Magma Feeding Research Articles

Shiveluch volcano: seismicity, deep structure and forecasting eruptions (kamchatka)

The deep structure, Wadati-Benioff zone (focal zone) geometry and the magma feeding system of Shiveluch volcano are investigated based on 1962–1994 detailed seismic surveillance. A focal zone beneath Shiveluch is dipping at an angle of 70° at depths of 100–200 km. Based on the revealed interrelations between seismicity at depths of 105–120 km and an extrusive phase of its eruptions in 1980 through 1994, it is inferred that primary magmas, periodically feeding the crustal chamber, are melted at depths of at least 100 km. An upsurge of extrusive-explosive activity at the volcano is preceded and accompanied by the increasing number and energy of both volcanic earthquakes beneath the dome and tectonic or volcano-tectonic earthquakes in the zones of NW-striking crustal faults near the volcano.The eruption of April 1993 has been the most powerful since 1964. It was successfully predicted based on interactive use of all seismic data. At the same time the influence of seismicity at depths of 105–120 km under the volcano on the style (and consequently on prediction) of its activity is decisive.

The influence of regional stresses on the mechanical stability of volcanoes: Stromboli (Italy)

Abstract An elastic model was developed to study the mechanical stability of a volcanic system that includes a volcanic edifice and a magmatic feeding system (conduits and reservoirs) with axial symmetry around a vertical axis. Stability depends critically on the horizontal regional stress which comes into the problem as a boundary condition imposed at large horizontal distance. We considered three types of boundary conditions: (1) zero horizontal displacement (NHD); (2) lithostatic regional stress (LITH); (3) non-axisymmetrical regional stress corresponding to a strike-slip regime (NAS). For the third case (NAS), we used a 3-D model. The stress distributions were calculated by a numerical finite element method. The development of tensile tangential stress in the elements adjacent to the walls of conduits and reservoirs is assumed to be a sufficient condition for the instability of the plumbing system. In compression, we have adopted the Navier-Coulomb criterion for failure. We apply this model to Stromboli volcano, which is characterized by a steep volcanic edifice and open conduits filled with magma. There is geochemical evidence for a deep reservoir ( c. 10 km depth) of unknown size and indirect evidence of a shallower reservoir (at the base of the edifice) with volume of the order of 1 km 3 . The persistent explosive activity has been steady for the last 2000 years: this implies that the feeding system is mechanically stable. It is found that the deep reservoir has no influence on the stability of the edifice. With NHD boundary condition, implying that the horizontal regional stress is one-third of the vertical component, Stromboli’s volcanic system is unstable. In general, any volcanic system with an open and magma-filled plumbing system, with NHD boundary conditions, is mechanically unstable. With lithostatic regional stresses (LITH), the magmatic feeding system is mechanically stable and the slopes of the subaerial edifice are unstable, being subject to landslides. Both NHD and LITH conditions imply that the regional horizontal stress is isotropic which is not appropriate for the Southern Tyrrhenian Sea. With the NAS boundary condition, the stability of the plumbing system of Stromboli volcano allows us to place an upper limit on the maximum shear stress Δ 0 = (|σ h | max − |σ h | min )/2 For Δ 0 = 10 MPa, the magmatic feeding system is at the limit of mechanical instability. The subaerial edifice has steep slopes and it is at the limit of gravitative instability independent of the boundary conditions.

Control of granite emplacement by regional deformation

A combination of structural measurements and gravity data is used to investigate the mode of granite emplacement in various tectonic settings. The deepest zones revealed by gravity data, provided those zones also show vertical lineations in outcrop, are interpreted as magma feeder channels. These structures, or root zones, reflect the flow pattern of the upwelling magma while being emplaced. They can be interpreted within the regional stress field active during that time. Granites intruded during transcurrent shearing have only a few feeder channels. These roots are not connected to the main shear zone. All of them lie within locally extensional areas of the regional stress field. This situation is also observed in granites emplaced during a compressional phase of deformation. When emplaced during a regionally extensional phase, the granitic plutons are very thin, with many small deeper zones in which variously evolved granite types may be intrusive. These regions, vertically differentiated, are also interpreted as feeder channels. The different morphology at depth, as well as the difference in the number of root zones suggest that deformation controls the emplacement of granitic plutons. The root zones are oriented at a high angle to the direction of maximum compression when magma is emplaced in a plastic crust. If the pluton is emplaced higher in a brittle crust, then the roots are aligned with the maximum compression, according to the Andersonian theory of fracture. The orientation of the root zone along the direction of extension in a plastic crust may be explained by shearing which acts as a valve mechanism for segregating magma. In all the examples, the geometry of the root zone relative to the major fault plane implies that the shear zone does not induce magma generation and that it does not serve as a guide for magma emplacement.

