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Magnetic stratigraphy of Vesuvius products. I. 1631 lavas
Journal o f Volcanology and Geothermal Research, 58 ( 1993 ) 2 1 1 - 2 1 5
211
Elsevier Science Publishers B.V., A m s t e r d a m
Magnetic stratigraphy of Vesuvius products. I. 1631 lavas Paola R. Gialanella, Alberto Incoronato, Filippo Russo and Giuseppina Nigro Dipartimento di Scienze della Terra, Universit?t degli Studi di Napoli Federico II, Largo S. Marcellino 10, 80138 Napoli, Italy (Received June 10, 1992; revised version accepted December 14, 1992)
ABSTRACT An extensive palaeomagnetic study has been undertaken to provide further elements leading to a better assessment of the volcanic history of the Vesuvius. This work refers to lavas for which the date of emplacement have recently been questioned; i.e. either during the period 968-1037 or in 1631. From 7 sites, 97 sun-orientated specimens have been collected. The dominant magnetic carrier for all sites consists of magnetite on the basis of an analysis of IRM acquisition. The linearity analysis carried out on at least 6 specimens per site subjected to PAFD has indicated the presence of singleand multi-component magnetizations. The within-site mean directions of 5 sites are close to each other suggesting that lavas from these sites can be ascribed to the same volcanic event. The remaining 2 sites can also be referred to this event on ground of stratigraphical considerations. The plotting of the between-site mean directions of the previously mentioned 5 sites on the Vesuvius secular variation curve suggests that the lavas from these sites could not be emplaced during the period 968-1037 but some hundred years later than 1301. The stratigraphic and historic dating of deposits and a building, respectively, at one of the 2 remaining sites allows to conclude that all the investigated flows had to be emplaced during the 1631 event. Therefore, this event was characterized not only by explosive activity but by an important effusive phase as well.
Introduction
The knowledge of the frequency and nature of events, together with the mapping of the areal distribution of products, plays an important role in assessing the hazard of a volcanic area. The knowledge of the volcanic history of the Vesuvius is far from satisfactory even with reference to events of some hundred years ago. In particular, recent investigations have cast doubts on the type of the 1631 eruption, the characteristics of which have been unanimously agreed upon for over a century. In fact, Burri and Di Girolamo (1975), Rolandi (1991), Rolandi and Russo (1987, 1989, 1993) consider the 1631 event as an explosive-effusive eruption, the lavas of which had already been mapped by Le Hon (1865) and Johnston Lavis (1884), while Rosi and Santacroce ( 1984, 1986), Arnb et al. (1987) consider the 1631 event solely as an explosive
eruption and suggest that lavas ascribed to the 1631 event by the above mentioned authors are much older having probably been erupted during the period 968-1037. The present work, referring to a palaeomagnetic study of these lavas, the date of which is controversial, is part of a large-scale investigation carried out at the Vesuvius as palaeomagnetism can provide further elements towards a better assessment of the nature, areal distribution and frequency of volcanic events. Method
Upon cooling from high temperature, lava behaves paramagnetically until the Curie temperature, at which spontaneous magnetization, aligned with the local EMF (Earth Magnetic Field) but characterized by a very short relaxation time (i.e. superparamagnetism), occurs. Further cooling to the blocking tem-
0 3 7 7 - 0 2 7 3 / 9 3 / $ 0 6 . 0 0 © 1993 Elsevier Science Publishers B.V. All rights reserved. SSDI 0 3 7 7 - 0 2 7 3 ( 9 3 ) E 0 0 2 6 - X
212
P.R.GIALANELLAETAL.
perature, which varies from grain to grain according to their composition and volume, resuits in an increase in the relaxation time. The magnetization acquired in this way is called TRM (Thermal Remanent Magnetization) and, as the cooling proceeds, the relaxation times increase exponentially. From the above it follows that each grain acquires its TRM independently of the other grains, and the total TRM is thus the sum of partial TRM acquired in different temperature intervals (Thellier, 1951 ). As the EMF is characterized by a secular variation, lavas emplaced at different times will exhibit TRMs the directions of which will be significantly different from one another. Therefore, the study of TRMs aimed at the identification of primary directions of magnetization allows correlations and discriminations of emplacement times of lava flows. It also allows absolute dating, if the secular variation curve for the area is known.
Direct field coring with a portable electric drill has been carried out at 7 sites (for loca4 Km L
• S.Selmstiano
14"19' l~.
l
).~
IF.............. t,/ .Y/It" N I / ..........
