Magma-filled Cracks Research Articles

Lunar floor-fractured craters: Modes of dike and sill emplacement and implications of gas production and intrusion cooling on surface morphology and structure

Lunar floor-fractured craters (FFCs) represent the surface manifestation of a class of shallow crustal intrusions in which magma-filled cracks (dikes) rising to the surface from great depth encounter contrasts in host rock lithology (breccia lens, rigid solidified melt sheet) and intrude laterally to form a sill, laccolith or bysmalith, thereby uplifting and deforming the crater floor. Recent developments in the knowledge of lunar crustal thickness and density structure have enabled important revisions to models of the generation, ascent and eruption of magma, and new knowledge about the presence and behavior of magmatic volatiles has provided additional perspectives on shallow intrusion processes in FFCs. We use these new data to assess the processes that occur during dike and sill emplacement with particular emphasis on tracking the fate and migration of volatiles and their relation to candidate venting processes. FFCs result when dikes are capable of intruding close to the surface, but fail to erupt because of the substructure of their host impact craters, and instead intrude laterally after encountering a boundary where an increase in ductility (base of breccia lens) or rigidity (base of solidified melt sheet) occurs. Magma in dikes approaching the lunar surface experiences increasingly lower overburden pressures: this enhances CO gas formation and brings the magma into the realm of the low pressure release of H2O and sulfur compounds, both factors adding volatiles to those already collected in the rising low-pressure part of the dike tip. High magma rise velocity is driven by the positive buoyancy of the magma in the part of the dike remaining in the mantle. The dike tip overshoots the interface and the consequent excess pressure at the interface drives the horizontal flow of magma to form the intrusion and raise the crater floor. If sill intrusion were controlled by the physical properties at the base of the melt sheet, dikes would be required to approach to within ∼300 m of the surface, and thus eruptions, rather than intrusions, would be very likely to occur; instead, dynamical considerations strongly favor the sub-crustal breccia lens as the location of the physical property contrast localizing lateral intrusion, at a depth of several kilometers. The end of lateral and vertical sill growth occurs when the internal magma pressure equals the external pressure (the intrusion just supports the weight of the overlying crust). Dynamical considerations lead to the conclusion that dike magma volumes are up to ∼1100 km3, and are generally insufficient to form FFCs on the lunar farside; the estimated magma volumes available for injection into sills on the lunar nearside (up to ∼800 km3) are comparable to the observed floor uplift in many smaller FFCs, and thus consistent with these FFCs forming from a single dike emplacement event. In contrast, the thickest intrusions in the largest craters imply volumes requiring multiple dike contributions; these are likely to be events well-separated in time, rather than injection of new magma into a recently-formed and still-cooling intrusion.We present a temporal sequence of 1) dike emplacement, 2) sill formation and surface deformation, 3) bubble rise, foam layer formation and collapse, 4) intrusion cooling, and a synthesis of predicted deformation sequence and eruption styles. Initial lateral injection of the sill at a depth well below the upper dike tip initiates upbowing of the overburden, leveraging deformation of the crater floor melt sheet above. This is followed by lateral spreading of the sill toward the edges of the crater floor, where crater wall and rim deposit overburden inhibit further lateral growth, and the sill grows vertically into a laccolith or bysmalith, uplifting the entire floor above the intrusion. Subsidiary dikes can be emplaced in the fractures at the uplift margins and will rise to the isostatic level of the initial dike tip; if these contain sufficient volatiles to decrease magma density, eruptions can also occur. This initial phase of intrusion, sill lateral spreading and floor uplift occurs within a few hours after initial dike emplacement. During the subsequent cooling of the sill, bubbles can rise hundreds of meters to the top of the intrusion to create a foam layer; when drainage of gas bubble wall magma occurs in the foam layer, a continuous gas layer forms above the foam. Gas formation and upward migration produces an increase in sill thickness, while subsequent cooling and solidification cause a thickness decreases and subsidence. The total topographic evolution history, following an initial 2 km thick sill intrusion and floor uplift (hours), includes further floor uplift by gas formation and migration (decades; ∼30 m), followed by cooling, solidification and subsidence (∼a century; ∼350 m). An initial 2 km thick sill is predicted to have a final thickness of ∼1.7 km. This predicted sequence of events can be compared with the sequence of floor deformation and volcanism in FFCs in order to test and refine this model.

