Magma Properties Research Articles

Rheology of the upper mantle: some geodynamic and magmatogenic implications

Basic magmatism originates by partial melting in the upper mantle. In order to place this process in a geodynamic context, this paper focuses on the long-term rheological properties of upper mantle materials and on their lateral variations. Geotherms vary between cases where the solidus temperature of wet peridotite is not reached at any depth (e.g., Precambrian Shields), and cases where the solidus is reached not far below the Moho (e.g., continental extension zones). Given the exponential temperature dependence of the rheology, lateral temperature variations result in viscosities in the upper mantle which vary in the horizontal direction by orders of magnitude. Two factors that affect the rheological properties are degree of partial melting and volatile content. While a small (less than a few percent) amount of melting may not significantly change the steady-state rheology, the presence of volatiles can have a considerable softening effect. If conditions are favourable, melt is produced in sufficient quantities and at a sufficient rate, and migrates upwards. The most likely form of magma transport across the lithosphere is through the propagation of fluid-filled fractures. The rheological properties of both magma and host rock are well within realistic limits for this process to be an efficient mechanism of mass and heat transfer

Microgranular enclaves in the Pan-African I-type granites from the Sinai Massif: petrology, mineralogy and geochemistry

A geological, petrological and geochemical study involving mineral chemistry and whole-rock major and trace element data has been carried out on the microgranular enclaves hosted in the Sinai Massif I-type granites. Enclave specimens were collected from two separated plutons along the western margin of the Sinai Massif: the northern pluton (NP) and southern pluton (SP). Enclaves consist of monzogabbros, monzodiorites and diorites, as well as, rare monzonites, syenogabbros and syenodiorites. They are characterized by wide compositional range (48–62% SiO 2). Mineral chemistry and whole-rock chemistries both suggest that enclaves from SP and NP were derived from two to three magmas. Enclaves provide microstructural evidence pertaining to magmatic source material. The origin of present microgranular enclaves hosted in the I-type metaluminous granitoids is best explained by mixing/mingling of hot basic magmas with relatively cooler acidic magma in a plutonic environment. Substantial changes in rheological properties of magmas occur due to thermal diffusion at the interfaces of these globules and increase the undercooling rate. Magma mingling occurs instead of through mixing and homogenization. The mafic globules are dispersed randomly or concentrated into swarms by flow of the hotter magma.

On the variations of flow rate in non-explosive lava eruptions

A physical model is developed to determine factors which influence the dynamics of non-explosive lava eruptions. In the model, magma rises in a laminar regime from an overpressured chamber surrounded by elastic rock and erupts at the surface, where the accumulation of lava may alter the vent pressure. In a flow rate versus volume diagram, an eruption follows a path which is sensitive to relative changes in flow variables such as viscosity, conduit dimensions and thickness of lava over the vent. The path does not depend on the unknown elastic properties of the country rock or the chamber dimensions. With appropriate volcanological data, an observed flow rate versus volume path can be interpreted. The approach is applied to three well-documented eruptions. In each case the eruption rate was partially influenced by decreasing chamber pressure. The 1988–1990 eruption of Lonquimay (Chile) exhibits an early phase of conduit shrinkage which lasted about 100 days, probably due to magma solidification against country rock. The 1979 eruption of La Soufrière de Saint Vincent (West Indies) was initially affected by lava dome growth and later passed through a phase possibly caused by constriction of the conduit. The 1943–1952 eruption of Paricutín (Mexico) was affected by changes of lava level in the cinder cone and magma properties. In these three eruptions, the pre-eruptive chamber overpressures were similar (10–20 MPa) even though the erupted volumes differed by as much as two orders of magnitude.

New visiting scientists in NSF's Earth sciences

The National Science Foundation's Division of Earth Sciences has hired two new rotators to serve as program directors, as part of the ongoing visiting scientists program. The new directors are Jonathan Fink in Geochemistry and Petrology, and L. Douglas James in Hydrological Sciences.Fink has exchanged roles for 1 year with NSF's John Snyder, who is on sabbatical at Arizona State University. Fink's current research includes studies of how the Theological properties of magma govern the emplacement of volcanic domes and lava flows, and the gravitational control on their mass movements. This research extends to the mechanisms of igneous intrusion and interpretation of volcanic features in extraterrestrial and submarine environments.

