonite and Fondo
Riccio-latite).
Viscosity of the two melts have been investigated in the high temperature/low viscosity
range at atmospheric pressure in dry samples and at 0.5 GPa in runs having water content
from nominally anhydrous to about 3 wt%. Data in the low temperature/high viscosity range
were obtained near the glass transition temperature at atmospheric pressure on samples
whose water contents vary from 0.3 up to 2.43 wt%.
The combination of high- and low-viscosity data permits a general description
of the viscosity as a function of temperature and water content using a modified
Tamman-Vogel-Fulcher equation.
logη = a+ –-b–-+ –-d–--
exp(g -
w-)
(T - c) (T - e) T
(1)
where η is the viscosity in Pa-
s, T the temperature in K, and w is the water content in wt%; a,
b, c, d, e, g are the Vogel-Fulcher-Tamman parameters. Each of the two compositions shows
its own VTF parameters. Following this equation we can now calculate viscosity
values for the two compositions under the condition inferred for Campi Flegrei
magma chambers, i.e., water content from 0.3 to 3 wt%, T=1393K (Mangiacapra et
al., 2008). For melt with 0.3 wt% water content we obtain viscosity values (η in
Pas) of 102.68and 102.24 for shoshonite and latite, respectively. At higher water
contents of about 3 wt% the viscosity difference decreases to 101.71 (shoshonite) and
101.51 (latite). One important application of these data is the estimate of flow regime
and magma rising velocity from deep to shallow reservoirs. Given the inferred
magma water contents (0.3 and 3 wt%), temperature (1393K) and depth of deep
and shallow reservoirs (9 and 4 km, respectively, Mangiacapra et al., 2008) and
assuming a 2 m dyke wide, we have calculated (Lister and Kerry, 1991) a rising
time from deep to shallow reservoir in the order of few minutes, 4.4 and 5.9 for a
shoshonitic magma with 3 and 0.3 wt% water content, respectively. The same order of
magnitude (4.1 and 5.2) has been obtained for latitic magma with similar amount of
water.
Lister and Kerry (1991) Fluid mechanical models of cracks propagation and their
application to magmatic transport and dyke. Journal of Geophysical Research 96,
10049-10077.
Mangiacapra A., Moretti, R., Rutherford L., Civetta L., Orsi G., Papale P. (2008) The
deep magmatic system of the Campi Flegrei caldera (Italy). Geophysical Research Letters 35,
L21304. |
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