Metamorphic carbonate ejecta from vesuvius plinian eruptions: Evidence of the occurrence of shallow magma chambers

Petrological, mineralogical and chemical data of 46 ejecta deriving from the sedimentary basement beneath Somma-Vesuvius volcano are reported. The ejecta samples were collected in pumice deposits formed during two major Plinian eruptions. One of these pumice deposits was formed during the well-known 79 A.D. eruption, and the other one — the so-called Avellino pumice — during an eruption occurred about 3,500 years B.P. Most of the ejecta from both the layers are fragments of contact-metamorphosed carbonate rocks. For the ejecta of the 79 A.D. Plinian eruption, the mineralogical parageneses of the metamorphosed carbonate rocks (dolomite-Mg calcite-periclase, and dolomite-Mg calcite-brucite) allow the evaluation of the conditions under which contact metamorphism developed. Temperatures, estimated by the Mg content in the calcite coexisting with dolomite, ranged from 360° to 790°C, whereas total fluid pressure would not have exceeded 1,500–2,000 bars with a maximum depth of metamorphism (and hence of the magma chambers) of 5,000–6,000 m. The ejecta from the so-called Avellino pumice layer (characzerized prevalently by a dolomite-Mg calcite assemblage) show that contact metamorphism occurred under the same temperature range as that of the 79 A.D. ejecta, but at an higherP _CO2 partial pressure and probably at an higher total fluid pressure. These differences in physico-chemical conditions of metamorphism seem to indicate that the two Plinian eruptions were fed probably by two different magma chambers. Comparison between chemical composition of the carbonate ejecta and carbonate samples of the Mesozoic sedimentary series outcropping near the volcano indicates that fragmentation of almost all the sequence were brought to the surface by the explosive Plinian eruptions. Although the data at our disposal do not provide any information on the size of the 79 A.D. eruption magma chamber, this probably had an important vertical length component of at least 2,000 m     

A statistical model for vesuvius and its volcanological implications

The last 300 years of Vesuvius history are reconstructed as a chronological succession of 4 phenomenological states: i) repose, ii) persistent activity, iii) intermediate eruption and iv) final eruption. It turns out that the times of permanence in each state are distributed according to the same exponential law. Vesuvius activity is then described by a Markov chain of these 4 states, with transition probabilities determined from the previous phenomenological analysis. The model reproduces the Vesuvius activity between 1694 and 1872 and possibly also in the 1872–1944 period. It turns out that, at least between 1694 and 1872, the volcano was behaving like a quasistationary system with 4 equilibrium states, perturbed by a stochastic noise responsible for occasional transitions from an equilibrium state to another. Major physical or structural changes of the volcanic system around 1872 and possibly in the whole subsequent period, are clearly shown by the statistical analysis.     

A thermal model of the magma reservoir feeding Plinian eruptions at Mt. Vesuvius (Italy)

A theoretical thermal model has been worked out for the magma reservoir that would have fed the two last Plinian eruptions of Mt. Vesuvius (Barberi et al., 1981). The effect of convective motions is discussed, and it is shown that the size of convective cells and the efficiency of the process in smoothing out temperature gradients evolves in time due to the progressive viscosity increase produced by the heat lost by conductive heat transfer through the host rock. Although convection will be important throughout the history of the reservoir, until very high viscosities are reached, the pure conductive model seems to account satisfactorily for the cumulative heat loss by the reservoir. Gravitative crystal settling can occur, even in presence of convective motions, mostly during several hundred years after the magma emplacement when viscosity is not yet increased to high values.     

Ground deformations on Mt. Vesuvius from 1977 to 1981

The results of a ground deformation study (1) of Vesuvius are reported. The study is a part of the geophysical program for the surveillance of this extremely dangerous active volcano. The results obtained in the period 1977–1981 show no significant changes in the dynamic state of the volcano. Corresponding to the southern Aapennines Nov. 23, 1980 earthquake a modification of the zenithal distance between the rim and bottom of the Vesuvius crater was observed which was likely connected with fairly shallow effects.     

Seismicity of Vesuvius on its present state of activity

Reference is hereby made to the characteristics of seismic recordings of volcanic origin as obtained at the Vesuvian Observatory up to date. The new seismic equipment recently put into operation and consisting of four three-component seismographs set up on the slopes of the Vesuvius Great Cone is also described. The characteristics of the records obtained by the above said equipment are hereby explained and related to the activity of the volcano.     