Crustal xenoliths in recent hawaiites from Mount Etna, Italy: evidence for alkali exchanges during magma-wall rock interaction

Detailed mineralogical and geochemical systematics were carried out on crustal xenoliths, the host hawaiite lavas erupted from Mount Etna (Sicily, Italy) in 1892, 1986 and 1989, as well as the flyschs from the volcano basement. The xenoliths suffered morphological, structural, mineralogical and geochemical alteration, induced by thermal and chemical exchange with the enclosing lava. High-temperature reactions and partial or complete melting, as well as different mechanisms of elemental interdiffusion, took place, their exact nature depending on the type of xenolith. An early and strong mobility of K, Rb and Cs and their selective migration towards the anatectic melts, the lava envelope, was observed at both mineral phase and whole-rock scales. This is closely connected to a release of H 2O, Cl, S and CO 2, associated with the dehydration and the decarbonation of the sediments. Thus the selective migration of the K, Rb and Cs towards the enclosing lava, as well as contamination of the host lava observed seems to be induced by a selective diffusion of alkalis away from the anatectic melts and especially by their selective transfer through the action of complex interstitial and chemically active fluids. Such processes might be connected to the strong enrichment in K, Rb and Cs of the recent hawaiites from Mount Etna. They might also substantiate the hypothesis that these enrichments result from selective contamination of the magma as a consequence of the percolation or circulation of a carrier fluid phase through the upper parts of the magmatic feeding system.

Gravity anomalies, isostasy, and mantle flow at the East Pacific Rise crest

Bathymetry and gravity data obtained during a detailed Hydrosweep survey of the southern East Pacific Rise from 7°S to 9°S are used to investigate isostasy at the axis of a fast spreading mid‐ocean ridge. In particular, we examine the manner in which the bathymetric crestal high is supported and how this support varies along the axis within the 160‐km‐long 7°12'S‐8°38'S ridge segment. The crestal high stands about 400 m above the adjacent ridge flanks and has a nearly constant minimum axial depth for a distance of 140 km. The summit is broad and flat, and an axial summit caldera is present for the entire length of the ridge segment. However, the width and cross‐sectional shape of the crestal high vary systematically along the axis. It is broad with gentle slopes in the center of the segment but becomes progressively narrower and steeper toward the ends of the ridge segment. The ridge crest is marked by a free‐air gravity anomaly high about 15–20 km wide with an amplitude of 10–15 mGal relative to the ridge flanks. Mantle Bouguer anomalies vary systematically along the axis with minimum values found near the center of the segment. The axial mantle Bouguer anomalies thus do not reflect the axial depth but are correlated with changes in the cross‐sectional area of the crestal bathymetric high. The effects of cooling and subsidence away from the axis were removed from the bathymetry and free‐air anomalies to isolate residual topographic and gravity anomalies associated with the ridge crest. The residual crestal bathymetric high was modeled as a flexural feature resulting from the upward buoyant load of a region of low density material centered beneath the axis. The lithosphere was treated as a broken plate, either with a constant flexural rigidity or an effective elastic thickness Te which grows at a rate proportional to the square root of distance from the axis. The best fitting values of Te for the constant rigidity case are in the range of 0.3–0.6 km. For the growing plate model, Te increases at a rate of 0.2–0.3 km½. The gravity constrains the mass deficiency to extend to a depth of 20–30 km for both lithospheric models. We interpret this low‐density material as a region of partial melt feeding magma to the ridge axis. The best fitting density anomalies imply that a 4–9% melt fraction is present beneath the crestal high. Upwelling of melt to the axis is thus confined to a narrow zone within about 10 km of the axis. The mass deficiency and thus the upwelling partial melt are not distributed evenly along the ridge axis but rather are concentrated in the central portion of the ridge segment. It thus appears that differences between the along‐axis gravity and depth patterns observed at slow spreading and at fast spreading ridges are not the result of a change from three‐dimensional, focused upwelling at slow spreading ridges to two‐dimensional sheetlike upwelling at fast‐spreading ridges. Rather, the differences in axial gravity and depth between fast and slow spreading ridges reflect differences in the efficiency of the shallow along‐axis magma distribution.

