tive effusion rate of Vesuvius between 1694 and 1944 allows a calculation of the magma production rate in the same period. Based on these data, a model of the ...
Journal o f Volcanology and Geothermal Research, 12 (1982) 393--400
393
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
THE FEEDING SYSTEM OF VESUVIUS BETWEEN 1754 AND 1944
M. CORTINI and R. SCANDONE
Istituto di Geologia e Geofisica, Largo S. Marcellino, 1 O, Napoli (Italy) Osservatorio Vesuviano, Ercolano, Napoli (Italy) (Received March 3, 1981; revised and accepted October 20, 1981)
ABSTRACT Cortini, M. and Scandone, R., 1982. The feeding system of Vesuvius between 1754 and 1944. J. Volcanoh Geotherm. Res., 12: 393--400. The occurrence of short-term variations of Sr isotopic ratios in the Vesuvian historical lavas, and the occurrence of different types of nodules, which formed in different P--T conditions, permit the identification of two different deep magmatic reservoirs. The cumulative effusion rate of Vesuvius between 1694 and 1944 allows a calculation of the magma production rate in the same period. Based on these data, a model of the Vesuvian feeding system is presented. Between 1754 and 1944 mixing of magmas probably occurred in two deep-seated magma reservoirs. We calculated the volume of such reservoirs (~ 0.1 km3). The increased effusion rate of Vesuvius after 1858 is nicely explained by the simultaneous activity of both reservoirs. Geometrical constraints on the Vesuvian conduit are also discussed.
INTRODUCTION
Volcanoes are complex thermodynamic systems which develop downwards up to hundreds of kilometers into the interior of the earth and encompass time intervals of up to hundreds of thousands of years. The parts of this system that are directly observable are only very limited, both in space and time. A thorough knowledge of their spatial and temporal evolution is useful in the forecasting of eruptions, and for a better understanding of the eruptive mechanisms and the exploitation of geothermal resources. Mostly within the framework of the Italian Geodynamic Project, several studies about Vesuvius, employing different approaches, have recently been published (Barberi and Leoni, 1980; Sheridan et al., 1981; Hermes and Cornell, 1981; Cortini and Hermes, 1981; Carta et al., 1981). As a result, a good deal of high-quality data now permit a reconstruction of the activity pattern of Vesuvius. The aim of our paper is to present such a reconstruction for the activity period 1631--1944, and to propose a first-order physical model of the Vesuvian volcanic system.
0377-0273/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company
394 EXPERIMENTAL DATA
The activity o f Vesuvius between 1631 and 1944
After a repose of at least 150 years, the latest period of activity of Vesuvius started in 1631, with a big explosive-effusive eruption. Since 1631 and until 1944 Vesuvius had a fairly continuous activity, characterized by short paroxysmal episodes that alternated with nearly continuous Strombolian activity and short periods of quiescence. Alfano and Friedlander (1929) proposed that cycles of activity could be recognized within this period, each cycle culminating with a paroxysmal eruption followed by a period of repose. Carta et al. (1981} described the 1694--1944 activity in terms of a Markov chain of states of activity (Fig. 1). The transitions between two such states are ruled by a
Fig. 1. Block diagram of the statistical model of the Vesuvian activity proposed by Carta et al. (1981). A, IE, FE and R, respectively, describe the activity, intermediate eruption, final eruption and repose states of activity. The arrows show the allowed transitions, which are restricted by statistically defined transition parameters (k).
statistical law described by the parameter ~. The time t spent in each state is a random variable, distributed according to the distribution function f(t) = e -xt. Carta et al. (1981) suggested that such states of activity are states of equilibrium with comparable energies, and that the transitions between two states are due to stochastic perturbations. An important conclusion of their study is that the Vesuvian activity pattern changed between 1858 and 1872; the events between 1694 and 1858--1872 and those from 1858--1872 to 1944 belong to two different statistical populations. Cortini and Hermes (1981) presented a Sr-isotope study of Vesuvian lavas of the same period. An important finding is that the Sr-isotopic compositions show a negative correlation with time along two different trends (Fig. 2). They interpreted this feature as due to magma mixing phenomena, which took place in two different deep magmatic reservoirs. In the 1861--1881 period, two such reservoirs were active contemporaneously. The coincidence between this finding and the activity change reported by Carta et al. (1981) is striking. The variation of the St-isotopic composition, dC/dt, with time is
395
07080117~ I
8'Sr/~Sr /
1794 o 41~.- 79 e D
~57
o
1834
07075 1858
895
o q l - - 1631
17 50
1800
1850
1900
1950 year of e r u p t i o n
Fig. 2. Sr isotope variations in historical Vesuvian lavas (from Cor tini and Hermes, 1981). The two trends with negative slopes were interpreted as due to magma mixing in two separate reservoirs. The variation of the Sr isotope composition with time in each trend is a measure of the rate of such a process.
similar in the t w o trends (--4.9, --5.6 X 10 .6 yr-1), and it is an important parameter that can be used to characterize the activity of Vesuvius in its latest active period.