Interpretation of gravity and magnetic anomalies near Naples,Italy, using computer techniques

Some aspects of the analytical procedure for an automatic fitting of gravity and magnetic anomalies are discussed. An example of application relative to the Campania volcanic district near Naples, Italy, is reported. Gravity anomalies in that area mainly reflect the depth to the carbonate basement rocks. A small gravity high over Vesuvius may indicate buried volcanic layers. Magnetic anomalies near L. Patria and Castel Volturno are interpreted to be caused by buried intrusive rocks of higher susceptibility. Computer models of subsurface rock units with appropriate densities and susceptibilities have been generated to match the observed anomalies.     

Average density of stratigraphic sequences at Vesuvius and Phlaegraean fields at different heights

In order to measure the density of rocks in the Phlacgraean and Vesuvius edifices, appropriately chosen indirect methods, based on a strict relationship between gravity and density, enabled useful data to be obtained. The data can be related to the particular lithological formations of the above volcanic complexes.     

Genesis of magmas producing pumice flow and fall deposits of towada caldera,Japan

Towada caldera, lying near the northern end of Honsyu, Japan was constructed by eruptions of lavas and pyroclastic materials in three separate periods. At the ends of the first and second periods, great amounts of pumice were erupted in the form of pumice flow and fall respectively. Each pumice cruption was followed by collapse of the center of the cones resulting in double calderas. The lavas of these three periods and the pumice of the first and second periods were chemically analysed. The result was plotted in several different types of variation diagrams. The points for the lavas and pumice lie generally on smooth curves, indicating that the magmas which caused the pumice cruptions belong to the same general differentiation series as do the lavas. If SiO₂/FeO+Fe₂O, is plotted against sodification index (MgO x 100/MgO+FeO+Fe₂O, +Na₂O+K₂O), points for the lavas lie on a straight line, whereas those for the pumice lie on another straight line branching from the former at some point in the middle stage of differentiation. The rate of increase of this ratio in the pumice is greater than in the lavas, implying that less SiO₂ and more iron were subtracted from the magmas producing the pumice than from those producing the lavas. This was probably caused by crystallization of a greater amount of magnetite in the former magmas possibly due to higher oxygen partial pressure which may be in turn related to higher water content. It is not necessary to postulate melting of the crust in order to generate magmas of the pumice eruptions of the central type.     

A comparison of the recent eruptions of Mt. Pelée,Martinique and Soufrière,St. Vincent

The simultaneous eruption of Mt. Pelée, Martinique and Soufrière, St. Vincent are regarded as the first recognized examples of Pelean-type and St. Vincent-type pyroclastic eruptions. Both produced nuées ardentes, the former usually laterally directed because of the presence of a dome and the latter vertically directed from an open crater. Both volcanoes have subsequently erupted for a second time this century. The 1902–05 and 1929–32 eruptions of Mt. Pelée produced andesite lava of almost identical composition and mineralogy. Both contain two generations of plagioclase, orthopyroxene, Fe-Ti oxide, corroded brown amphibole and olivine rimmed by pyroxene. In contrast, the Soufrière material is more basic in composition varying from basaltic andesite to basalt in 1902–03 and basaltic andesite in 1971–72. The Soufrière material contains two generations of plagioclase (with those of 1971–72 having additional zones of labradorite), clinopyroxene, orthopyroxene, olivine and Fe-Ti oxide. The pyroclastic deposits are strikingly different, those from the Pelean-type eruption are termed «block and ash deposits» being characterised by poorly vesicular lava blocks up to 7 m in diameter, while the St. Vincent-type eruption produced «scoria and ash deposits» containing vesicular ropey blocks or bombs no larger than 1 m in diameter. The differences in styles of eruption are attributed to differences in viscosity and mechanism of eruption of the magmas. Stratigraphic studies of Mt. Pelée reveal that the volcano has produced basaltic andesite scoria and ash deposits from St. Vincent-type eruptions. It is concluded that the recent eruptions of Pelée tapped a deep level magma during both eruptions releasing magma of similar composition, while the 1971 Soufrière magma is thought to be a remnant of the 1903 basaltic magma which remained at a high level within the volcano where it underwent enrichment in plagioclase and loss of olivine and oxide.     

Volcanic ash hazard in the Central Mediterranean assessed from geological data

Volcanic ash produced during explosive eruptions can have very severe impacts on modern technological societies. Here, we use reconstructed patterns of fine ash dispersal recorded in terrestrial and marine geological archives to assess volcanic ash hazards. The ash-dispersal maps from nine Holocene explosive eruptions of Italian volcanoes have been used to construct frequency maps of distal ash deposition over a wide area, which encompasses central and southern Italy, the Adriatic and Tyrrhenian seas and the Balkans. The maps are presented as two cumulative-thickness isopach maps, one for nine eruptions from different volcanoes and one for six eruptions from Somma-Vesuvius. These maps represent the first use of distal ash layers to construct volcanic hazard maps, and the proposed methodology is easily applicable to other volcanic areas worldwide.     