The role of transcurrent intra‐arc tectonics in the configuration of a volcanic arc

SUMMARYField investigations show that Plio‐Quaternary Mexican and Quaternary Ecuadorian volcanic arcs are coeval with intra‐arc transcurrent or transtensional tectonics with motions along faults parallel or subparallel to the arc. This relationship can also be observed in the Quaternary Kamchatka arc. These arcs have an anomalously wide distribution of volcanic centres which cannot be explained simply by a low dip in the subducting lithosphere. Comparison with the intra‐arc tectonics and geometry of other Quaternary arcs reveals that strike‐slip fault zones, parallel to the arc, favour magma rising in a broad surficial zone and may channel magma feeding mainly huge stratovolcanoes.

Nonequilibrium flow in volcanic conduits and application to the eruptions of Mt. St. Helens on May 18, 1980, and Vesuvius in AD 79

A steady-state, one-dimensional, and nonhomogeneous two-phase flow model was developed for the prediction of local flow properties in volcanic conduits. The model incorporates the effects of relative velocity between the phases and for the variable magma viscosity. The resulting set of nonlinear differential equations was solved by a stiff numerical solver and the results were verified with the results of basaltic fissure eruptions obtained by a homogeneous two-phase flow model, before applying the model to the eruptions of Mt. St. Helens and Vesuvius volcanoes. This verification, and a study of the sensitivity of several modeling parameters, proved effective in establishing the confidence in the predicted nonequilibrium results of flow distribution in the conduits when the mass flow rate is critical or maximum. The application of the model to the plinian eruptions of Mt. St. Helens on May 18, 1980, and Vesuvius in AD 79, demonstrates the sensitivity of the magma discharge rate and distributions of pressure, volumetric fraction, and velocities of phases, on the hydrous magma viscosity feeding the volcanic conduits. Larger magma viscosities produce smaller mass discharge rates (or greater conduit diameters), smaller exit pressures, larger disequilibrium between the phases, and larger difference between the local lithostatic and fluid pressures in the conduit. This large pressure difference occurs when magma fragments and may cause a rupture of the conduit wall rocks, producing a closure of the conduit and cessation of the volcanic eruption, or water pouring into the conduit from underground aquifers leading to phreatomagmatic explosions. The motion of the magma fragmentation zone along a conduit during an eruption can be caused by the varying viscosity of magma feeding the volcanic conduit and may cause intermittent phreatomagmatic explosions during the plinian phases as different underground aquifers are activated at different depths. The variation of magma viscosity during the eruptions of Mt. St. Helens in 1980 and Vesuvius in AD 79 is normally associated with the tapping of magmas from different depths of the magma chambers. This variation of viscosity, which can include different crystal and dissolved water contents, can also produce conduit wall erosion, the onset and collapse of volcanic columns above the vent, and the onset and cessation of pyroclastic flows and surges.

Feldspar megacrysts and anorthosite xenolith-bearing dykes in the Narssarssuaq area, South Greenland

Numerous dyke intrusions are found in the Narssarssuaq area of the Gardar province, a Mid-Proterozoic intracontinental rift system. Ten to fifteen percent of these dykes, which range in composition from trachybasalt to phonolite and rhyolite, contain significant proportions of feldspar megacrysts and occasionally anorthosite xenoliths. Two groups of dykes are distinguished; the older group is more alkaline, richer in incompatible elements and contains more anorthosite xenoliths than the younger. It is probable that the main reason for the differences is variation in magma production through time and from one area to another. Chemical zonation in the dykes reflects compositional gradients in the feeding magma reservoirs; the magma reservoirs acting as open systems in which crystal fractionation was an important controlling process. The anorthosite xenoliths are not strictly cognate with their hosts, but were derived from comparable alkaline magmas with a composition roughly corresponding to the most primitive of the dykes. The plagioclase megacrysts were presumably formed at an early stage of the development of the magma chambers. Rb-Sr dating of one of the dykes from the older group of dykes gives an age of 1206 ± 20 Ma and an initial 87Sr/86Sr ratio of 0.7028 ± 0.0001 supporting a low degree of contamination with upper crustal Sr.

