Vent Geometry Research Articles

Vent geometry and eruption conditions of the mixed rhyolite–basalt Námshraun lava flow, Iceland

We describe the morphology and circumstances of eruption of the mixed rhyolite–basalt lava flow Námshraun in the Torfajökull–Vei∂ivötn area of central Iceland. The unusual location and exposure of the elongate fissure vent permits its length along strike (a total of ∼ 275 m) and the width of the dyke feeding it (up to ∼ 10 m) to be estimated in the field. Using analyses of the heat losses during the rise of the mixed magma through the shallow part of its conduit system, we are able to refine the absolute minimum dyke width estimate to ∼ 1.5 m. The lengths of the two main lava flow lobes, assuming that their advance was cooling-limited, imply that the volume effusion rate of the lava varied between ∼ 2.7 and ∼ 1.5 m 3 s − 1 as different parts of the fissure became active. Prior to its emergence at the surface the magma had at most a small yield strength (probably significantly less than ∼ 3000–4000 Pa) and a near-Newtonian viscosity in the range ∼ 1 × 10 4 to ∼ 5 × 10 6 Pa s. After its eruption, the lava formed flows with marginal levées whose sizes imply a yield strength just less than 30 kPa. The lava in the central channels between the levées can be modeled either as a Newtonian fluid with a viscosity of between 3 × 10 7 and 6 × 10 7 Pa s or as a Bingham plastic. Estimates of the plastic viscosity from the two main flow lobes (< 10 4 to ∼ 6 × 10 5 and 1.2 × 10 7 to 1.8 × 10 7 Pa s) differ by a very large factor (at least 30) and are regarded as unreliable; however, they lead to a much smaller range of apparent viscosities, from ∼ 1.5 × 10 7 to ∼ 5.5 × 10 7 Pa s, values very similar to the viscosities found when the rheology is assumed to be Newtonian. If the field estimate of the dyke width is reliable, these results imply that the viscosity (and yield strength) of the magma averaged over the path from its source to the surface had increased by a factor close to 10 by the time that it emerged from the vent; alternatively the feeder dike may have been almost twice as wide during the eruption and relaxed to the presently exposed width as the eruption ended. The typical advance speeds of the two main flow lobes were less than 4 mm s − 1 and their emplacement times were ∼ 2.5 and ∼ 5 days. The implications for the sizes of the conduits feeding other rhyolitic and mixed lavas in central Iceland are discussed.

Instability threshold of a negatively buoyant fountain

Experimental simulations were carried out to investigate the onset of instability in negatively buoyant fountains by injecting glycerin–water mixtures into silicon oil. The transition from a stable to an unstable fountain structure is primarily governed by the Richardson number, and to a lesser extent, Reynolds number, viscosity ratio, Weber number and vent geometry. Transition nominally occurs at a Ri = 1.0. For a fountain issuing from a cylindrical pipe, the major effect of the Reynolds number is in determining whether or not the fountain is laminar or turbulent. The Reynolds number effect can be largely accounted for by basing a corrected Richardson number on the root mean square of the mean velocity. Viscosity ratio deviating from unity has the effect of stabilizing the flow structure and thereby reducing the transition Richardson number. Similarly, interfacial tension stabilizes the flow pattern resulting in a trend of increasing transition Richardson number with increasing Weber number. The results are valid in rectangular vents if the Richardson number and Reynolds number are based on the hydraulic diameter.

Volcanism in Iceland in historical time: Volcano types, eruption styles and eruptive history