The Vesuvian ejecta Recent petrological studies of the Vesuvian ejecta have contributed new and important evidence to the knowledge of Vesuvius. Several different types of ejecta were recognized by Hermes and CorneU (1978}. These are summarized as: (1) Contact-metamorphosed carbonatic rocks. This type of xenolith was studied in detail by Barberi and Leoni (1980), who determined the P--T conditions of their formation (3--5 km depth, 700 ° C). Based on this evidence and on the chemical features of the two latest Plinian eruptions of Vesuvius (3500 y. B.P. and 79 a.D.), Barberi et al. (1979) and Sheridan et al. (1981) concluded that the magmas of these eruptions are compatible with a fractional crystallization evolution of different parental liquids in a magma chamber 3-5 km deep, with a volume of 2--2.5 km 3. All the above-mentioned authors agree that interaction with carbonate wall rocks had no major influence on the chemistry of the magmas. (2) Cumulate rocks, mainly of pyroxenitic nature, that are interpreted as quenched crystal mush (Hermes and Cornell, 1981). These nodules contain interstitial glass and no phenocrysts of plagioclase or leucite, although these minerals are c o m m o n constituents both in the lavas and in the glass norms. Hermes and Cornell concluded that these types of nodules may have formed at depths exceeding the stability of feldspars, probably in the upper mantle, and were rapidly transported upward.
396
(3) Less c o m m o n l y occurring cumulate rocks, similar to those of type 2, in which plagioclase and leucite are present in the same textural setting as the glass in t y p e 2. Hermes and Cornell (1981) interpreted these rocks to mean that they initially crystallized at depth, but ascended less rapidly than type 2, permitting the subsequent crystallization of leucite and plagioclase. Hermes and Cornell based their studies on the products emitted in the 1944 eruption and in one eruption between 1631 and 1760. The occurrence of type-2 nodules in both eruptive events is significant, and demonstrates that, at least in these two studied cases, new magma came rapidly from depth during the period 1631--1944. DISCUSSION AND CONCLUSIONS
Magma production rate at Vesuvius and geometrical constraints In order to evaluate rates of magma production, we make the following assumptions: (1) As interpreted by Cortini and Hermes (1981), the variation of the Sr isotopic composition between 1754 and 1944 is due to mixing of magmas in t w o reservoirs with constant volume V0. (2) Such reservoirs are full of a magma A, with Sr isotope composition CA, at the beginning of the mixing process. In these conditions, when a volume V of a new magma B, with Sr isotopic composition CB, is fed to one system from below, then a volume V of a mixed magma will be erupted. It is important to note that, once assumption (1) is made, assumption (2) is the simplest w a y to obtain a linear variation of the isotopic composition C of Sr with time. The isotopic composition C of the erupted magma volume V is then:
C=
CA (Vo -- V) + Cs V
v0
By differentiating with respect to time, we obtain the following relationship: dC dt
CA - - C B d V V0 dt
(1)
where dV/dt is the magma feeding rate. The isotopic composition of the parent magma B is well identified: C B = 0.70720. This value corresponds to the final values of the two trends in Fig. 3 (1944 = 0.70719; 1881 = 0.70722) and it is also very similar to that of the 1631 lava (0.70723). For C A we will use the isotopic composition of the 1754 lava, C A = 70793, which is the highest value that Cortini and Hermes (1981) measured in any Vesuvian product. If dV]dt can be evaluated independently, then V0 can be calculated from eq. 1.
397
CUMULATIVE EFFUSION RATE OF MT VESUVIUS
V(Km3x 10 3)
O /
400
! i
H
o i / °o
~,"
/
300i o o"
.0"
9 /o /0 0 200"
0
"
~j.£-
o,, / o o
•
.1-o •
loo!
o~,0 7,,o 0 I,,~.
