Calc-alkaline volcanic rocks from NW Sardinia: Evaluation of a fractional crystallization model

The volcanic centre of Monte Seda Oro, N. W. Sardinia, representative of a Cenozoic calc-alkaline andesitic suite of rocks is composed of a variety of rocks ranging from high alumina basalts to dacites. The minerals of basaltic, andesitic and dacitic rocks show only limited variation in chemical composition. The geochemical data suggest that the various rock-types are related by a crystal-liquid fractionation. Least-squate numerical calculations, using major element data, support the derivation of andesites with SiO₃ content ranging from 53.8 to 59.0% from basalts having about 48.7% of SiO₂ by low pressure crystal fractionation of the phenocryst phases present in these rocks. However, the origin of dacites cannot be readily explained by this mechanism.     

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.     

Eruption of Piip Crater (Kamchatka)

In autumn of 1966 on the northern slope of Kliuchevskoy volcano a chain of new adventive craters broke out at the height of about 2200 m. Eighty-four hours before the beginning of the eruption a swarm of preliminary volcanic earthquakes had appeared. The number of preliminary shocks was 457 with total energy of 4 × 10¹⁷ erg. With the beginning of the lava flow the earthquakes stopped and a continuous volcanic tremor appeared. The total energy of volcanic tremor amounts to 10¹⁶ erg. During the eruption numerous explosive earthquakes with the energy of 10¹⁵–10¹⁶ erg were recorded and besides the microbarograph of the Volcanostation recorded 393 explosions with an energy more than 10¹³ erg and their total energy was equal to 10¹⁷ erg. All together it has been formed 8 explosive craters and the lowest 9th crater was effusive. The slag cone was formed round this effusive crater, the lava effusion of basaltic-andesite composition (52,5% SiO₂) tooke place from the lava boccas at the cone base and from the crater. The lava flow covered a distance of 10 km along the valley of the Sopochnoy river and descended to a height of about 800 m. The lava flow velocity at the outflow reached 800 m/hr, the lava temperature was 1050°C. The effused lava volume amounts to 0.1 km³. The eruption stopped on December 25–26, 1966.     

Major-element petrochemistry of some extrusive rocks from the volcanically active Mariana Islands

Volcanic rocks from six of the currently or recently active volcances of the Mariana Island are show little variation in major element abundances. SiO₂ content averages 51.5 wt.%. The flows are high in Al₂O (mean 17.7 wt.%) and Fe oxides (mean 10.1 wt.% calculated as FeO only), and moderate in MgO content (mean 4.7 wt.%), Na₂O (mean 2.7 wt.%), and K₂O (mean 0.7 wt.%). Only the rocks from Farallon de Pajaros, the northernmost of the Mariana Islands, deviate slightly from the average of the analyses. Three analyses from this island are slightly higher in SiO₂ (about 54 wt.%) and Al₂O₃, and are lower in total Fe oxides and MgO. According to preferred classification, the lavas of the Mariana Islands can be termed mela-andesites, high-alumina basalts, or calc-alkaline (orogenic) basalts. The K₂O values (mean 0.7 wt.%) obtained from lavas of the Mariana Islands are significantly higher than the K₂O values (about 0.33 wt.%) from volcanics of the Izu chain to the north. Inasmuch as the substantial scatter in location of earthquake foci beneath both arcs prevents accurate delineation of the upper boundary of the Benioff zone, it presently cannot be determined whether this discrepancy in K₂O values reflects a difference in depth from the volcanic are to the dipping seismic zone or relates to other phenomena. The older volcanic islands within the Mariana-Bonin island chain apparently defined an island arc system during Eocene to Miocene time. This indicates that the present plane of convergence between the Pacific plate and the Philippine Sea plate has defined the convergence between these plates since Eocene time.     