Gravity instabilities in magma chambers: rheological modelling

The physical study of gravity instabilities in magma chambers provides a method to estimate the rheological properties of magmas. Many layered intrusions exhibit gravity instability structures, such as magmatic load casts. We present here a study based on structures occuring in veined diorites in the layered plutonic complex of Guernsey. Wavelengths of gravitational perturbations are determined by spectral analysis. We relate the wavelengths to the rheological properties of the magmas by a perturbation enhancement model. The time scale for the gravity instabilities to develop is calculated to be about 10 2 s, which is about the characteristic time scale for solidification in these rocks; this is why these structures are preserved in the rock. The calculated length and time scales are compatible with the geometry of the observed structures and the crystallization kinetics of magma chambers, showing that the structures are appropriately modeled as gravity instabilities. This mechanical approach, together with petrological study and direct computation of magma viscosity, is likely to provide several constraints on petrogenetic modelling. The fact that the observed structures can be interpreted as gravity phenomena is clear evidence that the observed layering is a primary igneous feature rather than the result of any metasomatic process.

The effect of H2O and CO2 on the viscosity of sanidine liquid at high pressures

As part of our continuing research on the nature of silicate liquids at high pressures and to provide insight into the flow properties of granitic magmas at depth, we determined the viscosity (η) of liquid KAlSi3O8, and the viscosities of liquids in the systems KAlSi3O8‐CO2 and KAlSi3O8‐H2O, as a function of pressure and temperature using the falling‐sphere method. At 1500°C, log η of KAlSi3O8 decreases from 5.0 at 1 bar to 4.36 and 3.96 at 15 and 20 kbar, respectively, consistent with the negative (∂η/∂P)T of other highly polymerized silicate liquids. However, log η isothermally increases to 4.23 at 25 kbar. Adding 0.50 wt % CO2 to the liquid produces an isothermal decrease in log η to 4.06, 3.79, and 3.41 at 15, 20, and 25 kbar and 1500°C. An equimolar amount of H2O (0.21 wt %) reduces log η to 3.45, 3.40, and 3.34 at the above conditions. At 25 kbar, 0.21 wt % H2O and 0.50 wt % CO2 produce nearly equal reductions in the viscosity of KAlSi3O8 at 1500° and 1600°C. These data indicate that H2O and CO2 disrupt the flow mechanism of KAlSi3O8 by breaking bridging‐oxygen bonds, either by reaction of the molecular volatile species with the tetrahedral network to form OH− and CO32−, or by CO2 reacting with a non‐bridging oxygen to form CO32−, which disrupts the network of the liquid by rearrangement of the local balance in charges. This is in accord with previous studies of the viscosities of H2O‐ and CO2‐bearing NaAlSi3O8 liquids. CO2 is more effective in reducing the viscosity of KAlSi3O8 relative to NaAlSi3O8 at 25 kbar, indicating that the former liquid may dissolve a larger percentage of total carbon as CO32−. This conclusion is consistent with our published feldspar‐CO2 phase equilibria. At 20 and 25 kbar and 1500°C, log η decreases to 2.30 as H2O increases to 2.00 wt %, with a highly positive (∂2(log η)/∂χ2)T,P. In contrast to KAlSi3O8‐H2O, KAlSi3O8‐CO2 exhibits a viscosity minimum between 0.50 and 1.00 wt % at 20 kbar and 1.00 and 1.50 wt % at 25 kbar, perhaps reflecting a concentration‐dependent speciation of the CO2 molecule similar to that for H2O, or a change toward non‐Newtonian, pseudoplastic behavior as the liquid becomes saturated with carbon.

Sticking to Physical Properties of Magma

Sticking to Physical Properties of Magma

Silicon Coordination and Speciation Changes in a Silicate Liquid at High Pressures

Coordination and local geometry around Si cations in silicate liquids are of primary importance in controlling the chemical and physical properties of magmas. Pressure-induced changes from fourfold to sixfold coordination of Si in silicate glass samples quenched from liquids has been detected with (29)Si magic-angle spinning nuclear magnetic resonance spectrometry. Samples of Na(2)Si(2)O(5) glass quenched from 8 gigapascals and 1500 degrees C contained about 1.5 percent octahedral Si, which was demonstrably part of a homogeneous, amorphous phase. The dominant tetrahedral Si speciation in these glasses became disproportionated to a more random distribution of bridging and nonbridging oxygens with increasing pressure.

