Historical measurements of the Earth’s magnetic field compared with remanence directions from lava flows in Italy over the last four centuries

Direct measurements of the Earth’s magnetic field in Italy since 1640 a.d. have been used to check the remanence directions derived from historically dated volcanic rocks of Etna and Vesuvius. Direct measurements consist of the records of L’Aquila and
Pola geomagnetic observatories, the repeat stations of the Italian Magnetic Network and the data base of the Historical Italian
Geomagnetic Data Catalogue. All have been relocated to the same reference site (Viterbo — lat. 42.45◦N, long. 12.03◦E) in order to draw a reference secular variation (SV) curve. The direction of the Earth’s field at Viterbo has also been calculated from the historical records (2000–1600) of ref. [Jackson, A., Jonkers, A.R.T., Walker, M.R., 2000. Four centuries of geomagnetic secular variation from historical records. Phil. Trans. R. Soc. London, Ser. A 358, 957–990] database. The remanence directions from Etna show a general agreement with the trend of the SV curve, although their inclination is usually lower than that from the direct measurement. The directions from Vesuvius are more scattered. Large discrepancies occur at both volcanoes and in some cases have been ascribed in the literature to poor geographic information, making it difficult to identify the flows actually emplaced during the eruptions reported in the chronicles. Closer examination shows that the great majority of the best-defined remanence directions (semi-angle of confidence α95 < 2.5◦) deviate significantly from the geomagnetic direction measured at the time of the emplacement, the angle between the two directions being larger than the α95 value. The value of 2.5–3.0◦ can thus be regarded as a conservative evaluation of the error when dealing with dating Etna and Vesuvius lava flows older than 17th century, even when the accuracy attained in remanence measurements is higher. In default of a SV curve for Italy derived from archaeological artefacts, a further error in dating is introduced when reference is made to SV curves of other countries, even if
well-established, as these are from regions too far from Italy (>600 km) to confidently relocate magnetic directions.

Alexandrescu, M., Courtillot, V., Le Mou¨el, J.-L., 1997. Highresolution
secular variation of the geomagnetic field in western
Europe over the last 4 centuries: comparison and integration of
historical data from Paris and London. J. Geophys. Res. 102,
20245–20258.
Angelino, A., Incoronato, A., 1996. Stratigrafia magnetica in aree
vulcaniche; esperienze al Vesuvio e all’Etna. 15◦ Convegno
GNGTS, CNR Roma, pp. 13–18.
Baag, C., Helsley, C.E., Xu, S.Z., Lienert, B.R., 1995. Deflection
of palaeomagnetic directions due to magnetisation of underlying
terrain. J. Geophys. Res. 100, 10013–10027.
Bucur, I., 1994. The direction of the terrestrial magnetic field in
France during the last 21 centuries. Phys. Earth Planet. Int. 87,
95–109.
Cafarella, L., De Santis, A., Meloni, A., 1992a. The Historical Italian
Geomagnetic Data Catalogue. ING, Rome, p. 160.
Cafarella, L., De Santis, A., Meloni, A., 1992b. Secular variation
from historical geomagnetic field measurements. Phys. Earth
Planet. Inter. 73, 206–221.
Carracedo, J.C., Principe, C., Rosi, M., Soler,V., 1993. Time correlation
by palaeomagnetism of the 1631 eruption of MountVesuvius
Volcanological and volcanic hazard implications. J. Volcanol.
Geotherm. Res. 58, 203–209.
Chiari, G., Lanza, R., 1999. Remanent magnetization of mural paintings
from the Bibliotheca Apostolica (Vatican Rome). J. Appl.
Geophys. 41, 137–143.
Cioni, R., Gurioli, L., Lanza, R., Zanella, E., 2004. Temperatures of
the A.D. 79 pyroclastic density current deposits (Vesuvius Italy).
J. Geophys. Res. 109, B02207, doi: 10.1029/2002JB.002251.
Coticchia, A., De Santis, A., Di Ponzio, A., Dominici, G., Meloni,
A., Pierozzi, M., Sperti, M., 2000. Italian Magnetic Network and
Geomagnetic Field Maps of Italy at year 2000. Boll. Geod. Sci.