Abstract The large-scale volcanic lineaments in Iceland are an axial zone, which is delineated by the Reykjanes, West and North Volcanic Zones (RVZ, WVZ, NVZ) and the East Volcanic Zone (EVZ), which is growing in length by propagation to the southwest through pre-existing crust. These zones are connected across central Iceland by the Mid-Iceland Belt (MIB). Other volcanically active areas are the two intraplate belts of Oraefajokull (OVB) and Snaefellsnes (SVB). The principal structure of the volcanic zones are the 30 volcanic systems, where 12 are comprised of a fissure swarm and a central volcano, 7 of a central volcano, 9 of a fissure swarm and a central domain, and 2 are typified by a central domain alone. Volcanism in Iceland is unusually diverse for an oceanic island because of special geological and climatological circumstances. It features nearly all volcano types and eruption styles known on Earth. The first order grouping of volcanoes is in accordance with recurrence of eruptions on the same vent system and is divided into central volcanoes (polygenetic) and basalt volcanoes (monogenetic). The basalt volcanoes are categorized further in accordance with vent geometry (circular or linear), type of vent accumulation, characteristic style of eruption and volcanic environment (i.e. subaerial, subglacial, submarine). Eruptions are broadly grouped into effusive eruptions where >95% of the erupted magma is lava, explosive eruptions if >95% of the erupted magma is tephra (volume calculated as dense rock equivalent, DRE), and mixed eruptions if the ratio of lava to tephra occupy the range in between these two end-members. Although basaltic volcanism dominates, the activity in historical time (i.e. last 11 centuries) features expulsion of basalt, andesite, dacite and rhyolite magmas that have produced effusive eruptions of Hawaiian and flood lava magnitudes, mixed eruptions featuring phases of Strombolian to Plinian intensities, and explosive phreatomagmatic and magmatic eruptions spanning almost the entire intensity scale; from Surtseyan to Phreatoplinian in case of “wet” eruptions and Strombolian to Plinian in terms of “dry” eruptions. In historical time the magma volume extruded by individual eruptions ranges from ∼1 m3 to ∼20 km3 DRE, reflecting variable magma compositions, effusion rates and eruption durations. All together 205 eruptive events have been identified in historical time by detailed mapping and dating of events along with extensive research on documentation of eruptions in historical chronicles. Of these 205 events, 192 represent individual eruptions and 13 are classified as “Fires”, which include two or more eruptions defining an episode of volcanic activity that lasts for months to years. Of the 159 eruptions verified by identification of their products 124 are explosive, effusive eruptions are 14 and mixed eruptions are 21. Eruptions listed as reported-only are 33. Eight of the Fires are predominantly effusive and the remaining five include explosive activity that produced extensive tephra layers. The record indicates an average of 20–25 eruptions per century in Iceland, but eruption frequency has varied on time scale of decades. An apparent stepwise increase in eruption frequency is observed over the last 1100 years that reflects improved documentation of eruptive events with time. About 80% of the verified eruptions took place on the EVZ where the four most active volcanic systems (Grimsvotn, Bardarbunga–Veidivotn, Hekla and Katla) are located and 9%, 5%, 1% and 0.5% on the RVZ–WVZ, NVZ, OVB, and SVB, respectively. Source volcano for ∼4.5% of the eruptions is not known. Magma productivity over 1100 years equals about 87 km3 DRE with basaltic magma accounting for about 79% and intermediate and acid magma accounting for 16% and 5%, respectively. Productivity is by far highest on the EVZ where 71 km3 (∼82%) were erupted, with three flood lava eruptions accounting for more than one half of that volume. RVZ–WVZ accounts for 13% of the magma and the NWZ and the intraplate belts for 2.5% each. Collectively the axial zone (RVZ, WVZ, NVZ) has only erupted 15–16% of total magma volume in the last 1130 years.

The Fekete-hegy (Balaton Highland Hungary) “soft-substrate” and “hard-substrate” maar volcanoes in an aligned volcanic complex – Implications for vent geometry, subsurface stratigraphy and the palaeoenvironmental setting

The Fekete-hegy volcanic complex is located in the centre of the Bakony Balaton Highland Volcanic Field, in the Pannonian Basin, which formed from the late Miocene to Pliocene period. The eruption of at least four very closely clustered maar volcanoes into two clearly distinct types of prevolcanic rocks allows the observation and comparison of hard-substrate and soft-substrate maars in one volcanic complex. The analyses of bedding features, determination of the proportion of accidental lithic clasts, granulometry and age determination helped to identify and distinguish the two types of maar volcanoes. Ascending magma interacted with meteoric water in karst aquifers in Mesozoic carbonates, as well as in porous media aquifers in Neogene unconsolidated, wet, siliciclastic sediments. The divided basement setting is reflected by distinct bedding characteristics and morphological features of the individual volcanic edifices as well as a distinct composition of pyroclastic rocks. Country rocks in hard-substrate maars have a steep angle of repose, leading to the formation of steep sided cone-shaped diatremes. Enlargement and filling of these diatreme is mainly a result of shattering material by FCI related shock waves and wall-rock collapse during downward penetration of the explosion locus. Country rocks in soft-substrate maars have much shallower angles of repose, leading to the formation of broad, bowl shaped structures or irregular depressions. Enlargement and filling of these diatremes is mainly the result of substrate collapse, for example due to liquefaction of unconsolidated material by FCI-related shock waves, and its emplacement by gravity flows. The Fekete-hegy is an important example illustrating that the substrate of a volcanic edifice has to be taken into account as an important interface, which can have major control on phreatomagmatic eruptions and the resulting characteristics of the volcanic complex.

