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1.
An approximately 20-m-thick alkali basalt flow on the Penghu Islands contains ∼20 cm thick, horizontally continuous (>50 m), vesicular layers separated by ∼1.5 m of massive basalt in its upper 8.5 m. The three layers contain ocelli-like "vesicles" filled with nepheline and igneous carbonate. They are coarse grained and enriched in incompatible elements relative to the massive basalt with which they form sharp contacts. These vesicular layers (segregation veins) formed when residual liquid in the underlying crystal mush was forced (gas filter pressing) or siphoned into three thermally induced horizontal cracks that opened successively in the advancing crystal mush of the flow's upper crust. Most vesicular layer trace elements can be modelled by residual melt extraction after 25–40% fractional crystallization of massive basalt underlying each layer. Sulphur, Cl, As, Zn, Pb, K, Na, Rb, and Sr show large concentration changes between the top, middle, and bottom layers, with each vesicular and underlying massive basalt forming a chemically distinct "pair." The large changes between layers are difficult to account for by crystal fractionation alone, because other incompatible elements (e.g., La, Sm, Yb, Zr, Nb) and the major elements change little. The association of these elements (S, Cl, etc.) with "fluids" in various geologic environments suggests that volatiles influenced differentiation, perhaps by moving alkali, alkaline earth, and chalcophile elements as magma-dissolved volatile complexes. Volatiles may have also led to large grain sizes in the segregation veins by lowering melt viscosities and raising diffusion rates. The chemical variability between layers indicates that a convection and concentration mechanism acted within the flow. The specific process cannot be determined, but different rates of vesicle plume rise (through the flow) and/or accumulation in the upper crust's crystal mush might account for the chemical pairing and extreme variations in Cl, S, As, and C. This study emphasizes the importance of sampling vesicular rocks in flows. It also suggests that volatiles play important physical and chemical roles in rapidly differentiating mafic magmas in processes decoupled from crystal fractionation. Received: 11 November 1996 / Accepted: 20 September 1998  相似文献   

2.
Using constraints from an extensive database of geological and geochemical observations along with results from fluid mechanical studies of convection in magma chambers, we identify the main physical processes at work during the solidification of the 1959 Kilauea Iki lava lakes. In turn, we investigate their quantitative influence on the crystallization and chemical differentiation of the magma, and on the development of the internal structure of the lava lake. In contrast to previous studies, vigorous stirring in the magma, driven predominately by the descent of dense crystal-laden thermal plumes from the roof solidification front and the ascent of buoyant compositional plumes due to the in situ growth of olivine crystals at the floor, is predicted to have been an inevitable consequence of very strong cooling at the roof and floor. The flow is expected to have caused extensive but imperfect mixing over most of the cooling history of the magma, producing minor compositional stratification at the roof and thermal stratification at the floor. The efficient stirring of the large roof cooling is expected to have resulted in significant internal nucleation of olivine crystals, which ultimately settled to the floor. Additional forcing due to either crystal sedimentation or the ascent of gas bubbles is not expected to have increased significantly the amount of mixing. In addition to convection in the magma, circulation driven by the convection of buoyant interstitial melt in highly permeable crystal-melt mushes forming the roof and the floor of the lava lake is envisaged to have produced a net upward flow of evolved magma from the floor during solidification. In the floor zone, mush convection may have caused the formation of axisymmetric chimneys through which evolved magma drained from deep within the floor into the overlying magma and potentially the roof. We hypothesize that the highly evolved, pipe-like ‘vertical olivine-rich bodies’ (VORBs) [Bull. Volcanol. 43 (1980) 675] observed in the floor zone, of the lake are fossil chimneys. In the roof zone, buoyant residual liquid both produced at the roof solidification front and gained from the floor as a result of incomplete convective mixing is envisaged to have percolated or ‘leaked‘ into the overlying highly-permeable cumulate, displacing less buoyant interstitial melt downward. The results from Rayleigh fractionation-type models formulated using boundary conditions based on a quantitative understanding of the convection in the magma indicate that most of the incompatible element variation over the height of the lake can be explained as a consequence of a combination of crystal settling and the extensive but imperfect convective mixing of buoyant residual liquid released from the floor solidification front. The remaining chemical variation is understood in terms of the additional influences of mush convection in the roof and floor on the vertical distribution of incompatible elements. Although cooling was concentrated at the roof of the lake, the floor zone is found to be thicker than the roof zone, implying that it grew more quickly. The large growth rate of the floor is explained as a consequence of a combination of the substantial sedimentation of olivine crystals and more rapid in situ crystallization due to both a higher liquidus temperature and enhanced cooling resulting from imperfect thermal and chemical mixing.  相似文献   