Pressure waves in a supersaturated bubbly magma

SUMMARY We study the interaction of acoustic pressure waves with an expanding bubbly magma. The expansion of magma is the result of bubble growth during or following magma decompression and leads to two competing processes that affect pressure waves. On the one hand, growth in vesicularity leads to increased damping and decreased wave amplitudes, and on the other hand, a decrease in the effective bulk modulus of the bubbly mixture reduces wave velocity, which in turn, reduces damping and may lead to wave amplification. The additional acoustic energy originates from the chemical energy released during bubble growth. We examine this phenomenon analytically to identify conditions under which amplification of pressure waves is possible. These conditions are further examined numerically to shed light on the frequency and phase dependencies in relation to the interaction of waves and growing bubbles. Amplification is possible at low frequencies and when the growth rate of bubbles reaches an optimum value for which the wave velocity decreases sufficiently to overcome the increased damping of the vesicular material. We examine two amplification phase-dependent effects: (1) a tensile-phase effect in which the inserted wave adds to the process of bubble growth, utilizing the energy associated with the gas overpressure in the bubble and therefore converting a large proportion of this energy into additional acoustic energy, and (2) a compressive-phase effect in which the pressure wave works against the growing bubbles and a large amount of its acoustic energy is dissipated during the first cycle, but later enough energy is gained to amplify the second cycle. These two effects provide additional new possible mechanisms for the amplification phase seen in Long-Period (LP) and Very-Long-Period (VLP) seismic signals originating in magma-filled cracks.

Microstructural evidence for magma confluence and reusage of magma pathways: implications for magma hybridization, Karakoram Shear Zone in NW India

In the Karakoram Shear Zone, Ladakh, NW India, Miocene leucogranitic dykes form an extensive, varied and complex network, linking an anatectic terrane exposed in the Pangong Range, with leucogranites of the Karakoram Batholith. Mineral paragenesis of the heterogeneous anatectic source rocks suggests melting has resulted from water influx into rocks at upper amphibolite facies conditions, and microstructures suggest anatexis was contemporaneous with shearing. The network is characterized by continuous and interconnected dykes, with only rare cross-cutting relationships, forming swarms and chaotic injection complexes where magmatic rocks cover up to 50% of the outcrop area. Despite this volume of magma, the system did not lose continuity, suggesting that it did not flow en masse and that the magma network was not all liquid simultaneously. Leucogranites in this network, including leucosomes in migmatites, carry an isotopic signature intermediate between the two main anatectic rocks in the source, suggesting efficient homogenization of the magmatic products. Here, we describe a number of microscopic features of these magmatic rocks which suggests that several pulses of magma used the same pathways giving rise to textural and chemical disequilibrium features. These include: (i) narrow, tortuous corridors of fine-grained minerals cutting across or lining the boundaries of larger grains, interpreted to be remnants of magma-filled cracks cutting across a pre-existing magmatic rock; (ii) corrosion of early formed grains at the contact with fine-grained material; (iii) compositional zoning of early formed plagioclase and K-feldspar grains and quartz overgrowths documented by cathodoluminescence imaging; (iv) incipient development of rapakivi and anti-rapakivi textures, and (iv) different crystallographic preferred orientation of early formed quartz and fine-grained quartz. Mapping of the fine-grained corridors interpreted to represent late melt channels reveal an interlinked network broadly following the S-C fabric defined by pre-existing magmatic grains. We conclude that early formed dykes provided a pathway exploited intermittently or continuously by new magma batches. New influxes of magma opened narrow channels and migrated through a microscopic network following predominantly grain boundaries along an S-C fabric related to syn-magmatic shearing. A mixed isotopic signature resulted not from the mixing of magmas, but from the micro-scale interaction between new magma batches and previously crystallized magmatic rocks, through local equilibration.

The Huygens-Hellas giant dike system on Mars: Implications for Late Noachian–Early Hesperian volcanic resurfacing and climatic evolution

Two narrow, broadly arcuate, low ridges extend for 600–700 km in western Terra Tyrrhena, Mars, crosscut ancient Noachian terrain, and are associated with Early Hesperian plains, which cover ∼30% of Mars. Geological relationships suggest that the ridges represent near-surface erosional remnants of subsurface dikes, solidified magma-filled cracks that were responsible for the volcanic emplacement of the plains. Ridge width and geometry are consistent with very high-effusion-rate flood basalt eruptions, emplacement events that would form smooth featureless plains and input significant volcanic gas into the atmosphere. Geological relationships suggest that the ridges were exposed by erosion (fluvial, sublimation, eolian) and partial removal of a regional volatile-rich dust layer.