Eruptive dynamics and caldera collapse during the Onano eruption, Vulsini, Italy

The Onano explosive eruption of the Latera Volcanic Complex (Vulsini Volcanoes, Quaternary potassic Roman Comagmatic Region, Italy) provides an interesting example of multiple changes of eruptive style that were concomitant with a late phase of collapse of the polygenetic Latera Caldera. This paper reports a reconstruction of the event based on field analysis, laboratory studies of grain size and density of juvenile clasts, and re-interpretation of available subsurface geology data. The Onano eruption took place in a structurally weak area, corresponding to a carbonate substrate high bordered by the pre-existing Latera caldera and Bolsena volcano-tectonic depression, which controlled the ascent and eruption of a shoshonitic-phonotephritic magma through intersecting rim fault systems. Temporal changes of magma vesiculation, fragmentation and discharge rate, and consequent eruptive dynamics, were strongly controlled by pressure evolution in the magma chamber and changing vent geometry. Initially, pumice-rich pyroclastic flows were emplaced, followed by spatter- and lithic-rich flows and fallout from energetic fire-fountaining. The decline of magma pressure due to the partial evacuation of the magma chamber induced trapdoor collapse of the chamber roof, which involved part of the pre-existing caldera and external volcano slopes and eventually led to the present-day caldera. The widening of the vent system and the emplacement of the main pyroclastic flow and associated co-ignimbrite lag breccia marked the eruption climax. A sudden drop of the confining pressure, which is attributed to a pseudo-rigid behaviour of the magma chamber wall rocks during a phase of rapid magma drainage, led to extensive magma vesiculation and fragmentation. The disruption of the magma chamber roof and waning magma pressure in the late eruption stage favoured the explosive interaction of residual magma with groundwater from the confined carbonate aquifer. Pulsating hydrostatic and magma pressures produced alternating hydromagmatic pyroclastic surges, strombolian fallout and spatter flows.

Predicting Airflow Rates in the Coarse Layer of Passive Dry Barriers

Laboratory experiments were conducted with steady wind velocities to measure pressure drops in pipes used to ventilate passive dry barriers. Two vent pipe diameters (100 and 150 mm) and several pipe end geometries were tested. Nondimensional ventilator equations were developed using the pressure drop data that, when combined with Darcy’s law, can be used to estimate airflow rates in passive dry barriers. A field experiment was conducted to verify the laboratory results. Comparisons were also made between flow rates predicted using the airflow rate equations and flow rates measured in two field-scale tests of dry barriers, one of which was constructed by the writers. Results of the laboratory tests indicate that a unique ventilator equation exists for each pipe end geometry, and that more than one equation may exist if the pipe end is not symmetrical (i.e., the pressure drop depends on wind direction relative to the pipe end). The field experiment indicated that the equations developed based on the laborat...

Development of Large Volcanoes on Venus: Constraints from Sif, Gula, and Kunapipi Montes

Sif, Gula, and Kunapipi Montes are large volcanoes on Venus. They have differing morphologies reflecting their different eruptive histories and plumbing systems. All have a steeper upper cone surrounded by more shallowly sloping flanks, interpreted as resulting from histories of more frequent, shorter, lower effusion rate summit eruptions accompanied by infrequent, more voluminous flank eruptions. All three volcanoes have multiple flow field types (digitate, sheet, and fan), which probably reflect variations in effusion rate, eruption duration, and vent geometry. Sif Mons' summit has a large caldera, Gula Mons has summit calderas at either end of a summit rift, and the summit of Kunapipi Mons is composed of two volcanic centers on a summit plateau. The summit morphologies of Sif, Gula, and Kunapipi Montes are interpreted to reflect differences in magma chamber location over time, and differing internal plumbing. We find no positive evidence that neutral buoyancy has governed magma chamber position. Instead, magma chambers at the three volcanoes appear to have changed position over time, with lithospheric stresses as well as neutral buoyancy affecting chamber location. All three volcanoes have had episodes of diking at their summits, followed by episodes of extrusion. We interpret this to be caused by variations in either magma supply rate or magma volatile content, or both, over time. Fractures on the flows of the volcano indicate that dike intrusion has occurred throughout the visible history of the volcanoes, indicating that the local stress field within the volcano favored intrusion as well as extrusion over much of its history. The overall shape and history of the venusian volcanoes is similar to the histories of terrestrial volcanoes. However, some aspects of these volcanoes, including the rift at Gula, the large size and very long flows, differ significantly from terrestrial volcanoes.