3.
Internal differentiation processes in a solidifying lava flow were investigated for the Kutsugata lava flow from Rishiri Volcano in northern Japan. In a representative 6-m thick lava flow that was investigated in detail in this study, segregation products darker than the host lavas manifested mainly in the form of pipes (vesicle cylinders) and layers (vesicle sheets), occurring around 0.5–2.3 m and 2.0–4.0 m above the base, respectively. Both the cylinders and sheets are significantly richer in incompatible elements such as TiO2 and K2O than the host lavas, which suggest that these products essentially represent residual melt produced during solidification of the lava flow. Field observation and the geochemical features of the lavas suggest that the vesicle cylinders grew upward from near the base of the flow by continuous feeding of residual melt from the neighboring host lavas to the heads of the cylinders. On the other hand, the vesicle sheets were produced in situ in the solidifying lava flow as fracture veins caused by horizontal compression. The vesicle cylinders have a remarkably higher MgO content (up to 8 wt.%) than the host lava (< 6 wt.%), whereas the vesicle sheets display MgO depletion (as low as 3.5 wt.%). The relatively high MgO content of the vesicle cylinders cannot be explained solely by the mechanical mixing of olivine phenocrysts with the residual melt. It is suggested that the vesicle cylinders were produced by the extraction of olivine-bearing interstitial melt from an augite-plagioclase network in the host lava, whereas the vesicle sheets were formed by the migration of the residual melt from a crystal network consisting of plagioclase, augite, and olivine in the host lava into platy fractures. We infer that this selective crystal fractionation for forming the vesicle cylinders resulted from processes in which abundant vesicles rejected from the upward-migrating floor solidification front prevented olivine crystals from being incorporated into the crystal network in the host lava. The vesicle cylinders are considered to have formed in ∼ 1 day after the lava flow came to rest, while relatively large vesicle sheets (> 1 cm thick) appeared much later (after ∼ 9 days). The formation of these segregation products was essentially complete within 20 days after the lava emplacement.  相似文献   

4.
Gas accumulation in magma may be aided by coalescence of bubbles because large coalesced bubbles rise faster than small bubbles. The observed size distribution of gas bubbles (vesicles) in lava flows supports the concept of post-eruptive coalescence. A numerical model predicts the effects of rise and coalescence consistent with observed features. The model uses given values for flow thickness, viscosity, volume percentage of gas bubbles, and an initial size distribution of bubbles together with a gravitational collection kernel to numerically integrate the stochastic collection equation and thereby compute a new size spectrum of bubbles after each time increment of conductive cooling of the flow. Bubbles rise and coalesce within a fluid interior sandwiched between fronts of solidification that advance inward with time from top and bottom. Bubbles that are overtaken by the solidification fronts cease to migrate. The model predicts the formation of upper and lower vesicle-rich zones separated by a vesicle-poor interior. The upper zone is broader, more vesicular, and has larger bubbles than the lower zone. Basaltic lava flows in northern California exhibit the predicted zonation of vesicularity and size distribution of vesicles as determined by an impregnation technique. In particular, the size distribution at the tops and bottoms of flows is essentially the same as the initial distribution, reflecting the rapid initial solidification at the bases and tops of the flows. Many large vesicles are present in the upper vesicular zones, consistent with expected formation as a result of bubble coalescence during solidification of the lava flows. Both the rocks and model show a bimodal or trimodal size distribution for the upper vesicular zone. This polymodality is explained by preferential coalescence of larger bubbles with subequal sizes. Vesicularity and vesicle size distribution are sensitive to atmospheric pressure because bubbles expand as they decompress during rise through the flow. The ratio of vesicularity in the upper to that in the lower part of a flow therefore depends not only on bubble rise and coalescence, but also on flow thickness and atmospheric pressure. Application of simple theory to the natural basalts suggests solidification of the basalts at 1.0±0.2 atm, consistent with the present atmospheric pressure. Paleobathymetry and paleoaltimetry are possible in view of the sensitivity of vesicle size distributions to atmospheric pressure. Thus, vesicular lava flows can be used to crudely estimate ancient elevations and/or sea level air pressure.  相似文献   