Aff. LX/4, 262–291.
De Santis, A., 1992. Conventional spherical harmonic analysis for
regional modelling of the geomagnetic field. Geophys. Res. Lett.
19, 1065–1067.
Hoye, G.S., 1981. Archaeomagnetic secular variation record of
Mount Vesuvius. Nature 291, 216–218.
Iliceto, V., Napoleone, G., Pinna, E., 1973. Geomagnetic changes
during the cooling process of the Mount Etna 1971 lava
flow: preliminary report. Boll. Assoc. Geof. Ital. 22, 89–
94.
Incoronato, A., Angelino, A., Romano, R., Ferrante, A., Sauna, R.,
Vanacore, G., Vecchione, C., 2002. Retrieving geomagnetic secular
variations from lava flows: evidence from Mounts Arso,
Etna and Vesuvius (southern Italy). Geophys. J. Int. 149, 724–
730.
INGV, 2003. Risultati delle osservazioni magnetiche, L’Aquila, 55
pp.
Jackson, A., Jonkers, A.R.T., Walker, M.R., 2000. Four centuries
of geomagnetic secular variation from historical records. Phil.
Trans. R. Soc. London, Ser. A 358, 957–990.
Knudsen, M.F., Jacobsen, B.H., Abrahamsen, N., 2003. Paleomagnetic
distortion modelling and possible recovery by inversion.
Phys. Earth Planet. Inter. 135, 55–73.
Lanza, R., Zanella, E., 2003. Paleomagnetic secular variation at Vulcano
(Aeolian Islands) during the last 135 kyr. Earth Planet. Sci.
Lett. 213, 321–336.
Noel, M., Batt, C.M., 1990. A method for correcting geographically
separated remanence directions for the purpose of archeomagnetic
dating. Geophys. J. Int. 102, 753–756.
Principe, C., Tanguy, J.C., Arrighi, S., Paiotti, A., Le Goff, M., Zoppi,
U., in press. Chronology of Vesuvius’ activity from A.D. 79 to
1631 based on archeomagnetism of lavas and historical sources.
Bull. Volcanol.
Rolph, T.C., 1997. An investigation of the magnetic variation within
two recent lava flows. Geophys. J. Int. 130, 125–136.
Rolph, T.C., Shaw, J., Guest, J.E., 1987. Geomagnetic field variations
as a dating tool: application to Sicilian lavas. J. Archaeological
Sci. 14, 215–225.
Talamo, R., 1975. Le carte magnetiche d’ Italia delle isodinamiche
dellaHe delle isogone dell’ Istituto Geografico Militare e loro aggiornamento
al. 1973.0. Boll. Geod. Sci. Aff. XXXIV/1, 73–77.
Tanguy, J.C., Le Goff, M., Principe, C., Arrighi, S., Chillemi, V.,
Paiotti, A., La Delfa, S., Patan`e, G., 2003. Archeomagnetic dating
of Mediterranean volcanics of the last 2100 years: validity and
limits. Earth Planet. Sci. Lett. 211, 111–124.
Tanguy, J.C., Le Goff, M., 2004. Distortion of the geomagnetic field
in volcanic terrains: an experimental study of the Mount Etna
stratovolcano. Phys. Earth Planet. Inter. 141, 59–70.
Tarling, D., 1988. Secular variation of the geomagnetic field —
the archaeomagnetic record. In: Stephenson, F.R., Wolfendale,
A.W. (Eds.), Secular Solar and Geomagnetic Variations in
the Last 10,000 years, NATO ASI Series, vol. 236, pp. 349–
366.
Thellier, E., 1981. Sur la direction du champ magnetique terrestre, en
France, durant les deux derniers millenaires. Phys. Earth Planet.
Inter. 13, 89–132.
von Kesslitz, W., 1911. Ergebnisse aus den Erdmagnetischen
Beobachtungen in Pola (1847–1909). HydrographischenAmtder
Kaiserlichen und K¨oniglichen Kriegsmarine, Pola.