Numerical modelling of the transition from fissure- to central-type activity on volcanoes: a case study from Salina Island, Italy

The evolution of the Rivi–Capo volcanic complex (Aeolian Archipelago) is characterised by a two-stage activity. Early phases of activity developed along a 1.5 km long eruptive fissure, whereas the later phase occurred from a central conduit. Geological and structural data indicate that the transition from fissure- to central-type activity was almost continuous in time, and no changes in the composition of the erupted products has been observed. Such a transition cannot be explained as a consequence of (a) flow localisation along a dike due to viscosity variation, or (b) emplacement of a magma reservoir by intersecting propagating cracks, or (c) re-orientation of the stress within the volcano due to gravity processes. In order to explain the change in vent geometry, a finite-element model has been developed which, using reliable parameters for basaltic rocks, accounts for a progressive melting of the host rocks enhanced by the flow of hot magma in the partially molten region. The proposed model allows one to explain the transition from fissure to central vent activity observed at Rivi–Capo as well as on other volcanoes.

Modeling of air back pressure in die-casting dies

In high-pressure die-casting, molten metal is injected into a die cavity at a high velocity. The metal entering into the cavity expels the air from the cavity through the vents. Air back pressure will be created when the volume of metal flow exceeds the volume of air escaping out of the die cavity. A new mathematical model is proposed for the calculation of the creation of the back pressure in a die cavity. Air discharge through vents is modeled by the Poiseuille flow. The differential equation of the time rate of change of the back pressure is derived. The creation of the back pressure is correlated with the configuration sizes of the vent, and computations are carried out for different configurations of the vent geometry.

Eruption of lava flows on Europa: Theory and application to Thrace Macula

Europa, on the basis of its spectral characteristics and its apparently young surface age, is likely to be at least partially resurfaced by liquid water volcanic processes; however, convincing evidence for lava flow morphology in Voyager data has not previously been found, and the lower density of liquid water relative to an overlying solid water ice crust suggests that extrusive volcanism could be uncommon. We examine a candidate lava‐flow‐like feature (Thrace Macula) and compare its characteristics with theoretical predictions for the ascent and eruption of water magma on Europa, outlining a range of conditions under which water ice magma could plausibly be erupted to the surface. Thrace Macula (45°S, 171°W) is a dark lobate feature extending NNW to SSE; its northern part is centered in a 140 km diameter arcuate to circular lineament, and within this zone are radial and concentric lineaments and three dark lobes similar in appearance to flow lobes extending away from a central region for distances of 40, 50, and 160 km. The 160 km lobe appears to follow local topographic features, producing lobate margins at subdued structures, broadening locally parallel to structural trends, and terminating against a gray band. Models for the present structure and state of the predominantly H2O exterior of Europa range from solid water ice to a thin ice layer overlying a liquid water layer, and these conditions may even vary as a function of space and time. Ascent and eruption of H2O magma through a less dense water ice crust means that sustained effusive eruptions are very difficult to produce if a globally continuous liquid water (or salt‐water) layer exists [e.g., Crawford and Stevenson, 1988; Kargel, 1991]. For flows as extensive as the Thrace Macula feature, we thus require magma source regions sufficiently localized that excess pressure can be built up in the source region. Given the elastic properties of ice at these source depths, the most likely scenario is intrusion of silicate magma into the base of an ice crust, and melting and mobilization of water magma. With this range of magma source settings, we model plausible surface vent geometries for a water lava flow interpretation of Thrace Macula. Modeling calculations show that flow lobes with the above geometries, and thicknesses on the order of 10 m, could be formed in water eruptions on Europa involving discharge rates of ∼2×105 m3/s through dikes with widths in the range 5–10 m. These discharge rates (about 3 times larger than those of terrestrial flood basalts) should not be unusual for water eruptions on Europa under these conditions and suggest that water volcanic resurfacing could be common on the satellite if source regions meeting these criteria were readily available. The lack of abundant similar dark flows could be due to brightening with time or might imply that conditions for their formation (grounded or nearly solid ice crust) only evolved in the latter part of Europan history.