5.
天池火山东北侧造盾玄武岩可划分出8个流动单元,熔岩流的流动距离主要集中在30~50km,熔岩流宽度以5km左右为主。通过由野外调查获得的天池火山东北侧不同熔岩流单元的地表坡度、熔岩流厚度等,结合温度、密度与黏度等物理参数,按照熔岩流速度公式恢复的头道组和早白山组0.5m厚晶体含量5%的玄武岩熔岩流流速集中在0~1m/s之间。晶体含量为30%、厚度为0.5m的晚白山组和老房子小山组玄武岩熔岩流的流动速度集中在0~0.12m/s之间。厚度增大至2m左右,晶体含量不变的头道组和早白山组的玄武岩熔岩流流动速度可加快至11m/s。天池火山2m厚的碱性熔岩流在12h内达到或接近了它的最远距离,而各组内2m厚拉斑玄武岩熔岩流在20h内接近了最远距离。0.5m厚的熔岩流在10d内接近最大距离。50km是预计的熔岩流长度,在未来制定减灾措施时,可将此长度作为重要依据之一。天池火山熔岩流灾害主要表现为熔岩流动时对房屋建筑、农田、道路、林地、电站的毁坏,火灾及大量的人口伤亡  相似文献   

6.
A Pleistocene subaqueous, volcanic sequence in South Iceland consists of flows of basaltic hyaloclastite and lava with interbedded sedimentary diamictite units. Emplacement occurred on a distal submarine shelf in drowned valleys along the southern coast of Iceland. The higher sea level was caused by eustatic sea-level change, probably towards the end of a glaciation. This sequence, nearly 700 m thick, rests unconformably on eroded flatlying lavas and sedimentary rocks of likely Tertiary age. A Standard Depositional Unit, describing the flows of hyaloclastite, starts with compact columnar-jointed basalt overlain by cubejointed basalt, and/or pillow lava. This in turn is overlain by thick unstructured hyaloclastite containing aligned basalt lobes, and bedded hyaloclastite at the top. A similar lithofacies succession is valid for proximal to distal locations. The flows were produced by repeated voluminous extrusions of basaltic lava from subaquatic fissures on the Eastern Rift Zone of Iceland. The fissures are assumed to lie in the same general area as the 1783 Laki fissure which produced 12 km3 of basaltic lava. Due to very high extrusion rates, the effective water/melt ratio was low, preventing optimal fragmentation of the melt. The result was a heterogeneous mass of hyaloclastite and fluid melt which moved en masse downslope with the melt at the bottom of the flow and increasingly vesicular hyaloclastite fragments above. The upper and distal parts of the flow moved as low-concentration turbulent suspensions that deposited bedded hyaloclastite.  相似文献   

7.
The anisotropy of magnetic susceptibility (AMS) of lava flows is an innovative method which has been proved to be directly related to the shear history of lava. One of the advantages of this method is that it can be used in the absence of other morphological features commonly employed to study the mechanism of emplacement of lava flows. This feature of the AMS method makes it very attractive to gain insight into the mechanism of emplacement of massive, relatively featureless, long lava flows such as those forming flood basalt provinces. In this work, we report the results of the measurement of AMS as a function of vertical position within the Birkett lava flow, one of the Columbia River Basalt Group flows. The observed variation of AMS allows us to identify at least 16 discrete events of lava injection and to estimate the thickness of individual injection events. The AMS-estimated thickness of each injection event (in the range of 0.5-4.0 m) coincides with the range inferred for injected lava pulses in modern Hawaiian lava flows. Thus, the evidence provided by the AMS method supports the notion that at least some flood basalt lava flows were emplaced by the same mechanism as many present-day inflated pahoehoe flows. Regarding the orientation of the principal susceptibilities, in the central part of the flow they define a preferred orientation along an E-W trend, whereas in the outer parts of the flow they have a NNE-SSW trend. This difference in the orientation of the principal susceptibilities is interpreted as the result of a change of flow direction of the lava as emplacement progressed. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00445-002-0203-8.  相似文献   