Slit' '
I
"
.
r .....
i[
V
Silk' "~
' O.5
1
+},g
/
3t [1,'~/
" site
! i
~'t/ ' .......... (I.5
I
().~
~l ,r,
.+~'llt' [,-"
Specimen collection and preparation, measurements and data analyses
z
Silt' l
14" 3~" I~
Fig. 1. Location map of the studied Vesuvian lava flows (circles), the site numbering of which is indicated in squares. Dotted area, modified from Burri and Di Girolamo ( 1975 ), refers to lavas, the date of emplacement of which is controversial (see text).
(}.g
1
Fig. 2. IRM acquisition curve of one specimen from each site. Note that the saturation occurs at 0.1 T indicating that the magnetic carrier consists of magnetite. Vertical axis=intensity of magnetization (A/m). Horizontal axis = magnetic field (T).
tion refer to Fig. 1 ) where lavas the date of which is controversial outcrop. The number of specimens amounts to 97 with a minimum of 9 specimens for site 1 and with a maximum of 22 specimens for site 12. Specimens were oriented by solar compass prior to removal and sliced into standard cores (¢ 2.54 cm X h 2.10 cm) in the laboratory. IRM (Isothermal Remanent Magnetization) acquisition curves (Fig. 2 ) indicate that saturation occurs by 0.1 T suggesting that the magnetic carriers essentially consist of magnetite. The PAFD (Progressive Alternating Field Demagnetization) has been carried out up to 90 mT, at 5-roT intervals, with a Molspin AF
MAGNETIC STRATIGRAPHY OF VESUVIUSPRODUCTS• 1. 1631 LAVAS W.-/
specimens has been carried out both subjectively and objectively using the linearity test suggested by Kirschvink (1980). The linearity analysis, carried out by using the d.a. (diagonal angle) value less than 5 °, currently adopted in palaeomagnetic investigations, defines one component for both 81/8 (dec= 18.1, inc=64.0, d.a.= 1.5) and 120/11 (dec=346.0, inc= 65.8, d.a. = 3.0) specimens for the entire coercivity spectrum (Fig. 3). As far as the latter specimen is concerned, although its magnetic component is very well defined by the linearity analysis, the visual inspection of the horizontal projection of the magnetization (Fig. 3, right) clearly suggests the occurrence of two components of magnetization: one below 45 mT and another one above 45 mT. In fact, the linearity analysis carried out for these two distinct portions of the coercivity spectrum identifies two components: dec=4.2, inc=65.5, d.a.=2.5 (above 45 mT) and dec=341.0, inc=65.5, d.a.=2.6 (below 45 mT). These two components are well defined and differ from one another by 23.2 ° . It is worth mentioning how strongly the component defined for the entire coercivity spectrum is affected by the low coercivity spectrum component. On the ground of such analysis, on this and other specimens as well, the linearity analysis of all specimens subjected to PAFD has been
W.-~ / I
N~M
-
120 I 1
N 1 ,';','
\ <,, 1',4/
O. 2
213
,~/ m IL+Z
Fig. 3. Cartesian projections of some lava flows specimens subjected to P A F D up to 90 mT, at 5-roT intervals, starting from N R M ( N a t u r a l R e m a n e n t M a g n e t i z a t i o n ) . C o n t i n u o u s ( d a s h e d ) lines = vertical ( h o r i z o n t a l ) projection. Note that the choice of d.a. (Kirschvink, 1980) lower t h a n 3 ° (see text) allows the identification of multic o m p o n e n t magnetizations (right) that intersect at 45 mT.
demagnetizer and remanence has been measured with a Molspin magnetometer. All treatments and data analyses have been performed at the Palaeomagnetic Laboratory of the Dipartimento di Scienze della Terra, Universit/t degli Studi di Napoli Federico II. The analysis of magnetization components for individual TABLE1
Meandirecti•n•fmagnetizati•n•fVesuvius•avaspecimensc•redat7sites.Thesite••cati•nsaresh•wninFigure•. Site
No/Nm
Dec
Inc
a95
k
1
(5/9)
12 14 14 16 13 4 14 13
65 64 64 57 63 66 63 64
2.3 1.6 1.2 3,2 2.2 4.8 19 0.9
1100 1675 3120 ~59 %1 o66 585 7703
2 8 9 10 I1 12 1+2+8+10+
(6/8) (6/7) (4/12) (6/7) (3/12) (11/15) 12
Abbreviations: Nc/Nm= number of specimens from which the mean at site level has been computed over number of spectmens measured for that site (for explanation see text); Dec = declination; Inc = inclination; a95 and k = statistical parameters (Fisher, 1953).
