Magma pressure discharge induces very long period seismicity

Volcano seismicity is one of the key parameters to understand magma dynamics of erupting volcanoes. However, the physical process at the origin of the resulting complex and broadband seismic signals remains unclear. Syn-eruptive very long period (VLP) seismic signals have been explained in terms of the sudden expansion of gas pockets rising in the liquid melt. Their origin is linked to a magma dynamics which triggers the explosive process occurring before the explosive onset. We provide evidence based on acoustic, thermal, and ground deformation data to demonstrate that VLP signals at Stromboli are generated at the top of the magma column mainly after the explosion onset. We show that VLP amplitude and duration scale with the eruptive flux which induces a decompression of 103–104 Pa involving the uppermost ~ 250 m of the feeding conduit. The seismic VLP source represents the final stage of a ~ 200 s long charge and discharge mechanism the magma column has to release excess gas accumulated at the base of a denser and degassed magma mush. The position of the VLP seismic source coincides with the centroid of the shallow mush plug and tracks elevation changes of the magma free surface.

Lateral variations in plagioclase compositions, Main Zone, Bushveld Complex, South Africa: Evidence for slow mixing of magmas in basinal structures

Many layered intrusions are considered to have been repeatedly inflated by magma additions, but rates of magma mixing relative to rates of layer accumulation are difficult to model. The nature of magma recharge through the interval including the Pyroxenite Marker (PM), Main Zone, Bushveld Complex, South Africa, is examined with regard to such processes. The plagioclase compositions (An value) in five previously published and three new profiles (presented here and focusing on the core compositions) that are at least 600 m in vertical extent and spread along a strike length of 110 km are evaluated. The compilation of the eight profiles shows the following trends. Upward reversals in compositions show considerable lateral as well as vertical variations. Lateral variations show a range in: (1) the minimum An value reached in each profile prior to the onset of magma recharge (An51 to An59); (2) the depth below the PM at which the minimum value is observed (50 to 575 m); (3) the An value close to the PM (An54 to An75); (4) the maximum value recorded above the PM (An63 to An76); (5) the height above the PM at which this maximum value is reached (0 to 300 m) – in all cases, the highest values of An occur at the northern end of the studied sections; and (6) the vertical extents over which the reversals occur range from 150 to over 600 m indicating very protracted magma additions and/or slow mixing. The PM terminates toward the south, and close to this termination the immediate footwall rocks to the PM change from north to south from gabbronorite to magnetite gabbronorite. A cross-section through these profiles defines two basins, with an intervening structural upwarp. The magma pulses that were added to produce very gradual and protracted reversals in mineral compositions through the PM interval ponded initially at the base of the northern basin, and did not homogenize the entire magma column. These added magmas did not overflow and have an effect on mineral compositions in the southern basin until after considerable replenishment and crystallization (including the PM) had taken place in the northern basin. We emphasize the prolonged period(s) of magma input and slow rate of vertical homogenization of the magma column during the formation of this sequence of as much as 400 m of the Main Zone.

AIRBORNE ACOUSTICS OF EXPLOSIVE VOLCANIC ERUPTIONS

A recently developed theoretical model of the airborne acoustic field from an explosive volcanic eruption of the Strombolian type is described in this article. The magma column is assumed to be a circular cylinder, which is open to the atmosphere at the top, and which opens into a large magma chamber below. The magma itself is treated as a fluid, and the surrounding bedrock is taken to be rigid. An explosive source near the base of the magma column excites the natural resonances of the conduit. These resonances result in displacement of the magma surface, which acts as a piston radiating sound into the atmosphere. The source is modeled in much the same way as an underwater explosion from a high-explosive chemical such as TNT, although in the case of the volcano the detonation mechanism is the ex-solution of magmatic gases under extremely high hydrostatic pressure. The new theory shows compelling agreement with airborne acoustic signatures that were recorded in July 1994 at a distance of 150 m from the western vent of Stromboli volcano, Italy. The theoretical and observed power spectra both display the following features: (1) four energetic peaks below 20 Hz, identified as the first four longitudinal resonances of the magma column; (2) a broad minimum around 30 Hz, interpreted as a source-depth effect, occurring because the source lay close to nulls in the fifth and sixth longitudinal resonances and thus failed to excite these modes; and (3) radial resonance peaks between 35 and 65 Hz. On the basis of the theory, an inversion of the acoustic data from Stromboli yields estimates of the depth (≈100 m) and radius (≈16 m) of the magma column as well as the depth (≈83 m), spectral shape and peak shock wave pressure (≈1 GPa) of the explosive source. Most of the parameters estimated from the acoustic inversion compare favorably with the known geometry and source characteristics of Stromboli.