8.
Causes and consequences of pressurisation in lava dome eruptions   总被引：3，自引：0，他引：3  
High total and fluid pressures develop in the interior of high-viscosity lava domes and in the uppermost parts of the feeding conduit system as a consequence of degassing. Two effects are recognised and are modelled quantitatively. First, large increases in magma viscosity result from degassing during magma ascent. Strong vertical gradients in viscosity result and large excess pressures and pressure gradients develop at the top of the conduit and in the dome. Calculations of conduit flow show that almost all the excess pressure drop from the chamber in an andesitic dome eruption occurs during the last several hundred metres of ascent. Second, microlites grow in the melt phase as a consequence of undercooling caused by gas loss. Rapid microlite growth can cause large excess fluid pressures to develop at shallow levels. Theoretically closed-system microlite crystallization can increase local pressure by a few tens of MPa, although build up of pressure will be countered by gas loss through permeable flow and expansion by viscous flow. Microlite crystallization is most effective in causing excess gas pressures at depths of a few hundred metres in the uppermost parts of the conduit and dome interior. Some of the major phenomena of lava dome eruptions can be attributed to these pressurisation effects, including spurts of growth, cycles of dome growth and subsidence, sudden onset of violent explosive activity and disintegration of lava during formation of pyroclastic flows. The characteristic shallow-level, long-period and hybrid seismicity, characteristic of dome eruptions, is attributed to the excess fluid pressures, which are maintained close to the fracture strength of the dome and wallrock, resulting in fluid movement during formation of tensile and shear fractures within the dome and upper conduit.  相似文献   

9.
Observation and measurement of vertical sections of thin (< 10 m) basaltic lava flows show that the vertical structure of basalt flows, regardless of variation in chemical composition or thickness, can be divided into three, previously unrecognized, zones consisting of a fundamental and regular pattern in vesicle size and abundance. These zones can be characterized as follows: (1) an upper vesicular zone, (2) a middle nonvesicular or dense zone, and (3) a lower vesicular zone. The thickness of the upper vesicular zone is generally about one-half of the total vertical section, and the thickness of the lower vesicular zone is generally 30–40 cm regardless of the total flow thickness. In the upper vesicular zone, vesicles increase in diameter and decrease in number per unit cross-sectional area downward attaining a maximum diameter near the base of the upper vesicular zone. In the lower vesicular zone, vesicles increase in diameter and decrease in number per unit cross-sectional area upward attaining a maximum diameter at the top of the lower vesicular zone.Numerical simulations, performed for this study, suggest that these characteristic patterns of vesicle zonation are the result of the growth and rise of gas bubbles in cooling lavas rather than the result of dynamic conditions such as flow movement or convection. As a bubble grows, it begins to ascend, and continues to ascend until it is overtaken by solidification progressing inward from either the upper or lower cooling surfaces of the flow. Bubbles that start out high in the flow will ascend ahead of the lower solidification front and cease rising only after encountering the downward-advancing upper solidification front, and bubbles near the base of a flow will be entrapped by the upward-advancing lower solidification front. Bubbles that start and rise just above the lower solidification front form the lower part of the upper vesicular zone. Such bubbles will also have longer times in which to grow than bubbles that are either higher or lower and are therefore among the largest in the flow. A zone free of vesicles will remain between the last bubbles to ascend to the upper solidification front and the last bubbles to be overtaken by the lower solidification front.  相似文献   

10.
Observations in the Montgenevre Massif (French-Italian Alps) and in several other pillow basalt localities indicate that most pillowy flows are not made up of isolated spheres or sacks. In sections more or less parallel to the surface of the flow, it can be seen that pillows are in fact part of a system of interconnected fingers or tubes of lava that divide and bifurcate in the direction of flow. In transverse sections however the pillows appear to be isolated because of geometrical reasons. If this pillow lava structure seems to be the general rule, it is nevertheless likely that in the case of very fluid lavas and on a steep slope, the end of a tube or a new bud may become detached and roll down the bank in front of the flow to form an isolated spheroid.  相似文献   

11.
The Bouzentès lava flow is a 20-m-thick alkali basalt flow emplaced during the last stage of formation of the Cantal stratovolcano at 4.2 Ma. Its upper part has 1- to 20-cm-thick vesicle-rich segregation sheets which recur every 0.1–2 m. These horizontal veins are hawaiitic in composition. They are characterized by hypertrophic development of their minerals (‘pegmatoids’) and by glassy phonolitic segregation vesicles. Internal differentiation within the Bouzentès lava flow was triggered by an unusually high water content, as suggested by pre-emptive iddingsite alteration of olivine phenocrysts. The proposed model of formation of the segregation sheets includes the upward motion of diapirs of residual melt plus addition of vapor from the bottom of the central liquid lens to the base of the upper solidified crust of the cooling lava flow. Olivine settling appears to have been inhibited or at least retarded by upward migration of melt plus vesicles. Most of the features observed in Bouzentès recall the internal differentiation processes usually described within thick Hawaiian lava lakes. The segregation vesicles are believed to result from an increase of gas solubility in residual melt during the crystallization process.  相似文献   