Melting of Mg2GeO4 under pressure

Melting point of germanate forsterite, Mg₂GeO₄, was raised by compression at the rate of 30°C/GPa. The triple point, at which three phases of olivine- and spinel-type solids and liquid coexisted, was fixed at 1950°C and 3.5GPa. Wen these results are combined with the thermodynamical data of forsterite, Mg₂SiO₄, it is estimated that the triple point of forsterite lies in a region ranging from 2700° to 3000°C in temperature and from 20 to 30GPa in pressure.     

Expressions for the Global Gravimetric Moho Modeling in Spectral Domain

We apply a newly developed numerical method to improve the Moho geometry by the implementation of gravity data. This method utilizes expressions for the gravimetric forward and inverse modeling derived in a frequency domain. Methods for a spectral analysis and synthesis of the gravity field and crust density structures are applied in the gravimetric forward modeling of the consolidated crust-stripped gravity disturbances, which have a maximum correlation with the (a priori) Moho model. These gravity disturbances are obtained from the Earth’s gravity disturbances after applying the topographic and stripping gravity corrections of major known anomalous crust density structures; in the absence of a global mantle model, mantle density heterogeneities are disregarded. The isostatic scheme applied is based on a complete compensation of the crust relative to the upper mantle density. The functional relation is established between the (unknown) Moho depths and the complete crust-stripped isostatic gravity disturbances, which according to the adopted isostatic scheme have (theoretically) a minimum correlation with the Moho geometry. The system of observation equations, which describes the relation between spherical functions of the isostatic gravity field and the Moho geometry, is defined by means of a linearized Fredholm integral equation of the first kind. The Moho depths are determined based on solving the gravimetric inverse problem. The regularization is applied to stabilize the ill-posed solution. This numerical procedure is utilized to determine the Moho depths globally. The gravimetric result is presented and compared with the seismic Moho model. Our gravimetric result has a relatively good agreement with the CRUST2.0 Moho model by means of the RMS of differences (of 3.5 km). However, the gravimetric solution has a systematic bias. We explain this bias between the gravimetric and seismic Moho models by the unmodelled mantle heterogeneities and uncertainties in the CRUST2.0 global crustal model.     

Petrology of tholeiitic lavas from Tanna Island (New Hebrides): Importance of cumulative processes in Island arc magmatism

Tanna, one of the southernmost islands of the New Hebrides volcanic arc, is made of Late Pliocene to Recent island arc tholeiitic basalts and andesites, with SiO₂ contents ranging from 45 to 57%. These lavas are highly porphyritic (30–50% in volume): phenocrysts of plagioclase are the most abundant, together with olivine and clinopyroxene. The groundmass contain plagioclase, augite, olivine, magnetite and glass; pigeonite, tridymite, sanidine and, rarely, biotite may also occur. The olivines and clinopyroxenes show an iron enrichment from the cores of phenocrysts to their rims and the groundmass crystals, but their compositional variations are not correlated with the Mg/Fe ratio of bulk host rocks, the most Fe-rich compositions being found in Mg-rich lavas. Plagioclase compositions range from An₉₅ to An₆₀ in the basalts and An₆₀ to An₅₀ in the andesites, but, within each group, they are not correlated with SiO₂ or Na₂O contents of host lavas. Consequently, the bulk major element compositions of Tanna volcanic rocks cannot be considered as primarily controlled by crystal separation from successive liquids. The oxyde-SiO₂ variations diagrams, and the modal compositions and mineral chemistry show that crystal accumulation is the predominant mechanism accounting for bulk rock compositions. However, this does not exclude fractional crystallization: the variation of the calculated groundmass mineralogy strongly suggest the occurrence of crystal removal mainly clinopyroxene and magnetite.     

Petrology of filicudi,aeolian archipelago

Volcanic rocks on the island of Filicudi are typical of the cale-alkaline association of the Aeolian archipelago, and range in composition from basalt to high-K andesite. All specimens contain abundant phenocrystic plagioclase (cores of An_82–97, rims becoming progressively more sodic with increasing Differentiation Index), clinopyroxene, and magnetite; the most basic samples contain olivine, which is replaced by orthopyroxene at around D.I.=44. SiO₂ generally increases with time, and this in conjunction with the continuity in mineralogy suggests that the lavas are the differentiation products of a single batch of magma. Least-squares mixing calculations are consistent with a shallow fractional model, as is the variation in trace element abundances.     