Pyroclastic phases of a rhyolitic dome-building eruption: Puketarata tuff ring, Taupo Volcanic Zone, New Zealand

The 14 ka Puketarata eruption of Maroa caldera in Taupo Volcanic Zone was a dome-related event in which the bulk of the 0.25 km3 of eruption products were emplaced as phreatomagmatic fall and surge deposits. A rhyolitic dike encountered shallow groundwater during emplacement along a NE-trending normal fault, leading to shallow-seated explosions characterised by low to moderate water/magma ratios. The eruption products consist of two lava domes, a proximal tuff ring, three phreatic collapse craters, and a widespread fall deposit. The pyroclastic deposits contain dominantly dense juvenile clasts and few foreign lithics, and relate to very shallow-level disruption of the growing dome and its feeder dike with relatively little involvement of country rock. The distal fall deposit, representing 88% of the eruption products is, despite its uniform appearance and apparently subplinian dispersal, a composite feature equivalent to numerous discrete proximal phreatomagmatic lapilli fall layers, each deposited from a short-lived eruption column. The Puketarata products are subdivided into four units related to successive phases of:(A) shallow lava intrusion and initial dome growth; (B) rapid growth and destruction of dome lobes; (C) slower, sustained dome growth and restriction of explosive disruption to the dome margins; and (D) post-dome withdrawal of magma and crater-collapse. Phase D was phreatic, phases A and C had moderate water: magma ratios, and phase B a low water: magma ratio. Dome extrusion was most rapid during phase B, but so was destruction, and hence dome growth was largely accomplished during phase C. The Puketarata eruption illustrates how vent geometry and the presence of groundwater may control the style of silicic volcanism. Early activity was dominated by these external influences and sustained dome growth only followed after effective exclusion of external water from newly emplaced magma.

Electronic cabinet cooling by natural convection: Influence of vent geometry

An experimental investigation on the natural air cooling of vertical circuit cards, packaged within a ventilated enclosure has been carried out. Tests have been performed on a box provided with two vents and containing six uniformly heated circuit boards. The size and shape of each opening have been examined and their influence on the cooling efficiency of the box has been investigated. In particular three differently shaped openings have been used: circulars holes, vertical slots, and horizontal slots. For each case, the coefficient of friction loss has been evaluated as a function of the Reynolds number by mean of a simple relationship which takes into account the parameters of the volumetric air flow rate through the box.

A study of carbon monoxide and smoke yields from compartment fires with external burning

Hexane fires were burned in a 2.2 m 3 compartment with a window-style exhaust vent to study the effects of external burning on carbon monoxide and smoke yields downstream of a fire compartment. External burning was observed to occur in several modes: 1) intermittent flashes, 2) bursts that lasted for only a few seconds and 3) sustained external burning. Results showed that the flammability of the compartment fire effluent was a function of the equivalence ratio and that distinguishable equivalence ratios exist that determined which mode of external burning can be obtained for a given compartment fire. Results also showed that the reduction of carbon monoxide and smoke only occurred in the presence of sustained external burning. At plume equivalence ratios above 1.7, sustained external burning always occurred and the downstream carbon monoxide yield was reduced to 10 to 25 percent of the upper layer yield and the downstream smoke yield was reduced to approximately 0 to 50 percent of the level observed prior to sustained external burning. For fires with no sustained external burning, carbon monoxide yields downstream of the compartment fire were the same as upper layer yields even when flashes or short bursts of external burning occurred. Beyler's ignition criterion for layer burning was applied to the mixing of the hot fuel-rich, upper layer gases and ambient air outside of the fire compartment. The results suggest that the ignition index predicts the potential for external burning at equivalence ratios of 1.2 and greater. However, the occurrence of external burning is dependent on the presence of an ignition source which is dictated by the vent geometry and fire size.

Fluid volcanism on Miranda and Ariel: Flow morphology and composition

Several types of volcanic units have been recognized on the icy Uranian satellites Miranda and Ariel. On Miranda, ridges characterized by crest grooves are up to 10 km wide and 500 m high. A continuous flat‐topped flow band also 10 km wide and 500 m high forms the outer southern margin of Elsinore Corona. Elsinore Coronae at least appears to comprise coalesced flow bands and ridges. On Ariel, in addition to at least one ridge unit similar to these on Miranda, flood plains material has covered the floors of deep chasmata (grabens) and an irregular depression. Flows on both satellites are characterized by linear vent geometries and distinct topographic margins, which indicate extrusion of a relatively viscous material. The topography of the flows can be used to estimate flow viscosity or yield strength using a Bingham plastic model. Extrusion viscosity estimates, incorporating plausible volcanologically based emplacement time scales and a rigid crust correction, range from 107 to 109 P (1013 P in the unlikely absence of a chilled crust). Viscosity estimates are dependent on the assumed emplacement time scale, however, and could be as high as 1016 P, if a solid‐stale‐based time scale is assumed. Bingham models indicate that it is yield strength that controls the final flow width and thickness that we see. Yield strength estimates are probably more reliable because of uncertainties in the viscous flow time scale. Yield strength estimates range from 10−1 to 10−2 bars, consistent with yield strengths of partially crystallized silicate melts on other planets. Nevertheless, both the viscosities and yield strengths are well within the range of (and in some cases lower than) silicate flows. Silicate compositions are not possible for these flows because of the ice‐rich composition of these satellites. Measured properties of ammonia‐water liquid‐crystal mixtures are consistent with the observed and estimated flow rheologic properties, however, and extrusion of such material(s) is a viable alternative to solid‐state extrusion for the origin of these flow features.