214
P.R. GIALANELLAET AL
340
350
Declination 000
010
020
"-\,
! t
"G
1794.
( 1754
'
~-.... i~"~
./
/
zl
~
o Fig. 4. The secular variation curve for the Vesuvius (Hoye, 1981; only same dated points, in italics, are shown) with the mean directions of magnetization of sites 1, 2, 8, 9, 10, 11, 12 and their circles of confidence (Table 1 ). The square refers to the direction of the axial geocentric magnetic field at the sampling sites.
carried out by choosing a d.a. < 3 °. The PAFD has been initially carded out on 6 specimens per site and when the linearity analysis has not allowed to define any component of magnetization, further specimens from the same site have been subjected to the demagnetization procedure. The total number of specimens from each site subjected at PAFD is indicated in Table 1 as Nm, while the number of specimens, from which the site mean direction of magnetization has been calculated, is indicated as N~. In the case of multi-component magnetization only the high component of the coercivity spectrum has been considered as it is characterized by high relaxation times and, therefore, is likely to reflect the primary direction of magnetization. This can be illustrated by specimen 120/11. In fact, with reference to the secular variation curve of the Vesuvius (Fig. 4; modified from Hoye, 1981 ), the high and low components of the coercivity spectrum (not plotted in Fig. 4) will refer to timely different directions of the local EMF; in particular, the former results to be older than the latter. Discussion and conclusion
The mean direction of magnetization at site level, with ol95 and k (Fisher, 1953), Nm and
No, are shown in Table 1. These directions and their a95 have been referred to the secular variation curve of the Vesuvius (Fig. 4). Regardless of the Arc for sites 9 and l I that is lower than the one generally adopted in palaeomagnetic work, the cone of confidence of these sites (Table l) and the angular displacement of their site mean directions in respect to those of the other sites would not be considered high. As the object of the present study is to date lavas from a volcano the recent history of which is characterized by very frequent eruptions, it is necessary to be more restrictive in palaeomagnetically correlating the products of this volcano. Therefore, only the lavas from sites 1. 2, 8, 10 and 12, the within site mean direction of which are plotted close to one another with very small overlapping circles of confidence (Fig. 4), have been considered coeval. However, stratigraphic data (Rolandi and Russo, 1987, 1989, 1993) deafly indicate that lavas sampled at sites 9 and 11 are part of the same flows as sites 8 and 10, respectively. The discrepancy of the mean directions for sites 9 and 11 in respect to the other investigated sites, can be the result either of later displacement of their outcrops or changes in their magnetization (this problem is the object of further palaeomagnetic investigations planned in surrounding areas). The between site mean direction for sites 1, 2, 8, 10 and 12 is equal to: dec= 13, inc=64, a95=0.9 (Table 1 ). This mean direction plotted on the secular variation curve indicates that the investigated lava flows could not be emplaced during the period 968-1037 as suggested by Rosi and Santacroce (1984) but some hundred years later that 1301 (Fig. 4). In addition, lava sampied at site 9 rests above pyroclastics stratigraphically attributed to 1631 (Rolandi and Russo, 1989) and underlies a building, Masseria di Donna Chiara, historically dated 1698 (Rolandi and Russo, 1989 ). As in this time interval, 1631-1698, the only volcanic eruption affecting the investigated locality occurred in
215
MAGNETIC STRATIGRAPHY OF VESUVIUS PRODUCTS. I. 1631 LAVAS
1631. it can be concluded that the lava sampled at site 9 has to be related to the 1631 event. The same date can be assigned to the remaining sites as they are all stratigraphically correlated (Rolandi and Russo, 1987, 1989, 1993). Therefore, the mean direction equal to: d e c = 13, i n c = 6 4 , o~95---0.9 (Table 1 ), reflects the mean direction of the geomagnetic field at Vesuvius in 1631. This mean direction is very close to the one determined by Hoye ( 1981 ) ( d e c = 15, i n c = 65, a95= 1,7) who carried out an archaeomagnetic study at Vesuvius encompassing two sampling sites of lava flows now controversially ascribed to 1631 event, but the location of the sampling sites is not known to US.