12.
Compound lava flows, defined as those lavas which are divisible into flowunits, commonly have a shield-like form and are thought to develop when the rate of extrusion of lava is relatively low.Simple lava flows, defined as those lavas which are not divisible into flow-units, are thought to form when the rate of extrusion of lava is relatively high. A logical definition oflava flow must embrace both simple lava sheets and substantial lava shields (compound lava flows) up to 600 m high. Flood basalt piles include both compound and simple flows, but the most extensive and far-reaching flows are simple and they are believed to form when the rate of extrusion of lava is particularly high.  相似文献   

13.
The present study is probably the first of its kind in the Deccan Volcanic Province (DVP) that deals in detail with the morphology and emplacement of the Deccan Trap flows, and employs modern terminology and concepts of flow emplacement. We describe in detail the two major types of flows that occur in this province. Compound pahoehoe flows, similar to those in Hawaii and the Columbia River Basalts (CRB) constitute the older stratigraphic Formations. These are thick flows, displaying the entire range of pahoehoe morphology including inflated sheets, hummocky flows, and tumuli. In general, they show the same three-part structure associated with pahoehoe flows from other provinces. However, in contrast to the CRB, pahoehoe lobes in the DVP are smaller, and hummocky flows are quite common. 'Simple' flows occur in the younger Formations and form extensive sheets capped by highly vesicular, weathered crusts, or flow-top breccias. These flows have few analogues in other provinces. Although considered to be a'a flows by previous workers, the present study clearly reveals that the simple flows differ considerably from typical a'a flows, especially those of the proximal variety. This is very significant in the context of models of flood basalt emplacement. At the same time, they do not display direct evidence of endogenous growth. Understanding the emplacement of these flows will go a long way in determining whether all extensive flows are indeed inflated flows, as has recently been postulated.Most of the studies relating to the emplacement of Continental Flood Basalt (CFB) lavas have relied on observations of flows from the CRB. Much of the current controversy surrounding the emplacement of CFB flows centers around the comparison of Hawaiian lava flows to those from the CRB. We demonstrate that the DVP displays a variety of lava features that are similar to those from the CRB as well as those from Hawaii. This suggests that there may have been more than one mechanism or style for the emplacement of CFB flows. These need to be taken into account before arriving at any general model for flood basalt emplacement.Editorial responsibility: T. Druitt  相似文献   

14.
The initial cooling of pahoehoe flow lobes   总被引：1，自引：0，他引：1  
 In this paper we describe a new thermal model for the initial cooling of pahoehoe lava flows. The accurate modeling of this initial cooling is important for understanding the formation of the distinctive surface textures on pahoehoe lava flows as well as being the first step in modeling such key pahoehoe emplacement processes as lava flow inflation and lava tube formation. This model is constructed from the physical phenomena observed to control the initial cooling of pahoehoe flows and is not an empirical fit to field data. We find that the only significant processes are (a) heat loss by thermal radiation, (b) heat loss by atmospheric convection, (c) heat transport within the flow by conduction with temperature and porosity-dependent thermal properties, and (d) the release of latent heat during crystallization. The numerical model is better able to reproduce field measurements made in Hawai'i between 1989 and 1993 than other published thermal models. By adjusting one parameter at a time, the effect of each of the input parameters on the cooling rate was determined. We show that: (a) the surfaces of porous flows cool more quickly than the surfaces of dense flows, (b) the surface cooling is very sensitive to the efficiency of atmospheric convective cooling, and (c) changes in the glass forming tendency of the lava may have observable petrographic and thermal signatures. These model results provide a quantitative explanation for the recently observed relationship between the surface cooling rate of pahoehoe lobes and the porosity of those lobes (Jones 1992, 1993). The predicted sensitivity of cooling to atmospheric convection suggests a simple field experiment for verification, and the model provides a tool to begin studies of the dynamic crystallization of real lavas. Future versions of the model can also be made applicable to extraterrestrial, submarine, silicic, and pyroclastic flows. Received: 26 November 1994 / Accepted: 1 December 1995  相似文献   