Volcanic and structural morphology of the south flank of Axial Volcano, Juan de Fuca Ridge: Results from a Sea MARC I side scan sonar survey

A 1500 km2 Sea MARC I side scan sonar survey south of Axial volcano investigated the geological structure and constitution of the South Axial Rift Zone (SARZ) and the northern 30 km of the Vance spreading segment. Relative age assignments based on structural relationships indicate that recent volcanism on the southern flank of Axial volcano has been restricted to the SARZ. The surficial volcanic morphology of the SARZ changes downrift, from linear volcanic ridges (north) to small (1‐km diameter) cratered cones (south), perhaps indicating a variation in eruptive vent geometry from fissures to point sources downrift. Recent lavas on the northern and central SARZ erupted along preexisting faults in the underlying crust. By acting as pathways for SARZ magmas, these faults may have controlled the orientation of SARZ volcanic ridges and perhaps the orientation of the entire SARZ edifice. The Vance spreading segment terminates at 45°40′N and is not linked with a transform fault. The axis of spreading between 45°40′N and 46°00′N is offset west of the Vance segment and is constrained to lie between 129°54′W and 130°06′W. A discrete, structurally defined spreading axis, however, is not evident over the southern flank of Axial volcano. The divergent plate boundary may now underlie the SARZ massif, although South Helium Basin (an embayment in the southeast flank of Axial volcano) has undergone, and may continue to accommodate, crustal extension. A 48 km2 lava field discovered east of the SARZ is composed of lavas that are inferred to have erupted from the SARZ. These lavas can be traced eastward from the SARZ into the axial valley of the northern Vance segment and partially fill the valley with lavas up to 60 m thick. The eruption of these lavas, which occupy an estimated volume of 1.8 km3, may have contributed to the formation of Axial volcano's summit caldera.

RÜZGAR HIZI VE YÖNÜ İLE HAVA ÇIKIŞ AÇIKLIĞI TİPİNİN HAVALANDIRMAYA OLAN ETKİStNÎN KAPALI SIĞIR BARINAĞI MODELİNDE İNCELENMESİ ÜZERİNE BİR ARAŞTIRMA

OZET Bu arastirma, 30 baslik kapali ve durakli sut sigiri ahirinin 1/20 olcekli modeli uzerinde yapilmistir. Ruzgar hizi ve yonu ile farkli 4 tipte planlanan mahya hava cikis acikliklarinin model ici hava dagilimina ve havalandirma miktarina olan etkisi arastirilmistir. Arastirma laboratuvar kosullarinda yapilmis olup ruzgar tunelinden saglanan degisik yonlerden esen farkli ruzgar hizlari ile mahya hava akim hizlan ve model ici ve disi arasindaki sicaklik farklariyla olan iliskileri belirlenmistir. Arastirmadan elde edilen sonuclara gore, mahya acikliginin sekli, ruzgar yonu ve yapinin konumu mahya hava akim hizini onemli olcude etkilemektedir. Dikdortgen orifis seklinde planlanan mahya aciklik tipi, diger tiplere oranla daha uygun havalandirma saglamaktadir. RIDGE VENT, WIND DIREKTION AND WIND VELOCITY EFFECTS ON CLOSED, NATURALLY VENTTLATED, CATTLE BUILDlNG VENTILATION SUMMARY Naturally ventilated buildings used for cattle production hava various ridge vent configurations. Information pertaining to the ventilation characteristics ofdifferent geometrically shaped ridge vent is limited. Therefore, the effects ofwind speed, wind direction and ridge vent shape were studied in a model (1/20 scale) on ventilation characteristics, naturally ventilated, dairy cattle building. A simple wind tunnel was built to develop an airflow pattern. Wind was simulated by variable speedfans. Wind speed varied between 0 to 7 mJs. The construction of the model was based on a geometric lenght scale of 20, and the assumption was t hat the samefluid and material would be used in the model and prototype, Trusses, purlins, poles, sides and roof were made of wood. The end walls were constructed with plexiglass for the purpose ofvisual observation of airflow patterns. The four ridge vents were constructed by using galvanized metal. Principles of similitude and dimensional analysis were utilized in deriving a set of independent and dependent Pi terms, From the defination of the Buckingham Pi Theorem a set of14 dimensionless Pi terms were obtained (Langhaar, 1983). The sdudy descr 'ibed in this paper was conducted to determine which type of ridge vent would be suitable for naturally ventilated dairy barns in cold regions. Resuits indicated that ridge vent design has a significant effect on outled velocity and wind velocity. Temperature difference was affected by ridge vent geometry and vind velocity. The baffled (Type II) ridge vent had the highest outlet velocity when ali wind directions were considered, while covered (Type III) ridge vend produced the lowest outlet velodties. AUhough the covered ridge vent produced the largest temperature differences, the associated low outlet velodties make it the least desirable of the four ridge vents tested.