The intensity of plinian eruptions

Peak intensities (magma discharge rate) of 45 Pleistocene and Holocene plinian eruptions have been inferred from lithic dispersal patterns by using a theoretical model of pyroclast fallout from eruption columns. Values range over three orders of magnitude from 1.6 × 106 to 1.1 × 109 kg/s. Magnitudes (total erupted mass) also vary over about three orders of magnitude from 2.0 × 1011 to 6.8 × 1014 kg and include several large ignimbrite-forming events with associated caldera formation. Intensity is found to be positively correlated with the magnitude when total erupted mass (tephra fall, surges and pyroclastic flows) is considered. Initial plinian fall phases with intensities in excess of 2.0 × 108 kg/s typically herald the onset of major pyroclastic flow generation and subsequent caldera collapse. During eruptions of large magnitude, the transition to pyroclastic flows is likely to be the result of high intensity, whereas the generation of pyroclastic flows in small magnitude eruptions may occur more often by reduction of magmatic volatile content or some transient change in magma properties. The correlation between plinian fall intensity and total magnitude suggests that the rate of magma discharge is related to the size of the chamber being tapped. A simple model is presented to account for the variation in intensity by progressive enlargement of conduits and vents and excess pressure at the chamber roof caused by buoyant forces acting on the chamber as it resides in the crust. Both processes are fundamentally linked to the absolute size of the pre-eruption reservoir. The data suggest that sustained eruption column heights (i.e. magma discharge rates) are indicators of eventual eruption magnitude, and perhaps eruptive style, and thus are key parameters to monitor in order to assess the temporal evolution of plinian eruptions.

Rheology of melts and magmatic suspensions: 1. Design and calibration of concentric cylinder viscometer with application to rhyolitic magma

Accurate rheological data for silicate melts and crystal‐melt, melt‐vapor, and crystal‐melt‐vapor suspensions of both natural compositions and simple binary and ternary systems are useful in constraining theories of momentum transport in magmatic systems and theories of melt structure. Data addressing these problems are presented in a series of papers of which this is the first. A high‐torque wide‐gap concentric cylinder viscometer has been constructed and used for characterization of the rheometric properties of rhyolitic magma under conditions of varying temperature (1100°–1350°C) and shear rate (0.05–13 s−1) at 105 Pa total pressure. Two natural rhyolitic starting materials were used in these experiments. Four independent temperature‐shear rate cycles were performed on composition RlA (∼76% SiO2) and two independent cycles on composition R2A (∼74% SiO2). Each run had a small (3–8 vol %) fraction of vapor present during viscometric testing. A power law model of the form τ = m (T) n(T) was found to fit the data. The temperature dependencies of m and n are m (т = a exp (b/T) and n (T) = c + dT, where a, b, c, and d are constants for fixed melt composition and bubble content. The viscosity function is given by η = a exp (b/T) c−1 + d T. Representative values of the constants for composition R2A are a = 5.579 ± 0.034 × 10−7 Pa s−n, b = 41491 ± 9.5 K, c = 3.039 ± 0.003 and d = −1.516 ± 0.002 K−1. The power law index coefficient n increases as temperature increases in the range 1100°–1350°C due to both volumetric expansion of vapor bubbles with increasing temperature and because of the effects of high shear rate on vapor bubble shapes. The Capillary number (Ca = ηa/ϕ), the ratio of stress tending to distort bubbles and stress tending to maintain sphericity, is high for these experiments, indicating substantial deviations from bubble sphericity. The activation energy for viscous flow, defined according to ∂1nn/∂(1/T) = b ‐ dT21nϒ, depends strongly and monotonically on shear rate. The Weissenberg effect, a phenomenon indicative of the presence of nonzero normal stresses has been observed. Melt climbs along the inner rotating rotor due to a positive first normal stress difference defined according to τΘΘ ‐ τrr = ‐Ψ1 r Θ2, where ‐Ψ1 is the first normal stress coefficient. Based on a plot of climb height Ch versus rotor angular frequency ω, ‐Ψ1 is found to be ∼2.5 × 103 Pa s2. Viscoelastic effects may be important in certain high shear rate flows of petrologic significance in which the Deborah number, defined according to De = Ψ1 /2η, is of order unity. To our knowledge, this is the first reported measurement of a normal stress coefficient for magma.