Petrology, petrography, and crystallization history of intrusive phases related to the Hall (Nevada Moly) molybdenum deposit, Nye County, Nevada

Each of the two quartz monzonite porphyry intrusions that form the Hall stock contains four phases concentrically zoned from fine-grained, groundmass-rich, silicic (70–73 wt. % SiO2) phases at the top and margin toward deeper phases that are progressively coarser, more equigranular, and richer in plagioclase and biotite (68–72 wt. % SiO2). In each stock, the depthwise decrease in groundmass is not continuous but is interrupted by flow-foliated, gradational contacts (at 60–70, 50, and 10 vol. % groundmass) at or above which concentrations of quartz–molybdenite veins and (or) other SiO2-rich features are common. Magma supercooling is documented by quartz–K-feldspar dendrites and crenulate quartz layers at phase contacts. Rare sharp contacts and xenoliths document that all phases are temporally distinct, with earliest phases at the top and margin and progressively later phases inward and with depth. However, gradational contacts, concentricity of phases, and unidirectionality of textural–compositional zoning argue that each stock developed from a single magma column whose progressively inward crystallization was episodically interrupted by the release of molybdenum-bearing fluids to produce stacked orebodies. Conductive heat-loss modeling indicates that each stock took ≤ 1130 years to (i) cool to solidus temperature (740–750 °C) and (ii) form three distinct molybdenum shells. Not only is progressively deeper fluid release from concentrically zoned textural phases of a single magma column previously undocumented, but also the short cooling interval in each stock implies very rapid rates of volatile migration in these systems and thus very rapid development of vertical compositional gradients.

Two-dimensional study of a layered intrusion — the Hyllingen Series, Norway

The Hyllingen Series, comprising the southern part of the 160 km2 Caledonian, synorogenic, layered, mafic Fongen-Hyllingen intrusion, southeast of Trondheim, Norway, crystallized from a basaltic parent magma at 5–6 kb. Well developed modal layering strikes directly towards the magma chamber wall at the southern margin. The lithologies of inclusions match those of the adjacent country rock envelope. Eleven sample profiles (274 samples; over 1000 mineral analyses) allow the Hyllingen Series to be subdivided into four stages which generally decrease in thickness from north to south. A sporadically developed Stage I (< 100 m thick) of mostly unlayered ferrodiorites with relatively evolved compositions occurs at the base. Stage II (400–1500m) consists of layered, broadly ferrodioritic rocks. Modal layering is undisturbed by numerous, plate-like, metabasaltic inclusions which occupy about 22% of Stage II. Cryptic variation is slight except at the top where more evolved compositions are developed. Stage III (350–550 m) has fewer inclusions and is characterized by a gradual regression to more primitive compositions. Stage IV (800–2300 m) shows normal fractionation patterns, and a few minor reversals, with extremely evolved rocks in contact with the roof. Occasional metapelitic, platy inclusions occur near the southern margin.The rocks gradually become more evolved along the strike of modal layering towards the southern margin. Olivine and plagioclase vary systematically over about 7 km from Fo23:An46 to Fo7:An35 along the Stage II/III boundary and from Fo75:An63 to Fo13:An42 along the Stage III/IV boundary. While the apparent angle of discordance between modal and cryptic layering is usually less than 20°, in Stage IV near the wall of the magma chamber they are highly discordant (approaching 90°) over a thickness of about 900 m.In situ crystallization along an inclined floor took place from magma which became stratified by double-diffusive convection during Stage II. Modal layering developed concordant with the crystallization front while cryptic layering developed essentially parallel to the liquid stratification. Gradual influx of dense hot primitive magma caused elevation of the stratified magma column in Stage III. During this uplift, progressively more primitive liquid came into contact with earlier crystalline products along the inclined crystallization front. Crystallization during uplift of the magma column gave rise to the gradual regression of Stage III. Highly discordant modal/cryptic layering relations in Stage IV require that the magma became zoned with a horizontal, as well as a vertical, component. This may have occurred in response to lateral cooling establishing horizontal thermal gradients within individual magma layers, the resulting tendency towards density increase being compensated by material diffusion to more evolved compositions.The original roof during the early period of magma chamber evolution is possibly among the inclusions in Stage II. Repeated magma addition during Stage II resulted in the top magma layer having an extremely evolved composition. During the later stages this buoyant, evolved liquid ultimately crystallized to produce the quartz-bearing syenite in contact with the roof.

Mount Etna and the 1971 eruption - Some consideration on the magma of the 1971 eruption

Lava samples, collected periodically during the 1971 eruption of Mt Etna, have been analysed. A certain evolution of their composition has been observed: the first lavas are phonolitic tephrites, while the last ones are mugearites. This evolution can be explained by assuming a pneumatolytic differentiation in the uppermost parts of the magma column and a subtraction of femic phenocrysts by gravitational differentiation in its deeper parts, where the last products originated. Furthermore, the analyses of the 1971 lavas are compared with all available data of ancient products of this complex volcano and, particularly, with those of its historical eruptions.