The figure of the earth — Changes in concepts

The search for the Figure of the Earth is rooted in the age-old wonder about the world we live in. The ancient mythical notions have developed, under the tutelage of Greek systematic thought, into specific questions of the shape and size of the Earth. The history of these questions, now spanning more than two millenia, shows a continuous refinement in formulation, as each bona fide answer prompted refined measurements which, in turn, started a new round of problems. The change in concept of what one was looking for was, and still is, closely connected with the technical capability and its potential for further development. This change in concept is traced here from ancient times to the present throungh some of its milestones: Anaximander's partly invisible celestial sphere and Pythagoras' spherical Earth; Eratosthenes' exemplary combination of astronomy and land surveying for the dual purpose of determining the size of the spherical Earth and making a map of the habitable world; the identical dual purpose followed by the Académie Royale des Sciences in the 17th century, but on a more sophisticated technical level; the challenge to the spherical concept by Newton's gravitational theory and the absorption of that theory into a wider concept of the Figure of the Earth; the acknowledgment of the irregular geoid by Gauss, Bessel, and Helmert, and the changed significance of the ellipsoid; Heiskanen's and Vening Meinesz' ‘Basic Hypothesis of Geodesy’ concerning the hydrostatic equilibrium of the Earth, and O'Keefe's contradiction on the basis of satelite data; the dream of global surveys come true through satellite geodesy and the accompanying inclusion of the outer gravity field of the Earth into the concept of its Figure; and lastly, the present and projected capability of high precision measurement and the widening of the concept of the Figure of the Earth from 3-dimensional rigidity to 4-dimensional time-dependence.     

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.     

Devonian tioga tuff in Northeastern United States

The Tioga Bentonite ranges from a coarse crystal tuff to a tuffaceous shale enclosed in marine strata. It originally was deposited over the entire northeastern United States during the Middle Devonian. Trends in grain size, thickness, and numbers of preserved ash layers indicate a source volcano in west-central or central Virginia.     

Volcan ceboruco: A major composite volcano of the mexican volcanic belt

Ceboruco is a major composite volcano at the western end of the Mexican Volcanic Belt, near the junction between the North American and Pacific plates. The volcano is built from successive eruptions of andesite lavas and pyroclastic rocks, and major eruptions during its history have resulted in the formation of two concentric calderas. The youngest volcanic activity has included the extrusion of dacites within the inner caldera and a voluminous flank eruption of andesite during 1870–72. Fumarolic activity persists to the present day. Chemical analyses show that the lavas are of cale-alkaline type and rangs from andesite (SiO₂=58–61%) to acid dacite (SiO₂=68%) in composition. The rate of increase of K₂O relative to SiO is greater than that in volcanic rocks from the Mexican Volcanic Belt as a whole. This indicates that simple models based on the application of such relationships may not be adequate to explain the petrogenesis of calc-alkaline lavas.     

Geochemistry of the volcanics of central Mexico

The Tertiary and Recent volcanics of Mexico occur in two provinces. The Cordillera Province is made up of about 1700 m of ignimbrite sheets overlain and intercalated in the upper part by olivine basalt and basaltic andesite. The Rio Lerma Province extends transversely across Mexico and in the Valley of Mexico the lavas consist mainly of andesite and dacite, 68 % of those analysed having 62 | 4.7 % SiO₂. A total of 108 chemical analyses were made for the major elements, 90 of these including determinations of Cr, Ni, Cu, Zn, Rb, Sr, Ba, Th and Pb for two areas, the Valley of Mexico in the Rio Lerma Province and the Guadalajara region which lies at the intersection of the two provinces. Computer constructions of normative components in the basalt tetrahedron and other projections support an origin of partial melting of tholeiitic to pyrolitic material for the production of andesite. The Guadalajara lavas have consistently higher K/SiO₂ and K/Rb ratios and lower Mg/SiO₂ ratio than the Valley of Mexico rocks suggesting generation at greater depth.     