The May 1915 eruptions of Lassen Peak, II: May 22 volcanic blast effects, sedimentology and stratigraphy of deposits, and characteristics of the blast cloud

Abstract The May 22, 1915 eruptions of Lassen Peak involved a volcanic blast and the emplacement of three geographically and temporally distinct lahar deposits. The volcanic blast occurred when a Vulcanian explosion at the summit unroofed a shallow magma source, generating an eruption cloud that rose to an estimated height of 9 km above sea level. The blast cloud was probably caused by the collapse of a small portion of the eruption column; absence of a flank vent associated with these eruptions argues against it originating as an explosion that has been directed by vent geometry or location. The volcanic blast devasted ∼7 km2 of the northeast flank of the volcano, and emplaced a deposit of juvenile tephra and accidental lithic and mineral fragments. Decrease in blast deposit thickness and median grain size with increasing distance from the vent suggests that the blast cloud lost transport competence as it crossed the devastated area. Scanning electron microscope examination of pyroclasts from the blast deposit indicates that the blast cloud was a dry, turbulent suspension that emplaced a thin deposit which cooled rapidly after deposition. Lahar deposits were emplaced primarily in Lost Creek, with minor lahars flowing down gullies on the west, northwest and north flanks of the volcano. The initial lahar was apparently triggered early in the eruption when the blast cloud melted the residual snowpack as it moved down the northeast flank of the peak. The event that triggered the later lahars is enigmatic; the presence of approximately five times more juvenile dacite bombs on the surface of the later lahars suggests that they may have been triggered by a change in eruption style or dynamics.

Mixed deposits of simultaneous strombolian and phreatomagmatic volcanism: Rothenberg volcano, east Eifel volcanic field

Abstract The Pleistocene basanite-tephrite Rothenberg cone complex in the East Eifel was constructed by alternating dominantly Strombolian (S1–3) and dominantly phreatomagmatic (P1–3) phases of volcanism along a NNE-SSW linear vent system. Strombolian eruptions, from the central vent of the S1 scoria cone, and phreatomagmatic eruptions, from a vent on the southern margin of the cone, occurred simultaneously during the second phreatomagmatic phase (P2). The P2 deposits are a complex sequence in which Strombolian fallout ejecta is intimately admixed with phreatomagmatic fallout and pyroclastic surge material. Every bed contains at least trace amounts of ejecta from both sources but, at every site, an alternation of Strombolian-dominant and phreatomagmatic-dominant units is recorded. Each bed also shows marked lateral changes with a progressive northward increase in the proportion of Strombolian material. The two eruptive styles produced morphologically distinct clast populations often with widely separated (5–7 φ) grain size modes. The phreatomagmatic component of the P2 deposits is inferred to be the result of shallow interaction of external water and cool, partially degassed magma which reached the surface at a time when the magma column was retreating from the northern Strombolian central vent. The Rothenberg deposits illustrate the complexity and sensitivity of controls on Strombolian and associated phreatomagmatic volcanism, and the shallow depth of fragmentation during such eruptions. During such shallow eruptions minor, ephemeral and localised variations in the rate of rise and discharge of magma, and vent geometry and hydrology significantly influence the magma:water ratio and hence eruptive style.