Buoyancy-driven crack propagation from an over-pressured source

The propagation of a liquid-filled crack from an over-pressured source into a semi-infinite uniform elastic solid is studied. The fluid is lighter than the solid and propagates due to its buoyancy and to the source over-pressure. The role of this over-pressure at early and late times is considered and it is found that the combination of buoyancy and over-pressure leads to significantly different behaviour from buoyancy or over-pressure alone. Lubrication theory is used to describe the flow, where the pressure in the fluid is determined by the elastic deformation of the solid due to the presence of the crack. Numerical results for the evolution of the crack shape and speed are obtained. The crack grows exponentially at early times, but at later times, when buoyancy becomes important, the crack growth accelerates towards a finite-time blow-up. These results are explained by asymptotic similarity solutions for early and late times. The predictions of these solutions are in close agreement with the full numerical results. A different case of crack geometry is also considered in order to highlight connections with previous work. The geological application to magma-filled cracks in the Earth's crust, or dykes, is discussed.

Analog experiments on magma-filled cracks: Competition between external stresses and internal pressure

We have performed two series of analog experiments using gelatin to study the propagation of liquid-filled cracks in stressed medium. The first series was designed to study the competition between the external stress and the liquid excess pressure in controlling the propagation direction. We systematically controlled the external stress and the liquid excess pressure by changing the surface load and the liquid volume. An ascending crack progressively deflected to be perpendicular to the maximum tensile direction of the external stress. The degree of deflection depends on the ratio of the shear stress on a crack plane to the average liquid excess pressure. More deflection was observed for a crack with a larger ratio. No significant deflection was observed for the ratio less than 0.2. The volcanic activity in a compressional stress field might be understood in the context of this competition. The first series also demonstrated the importance of the gradient of the crack normal stress as a driving force for propagation. The vertical gradient of the gravitational stress generated by a mountain load can control the emplacement depth of magmas, and it might lead to the evolution of eruption style during the lifetime of a volcano. The second series was designed to study the three-dimensional interaction of two parallel buoyancy-driven cracks. The deflection of the second crack takes place, when the ratio of the shear stress generated by the first one to the average excess pressure of the second crack is larger than 0.2. If the second crack reaches the first one, the interaction can lead to the coalescence of two cracks. It has directivity: the region of coalescence extends more in the direction perpendicular to the first crack than in the direction parallel to it. It reflects the stress field around the first crack. This directivity might cause a characteristic spatial variation of magma chemistry through magma mixing.

Propagation of Magma-Filled Cracks

Carbonatites are igneous rocks formed in the crust by fractional crystallization of carbonate-rich parental melts that are mostly mantle derived. They dominantly consist of carbonate minerals such as calcite, dolomite, and ankerite, as well as minor ...Read More

Propagation of Magma-Filled Cracks

Propagation of Magma-Filled Cracks

The influence of regional stress and magmatic input on styles of monogenetic and polygenetic volcanism

A new model is proposed to relate the development of monogenetic and polygenetic volcanoes to magmatic input and regional stress in the lithosphere. Output‐stress diagrams relate magmatic output rate and crustal deformation rate (or strain rate). The magmatic output rates of polygenetic volcanoes and monogenetic volcano fields, including lava fields and central (axial) volcanoes on the mid‐oceanic ridge systems, are estimated. Crustal deformation rates obtained from the literature are used as indicators of differential stress (Δσ). These data are normalized over time periods of 104 years, and over areas of 103 km2. The ratio of output rate to input rate, inferred from ophiolite sections, and from crustal deformation around volcanoes, is about 0.1 to 0.3. Based on the average ratio, an input‐stress diagram may be obtained at some volcanoes. Polygenetic volcanoes are plotted in the region of both larger output rate and smaller Δσ. Monogenetic volcano fields are plotted on a rift trend, or in the region of a small output (≤1 km3/104 yr/103 km2). In some regions, polygenetic volcanoes and monogenetic volcanoes coexist, for example, during 0.1 m.y. periods, and within the area of 30 km × 30 km. The output‐stress diagram indicates that coexistence of these volcano types results from variation in differential stress, or variation in production rate of magma in the mantle at different levels over periods of time less than 0.1 m.y. This is supported using extrusive volumes and crustal deformation data from the eastern volcanic zone, Iceland, Taupo Volcanic Zone, New Zealand, TransMexican Volcanic Belt, Mexico, and the volcanic field on and around the Izu peninsula, Japan. The observed relationship between deformation rate, magmatic output rate, and volcano type supports the crack interaction theory of magma‐filled cracks. The output‐stress diagram also indicates that the balance between local stress induced by magma accumulation and regional stress, and stress relaxation, govern the structure of a volcano.