The significance of Mo for the geochemistry of the upper mantle

Northwest Africa 6693 is a new type of achondrite, with a unique combination of oxygen-isotopic composition (low Δ17O: −1.08‰; also δ17O = 1.19‰) and FeO-rich, low mg bulk composition. A mode (in vol%) shows 70% pyroxene, 16% olivine and 13% feldspar, along with 0.6% Cr-spinel, and 0.4% NiFe metal (awaruite). Its coarse-poikilitic texture, with pigeonite oikocrysts up to 14 mm, as well as the subchondritic MgO/SiO2 of the rock’s bulk composition, indicate origin as an igneous cumulate. The cumulus phases included pigeonite and olivine, and the parent magma was probably also saturated with feldspar, which occurs mainly as anhedral, yet optically continuous, grains intergrown with the pyroxene. The mafic silicates are uniformly ferroan: pigeonite near En57Wo3.2 and olivine near Fo49. The feldspar is uniformly albitic, near Ab92, except for a single tiny grain of Ab57Or43. However, the albite features diverse K/Ca (Or/An) ratios: ranging from consistently ∼0.46 in one end of the oblong NWA 6693 stone, to 5.2 in an olivine-rich enclave that consists mostly of micrographic olivine–feldspar intergrowth. Also, siderophile and incompatible element data show heterogeneity among samples from different regions of this large cumulate. The rock was probably neither an orthocumulate nor an adcumulate, and the proportion of “trapped liquid” probably varied from place to place. After initial crystallization, a shock event caused very minor brecciation, and pervasively mobilized linear-arcuate trails of microinclusions (minute oxides, mostly) and bubbles. A minor proportion of additional melt was formed within, and/or infiltrated into, the rock and formed discrete overgrowth mantles, recognizable based on unusual scarcity of microinclusions, on some pyroxenes. Final cooling, based on mineral-equilibration temperatures, occurred at a moderate rate by intrusive-igneous standards.Olivine, metal, and sulfide phases are all very Ni-rich (e.g., olivine NiO averages 0.77 wt%). Evidently the partitioning of NiO into the parent melt was extraordinarily high, which suggests a commensurately high oxygen fugacity. The V/(Al + Cr) ratio within spinel suggests that fO2 was IW + 2. The bulk-rock composition features strong depletions in sulfur and chalcophile elements, but nonvolatile lithophile elements are only subtly fractionated from chondritic. Even siderophile element concentrations are near-chondritic; Co, Ni, Ir and Os are all at 0.7–1.0 × CI chondrites, and Au at 0.55 × CI. The limited fractionation of the siderophile elements may reflect igneous processing in a parent body so pervasively oxidizing that FeNi metal did not play its usual planetary role as agent for efficient sequestration of siderophile elements. More generally, the limited fractionation among nonchalcophile, nonvolatile elements suggests that the parent melt was not produced by extensive fractional crystallization; i.e., the high FeO and low mg of NWA 6693 were probably in large measure already properties of the original primary magma, produced from an extremely oxidized (FeO-rich) variety of primitive material. NWA 6693 indicates that high oxygen fugacity inherited from oxidized-chondritic building blocks may persist within small bodies, despite melting extensive enough to engender igneous cumulates.

Prediction of the activity of FeO in multicomponent magma from known values in [SiO 2-KAlO 2-CaAl 2Si 2O 8]-FeO liquids