15.
The trachytic Tanetomi lava from Rishiri Volcano, northern Japan, provides useful information concerning how a replenished mafic magma mixes with a compositionally zoned felsic magma in a magma chamber. The Tanetomi lava was erupted in the order of Lower lava 1 (LL1, 59.2-59.8 wt.% in SiO2), Lower lava 2 (LL2, 58.4-59.1 wt.%), and Upper lava (UL, 59.9-65.1 wt.%). Evidence for mixing with a mafic magma is observed only in the LL2, in which a greater amount of crystals derived from the mafic magma occurs in rocks with higher SiO2 content. The whole-rock compositional trend of the Tanetomi lavas is fairly smooth except for the LL2 lava composition, which scatter along the main composition trend. There is no reasonable composition of basaltic magma on the extrapolation of the LL2 composition trend, and the trend cannot be explained by a simple two-component magma mixing. Before the replenishment, the felsic magma was zoned in composition (58-65 wt.% in SiO2) and temperature (1030-920°C) in the magma chamber located at the pressure of ~2 kbar. The compositional variation of the main felsic magma was produced by extraction of a fractionated interstitial melt from mush zones along the chamber walls and its subsequent mixing with the main magma (boundary layer fractionation). The LL1 magma tapped the magma chamber soon after the replenishment, before the mafic magma mixed with the overall felsic magma. Then the basalt magma mixed heterogeneously with the upper part of the felsic magma by forced convection as a fountain during injection. The mixing of the basalt magma with compositionally zoned felsic magma resulted in the characteristic composition trend of the LL2. The fraction of basaltic magma in the LL2 magma is estimated to be at most 10%. Despite such a small proportion, the basalt magma was mixed completely with the felsic magma, probably because the crystallinity of undercooled basalt magma was low enough to behave as a liquid.  相似文献   

16.
Newly identified ??a?? lava flows outcrop intermittently over an area of ~110?km2 in the western Deccan Volcanic Province (DVP), India. They occur in the upper Thakurvadi Formation in the region south of Sangamner. The flows, one of which is compound, are 15?C25?m thick, and exhibit well-developed basal and flow-top breccias. The lavas have microcrystalline groundmasses and are porphyritic or glomerocrystic and contain phenocrysts of olivine, clinopyroxene or plagioclase feldspar. They are chemically similar to compound p??hoehoe flows at a similar stratigraphic horizon along the Western Ghats. Petrographic and geochemical differences between ??a?? flows at widely spaced outcrops at the same stratigraphic horizon suggest that they are the product of several eruptions, potentially from different sources. Their presence in the DVP could suggest relative proximity to vents. This discovery is significant because ??a?? lavas are generally scarce in large continental flood basalt provinces, which typically consist of numerous inflated compound p??hoehoe lobes and sheet lobes. Their scarcity is intriguing, and may relate to either their occurrence only in poorly preserved or exposed proximal areas or to the flat plateau-like topography of flood basalt provinces that may inhibit channelization and ??a?? formation, or both. In this context, the ??a?? flow fields described here are inferred to be the products of eruptions that produced unusually high-effusion-rate lavas compared to typical flood basalt eruptions. Whether these phases were transitional to lower intensity, sustained eruptions that fed extensive low effusion rate p??hoehoe flow fields remains unclear.  相似文献   

17.
Komatiites of the 3.5-Ga Komati Formation are ultramafic lavas (>23% MgO) erupted in a submarine, lava plain environment. Newly discovered vesicular komatiites have vesicular upper crusts disrupted by synvolcanic structures that are similar to inflation-related structures of modern lava flows. Detailed outcrop maps reveal flows with upper vesicular zones, 2-15 m thick, which were (1) rotated by differential inflation, (2) intruded by dikes from the interior of the flow, (3) extended, forming a flooded graben, and/or (4) entirely engulfed. The largest inflated structure is a tumulus with 20 m of surface relief, which was covered by a compound flow unit of spinifex flow lobes. The lava that inflated and rotated the upper vesicular crust did not vesiculate, but crystallized as a thick spinifex zone with fist-size skeletal olivine. Instead of representing rapidly cooled lava, the spinifex zone cooled slowly beneath an insulating upper crust during inflation. Overpressure of the inflating lava may have inhibited vesiculation. This work describes the oldest vesicular komatiites known, illustrates the first field evidence for inflated structures in komatiite flows, proposes a new factor in the development of spinifex zones, and concludes that the inflation model is useful for understanding the evolution of komatiite submarine flow fields.  相似文献   