] h e present study indicates that in carrying out volcanic magnetic stratigraphy surveys more stringent criteria for magnetization component analyses must be applied. In particular, the choice of the d.a. (Kirschvink, 1980) threshold value appears to be very critical in order to separate one component from another. This study also indicates how valuable palaeomagnetism can be in assessing the nature, areal distribution, frequency of volcanic events, and, in turn, of volcanic hazard of a volcanic area. As far as the Vesuvius is concerned, it can be concluded that the 1631 event cannot be considered only as an explosive eruption as it is also characterized by an important effusive phase.
Acknowledgment Biagio Fioretti helped with field works. Don Tarling and Roberto Scandone provided very helpful comments. This work was supported by 60% MURST (Ministero per l'Universith e la Ricerca Scientifica e Tecnologica).
References Arn6, V., Principe, C., Rosi, M., Santacroce, R., Sbrana, A. and Sheridan, M.F., 1987. Eruptive history. In: R. Santacroce (Editor), Somma-Vesuvius. Quad. Ric. Sci., CNR-Roma, 114: 53-103. Burri, C. and Di Girolamo, P., 1975. Contributo alla conoscenza delle lave della grande eruzione del Vesuvio de11631. Rend. Soc. Ital. Miner. Petrogr., 30: 705-740. Fisher, R.A., 1953. Dispersion on a sphere. Proc. R. Soc. Ser. A, 217: 295-305. Hoye, G.S., 1981. Archaeomagnetic secular variation record of Mount Vesuvius. Nature, 291 : 216-2 l 8. Johnston Lavis, H.J., 1884. The geology of the Mr. Somma and Vesuvius: being a study of volcanology. Q.J. Geol. Soc. London, 40: 135-149. Le Hon, M., 1865. Histoire compl6te de la grande eruption du Vesuve de 1631. Bull. Acad. Sci. Lett. Beaux Arts, Belg., 20: 483-538. Kirschvink, J.L., 1980. The least-squares line and plane and the analysis of palaeomagnetic data. Geophys. J. R. Astron. Soc., 62: 699-718. Rolandi, G., 1991. L'eruzione vesuviana del 1631 ricostruita dall'analisi dei documenti coevi. In: A. Pozzuoli (Editor), Colloquio sulle scienze della terra in onore di Nicola Covelli, Caiazzo 27 maggio 1989. Proc. Assoc. Stor. Caiatino, 8:107-134. Rolandi, G. and Russo, F., 1987. Contributo alla conoscenza dell'attivita storica del Vesuvio. La stratigrafia di Villa Inglese (Torre del Greco). Rend. Accad. Sci. Fis. Mat. Napoli, 54: 123-27. Rolandi, G. and Russo, F., 1989. Contributo alia conoscenza dell'attivith storica del Vesuvio: dati stratigrafici e vulcanologici nel settore meridionale tra Torre del Greco, localith Villa Inglese, e Torre Annunziata. Boll. Soc. Geol. Ital., 108: 521-536. Rolandi, G. and Russo, F., 1993. L'eruzione del Vesuvio del 1631. Boll. Soc. Geol. Ital., 112: 315-332. Rosi, M. and Santacroce, R., 1984. The famous AD 1631 eruption of Vesuvius: a revised interpretation in light of historical and volcanological data. Workshop on Volcanic Blast, Mount St. Helens, abstr. Rosi, M. and Santacroce, R., 1986. L'attivita del SommaVesuvio precedente l'eruzione del 1631. Dati stratigrafici e vulcanologici. In: C. Albore Livadie (Editor), Tremblement de terre, eruptions volcaniques et we des hommes dans le Campanie antique. Bibl. Inst. Franq. Naples, 7:15-33. Thellier, E., 1951. Propri6t6s magn6tiques des terres cuites et des roches. (3. Phys. Radium, 12: 205-218.