The complex filling of alae crater,Kilauea Volcano,Hawaii

Since February 1969 Alae Crater, a 165-m-deep pit crater on the east rift of Kilauea Volcano, has been completely filled with about 18 million m³ of lava. The filling was episodic and complex. It involved 13 major periods of addition of lava to the crater, including spectacular lava falls as high as 100 m, and three major periods of draining of lava from the crater. Alae was nearly filled by August 3, 1969, largely drained during a violent ground-cracking event on August 4, 1969, and then filled to the low point on its rim on October 10, 1969. From August 1970 to May 1971, the crater acted as a reservoir for lava that entered through subsurface tubes leading from the vent fissure 150 m away. Another tube system drained the crater and carried lava as far as the sea, 11 km to the south. Much of the lava entered Alae by invading the lava lake beneath its crust and buoying the crust upward. This process, together with the overall complexity of the filling, results in a highly complicated lava lake that would doubtless be misinterpreted if found in the fossil record.     

Fractional Tajimi–Kanai model for simulating earthquake ground motion

The ground acceleration is usually modeled as a filtered Gaussian process. The most common model is a Tajimi–Kanai (TK) filter that is a viscoelastic Kelvin–Voigt unit (a spring in parallel with a dashpot) carrying a mass excited by a white noise (acceleration at the bedrock). Based upon the observation that every real material exhibits a power law trend in the creep test, in this paper it is proposed the substitution of the purely viscous element in the Kelvin Voigt element with the so called springpot that is an element having an intermediate behavior between purely elastic (spring) and purely viscous (dashpot) behavior ruled by fractional operator. With this choice two main goals are reached: (i) The viscoelastic behavior of the ground may be simply characterized by performing the creep (or the relaxation) test on a specimen of the ground at the given site; (ii) The number of zero crossing of the absolute acceleration at the free field that for the classical TK model is \(\infty \) for a true white noise acceleration, remains finite for the proposed model.     

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.     

Neutron activation analysis of the 1965 taal volcanic ash

Neutron activation analysis of the Taal volcanic ash revealed the presence of unusual amount of scandium in the volcanic ash as compared to the standard basalt BCR-1. The BCR-1 value for Fe/Sc is 2760 while that of Taal ash is about 2649. It is suggested that the eruption was probably characterized by the ejection of scandium-rich materials. Scandium may be used as supplementary evidence in evaluating an impending future Taal volcanic activity.     

Rapakivi texture from the O’Leary Porphyry,Arizona (U.S.A.)