Voluminous acid volcanism in the Busheveld Complex: A review of the Rooiberg Felsite

The 2.1 b.y. old Rooiberg Felsite roofs and is intruded by the mafic layered rocks and granites of the Bushveld Complex. The felsite unit, which locally exceeds 5 km in thickness and may represent an originally erupted volume of more than 300,000 km³, is dominated by rhyolitic to dacitic lavas with minor pyroclastic and sedimentary rock types. Volcanic rocks of more mafic composition occur towards the base of the sequence. The Rooiberg episode essentially terminated the volcanic activity in the Transvaal basin and heralded the emplacement of the Bushveld Complex. Despite the close spatial and temporal relationships between the Rooiberg Felsite and the Bushveld Complex, the precise nature of the petrogenetic link is obscure. Chemical analyses of felsite have been variously interpreted to suggest cyclic differentiation along a comagmatic trend or to demonstrate anomalous enrichment in SiO₂. Several characteristics delineate the Rooiberg Felsite as a possibly unique occurrence of rhyolitic magmatism, notably the immense volume of the unit, the marked preponderance of lavas over pyroclastic types, and the unusually great thickness and lateral extent of the flows. The thesis that the Rooiberg Felsite represents a shock-produced, meteorite-impact melt cannot be supported on the available evidence.     

Fractional crystallization trends in the system Mg2SiO4-CaAl2Si2O8-FeO-Fe2O3-SiO2 over the range of oxygen partial pressures of 10−11 to 10−0.7 atm

A brief report is made of current laboratory investigations on phase relations among olivine, pyroxene, anorthite, magnetite, tridymite, liquid and gas in the system Mg₂SiO₄-CaAl₂Si₂O₈-FeO-Fe₂O₂-SiO₂ over a wide range of oxygen partial pressures. Courses of fractional crystallization under various conditions of oxygen partial pressure are depicted using an anorthite saturation diagram. Starting with a basalt-like composition in the system, fractional crystallization at a moderate oxygen partial pressure (10 atm.) results in an andesite-like residual liquid of composition 55 SiO₂, 14 iron oxide, 6 MgO, 9 CaO, 16 Al₂O₃ at a temperature of 1155°C. With fractional crystallization in a closed system, the end liquid approaches the composition of 45 SiO₂, 38 iron oxide, 6 CaO and 11 Al₂O₃, at a temperature of 1050°C and oxygen partial pressure of about 10^?12 atm. The andesitic final liquid in this system would be expected to further differentiate toward dacitic and rhyolitic compositions if alkalies and water were present in the system. On the basis of these studies, the derivation of liquids of andesitic, dacitic or rhyolitic composition from primary basalts by fractional crystallization seems entirely possible if the oxygen partial pressure is maintained at a moderate or high level.     

Pumice eruptions of the lesser Antilles

Stratigraphic studies on the active and potentially active volcanoes of the Lesser Antilles have revealed two main types of andesitic pyroclastic deposit. One with dense clasts in a poorly vesicular ash represents nuée ardente eruptions of Pelean type and the other group of vesicular pumice and ash represent both Plinian airfall and ash-pumice flow eruptions. The pumiceous deposits can be divided into airfall lapilli, airfall ash, crystal-pumice surge, ashpumice flow and ash hurricane types. No pumice eruptions have been witnessed in the Lesser Antilles during the period of written history although the stratigraphy of archaeological sites shows they occurred in pre-Columbian times. Detailed stratigraphic studies of Mt. Pelée, Martinique, and the Quill, St. Eustatius, show that, throughout their history, pumice eruptions have alternated with nuée ardente eruptions with approximately equal frequency. The widespread occurrence of pumiceous deposits on many of the West Indian volcanoes and the frequent alternations in the stratigraphic sections suggest the high probability that they will be witnessed in the future. On Martinique, some on the late prehistoric pumiceous pyroclastic flow deposits (the ash hurricanes) have been traced 20 km from the central vent to the out-skirts of Fort de France, indicating that they are the major hazard in the Lesser Antilles. Measured stratigraphic sections show that the Pelean type nuée ardente deposits are separated from the pumiceous pyroclastic deposits by others of intermediate vesicularity and appearance. The presence of such deposits of intermediate vesicularity could provide a future warning of impending change in pyroclastic style. As no such deposits formed on Mt. Pelée this century the present «safer» episode of nuée ardente (Pelean type) activity is expected to continue.     