Development of a subvolcanic structure by the interaction of liquid-filled cracks

The interaction of liquid-filled cracks may be an important process for magma accumulation and the development of subvolcanic structures. The horizontal interaction was studied through the stress analyses around pressured cracks in various regional stress fields. The vertical interaction was studied using gelatin experiments. Buoyaney-driven cracks coalesce more easily than pressure cracks. The theory of the three-dimensional crack interaction was discussed. There are two types of crack interaction. The interaction between liquid-filled cracks may cause coalescence of cracks. The interaction between a liquid-filled crack and a solid-filled crack never causes coalescence. The stress relaxation by ductile or brittle behavior, or by the driving-out of magma, decreases the local stress field generated by the accumulation of magma-filled cracks. As the regional differential stress increases and the minimum compressive principal stress becomes tensile, magma-filled cracks become parallel to each other. As the supply rate of magma-filled cracks becomes relatively large, or as the regional differential stress decreases and the minimum compressive stress increases, the arrangement of magma-filled cracks varies from an offset pattern to a radial one, because the local compressive stress field cannot be decreased. This process leads to the development of subparallel or radial dikes in two dimensions, or a dike-sheet complex in three dimensions. A large magma chamber, such as a sphere or a cylinder, is not always necessary for the development of a basaltic volcano. In case of the interaction between liquid-filled cracks, magma-filled cracks coalesce easily into one large crack and diminish the local compressive stress field. Last, the theory of the crack interaction is applied to developments of various volcanoes. The fissure pattern of Hekla volcano, Iceland is explained in terms of the variation of the stress relaxation. The difference in fissure pattern between Izu-Oshima and Miyakejima volcanoes, Japan is interpreted by the variation of the differential stress. The local stress balance including the stress relaxation is kept in the developments of Etna volcano, Italy and Kilauea volcano, Hawaii.

The tectonic setting of Mount Vesuvius and the correlation between its eruptions and the earthquakes of the Southern Apennines

Mount Vesuvius is emplaced on a regional NE-SW-trending fault that accommodates the stretching of the lithosphere caused by a backward retreat of the Calabrian arc. The dynamics of the Calabrian arc controls the temporal occurrence of earthquakes in the Southern Apennines and in Sicily. By means of a detailed statistical approach, we identified a significant correlation between seismic events occurring in different subsets of this geodynamic domain: seismicity changes in the Southern Apennines follow those in the Calabrian arc after 18–21 years, while seismicity changes in Sicily follow those in the Calabrian arc after 8–10 years. The seismicity changes in these three areas appear also to have affected the eruptive activity of Vesuvius in the period 1631–1944. The major effusive-explosive eruptions of this period followed the seismicity changes in the Southern Apennines after 6–13 years and those in the Calabrian arc after 36–39 years. From a tectonic point of view, this indicates a direct link between the eruptive activity of Vesuvius and the dynamics of the Calabrian arc. The backward retreat of the arc produces strain pulses propagating to adjacent areas. From a volcanological point of view, we speculate that the arrival of an extension strain pulse in the area of Vesuvius may trigger the fast movement of magma-filled cracks that stay in unstable equilibrium in the roots of the volcano.

Ice age as a trigger of active Quaternary volcanism and tectonism

The stress accumulation within the crust, caused by the surface mass redistribution associated with the glaciation-deglaciation cycle during the Quaternary, was numerically evaluated in order to examine the relationship between active Quaternary volcanism and tectonism in island-arc areas and ice age. The vertical gradient of horizontal stress difference in the lithosphere for a meltwater of 130 m in equivalent sea-level reaches a maximum value of 0.8 MPa/km, which is corresponding to the equivalent buoyancy of about 100 kg/m 3 for magma-filled cracks, for an earth model with a lithospheric layer of 20–30 km thickness and with a viscosity greater than 10 23 Pa s. The changes in stress difference during the stages of deglaciation of 10,000 years amount to 13 MPa for both the top and bottom of the thin lithosphere. Thus, the additional stress difference within the crust may be effective for island-arc areas with thin lithospheric thickness. We, therefore, speculate that the stress accumulation associated with ice age may be an important trigger and/or accelerator on the active Quaternary volcanism and tectonism for the areas along the circum-Pacific.