Volcano monitoring by microgravity and energy budget analysis

Microgravity monitoring of active volcanoes can provide evidence of sub-surface mass and/or density changes that precede eruptions. If, in addition, precursory increases in thermal emissions are observed, an integrated mechanistic model for volcanic activity may be developed with the potential for forecasting eruptions. Crater-lake volcanoes provide an interesting target for such studies since thermal output can be monitored simply through lake water calotimetry. Here we summarize 10 years of gravity and thermal data from Poás volcano, Costa Rica. Mass/energy balance calculations demonstrate that, in the steady-state, the large thermal inertia of the crater lake acts as a buffer to short-term changes in the energy input from the cooling magma feeder pipe. Since February 1986, it is postulated that there has been gradual emplacement of a shallow magma intrusion associated with vesiculation and gas loss to the surface. This follows from unambiguous, gravity increases, constrained by elevation control, that are coincident in time with a period of long-term increased energy input to the crater lake. Progressive reduction of the lake volume by evaporation/seepage culminated in an (April 1989) ash eruption providing a good documented record of combined gravity and thermodynamic precursors to volcanic activity.

Use and misuse of geophysical data to determine the shape at depth of granitic intrusions

AbstractGeological data are seldom reliable enough to quantify the shape of granitic bodies at depth. Remote sensing measurements and geophysical methods may allow hidden features inside plutons to be recognized. Seismic methods can be employed to display the interface between the granite and its sedimentary cover. Unfortunately, impedance contrasts are very low inside the intrusive body resulting in a transparency to seismic waves. In addition, the high cost of seismic lines with sufficient resolution excludes full 3D shaping. The use of magnetic methods is limited by the low content of magnetic minerals in acidic rocks and the observed magnetic field is often flat over plutons. Some high amplitude magnetic anomalies are restricted to the contact metamorphic aureoles surrounding granites. Acidic rocks generally have a lower density than the surrounding country rocks. A negative gravity signature is therefore associated with the intrusion. To assist with geological interpretation, a detailed gravity survey should be made over the pluton with a coverage density of about 1 measurement per km2. This is necessary to register the effects of facies variations. Interpretation should be restricted to 3D methods rather than 2D modelling on profiles because granitic bodies are not uniform along any one direction. However, 3D inversion of data must take into account the regional gravity field and variations of density in and outside the pluton. With these assumptions, models are presented which can be correlated with the structures acquired during the magmatic stage of pluton emplacement. The zones where magmatic foliations are vertical correlate with deep zones identified from gravity data. They are interpreted as magma feeding zones. The information provided by these methods represents a valuable contribution to determining the mode of emplacement of granitic intrusions.

Surface mercury geochemistry as a guide to volcanic vent structure and zones of high heat flow in the valley of ten thousand smokes, Katmai National Park, Alaska

A reconnaissance survey of Hg° was designed to model the 1912 Novarupta vent structure and delineate zones of near-surface high heat flow. Statistical analysis of 294 regolith samples collected at 127 sites from fossil/active fumaroles and relatively unaltered ash-flow tuff and air-fall ejecta indicates two Hg° sample populations; one associated with fumarole deposits and the other corresponding to the relatively unaltered regolith. Measured Hg° concentrations range from 12 to 6913 ppb. Sample threshold (upper limit of background Hg° concentrations; fumarole deposits: 351 ppb, unaltered regolith: 110 ppb) and contrast (sample value/threshold) determinations enable differentiation between relict and actively accumulating Hg° contents. All significant Hg° anomalies (contrast values > 1.5) occur within Novarupta Basin. High-magnitude, active Hg 2 enrichment was found on the Turtle, a dome-like feature northeast of the Novarupta extrusive dome. The Hg° data, coupled with available geologic and geophysical evidence, suggest a shallow intrusion beneath the Turtle. Results from two Novarupta Basin sample traverses do not support the presence of a near-surface magma feeder dike connecting the proposed intrusion beneath the Turtle with the Novarupta dome. Based on the Hg° data, the preferred vent model is one generated by collapse of supporting walls after the major eruptive phase into a cored-out explosive vent. Collapse of vent walls is initiated along a series of deep-extending faults which subsequently serve as migration pathways for Hg° to the surface. The overall vent morphology is funnel-like with subsidence concentrated in the narrow funnel center. Results of this study show that Hg° surveys can further constrain the morphology of the 1912 vent and aid in delineating zones of near-surface high heat flow in this region.

