Emplacement Temperature of the Lithic-rich Pyroclastic Flow of Mt. Vesuvius.1, AD Eruption: Paleomagnetic Measurement

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1 Bull. Earthq. Res. Inst. Univ. Tokyo Vol. 2,,**1 pp , * +,,, +, Emplacement Temperature of the Lithic-rich Pyroclastic Flow of Mt. Vesuvius.1, AD Eruption: Paleomagnetic Measurement Keiko Suzuki-Kamata *, Kenji Niihori, Takayuki Kaneko Faculty of Science, Kobe University, +, and Toshitugu Fujii +,,, Earthquake Reserch Institute, University of Tokyo Abstract The.1, AD eruption of Mt.Vesuvius in Italy started from the explosive eruption which formed air-fall scoria, and was followed by pyroclastic-flows. The pyroclastic-flows changed from scoria flows in the early stage to pyroclastic surge, and lithic-rich pyroclastic flows at the last stage. The emplacement temperature of the flow deposits were estimated applying the progressive thermal demagnetization experiments for essential fragments and lithic fragments. The experiments show that the most of specimens exhibit stable magnetizations composed commonly of one or two components. Temperatures recorded in essential fragments of pyroclastic flows could be separated into several groups; more than /3*, around -** and normal ambient temperature. Some lithic fragments show,** indicating heating after entrapment within a flow. The estimated emlacement temperature of lithic-rich pyroclastic flow varies in di# erent flow unit. It was presumed that the relatively cool lithic-rich pyroclastic flows were formed by the collapse of the eruption columns of explosive phreatomagmatic eruptions which eroded the volcanic conduit entraining a large amount of lithic fragments. Water-magma interaction during the eruption may have caused cooling of some essential fragments. Key words: Vesuvius volcano, lithic-rich pyroclastic flow, phreatomagmatic eruption, remnant magnetization +.1, Kaneko et al.,,**/ Niihori et al.,**1,.1, * kamata@kobe-u.ac.jp 0/12/*

2 , -.1, BC+0** AD13 AD.1, Hoblitt and Kellogg +313 Rolandi et al., +332 AD.1, 13 VEI. Newhall and Self, +32,. AD.1, : ++ 0, TRM m, +,,** km 2 mm Rosi and Santacroce, +32- : TRM.1, :.1,, : Hoblitt and Kellogg +313 Fig. +. Idealized columnar section of the deposit of AD.1, eruption, Mt. Vesuvius. 180

3 .1, 1 Fig. + Fig. + Fig. + Fig. + S, coarse Fig. + S, fine +, km, / km +* m AD.1, Fig. + Kaneko et al.,,**/, Fig.,. Location map of sampling localities. +* +* + Fig. -b Fig. -c 1 32/,* Fig., Fig., +* AD.1, /, +* + +* Fig. - d +* Fig. - a. cm,* cm 181

4 Fig. -. Photographs of the sampled exposures at location (a), location (b), location (c) and location (d).. /* /** /** -*.+ 0,*,/ mm,, mm TDS-+ NRM / SMM-2/ / nt natural remanent Zijderveld, magnetization; NRM +301 Fig.. Kirschvink +32* +** 182

5 .1, Fig... Vector end-point diagram of the progressive thermal demagnetization results. The solid and open circles represent projection on the horizontal and vertical plane, respectively. Numbers adjacent to data points indicate temperature in degrees. 183

6 MAD Maximum Angular Deviation +*.* Zijderveld, +301 Fig.. Table +., 30,3 AD.1, + 0*, Table + + Table +. Paleomagnetic results from the pyroclastic-flow deposit of.1, AD eruption. 184

7 .1,, /** /0* /3* 0,* 0/* Table +. (continued) 185

8 +,0*+ +** Fig. / + Pullaiah et al. +31/ +**,,0*, Fig. 0 -** Table +. (continued) 186

9 .1, Fig. / -** -** Fig. 0, Lithic,,0*, L -**,,** Table +. (continued) min-max : interval of demagnetization level used to calculating deirection of characteristic remanent magnetization (TRM). dec. : declination of TRM. Inc. : inclination of TRM. T : thermal demagnetizaion which level is shown in degree C. NRM : natural remanent magnetization. 187

10 Fig. /. Equal area projections showing site-mean directions of ++ exposures of 1 localities. Solid and double open circles represent projection on the lower and upper hemisphere, respectively. Squares and ovals represent the mean direction and the 3/ confidence region with radius a3/, respectively. 188

11 .1, Fig. /. (continued). 189

12 Fig. /. (continued). 190

13 .1, Fig. /. (continued).,** Fig. 0,** + /+*, +,,** Fig. /,,1*- +, Fig. / *+*, +, Fig. 0. Relaxation time vs Temperature of acquisition of remanence for magnetite after Pullaiab, Irving, Bucban and Dunlop ( +31/ ). 191

14 ,**, -, + 0, + -** -** +, Fig. / + + 0, /3* +.** Fig. / + + 1,.** -** 0 AD.1, Fig. / /3* +,,, -** Fig. /. + /, -/* + -** Fig. / -** + /3* -** /3* AD.1, / -**,**,** -**.** -** AD.1,,* cm AD.1, -** 1 AD.1, 192

15 .1, /3* -**,** +0*23,*. Dunlop, D. and» Ozdemir,» O.,,**+, Rock Magnetism, Cambridge University Press, /30 p. Hoblitt, R.P. and Kellogg, K.S., +313, Emplacement temperatures of unsorted and unstratified deposits of volcanic rock debris as determined by paleomagnetic techniques. Geol. Soc. Am. Bull., 3*, 0--0.,. Kaneko, T., Nakada, S., Yoshimoto, M., Fujii, T., Yasuda, A., Yoneda, M. and Aoyagi, M.,,**/, Determination of burial age of the Augustus villa (Italy). Geochemical Journal, -3, /1- /12. Kirschvink, J.L., +32*, The least-squares line and plane and the analysis of paleomagnetic data. Geophys. J. Roy. Astron. Soc., 0,, Newhall, C.G. and Self, S., +32,, Volcanic explosivity index (VEI): an estimate of explosive magnitude for histrical volcanism. J. Geophys. Res., 21 C,, +,-+ +,-2. Niihori, K., Nagai, M., Kaneko,T., Fujii, T., Nakada,S., Yoshimoto, M., Yasuda, A. and Aoyagi, M.,,**1, Stratigraphy and geological characteristics of the volcanic deposits burying the Roman villa on the northern flank of Mt. Vesuvius (Italy). Bull. Earthq. Res. Inst. Univ. Tokyo, in press. Pullaiah, G., Irvinng, E., Buchan, K. and Dunlop, D. J., +31/, Magnetisation changes casused by burial and uplift., Earth. Planet. Sci. Lett.,,2, Rolandi, G., Petrosino, P. and Mc geehin, J., +332, The interplinian activity at Somma-Vesuvius in the last -/** years. J. Volcanol. Geotherm. Res., 2,, +3 /,. Rosi, M. and Santacroce, R., +32-, The A.D..1, Pollena eruptherm. Res., +1,,.3,1+. tion : volcanological and petrological data for this poorlyknown, Plinian-type event at Vesuvius. J. Volcanol. Geo- Zijderveld, J.D.A., +301, A.C. demagnetization of rocks : Analy- sis of results, in Methods in Paleomagnetism, pp.,/.,20, eds Collinson, Creer & Runcorn, Elsevier, New York. Received June +/,,**1 Accepted October +/,,**1 193

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