Igneous series in island arcs: The northeastern Caribbean compared with worldwide island-arc assemblages

Two major magma series are recognised in the northeastern Carribean island arcs and correlated with numerous examples elsewhere, mainly in the western Pacific. The Primitive Island Arc (PIA) series is apparently the oldest in each of its island-arc occurrences and is followed by the calcalkaline (CA) series, which locally co-occurs with a high-potassium (HK) variety, especially in topographically larger island areas. Also occurring in the northeastern Caribbean are tectonically emplaced examples of metamorphosed mid-ocean ridge basalt (MORB) and a diabase dike swarm possibly related to a rifting event. PIA series is distinguished from the CA by its lower content of LIL elements, including REE. Th, U, Zr, K, and Ba. The difference is most striking in the abundant siliceous differentiates of this series, in which these elements are scarcely higher in abundance than in mafic examples. The HK series is considered to be a subseries of the CA which is higher in K, Ba, Sr, Mg, and Ni, but has REE, Th, and other element levels very similar to the CA. The PIA series has been confused with MORB but has lower REE (with flatter patterns generally), much lower Ni, lower Ti, Zr, and Mg, and appears to be intrinsically hydrated. The abundance of siliceous differentiates also separates this series from MORB. PIA magma is believed to be generated by massive melting of hydrated mantle which rises in front of the descending snout of a newly subducting slab at the initial stage of island-arc genesis; i.e., before the island-arc platform has formed. CA magma has a different source and is probably the partially fused material of the slab itself, more or less modified by volcanic-sedimentary materials dragged to depth during subduction.     

Geochemistry and origin of basaltic lavas from Society Islands,French Polynesia (south central Pacific Ocean)

The Society Islands of French Polynesia (south-central Pacific Ocean) are formed predominatly by basaltic lavas of alkaline affinities. These intraplate rocks are either Ne- or Hy-normative. Ne-normative basaltic lavas are lower in Si and heavy rare earth elements and higher in Ti, V and Sr than Hy-normative. The differences are attributed to the contrasting behaviour of amphibole during melting of the upper mantle, which, in turn, may be related to the variation in the depth of melting. Compared to pyrolite, the upper mantle source of the lavas was enriched in incompatible elements.     

The composition and differentiation of the volcanic rocks of Carriacou,grenadines, West Indies

Tertiary volcanic rocks of Carriacou occupy two-thirds of the island. The volcanics include volcaniclastics, lava flows and dome lavas and range in composition from basalts to andesites. Carriacou basalts fall into two petrographic types (a) clinopyroxene-plagioclase-phyric basalts and (b) olivine microphyric basalts; the latter having higher MgO and lower Al₂O₃ than the clinopyroxene basalts. Both types are unusually rich in mafic minerals compared with Lesser Antilles basalts in general, although similar types have been reported from the nearby island of Grenada. The potash to silica ratios are relatively high and confirm the similarity between Carriacou and Grenada basalts and the differences between these basalts and basalts from other islands of the Lesser Antilles. The basaltic andesites and andesites from Carriacou correspond closely in mineralogical and chemical composition with typical andesites found elsewhere in the Lesser Antilles. The geochemistry of the volcanics shows that the olivine microphyric basalts display tholeiitic affinities whereas the clinopyroxeneplagioclase-phyric basalt, basaltic andesites and andesites are calcalkaline. The compositional gradation in both the geochemistry and mineralogy of these volcanics suggests that fractional crystallization played an important role in the derivation of the various magma.     

Phreatic eruption clouds: the activity of la Soufrière de Guadeloupe,F.W.I., August — October, 1976