Using both high- and low-iron starting materials, 129 liquids are equilibrated with metallic Fe under controlled oxygen fugacity at 1600 K to contour for a FeO the geologically relevant portion of the system [SiO 2-KAlO 2-CaAl 2Si 2O 8]-FeO (SKAnF). The data show changes of a FeO - X FeO relations caused by systematic variation of Matrix composition in pseudobinaries of the type Matrix-FeO. The results clearly demonstrate that decrement of X An Matrix (= 1 − X KAlO 2 Matrix − X SiO 2 Matrix ) and increment of X KAlO 2 Matrix X SiO 2 Matrix both cause the activity coefficient ( γ FeO ) to increase. Importantly, the a FeO − X FeO curves are flattened in the same compositional range where liquid immiscibility occurs at lower temperatures. A simple empirical ideal mixing model is proposed by assuming that magma consists of a variously interconnected polymeric network that is unaffected by the substitutions of Na for K and Mn, Mg and Ca for Fe. Furthermore, the components of magma are assumed to be SiO 2, (Na, K)AlO 2, CaAl 2Si 2O 8, TiO 2 and the divalent cation oxides. The model is used to predict a FeO at 1600 K in multicomponent magma from the above data for SKAnF liquids. The excellent agreement between the calculated and measured values of the activity of FeO supports this model. The above assumptions are thus good approximations to physical reality. The predictions of the model for other properties of magma ( i.e. activities of other components, molar volumes, viscosities) are remarkably simple.

Physical Properties of Magmas at High Pressure

Physical Properties of Magmas at High Pressure

Carbon in silicate liquids: the systems NaAlSi3O8-CO2, CaAl2Si2O8-CO2, and KAlSi3O8-CO2

To further our knowledge of the effects of volatile components on phase relationships in aluminosilicate systems, we determined the vapor saturated solidi of albite, anorthite, and sanidine in the presence of CO2 vapor. The depression of the temperature of the solidus of albite by CO2 decreases from ∼30° C at 10 kbar, to ∼10° C at 20 kbar, to about 0 at 25 kbar, suggesting that the solubility of CO2 in NaAlSi3O8 liquid in equilibrium with solid albite decreases with increasing pressure and temperature. In contrast, CO2 lowers the temperature of the solidus of anorthite by ∼30° C at 14 kbar, and by ∼70dg C at 25 kbar. This contrasting behavior of albite and anorthite is also reflected in the behavior of melting in the absence of volatile components. Whereas albite melts congruently to a liquid of NaAl-Si3O8 composition to pressures of ∼35 kbar, anorthite melts congruently to only about 10 kbar and, at higher pressures, incongruently to corundum plus a liquid that is enriched in SiO2 and CaO and depleted in Al2O3 relative to CaAl2Si2O8. The tendency toward incongruent melting with increasing pressure in albite and anorthite produces an increase in the activity of SiO2 component in the liquid ( $$a_{SiO_2 }^l $$ ). We predict that this increases the ratio of molecular CO2/CO 3 2− in these liquids, but the experimental results from other workers are mutually contradictory. Because of the positive dP/dT of the albite solidus and the negative dP/dT of the anorthite solidus, we propose that a negative temperature derivative of the solubility of molecular CO2 in plagioclase liquids may partly explain the decrease in solubility of carbon with increasing pressure in near-solidus NaAlSi3O8 liquids, which is in contrast to that in CaAl2Si2O8 liquid. Also, reaction of CO2 with NaAlSi3O8 liquid to form CO 3 2− that is complexed with Na+ must be accompanied by a change in Al3+ from network-former to network-modifier, as Na+ is no longer abailable to charge-balance Al3+ in a network-forming role. However, when anorthite melts incongruently to corundum plus a CaO-rich liquid, the complexing of CO 3 2− with the excess Ca2+ in the liquid does not require a change in the structural role of aluminum, and it may be more energetically favorable. The depression of the temperature of the solidus of sanidine resulting from the addition of CO2 increases from ∼50° C at 5 kbar to ∼170° C at 15 kbar. In marked contrast to the plagioclase feldspars, sanidine melts incongruently to leucite plus a SiO2-rich liquid up to the singular point at ∼15 kbar. Above this pressure, sanidine melts congruently, resulting in a decrease in the $$a_{SiO_2 }^l $$ with increasing pressure in the interval up to ∼15 kbar. Above this pressure, the congruent melting of sanidine results in a lower and nearly constant $$a_{SiO_2 }^l $$ relative to those of albite and anorthite, and CO2 produces a nearly constant freezing-point depression of about 170° C. Because of the low $$a_{SiO_2 }^l $$ at pressures above the singular point, we infer that most of the carbon dissolves as CO 3 2− , resulting in a low CO2/ CO 3 2− , but a high total carbon content. The principles derived from the studies of phase equilibria in these chemically simple systems provide some information on the structural and thermal properties of magmas. We propose that the $$a_{SiO_2 }^l $$ is an important parameter in controlling the speciation of carbon in these feldspathic liquids, but it certainly is not the only factor, and it may be relatively less significant in more complex compositions. In addition, our phase-equilibria approach does not provide direct thermal and structural information as do calorimetry and spectroscopy, but the latter have been used primarily on glasses (quenched liquids) and cannot be used in situ to derive direct information on liquids at elevated pressures, as can our method. Hopefully, the results of all of these approaches can be integrated to yield useful results.