Ii G-
0 LO"
1700
1750
1800
1850
1900
1950
years
Fig. 3. Cumulative effusion rate of Vesuvius between 1694 and 1944. A break in slope at approximately 1860 is apparent, which is consistent with an overlap of the two Sr trends (see Fig. 2) and with the conclusions of Carta et al. (1981) (see text).
Principe (1979) evaluated the volumes of the products erupted during some final and intermediate eruptions in the period 1631--1944. We calculated the mean effusion rate for such final and intermediate eruptions, based on the duration of the eruptions as given by Carta et al. (1981); then, by using the eruption catalogue given by these authors, we calculated the cumulative eruption rate for the period 1694--1944, which is represented in Fig. 3. A change of the slope near 1860 is clearly observable in this Figure and is a further confirmation that the activity of Vesuvius changed at that time. The change of slope in Fig. 3 is related to the contemporaneous activity of the two reservoirs between 1860 and 1900. The slope of the cumulative eruption rate before 1860 gives the mean rate of magma feeding for the first reservoir, which is d V / d t = 1.05 × 10 -3 km3/year. Therefore V0 = 0.13 km 3. This volume is one order of magnitude smaller than that inferred for the shallow magma chamber in which the magmas of the Plinian eruption evolved (~ 2 km 3, Barberi et al., 1979; Sheridan et al., 1981). The calculated V0 will be too small if CA is larger than the assumed value of 0.70793. However, this is the highest Sr isotope value recorded at Vesuvius (Cortini and Hermes, 1981); therefore it is unlikely that very large errors may affect our assumption. The depth of the reservoir, where magma mixing occurred, can be inferred from the data about pyrox-
398 enitic ejecta t hat were er upt e d in historical times (Hermes and Cornell, 1981). These authors suggested that some of these nodules formed at P--T conditions t h a t exceed t he stability range of leucite and feldspar, probabl y at mantle depth. Because the Sr isotope range o f the materials within the nodules is 0 . 7 0 7 1 2 - - 0 . 7 0 7 8 7 , and is similar t o the range in the lavas (0. 70719-0.70793; Cortini and Hermes, 1981), the reservoirs where these nodules f o r m e d can reasonably be identified as the same one in which mixing of magmas occurred. F u r t h e r m o r e , the g e o m e t r y of these reservoirs must have been such th at convection was an effective process, in order t hat mixing could be achieved. Our m ode l is summarized in Fig. 4. ~, La,
1861-1944 Sr trend reservoir
1754-1881 Sr trend ~reservoir
9
Fig. 4. The feeding system o f Vesuvius in its latest active period is schematically illustrated in this figure. Between 1754 and 1944 magmas formed in a heterogeneous source and mixed in two deep reservoirs. Crystallization of cumulate rocks occurred in such reservoirs, probably located at mantle depth (Hermes and Cornell, 1981). A shallow and larger magmatic chamber is also drawn, where the magmas of the Plinian eruptions evolved and where thermometamorphism of carbonatic rocks occurred (Barberi et al., 1979; Barberi and Leoni, 1980). Deep~seated nodules may have resided in this chamber for variable amounts of time.
399 The section of the conduit (S) can be constrained from: d V / d t = vS
(2)
where v is the velocity of magma ascent. This relationship expresses the conservation of mass, by assuming a constant density for the magma. Based on the Stokes law: 2 g r2 v -
(Pl--P2) 97
(3)
where g is the gravity acceleration, r is the maximum radius of the ejected nodules, p 1 and P2 are the densities of the nodule and of the magma, respectively, and ~ is the viscosity of the magma; a minimum value for v can be obtained in the case of a paroxysmal eruption, when pyroxenitic nodules up to 70 cm in diameter are ejected. However, this calculation is affected by large uncertainties: ~ may vary b y at least one order of magnitude in Vesuvian lavas between 1631 and 1944 (Imbb and Luongo, 1967); d V / d t may vary one or t w o orders of magnitude during the same eruption (Scandone, 1979). Furthermore, although d V / d t can be averaged over the 1631--1944 period of activity, short-term variations of the magma effusion rate probably are largely unrelated to the deep feeding regime of the magma (MaalOe, 1973). Accounting for all of these uncertainties, with r = 35 cm, Ap = 0.6 g/cm 3, ~ = 5 X 104 poise (Imbb and Luongo, 1967), we calculated v = 3.2 cm/sec. This is the minimum magma velocity required in order that large nodules can be ejected, and obviously refers to paroxysmal eruptions. Principe (1979) evaluated that the mean effusion rate during paroxysmal eruptions of Vesuvius is d V / d t = 1.1 X 10 -3 km3/day. We can therefore evaluate from (2) the section S of the conduit: S = 3.8 X 10 .4 km 2. The occurrence between 1631 and 1944 of deep-seated cumulitic nodules which were rapidly transported upwards verifies the arrival of new magma from below. The 79 a.D. products were formed by closed-system fractional crystallization in a shallow magma chamber (Barberi et al., 1979; Barberi and Leoni, 1980; Sheridan et al., 1981). It would be very important to know whether similar types of nodules occurred in the 79 a.D. products; in fact their presence would prove that new magma arrived, perhaps triggering the eruption by the mechanism proposed by Sparks et al. (1977). The shallow magma chamber of 79 a.D. does n o t seem to have influenced the chemistry of the lavas in the period 1631--1944 (Barberi et al., 1979; Cortini and Hermes, 1981). However, such a reservoir may have acted as a filter, where some of the deep nodules resided during variable intervals of time, permitting plagioclase and leucite to crystallize from the interstitial liquid. The filtering effect may be due to the increase of the effective section of the conduit, which causes the decrease of the velocity of ascent, and/or to pulsations of the magma feeding rate. When high magma velocities become operative along the upper part of the feeding system, the various kinds of nodules are erupted.