Volcanic tremor at Mt. Etna: State-of-the-art and perspectives

Volcanic tremor on Etna seems to have its origin within the main magma feeding system. On the basis of both spectral analyses at two permanent seismic stations and periodical measurements along the slopes of the volcano, two distinct sources are proposed. The former, characterized by low frequency contents (f<1.5 Hz), is located in a 2 km deep flat magma chamber, whereas the latter source seems to be linked to the upper part of the active vents.

Structural and volcanic expression of a fast slipping Ridge‐Transform‐Ridge‐Plate Boundary: Sea MARC I and photographic surveys at the Clipperton Transform Fault

A high‐resolution near‐bottom survey has been conducted of the Clipperton transform fault and adjoining segments of the East Pacific Rise (EPR), using the Sea MARC I side‐looking sonar system and the Lamont‐Doherty Geological Observatory Olympus‐based camera system. The transform fault zone (TFZ) is a narrow, well‐defined belt of transform‐parallel lineaments, which varies along strike from a single, sharp‐edged notch to a complex band of subparallel lineaments up to 1 km wide. The TFZ is set within a 5‐km‐wide band of unusually fine‐grained side scan texture, which could indicate nonbasaltic seafloor and/or pervasively sheared and mass‐wasted basaltic crust The fine‐grained swath is surrounded by constructional volcanic terrain with no hint of strike‐slip motion; this observation puts an upper limit of 5 km on the extent of lateral migration of the TFZ in the last 1.5 m.y. Both ridge transform intersections (RTIs) are dominated by bathymetric highs located on the old plate opposite the spreading center. A mantling of fresh‐looking constructional volcanic terrain on side scan images suggests that the highs are built in part by recent extrusive and intrusive volcanism; thermal expansion may also play a part. The EPR south of Clipperton has recently experienced extrusion of high effusion rate basalts, burial of faults and fissures by lava flows, and development of vigorous hydrothermal circulation. On the EPR north of Clipperton, the axial zone of faults and fissures tapers toward the transform fault; this may reflect a change in the shape or size of the underlying shallow level magma feeders as a function of distance from the site of magma upwelling or distance toward the transform fault.

An Archean molybdenite occurrence of possible porphyry type at Setting Net Lake, northwestern Ontario, Canada

Widespread, currently subeconomic molybdenite mineralization of possible porphyry type occurs in an oval, epizonal, late tectonic Archcan (2,643 m.y.) porphyritic granodiorite-quartz monzonite stock intruded into a sequence of mafic flows and sedimentary rocks. The stock has an area of 5.7 km 2 and is largely a single intrusive phase with an average composition of 32 percent quartz, 43 percent oligoclase, 20 percent potassium feldspar, 2.5 percent biotite and chlorite, and 2.5 percent accessory minerals. Most of the stock is medium grained (2-5 mm) with sparse, 1- to 2-cm, flow-oriented potassium feldspar phenocrysts, but a fine-grained chilled zone up to 500 m wide is preferentially developed on the north and west sides of the stock. Eccentric development of the chilled zone suggests that the stock was emplaced over an extended period of time with the main magma feeding system in the south part of the stock.The east-trending mineralized zone is confined to the more rapidly cooled, early north end of the stock and is up to 450 m wide and 2,500 m long. It consists of closely spaced, molybdenite-sheathed quartz veinlets that form a single set trending 068 degrees and dipping 75 degrees S. The veinlets have an average width of 1 to 2 mm. The average grade in the east part of the zone is 0.06 percent MoS 2 with traces of Cu. The mineralized zone is characterized by weak phyllic alteration in which oligoclase has been largely replaced by albite-sericite pseudomorphs and by minor carbonate and epidote; biotite has been replaced by chlorite and muscovite; pyrite has been introduced; and, in the west part of the zone, there has been some potassium metasomatism. Narrow alteration halos envelop some of the veinlets. Surrounding the phyllic alteration is a 1-km-wide zone of decreasing alteration intensity in which oligoclase is replaced by partial albite-sericite pseudomorphs and biotite partly chloritized.The occurrence has been classified as an Archcan porphyry deposit because of the wide-spread but low-grade mineralization, deposition of the mineralization in joints, zoned alteration of phyllic and propylitic character, and association with an epizonal porphyritic pluton.