211
Elsevier Science Publishers B.V., A m s t e r d a m
Magnetic stratigraphy of Vesuvius products. I. 1631 lavas Paola R. Gialanella, Alberto Incoronato, Filippo Russo and Giuseppina Nigro Dipartimento di Scienze della Terra, Universit?t degli Studi di Napoli Federico II, Largo S. Marcellino 10, 80138 Napoli, Italy (Received June 10, 1992; revised version accepted December 14, 1992)
ABSTRACT An extensive palaeomagnetic study has been undertaken to provide further elements leading to a better assessment of the volcanic history of the Vesuvius. This work refers to lavas for which the date of emplacement have recently been questioned; i.e. either during the period 968-1037 or in 1631. From 7 sites, 97 sun-orientated specimens have been collected. The dominant magnetic carrier for all sites consists of magnetite on the basis of an analysis of IRM acquisition. The linearity analysis carried out on at least 6 specimens per site subjected to PAFD has indicated the presence of singleand multi-component magnetizations. The within-site mean directions of 5 sites are close to each other suggesting that lavas from these sites can be ascribed to the same volcanic event. The remaining 2 sites can also be referred to this event on ground of stratigraphical considerations. The plotting of the between-site mean directions of the previously mentioned 5 sites on the Vesuvius secular variation curve suggests that the lavas from these sites could not be emplaced during the period 968-1037 but some hundred years later than 1301. The stratigraphic and historic dating of deposits and a building, respectively, at one of the 2 remaining sites allows to conclude that all the investigated flows had to be emplaced during the 1631 event. Therefore, this event was characterized not only by explosive activity but by an important effusive phase as well.
Introduction
The knowledge of the frequency and nature of events, together with the mapping of the areal distribution of products, plays an important role in assessing the hazard of a volcanic area. The knowledge of the volcanic history of the Vesuvius is far from satisfactory even with reference to events of some hundred years ago. In particular, recent investigations have cast doubts on the type of the 1631 eruption, the characteristics of which have been unanimously agreed upon for over a century. In fact, Burri and Di Girolamo (1975), Rolandi (1991), Rolandi and Russo (1987, 1989, 1993) consider the 1631 event as an explosive-effusive eruption, the lavas of which had already been mapped by Le Hon (1865) and Johnston Lavis (1884), while Rosi and Santacroce ( 1984, 1986), Arnb et al. (1987) consider the 1631 event solely as an explosive
eruption and suggest that lavas ascribed to the 1631 event by the above mentioned authors are much older having probably been erupted during the period 968-1037. The present work, referring to a palaeomagnetic study of these lavas, the date of which is controversial, is part of a large-scale investigation carried out at the Vesuvius as palaeomagnetism can provide further elements towards a better assessment of the nature, areal distribution and frequency of volcanic events. Method
Upon cooling from high temperature, lava behaves paramagnetically until the Curie temperature, at which spontaneous magnetization, aligned with the local EMF (Earth Magnetic Field) but characterized by a very short relaxation time (i.e. superparamagnetism), occurs. Further cooling to the blocking tem-
0 3 7 7 - 0 2 7 3 / 9 3 / $ 0 6 . 0 0 © 1993 Elsevier Science Publishers B.V. All rights reserved. SSDI 0 3 7 7 - 0 2 7 3 ( 9 3 ) E 0 0 2 6 - X
212
P.R.GIALANELLAETAL.
perature, which varies from grain to grain according to their composition and volume, resuits in an increase in the relaxation time. The magnetization acquired in this way is called TRM (Thermal Remanent Magnetization) and, as the cooling proceeds, the relaxation times increase exponentially. From the above it follows that each grain acquires its TRM independently of the other grains, and the total TRM is thus the sum of partial TRM acquired in different temperature intervals (Thellier, 1951 ). As the EMF is characterized by a secular variation, lavas emplaced at different times will exhibit TRMs the directions of which will be significantly different from one another. Therefore, the study of TRMs aimed at the identification of primary directions of magnetization allows correlations and discriminations of emplacement times of lava flows. It also allows absolute dating, if the secular variation curve for the area is known.
Direct field coring with a portable electric drill has been carried out at 7 sites (for loca4 Km L
• S.Selmstiano
14"19' l~.
l
).~
IF.............. t,/ .Y/It" N I / ..........
Slit' '
I
"
.
r .....
i[
V
Silk' "~
' O.5
1
+},g
/
3t [1,'~/
" site
! i
~'t/ ' .......... (I.5
I
().~
~l ,r,
.+~'llt' [,-"
Specimen collection and preparation, measurements and data analyses
z
Silt' l
14" 3~" I~
Fig. 1. Location map of the studied Vesuvian lava flows (circles), the site numbering of which is indicated in squares. Dotted area, modified from Burri and Di Girolamo ( 1975 ), refers to lavas, the date of emplacement of which is controversial (see text).