400
The conclusions reached in this work show that high-precision Sr isotope data on historical lavas, when correlated with results obtained by different approaches, may be very useful in investigating the eruptive mechanisms of active volcanism. In the case of Vesuvius the interpretation of these different data permits of determining some of the physical boundary conditions that dynamical models of Vesuvius must obey. ACKNOWLEDGEMENTS
The authors extend warm thanks to Fabrizio Innocenti for valuable discussions and advice and to O. Don Hermes, who kindly reviewed an earlier version of this manuscript. This work was supported by a CNR grant, Progetto Geodinamica.
REFERENCES Alfano, G.B. and Friedlander, I., 1929. La Storia del Vesuvio Illustrata da Documenti. Coevi. Karl Hohn, Ulm. Barberi, F. and Leoni, L., 1980. Metamorphic carbonate ejecta from Vesuvius Plinian eruptions. Evidence of the occurrence of shallow magma chambers. Bull. Volcanol., 43(1): 107--120. Barberi, F., Bizouard, H., Clocchiati, R., Metrich, N., Santacroce, R. and Sbrana, A., 1979. The Somma Vesuvius magma chamber: a petrological and volcanological approach. IUGG, 17th General Assembly, Canberra (Abstr.). Carta, S., Figari, R., Sartoris, G., Sassi, E. and Scandone, R., 1981. A statistical model for Vesuvius and its volcanological implications. Bull. Volcanol., pp. 44 (2). Cortini, M. and Hermes, O.D., 1981. Sr isotopic evidence for a multi-source origin of the potassic magmas in the Neapolitan area (S. Italy). Contrib. Miner. Petrol., 77: 47--55. Hermes, O.D. and Cornell, W.C., 1978. Petrochemical significance of xenolithic nodules associated with potash-rich lavas of Somma-Vesuvius volcano. NSF Final Tech. Rep., Univ. Rhode Island. Hermes, O.D. and Cornell, W.C., 1981. Quenched crystal mush and associated magma compositions as indicated by intercumulus glasses. J. Volcanol. Geotherm. Res., 9: 133-149. Imbb, G. and Luongo, G., 1967. Contribution to the knowledge of the magmatic evolution by the study of the variation of the coefficient of viscosity. (Paper presented at the IUGG General Assembly, Zurich, 1967.) Maal~be, S., 1973. Temperature and pressure relation of ascending primary magmas. J. Geophys. Res., 78(29): 6877--6886. Principe, C., 1979. Le eruzioni storiche del Vesuvio: riesame critico, studio petrologico dei p r o d o t t i ed implicazioni vulcanologiche. M. Sc. Thesis, Univ. Pisa, Ist. Mineral. Petrogr. Scandone, R., 1979. Effusion rate and energy balance of Paricutin eruption (1943-1952), Michoacan, Mexico. J. Volcanol. Geotherm. Res., 6: 49--59. Sheridan, M.F., Barberi, F., Rosi, M. and Santacroce, R., 1981. A model for Plinian eruptions of Vesuvius. Nature, 286 (5795): 282--285. Sparks, S.R.J., Sigurdsson, H. and Wilson, L., 1977. Magma mixing: a mechanism for triggering acid explosive eruption. Nature, 267 : 315--318.