Magma transport and reservoir formation by a system of propagating cracks

A system of propagating cracks may explain magma transport and the evolution of a volcano. This paper considers only a basaltic magma. The system is controlled by two boundary conditions: the stress field, and the production rate of the magma-filled cracks in the mantle. Numerical solutions of crack propagation for various stress conditions, with a constant production rate high enough to coalesce isolated cracks, were performed, and the results applied to different tectonic conditions. For the hydrostatic stress conditions, most magma-filled cracks beneath a polygenetic volcano become trapped either in the lower crust, because there the density difference between magma and the host rocks (Δρ) becomes suddenly small, compared with that in the mantle, or trapped in the upper crust, because there Δρ is near to zero. Magma traps composed of such cracks may grow into magma reservoirs if the production rate of cracks in the mantle is large. If horizontal stress with a vertical gradient is superimposed on the hydrostatic condition in the crust, that is, tensile stress which increases upward or compressional stress which increases downward, magmafilled cracks, even if the density of magma is higher than that of the crust, may ascend directly without trapping. When the crust undergoes relative tension, magma-filled cracks may become trapped. Then, the lower part of the trap may grow into a magma reservoir, while the upper part may become filled with dikes. When the production rate of cracks is small, an initial magma-filled crack can rise directly to the surface only when the stress with a gradient is superimposed as mentioned above, or when the average density in a crack decreases, owing to, for example, vesiculation of volatile components.

Simple two-dimensional model of propagation of magma-filled cracks

The direction of propagation of magma-filled cracks was theoretically examined for a two-dimensional model. The analytical result indicates that magma-filled cracks, which have magmatic pressures larger than a critical value, propagate in parallel with the maximum principal direction of far-field stress. This may give one of theoretical grounds to the dike method by which the regional stress field is estimated.

Output rate of magma from active central volcanoes

For part of their historic records, nine of the most active volcanoes on Earth have each erupted magma at a nearly constant rate. These output rates are very similar and range from 0.69 to 0.26 m3 s−1. The volcanoes discussed here—Kilauea, Mauna Loa, Fuego, Santiaguito, Nyamuragira, Hekla, Piton de la Fournaise, Vesuvius and Etna—represent almost the whole spectrum of plate tectonic settings of volcanism. A common mechanism of buoyantly rising magma-filled cracks in the upper crust may contribute to the observed restricted range of the rates of output.

Coalescence of Magma Pockets into Large Pools in the Upper Mantle

A theory of bubble coalescence in glacier ice (Weertman, 1968) can be used to explain why small pools of magma in the upper mantle can coalesce into large pools. Glacier ice contains entrapped gas bubbles and water bubbles. If glacier ice is sheared, it is inevitable that the plastic deformation will lead to coalescence of bubbles (Fig. 1). If the temperature is high enough so that the process that spheriodizes inclusions is rapid, the collision of two spherical bubbles leads to their coalescence into one larger spherical bubble. The theory that was developed for coalescence of bubbles in glacier ice (Weertman, 1968) can be applied to the problem of the collection of magma in the upper mantle into large pools. I draw attention to this possible application of a theory originally developed for glacier ice. I have shown (1968) that the concentration C of bubbles is given by the equation. where C o is the initial concentration of liquid-filled spheres, e is shear strain (understood to be always a positive quantity), and V is the volume fraction of liquid in a unit volume of material. For V = 0.1, large amounts of coalescence occur for a shear strain ∈ greater than 10. It is commonly assumed that liquid phase is present in the low-strength zone of the upper mantle. I have suggested (1967) that horizontal flow can occur in this zone in a manner similar to the flow of magma within a dike. The plate tectonic picture cannot avoid having this zone sheared to large strains. Suppose this zone is of the order of 100km thick and motion of matter in it occurs over horizontal distances of the order of 5,000 km. The average shear strain of a volume element that has traveled this distance thus is of the order of 100. For V = 0.1, the ratio C/C o predicted by equation (8) is 3 × 10 −6 , which is an extremely strong degree of coalescence (that is, about 3 × 10 5 bubbles have coalesced into one gigantic bubble). Thus shear flow in the low-strength zone of the upper mantle could lead to the concentration of small pools of magma into large ones. The magma in large pools subsequently could reach the earth9s crust in rising magma-filled cracks which turn into dikes (Weertman, 1971a, 1971b). Shaw (1969) discussed magma production in a mantle that is sheared. He stated: “The puzzling question of how magma accumulates in order to rise may be answered by the conclusion that viscous failure and flow are part of the same mechanism, and that the melt fraction is virtually kneaded from the crystalline source.” This note suggests a possible mechanism for “kneading out” the melt fraction in the upper mantle, regardless of how the melt fraction is produced.