(}.g
1
Fig. 2. IRM acquisition curve of one specimen from each site. Note that the saturation occurs at 0.1 T indicating that the magnetic carrier consists of magnetite. Vertical axis=intensity of magnetization (A/m). Horizontal axis = magnetic field (T).
tion refer to Fig. 1 ) where lavas the date of which is controversial outcrop. The number of specimens amounts to 97 with a minimum of 9 specimens for site 1 and with a maximum of 22 specimens for site 12. Specimens were oriented by solar compass prior to removal and sliced into standard cores (¢ 2.54 cm X h 2.10 cm) in the laboratory. IRM (Isothermal Remanent Magnetization) acquisition curves (Fig. 2 ) indicate that saturation occurs by 0.1 T suggesting that the magnetic carriers essentially consist of magnetite. The PAFD (Progressive Alternating Field Demagnetization) has been carried out up to 90 mT, at 5-roT intervals, with a Molspin AF
MAGNETIC STRATIGRAPHY OF VESUVIUSPRODUCTS• 1. 1631 LAVAS W.-/
specimens has been carried out both subjectively and objectively using the linearity test suggested by Kirschvink (1980). The linearity analysis, carried out by using the d.a. (diagonal angle) value less than 5 °, currently adopted in palaeomagnetic investigations, defines one component for both 81/8 (dec= 18.1, inc=64.0, d.a.= 1.5) and 120/11 (dec=346.0, inc= 65.8, d.a. = 3.0) specimens for the entire coercivity spectrum (Fig. 3). As far as the latter specimen is concerned, although its magnetic component is very well defined by the linearity analysis, the visual inspection of the horizontal projection of the magnetization (Fig. 3, right) clearly suggests the occurrence of two components of magnetization: one below 45 mT and another one above 45 mT. In fact, the linearity analysis carried out for these two distinct portions of the coercivity spectrum identifies two components: dec=4.2, inc=65.5, d.a.=2.5 (above 45 mT) and dec=341.0, inc=65.5, d.a.=2.6 (below 45 mT). These two components are well defined and differ from one another by 23.2 ° . It is worth mentioning how strongly the component defined for the entire coercivity spectrum is affected by the low coercivity spectrum component. On the ground of such analysis, on this and other specimens as well, the linearity analysis of all specimens subjected to PAFD has been
W.-~ / I
N~M
-
120 I 1
N 1 ,';','
\ <,, 1',4/
O. 2
213
,~/ m IL+Z
Fig. 3. Cartesian projections of some lava flows specimens subjected to P A F D up to 90 mT, at 5-roT intervals, starting from N R M ( N a t u r a l R e m a n e n t M a g n e t i z a t i o n ) . C o n t i n u o u s ( d a s h e d ) lines = vertical ( h o r i z o n t a l ) projection. Note that the choice of d.a. (Kirschvink, 1980) lower t h a n 3 ° (see text) allows the identification of multic o m p o n e n t magnetizations (right) that intersect at 45 mT.
demagnetizer and remanence has been measured with a Molspin magnetometer. All treatments and data analyses have been performed at the Palaeomagnetic Laboratory of the Dipartimento di Scienze della Terra, Universit/t degli Studi di Napoli Federico II. The analysis of magnetization components for individual TABLE1
Meandirecti•n•fmagnetizati•n•fVesuvius•avaspecimensc•redat7sites.Thesite••cati•nsaresh•wninFigure•. Site
No/Nm
Dec
Inc
a95
k
1
(5/9)
12 14 14 16 13 4 14 13
65 64 64 57 63 66 63 64
2.3 1.6 1.2 3,2 2.2 4.8 19 0.9
1100 1675 3120 ~59 %1 o66 585 7703
2 8 9 10 I1 12 1+2+8+10+
(6/8) (6/7) (4/12) (6/7) (3/12) (11/15) 12
Abbreviations: Nc/Nm= number of specimens from which the mean at site level has been computed over number of spectmens measured for that site (for explanation see text); Dec = declination; Inc = inclination; a95 and k = statistical parameters (Fisher, 1953).
214
P.R. GIALANELLAET AL
340
350
Declination 000
010
020
"-\,
! t
"G
1794.