Ascent of magma in a deformable vent

The ascent of magma in a deformable cylindrical vent is analyzed to reveal the character and extent of mechanical disturbances in vent geometry and associated effects on the magma ascent process. Both magma and surrounding country rock are assumed to behave as Newtonian fluids. According to the analysis, any disturbance involving the deformation of conduit is transmitted upward at approximately same speed as magma motion, while the disturbance hardly influences the state below. In particular, irregular vent geometry containing a bump or a constriction propagates upward like a wave, being accompanied by a pressure dipole that is composed of a pair of positive and negative pressure deviations. Since magma flux is strongly controlled by the pressure below, the persistence of high magma pressure in the magma reservoir favors expansion of the vent and can lead to a volcanic eruption. On the contrary, a low magma pressure tends to result in the closure of the conduit above. Therefore if the effective viscosity of the country rock is sufficiently low as might be the case when it is partially molten, a vent can be flexibly opened or closed in response to pressure in the magma reservoir.

The roots of ash flow calderas in western North America: Windows into the tops of granitic batholiths

Large‐volume ash flow eruptions and associated caldera collapses provide a direct link with subvolcanic granitic plutons of batholithic dimensions. The eruptive history, structural features, and petrologic evolution of ash flow calderas provide data on early stages of the evolution of an associated subvolcanic magmatic system. Broadly cogenetic, erosionally unroofed granitic plutons provide a record mainly of the late stages of emplacement and crystallization of silicic magmas. This review summarizes features of well‐studied calderas and ash flow volcanic fields in western North America, exposed at advantageous levels where both remnants of a Volcanic sequence and upper parts of the cogenetic intrusion are preserved, in comparison with similar rocks elsewhere in the worjd. Primary examples include San Juan, Mogollon‐Datil, Marysvale, Latir‐Questa, Chiricahua‐Turkey Creek, Challis, and Boulder Batholith‐Elkhorn Mountains. Most ash flows have erupted from sites of preceding volcanism that records shallow accumulation of caldera‐related magma. Structural boundaries of calderas are single ring faults or composite ring fault zones that dip vertically to steeply inward; outward dipping boundary faults favored by some models have not been identified in North American calderas. The area and volume of caldera collapse are roughly proportional to the amount of erupted material. Pyroclastic eruptions of relatively small volume (less than 50–100 km3) may cause incomplete hinged caldera subsidences or structural sags; larger systems are bounded by complete ring faults. Few ash flow vent structures have been related to major calderas; vent geometry, as determined by size analyses of pyroclastic materials, may shift complexly during caldera collapse. Scalloped topographic walls beyond the structural boundaries of most calderas are due to secondary gravitational slumping during subsidence. Most exposed floors are a structurally coherent plate or cylinder bounded by a ring fault or dike, indicating pistonlike caldera collapse; chaotically brecciated floors predicted by models of piecemeal collapse have not been identified. Deviations from circular shape commonly reflect influence of regional structures; some calderas in extensional terranes are elongate in the direction of extension. Large calderas (greater than 100 km3 of erupted material) collapse concurrently with eruption, as indicated by thick intracaldera ash flow fill and interleaved collapse slide breccias. Volumes of intracaldera and outflow tuff tend to be subequal; correlation between them is commonly complicated by contrasts in abundance and size of phenocrysts and lithic fragments, degree of welding, devitrification, alteration, and even chemical composition of magmatie material. Postcollapse volcanism may occur from varied vent geometries within ash flow calderas; ring vent eruptions are most common in resurgent calderas, reflecting renewed magmatic pressure. Large intrusions related to resurgence are exposed centrally within some calderas; ring dikes and other intrusions along bounding ring fractures are especially common in alkalic igneous systems in extensional environments. Subvolcanic magma chambers of calc‐alkaline affinities associated with plate‐convergent tectonic settings may rise to such high levels that deep cauldron subsidence structures are obliterated. Resurgence within calderas may result in a symmetrical dome or more geometrically complex forms; resurgence is most common in large calderas (greater than 10‐km diameter) in cratonic crust and is associated with large silicic intrusions. In addition to resurgence within single calderas, broader magmatic uplift occurs widely within silicic volcanic fields, reflecting isostatic adjustment to emplacement of associated subvolcanic batholiths. Much additional space for shallow batholith emplacement is probably accommodated by gravitationally driven down warping of wall rocks at lower structural levels. Hydrothermal activity and mineralization accompany all stages of ash flow magmatism, becoming dominant late during caldera evolution. Much rich mineralization is millions of years later than caldera collapse, where the caldera served primarily as a structural control for genetically unrelated intrusions and associated hydrothermal systems.