The rhyodactic O’Leary Porphyry which forms the Pleistocene (0.233±0.37 m.y.) volcanic domes of O’Leary Peak and Darton Dome in the San Francisco Volcanic Field (northern Arizona, U.S.A.) contains sanidine phenocrysts with oligoclase mantles (rapakivi texture). Rapakivi texture occurs worldwide in silicic rocks of many ages and has been attributed to various igneous and metamorphic processes. The O’Leary Porphyry contains both mantled and unmantled sanidine (both are Or_63–69 Ab_30–36An₁), oligoclase and quartz phenocrysts, labradorite (An₅₃Ab₄₅Or₂) and kaersutite xenocrysts and andesite xenoliths. The compositional range of oligoclase is the same (An_11–26Ab_70–80Or_r–10) for the rapakivi mantles, the oligoclase phenocrysts, and the oligoclase crystals poikilitic within sanidines. Most mantles are discontinuous. The sanidine appears to have been resorbed prior to mantling. Experimental melting studies on the O’Leary Prophyry show that, for a 15 wgt.% water system, plagioclase crystallized prior to sanidine and quartz crystallized last. The O’Leary Porphyry, although inhomogeneous, plots on a Q-Or-Ab-An diagram well within the plagioclase stability field. Poikilitic plagioclases within sanidines further support crystallization of plagioclase prior to sanidine in the O’Leary Porphyry. Exsolution of a ternary feldspar to form a plagioclase mantle is the most commonly accepted igneous theory of rapakivi texture formation but has been eliminated as the origin of the O’Leary Porphyry rapakivi. Petrologic models by Tuttle and Bowen and by Stewart are rejected for the O’Leary rapakivi because of inconsistencies with the O’Leary occurrences. Two theories are viable for the O’Leary rapakivi texture. First, is a decrease in water vapor pressure which would enlarge the plagioclase stability field possibility causing mantling of metastable sanidines. The second and preferred theory is that of an addition of sodium and calcium by basification (chemical assimilation without melting) of the xenoliths within the O’Leary Porphyry. This would move the bulk composition of the melt into the plagioclase field possibly resulting in crystallization of plagioclase on sanidine crystals. Diffusion of sodium and calcium from the xenoliths to sanidine would result in mantling only those crystals near to the xenoliths. Later, convection would result in distribution throughout the melt of rapakivi, unmantled sanidines, and xenolithic kaersutite as is seen in the porphyry. Basic xenoliths are extremely common in rapakivi-bearing rocks. Those within the O’Leary Porphyry are andesitic and show resorption, and in some areas of O’Leary Peak itself, have been drawn out into schlieren.     

Boninite as a possible calc-alkalic primary magma

Boninite is an unusual, plagioclase-free magnesian andesite, occurring as vesicular pillow lavas and hyaloclastites, accompanied by andesites and dacites in Chichi-jima, Bonin Islands. The Bonin Islands belong to the Izu-Mariana arc and consist of dominant volcanic rocks and subordinate sedimentary rocks of late Oligocene-early Miocene age. The chemistry of boninite is characterized by high contents of MgO. Cr and Ni similar to primitive basalts, but apparently in ill accord with its relatively high SiO₂ content of ? 55%. The relation of SiO₂ to total FeO/MgO ratio indicates that boninite belongs to the cale-alkalic rock suite. The mineralogy of boninite consists of olivine (Fo87-90), orthopyroxene (En87-90), clinopyroxene (Wo38-35En37-44Fs25-21), hydrous glass and Cr-spinel, Experimental studies show that the magma of boninite composition could be in equilibrium with upper mantle peridotite at pressures less than 17 kb and temperatures of 1200–1050°C under high P_H2_O. It is suggested that boninite is a sea-floor quenched product (900°C) of a direct partial melt of the upper mantle. Related andesites and dacites are considered to be probably fractional crystallization products from the same magma.     

Assessment of the statistical significance of global changes in the annual river runoff in XXI century due to possible anthropogenic warming of climate

A number of uncertainties of forecasts of changes in the annual runoff depths at global scale, obtained using information on results of integration of 21 IPCC climate models is studied. Following possible errors of these forecasts are calculated: errors of models; differences between main (IPCC) scenarios of emission of greenhouse gases in the atmosphere and resultant changes of global temperatures; mistakes in estimates of average long-term observed values of the runoff depths for the “control” period. Global maps of a “significance index” of forecasted changes in the runoff depths (estimations of changes in the annual runoff depths divided by mean square root values of errors of these estimations) for 2025, 2050, 2075 and 2100 are presented. It is shown that the most significant global changes of the runoff depths (growth in the north of Eastern Siberia, of the Russian Far East, of North America, falling in the “Greater Mediterranean Region”) are predicted for the second quarter of 21st century. Further changes of the runoff amplify only in the Amazon basin (reduction, by 2075). Almost everywhere else (including almost all European territory of Russia, Western Siberia, south of Eastern Siberia and of the Far East) the significance of changes in the runoff depths during 21st century is negligible.