( 1754
'
~-.... i~"~
./
/
zl
~
o Fig. 4. The secular variation curve for the Vesuvius (Hoye, 1981; only same dated points, in italics, are shown) with the mean directions of magnetization of sites 1, 2, 8, 9, 10, 11, 12 and their circles of confidence (Table 1 ). The square refers to the direction of the axial geocentric magnetic field at the sampling sites.
carried out by choosing a d.a. < 3 °. The PAFD has been initially carded out on 6 specimens per site and when the linearity analysis has not allowed to define any component of magnetization, further specimens from the same site have been subjected to the demagnetization procedure. The total number of specimens from each site subjected at PAFD is indicated in Table 1 as Nm, while the number of specimens, from which the site mean direction of magnetization has been calculated, is indicated as N~. In the case of multi-component magnetization only the high component of the coercivity spectrum has been considered as it is characterized by high relaxation times and, therefore, is likely to reflect the primary direction of magnetization. This can be illustrated by specimen 120/11. In fact, with reference to the secular variation curve of the Vesuvius (Fig. 4; modified from Hoye, 1981 ), the high and low components of the coercivity spectrum (not plotted in Fig. 4) will refer to timely different directions of the local EMF; in particular, the former results to be older than the latter. Discussion and conclusion
The mean direction of magnetization at site level, with ol95 and k (Fisher, 1953), Nm and
No, are shown in Table 1. These directions and their a95 have been referred to the secular variation curve of the Vesuvius (Fig. 4). Regardless of the Arc for sites 9 and l I that is lower than the one generally adopted in palaeomagnetic work, the cone of confidence of these sites (Table l) and the angular displacement of their site mean directions in respect to those of the other sites would not be considered high. As the object of the present study is to date lavas from a volcano the recent history of which is characterized by very frequent eruptions, it is necessary to be more restrictive in palaeomagnetically correlating the products of this volcano. Therefore, only the lavas from sites 1. 2, 8, 10 and 12, the within site mean direction of which are plotted close to one another with very small overlapping circles of confidence (Fig. 4), have been considered coeval. However, stratigraphic data (Rolandi and Russo, 1987, 1989, 1993) deafly indicate that lavas sampled at sites 9 and 11 are part of the same flows as sites 8 and 10, respectively. The discrepancy of the mean directions for sites 9 and 11 in respect to the other investigated sites, can be the result either of later displacement of their outcrops or changes in their magnetization (this problem is the object of further palaeomagnetic investigations planned in surrounding areas). The between site mean direction for sites 1, 2, 8, 10 and 12 is equal to: dec= 13, inc=64, a95=0.9 (Table 1 ). This mean direction plotted on the secular variation curve indicates that the investigated lava flows could not be emplaced during the period 968-1037 as suggested by Rosi and Santacroce (1984) but some hundred years later that 1301 (Fig. 4). In addition, lava sampied at site 9 rests above pyroclastics stratigraphically attributed to 1631 (Rolandi and Russo, 1989) and underlies a building, Masseria di Donna Chiara, historically dated 1698 (Rolandi and Russo, 1989 ). As in this time interval, 1631-1698, the only volcanic eruption affecting the investigated locality occurred in
215
MAGNETIC STRATIGRAPHY OF VESUVIUS PRODUCTS. I. 1631 LAVAS
1631. it can be concluded that the lava sampled at site 9 has to be related to the 1631 event. The same date can be assigned to the remaining sites as they are all stratigraphically correlated (Rolandi and Russo, 1987, 1989, 1993). Therefore, the mean direction equal to: d e c = 13, i n c = 6 4 , o~95---0.9 (Table 1 ), reflects the mean direction of the geomagnetic field at Vesuvius in 1631. This mean direction is very close to the one determined by Hoye ( 1981 ) ( d e c = 15, i n c = 65, a95= 1,7) who carried out an archaeomagnetic study at Vesuvius encompassing two sampling sites of lava flows now controversially ascribed to 1631 event, but the location of the sampling sites is not known to US.
] h e present study indicates that in carrying out volcanic magnetic stratigraphy surveys more stringent criteria for magnetization component analyses must be applied. In particular, the choice of the d.a. (Kirschvink, 1980) threshold value appears to be very critical in order to separate one component from another. This study also indicates how valuable palaeomagnetism can be in assessing the nature, areal distribution, frequency of volcanic events, and, in turn, of volcanic hazard of a volcanic area. As far as the Vesuvius is concerned, it can be concluded that the 1631 event cannot be considered only as an explosive eruption as it is also characterized by an important effusive phase.
Acknowledgment Biagio Fioretti helped with field works. Don Tarling and Roberto Scandone provided very helpful comments. This work was supported by 60% MURST (Ministero per l'Universith e la Ricerca Scientifica e Tecnologica).
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