Seismic data on magma chambers of the large tolbachik eruption

This paper discusses the methods and techniques of observation that can at present be applied to a seismic refraction study of active volcanoes with a view of determining the magma chamber location. A system of magma chambers has been outlined in the earth’s crust and transition layer between the crust and the mantle under the Tolbachik Volcano group. Magma chambers are dynamically related to each other. The feeding magma chambers of the newly-formed Tolbachik volcanoes and Plosky Tolbachik volcano are assumed to be interrelated through the transition zone between the crust and the mantle.

The Evolution of Santa María Volcano, Guatemala

Samples of 26 successive lavas from the high composite volcano of Santa María in southwestern Guatemala, demonstrate variations in magnetic declination and inclination which can be roughly-correlated with similar data from cores in the Gulf of Mexico and lake sediments near Tlapacoya, Mexico, and suggest that Santa María's constructive history goes back to approximately 30,000 years B.P. The basaltic andesites and associated pyroclastics, which make up this volcanic succession and comprise the latter 40% of the volcano's eruptive output, show systematic increases in , and Zr, and decreases in MgO and CaO, up the succession. These evolutionary changes anticipate the dacites which first appeared after a period of repose in Santa María, first as pyroclastic debris in the catastrophic flank eruption of 1902, and later as the endogenous dacite dome of Santiaguito which appeared in 1922 and has continued to grow to the present day. The increasing effects of crystal fractionation in the latter part of Santa María's history, as reflected in the compositional changes in the basaltic andesites and its culmination in the Santiaguito dacites, is thought to be due to the inhibiting effect which increasing cone height has had on the volcano's eruptive frequency through time, so progressively increasing the lengths of the repose periods when fractionation could proceed undisturbed in the top of the feeding magma column. Data from other high composite cones in Guatemala suggests that 3,500-4,200 meters represents a threshold cone height above which basaltic andesite is not erupted in this region, and it is believed that this limitation led to the historically documented dormancy of Santa María during which the Santiaguito dacites evolved. A peridotitic mantle source, rather than eclogite, for the basaltic andesites is consistent with their chemistry, provided that the primary liquids have been depleted in Ni and Cr by approximately 10% olivine and clinopyroxene fractionation.

Impact Craters and the Relative Ages of Earth and Moon

RECENT aerial photographic and ground studies of islands in the Lake of the Woods area of the Canadian Shield have revealed a series of arcuate ridges which together make up a conspicuous circular feature 16 km in diameter (Fig. 1). The appearance of these ridges suggests that they are igneous intrusions which have been constrained to conform to a circular pattern with predominantly outward dipping layers by distortions in the Earth's crust produced by an ancient meteorite impact. The ridges are composed almost entirely of gneiss or granite gneiss similar to rocks immediately outside the circle and in many other areas of the Canadian Shield. A “volcanic” phenomenon of this character could reasonably be explained by supposing that the magma feeding the intrusions was at a relatively shallow distance beneath the surface. This in turn suggests that the feature may have been formed at a very early period in the history of the Earth's crust before solidification had proceeded to any great depth.

TECTONIC FORMATION CONDITIONS OF THE TRAP INTRUSIONS OF THE SIBERIAN PLATFORM

Sills increase in dimensions and areal extent with depth in the Tychana river basin. Feeder dikes are very widely spaced in the lower sills. Discordant intrusives occur most abundantly in the shallowest rocks (massive Triassic pyroclastic rocks). These dikes have very diverse orientations and are considered to be emplaced along open fissures formed by tension during a uniaxial stretching not related to any compressive forces. The stretching also resulted in a decrease in cross-section area at depth with resulting thinning of some units. This thinning resulted in natural spaces suitable for the intrusion of sills. These were not open spaces, however. At greater depths the pressure difference between the discordant magma feeders and adjacent strata interfaces was insufficient for intrusion of sills. Certain theoretical aspects of the stretching versus compression are considered. J. A. Redden