The dynamics of magma withdrawal from a density stratified dyke

Eruptions from the top of a dyke containing two layers of magma can selectively withdraw the upper layer, leaving the dense lower layer undisturbed. Alternatively, if the upper layer is thinner than some critical depth, d, then both layers will be tapped simultaneously. Laboratory experiments yield an equation giving the draw-up depth, d, as a function of dyke geometry, eruption rate, and magma properties. This equation is valid for low to moderate Reynolds numbers and applies to dykes which are much longer than the draw-up depth. Short dykes will yield larger draw-up depths than are predicted by the equation. A large draw-up depth is favoured when the eruption rate, upper layer magma viscosity, or dyke length/breadth ratio is large or the density difference is small. Calculations show that rhyolite-capped dykes can contain several hundred metres thickness of rhyolite when a lower layer is first tapped. Draw-up depths in a dyke are as much as an order of magnitude greater than those for an identical eruption from a large cylindrical chamber tapped by a central vent. Nonetheless, for low effusion rate eruptions from small dykes, as at Inyo Domes, California, relatively small draw-up heights are calculated (e.g. 70 m). This is compatible with the small amounts of mixed magmas found at the transition between the two rhyolite magmas erupted there [11].

The origin of small-scale geochemical and mineralogic variations in a granite intrusion

A post-tectonic unzoned granite intrusion in the Meatiq Dome, a Late Proterozoic metamorphic complex in the Central Eastern Desert of Egypt, shows significant chemical and mineralogic heterogeneity on the scale of sampling (∼5 kg). Whole rock analyses of 21 samples indicate small variations in SiO2, Al2O3, Na2O and K2O, and larger systematic variations in the less abundant major elements such as FeO, TiO2, CaO, MnO and MgO, and trace elements such as Sc, Cr, Co, Rb, Sr, Zr, La, Ce, Nd, Sm, Tb, Yb, Lu, Ta, Th and U. These variations cannot be accounted for by processes such as marginal accretion, assimilation, alteration by late stage fluids, or multiple intrusion. Instead, we proposed a model involving 35 percent solidification of the granite magma followed by partial solid-liquid segregation during emplacement, resulting in rocks containing 7–71 volume percent early-formed solids. Such randomly distributed local segregation could have been caused by filter pressing, flow differentiation, and possibly gravity segregation, either singly or in combination. Thus, each sample is interpreted as a mixture of two end-members with nearly constant compositions: an early-formed solid assemblage of crystals and a complementary residual liquid. Early formed solids are enriched in TiO2, FeO, CaO, P2O5, Sc, Co, Cr, Sr, La, Ce and depleted in SiO2, Rb, Yb, Lu, Ta, Th, and U, while the residual liquid has complementary enrichments and depletions. This simple mixing model is consistent with field and petrographic observations, experimental studies pertaining to the crystallization sequence in the system Ab-Or-Qtz-H2O at 1 Kb, and physical properties of silicic magmas. Furthermore, it is quantitatively supported by trace-element data for minerals, computed endmember compositions at 35% crystallization using mineral analyses and reasonable Kd values, and internal consistency in the percent solid for each sample computed from each of 17 elements in the inferred end-members. We suggest that this model might also apply to other small epizonal granite intrusions that show small-scale chemical heterogeneities.

Volcanic activity: a review for health professionals.