From August to October, 1976, La Soufrière de Guadeloupe was observed, and recorded with an automated sequence camera and numerous handheld cameras. During the period of observation, the nature of volcanic activity ranged from mild steam emission to moderately energetic phreatic eruptions. Background fumarolic activity (steam emission) was characterized by the emission of generally tephra-free steam clouds 50 to 150 m above the summit. The clouds rose buoyantly above the vent and were blown downwind at prevailing wind velocities. Phreatic eruptions were well-documented on September 22, October 2, and October 4. In the latter two eruptions, small bursts of tephra-laden steam erupted at intervals of 30 to 45 min, and rose from 350 to 500 m above the summit. In the largest observed eruption, that of October 2, the steam and tephra cloud rose to a maximum height of 600 to 650 m in 20 min. A white vapor cloud and a medium gray, tephra-laden cloud were erupted simultaneously from the summit vent and both were surrounded by a vapor collar: the clouds were thoroughly mixed within 1 km downwind of the summit. The concurrent growth of clouds from separate vents (summit and flank) implies a common source. Simultaneous eruption of tephra-free and tephra-laden clouds from the same vent is puzzling and implies: (i) lateral changes in the degree of alteration of dome rocks along the elongate vent, hence erodability of the dome lavas, or (ii) differences in the gas velocities. These «mixed» clouds moved westward, downwind and downslope as a density current, along the watersheds of the R. Noire and R. des Pères with an approximate velocity of 10 to 25 m/sec. Upon reaching the sea the clouds continued to move forward, but at a decreased velocity, and spread laterally, having left behind the restrictions of valley walls. A thin gray veneer of moist tephra, ranging from several cm thick near the dome to less than 1 mm thick several km downwind, was deposited along a narrow corridor southwest of the summit. Tephra from the phreatic eruptions consisted mostly of hydrothermally altered lithic, mineral, and glass fragments derived from dome lavas; no fresh (juvenile) pyroclasts were present in the tephra. Absence of juvenile tephra at La Soufrière supports the view that activity was due to groundwater circulating in a vapor-dominated geothermal system, probably driven by a shallow heat source. At La Soufrière, most vapor-dominated systems are located in elevated areas of groundwater recharge where groundwater movement is downward and outward. The sporadic phreatic eruptions may be related to the rate of recharge of meteoric waters within the dome, the decrease in pore pressure during fortnightly tidal minimums or both. Whatever the triggering mechanism, vapor-dominated fluids eroded vent walls during phreatic eruptions and carried out fine-grained, hydrothermally altered, pre-existing dome material as tephra.     

Explosive felsic volcanism on El Hierro (Canary Islands)

The Canary Islands consist of seven basaltic shield volcanoes whose submerged portion is much more voluminous than the subaerial part of each island. Like so many other volcanic oceanic islands, the indicative deposits of explosive felsic volcanism are not a common feature on the Canary archipelago. Hitherto, they have only been documented from the central islands of Gran Canaria and Tenerife, which are the largest volcanic complexes of the islands. On the other Canary Islands, the presence of felsic rocks is mostly restricted to intrusions and a few lava flows, generally within the succession in the oldest parts of individual islands. In this paper, we present a detailed stratigraphic, lithological and sedimentological study of a significant felsic pumice deposit on the island of El Hierro, referred here as the Malpaso Member, which represents the only explosive episode of felsic volcanism found on the Canary Islands (outside of Gran Canaria and Tenerife). The products of the eruption indicate a single eruptive event and cover an area of about 15 km². This work provides a detailed stratigraphic and chronological framework for El Hierro, and four subunits are identified within the member on the basis of lithological and granulometric characteristics. The results of this study demonstrate the importance of an explosive eruption in a setting where the activity is typified by effusive basaltic events. Given the style and the spatial distribution of the Malpaso eruption and its products, a future event with similar characteristics could have a serious impact on the population, infrastructure and economy of the island of El Hierro.     

The publications of Gordon A. Macdonald, 1934–1978

Dr. Gordon Andrew Macdonald’s publication list reflects the developments in volcanology over more than 40 years. Of his publications, 227 are recorded in this bibliography, which is divided into three sections: Scientific articles, Books, and Miscellaneous papers.     

Geochemistry and petrogenesis of the kilimanjaro volcanic rocks of the Amboseli area,Kenya

The paper summarizes the geochemical and petrogenetic aspects of an investigation of late Tertiary to Recent volcanic rocks in the Amboseli area of southern Kenya. A study of chemical variations in the Amboseli and Kilimanjaro lavas distinguishes a mildly alkaline series (alkali olivine basalts, trachybasalts/trachyandesites, trachytes, rhomb porphyries and phonolites) from a strongly alkaline series embracing subordinate nephelinitic, phonolitic and tephritic lavas. The two series probably evolved independently from a source in the mantle. A comparison of Kilimanjaro with other East African volcanoes shows that the focus of strongly alkaline volcanicity moved from eastern Uganda and western Kenya to northern Tanzania at the end of Miocene times. The Pliocene to Recent centres near the Kenya-Tanzania border show evidence of decreasing alkalinity from a western zone of nephelinite-phonolite volcanoes to an eastern region in which central volcanoes are characterized by the association of strongly and mildly alkaline suites.