18.
Thermal–mechanical analyses of isotherms in low-volume basalt flows having a range of aspect ratios agree with inferred isotherm patterns deduced from cooling fracture patterns in field examples on the eastern Snake River Plain, Idaho, and highlight the caveats of analytical models of sheet flow cooling when considering low-volume flows. Our field observations show that low-volume lava flows have low aspect ratios (width divided by thickness), typically < 5. Four fracture types typically develop: column-bounding, column-normal, entablature (all of which are cooling fractures), and inflation fractures. Cooling fractures provide a proxy for isotherms during cooling and produce patterns that are strongly influenced by flow aspect ratio. Inflation fractures are induced by lava pressure-driven inflationary events and introduce a thermal perturbation to the flow interior that is clearly evidenced by fracture patterns around them. Inflation fracture growth occurs incrementally due to blunting of the lower tip within viscoelastic basalt, allowing the inflation fracture to pivot open. The final stage of growth involves propagation beyond the blunted tip towards the stress concentration at the tapered tip of a lava core, resulting in penetration of the core that causes quenching of the lava and the formation of a densely fractured entablature. We present numerical models that include the effects of inflation fractures on lava cooling and which support field-based inferences that inflation fractures depress the isotherms in the vicinity of the fracture, cause a subdivision of the lava core, control the location of the final portion of the lava flow to solidify, and cause significant changes in the local cooling fracture orientations. In addition to perturbing isotherms, inflation fractures cause a lava flow to completely solidify in a shorter amount of time than an identically shaped flow that does not contain an inflation fracture.  相似文献   

19.
The present paper examines magmatic structures in the Jizera and Liberec granites of the Krkonoše–Jizera Plutonic Complex, Bohemian Massif. The magmatic structures are here interpreted to preserve direct field evidence for highly localized magma flow and other processes in crystal-rich mushes, and to capture the evolution of physical processes in an ancient granitic magma chamber. We propose that after chamber-wide mixing and hybridization, as suggested by recent petrological studies, laminar magma flow became highly localized to weaker channel-like domains within the higher-strength crystal framework. Mafic schlieren formed at flow rims, and their formation presumably involved gravitational settling and velocity gradient flow sorting coupled with interstitial melt escape. Local thermal or compositional convection may have resulted in the formation of vertical schlieren tubes and ladder dikes whereas subhorizontal tubes or channels formed during flow driven by lateral gradients in magma pressure. After the cessation or deceleration of channel flow, gravity-driven processes (settling of crystals and enclaves, gravitational differentiation, development of downward dripping instabilities), accompanied by compaction, filter pressing and melt segregation, dominated in the crystal mush within the flow channels. Subsequently, magmatic folds developed in schlieren layers and the magma chamber recorded complex, late magmatic strains at high magma crystallinities. Late-stage magma pulsing into localized submagmatic cracks represents the latest events of magmatic history of the chamber prior to its final crystallization. We emphasize that the most favorable environments for the formation and preservation of magmatic structures, such as those hosted in the Jizera and Liberec granites, are slowly cooling crystal-rich mushes. Therefore, where preserved in plutons, these structures may lend strong support for a “mush model” of magmatic systems.  相似文献   

20.
A seafloor lava field was mapped within the summit caldera of Axial Volcano, Juan de Fuca Ridge, using SeaMARC I sidescan sonor and submersible observations. By analogy with similar subaerial features, we infer that several volcanic seafloor features here formed by the process of lava flow inflation. Flow inflation occurs within tube-fed lava flows when lava continues to be supplied to the interior of a flow that has ceased advancing, thus uplifting the flow's rigid surface and creating a suite of characteristic surface structures. Inflated lavas require a feeder lava tube or tube system connected to a remote lava source, and therefore we infer that inflated submarine lava flows contain lava tubes. Inflated flow features identified from sidescan sonar images elsewhere on Axial Volcano and within the axial valley of the southern Juan de Fuca ridge suggest that flow inflation is a widespread submarine volcanic process.  相似文献   

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