Volcanoes erupt magma (molten rock containing variable amounts of solid crystals, dissolved volatiles, and gas bubbles) along with pulverized pre-existing rock (ripped from the walls of the vent and conduit). The resulting volcanic rocks vary in their physical and chemical characteristics, e.g., degree of fragmentation, sizes and shapes of fragments, minerals present, ratio of crystals to glass, and major and trace elements composition. Variability in the properties of magma, and in the relative roles of magmatic volatiles and groundwater in driving an eruption, determine to a great extent the type of an eruption; variability in the type of an eruption in turn influences the physical characteristics and distribution of the eruption products. The principal volcanic hazards are: ash and larger fragments that rain down from an explosion cloud (airfall tephra and ballistic fragments); flows of hot ash, blocks, and gases down the slopes of a volcano (pyroclastic flows); "mudflows" (debris flows); lava flows; and concentrations of volcanic gases in topographic depressions. Progress in volcanology is bringing improved long- and short-range forecasts of volcanic activity, and thus more options for mitigation of hazards. Collaboration between health professionals and volcanologists helps to mitigate health hazards of volcanic activity.

Deformation of poorly consolidated sediment during shallow emplacement of a basalt sill, Coso Range, California

A 150-m-long, wedge-shaped unit of folded and faulted marly siltstone crops out between undeformed sedimentary rocks on the north flank of the Coso Range, California. The several-meter-thick blunt end of this wedge abuts the north margin of a basaltic sill of comparable thickness. Chaotically deformed siltstone crops out locally at the margin of this sill, and at one locality breccia pipes about one meter in diameter crosscut the sill. The sill extends about 1 km south up the paleoslope, where it merges through continuous outcrop with a lava flow that in turn extends 1.4 km to a vent area marked by more than 100 m of agglutinate and scoria. Apparently, lava extruded at this vent flowed onto unconsolidated sediments, burrowed into them, and fed a sill at about 40 m depth within the sedimentary sequence. The sill initially propagated by wedging between sedimentary beds, but eventually began to push some beds ahead of itself, forming a remarkable train of folds in the process. The sediments apparently were wet at the time of sill emplacement, because hydrothermal alteration is common near the contact between the two rock types and because the breccia pipes that crosscut the sill apparently resulted from phreatic explosions of pore water heated at the base of the cooling sill. Comparison of deformation of the host material at the Coso locality with that reportedly caused by emplacement of sills elsewhere indicates that the character of deformation differs greatly among the various localities. The specific response of host material depends upon such parameters as initial properties of magma and host material, rate of sill growth and attendant rate of strain of host material, and depth of sill emplacement. Some properties may change considerably during an intrusive-deformational episode, thus complicating accurate reconstruction of such an event.

The May 18, 1980 eruption of Mount St. Helens: 2. Modeling of dynamics of the Plinian Phase

The Plinian phase of the May 18, 1980, Mount St. Helens eruption is modeled as a steady state discharge of dacitic magma from a reservoir at 7–10 km depth at a rate of 1.94×107 kg/s. Properties of the magma, including preeruption volatile content (4.6% in the melt), temperature (920°–940° C), and confining pressure (190–250 MPa) are constrained by petrologic studies. Mass eruption rate, magma viscosity, and independent estimates of magma ascent velocity suggest a 95‐m‐diameter conduit at a depth below vapor saturation. Dispersal of pyroclasts indicate a minimum exit velocity of roughly 200 m/s during the Plinian phase. An upper limit of 330 m/s is obtained from the total amount of exsolved volatiles. Model‐derived vent diameters based on 0.1‐MPa exit pressure, petrologically inferred magma properties, and known mass eruption rate range from 105 to 135 m with a flared configuration. The calculated vent diameter, mass eruption rate, and exit velocity define conditions close to the transition between convective column rise and column collapse. These results are consistent with the sporadic generation of pyroclastic flows during a period characterized mainly by a sustained eruption column.

Contributions of Isotope Research to the Knowledge of the Distribution Coefficients of Rare Earth Elements between Natural Aluminosilicates and their Molten Phases

The distribution coefficients D of yttrium and the lanthanides between solid and liquid natural aluminosilicates can be described in terms of an extension of Schuetze's [1] incremental method for calculating oxygen isotope effects in silicate systems: log D = k + S · ΔI, where , ΔI the difference of the 18O indices of the liquid and the solid phase and k and S coefficients to be determined empirically. The coefficient k accounts for the effect of the aggregational state, particularly the water content, S accounts for the effect of the degree of linking of the (Si, Al)O4 tetrahedrons in the phases being in equilibrium on the distribution coefficients. The dependence of D, k and S on the atomic number can be understood in terms of the radii of the hydrated and not hydrated lanthanide ions. The constraints on the structural properties of magmas are discussed.