Pressure, gas content and eruption periodicity of a shallow, crystallising magma chamber

Giant radiating dyke swarms on Earth and Venus

The historical records of Kilauea and Mauna Loa volcanoes reveal that the rough-surfaced variety of basalt lava called aa forms when lava flows at a high volumetric rate (>5–10 m3/s), and the smooth-surfaced variety called pahoehoe forms at a low volumetric rate (<5–10 m3/s). This relationship is well illustrated by the 1983–1990 and 1969–1974 eruptions of Kilauea and the recent eruptions of Mauna Loa. It is also illustrated by the eruptions that produced the remarkable paired flows of Mauna Loa, in which aa formed during an initial short period of high discharge rate (associated with high fountaining) and was followed by the eruption of pahoehoe over a sustained period at a low discharge rate (with little or no fountaining). The finest examples of paired lava flows are those of 1859 and 1880–1881. We attribute aa formation to rapid and concentrated flow in open channels. There, rapid heat loss causes an increase in viscosity to a threshold value (that varies depending on the actual flow velocity) at which, when surface crust is torn by differential flow, the underlying lava is unable to move sufficiently fast to heal the tear. We attribute pahoehoe formation to the flowage of lava at a low volumetric rate, commonly in tubes that minimize heat loss. Flow units of pahoehoe are small (usually <1 m thick), move slowly, develop a chilled skin, and become virtually static before the viscosity has risen, to the threshold value. We infer that the high-discharge-rate eruptions that generate aa flows result from the rapid emptying of major or subsidiary magma chambers. Rapid near-surface vesiculation of gas-rich magma leads to eruptions with high discharge rates, high lava fountains, and fast-moving channelized flows. We also infer that long periods of sustained flow at a low discharge rate, which favor pahoehoe, result from the development of a free and unimpeded pathway from the deep plumbing system of the volcano and the separation of gases from the magma before eruption. Achievement of this condition requires one or more episodes of rapid magma excursion through the rift zone to establish a stable magma pathway.

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Pahoehoe and aa in Hawaii: volumetric flow rate controls the lava structure

On Earth, giant radiating dyke swarms are usually preserved as fan-shaped fragments which have been dismembered from their original configuration by subsequent plate tectonic rifting events. Analysis of the largest fragments and consideration of their original configuration has led to the idea that many swarms are plume related, and that dyke swarms radiate away from plume centers. Magellan radar data reveal abundant intact giant radiating swarms on Venus which are similar in scale and pattern to those on Earth. The absence of intense weathering and plate tectonic processes on Venus accounts for the preservation of the primary radiating patterns. It is characteristic of both Earth and Venus that giant radiating dikes are emplaced laterally for distances of at least 2000 km away from plume centers. At distances beyond the influence of the plume on both Earth and Venus, the radiating dyke pattern is often swept into a linear pattern aligned with the regional stress field. There is tremendous potential synergism between the characterization and analysis of terrestrial dyke swarms (where significant erosion has revealed their structure and emplacement directions at depth) and the giant swarms of Venus (where the complete circumferential structure is preserved, and the surface fracture systems above near surface dikes and the nature of the central source regions are revealed). In this study, we report on the characteristics of radial dyke swarms on Earth and Venus and draw some preliminary comparisons from the two perspectives. In summary, on both planets there is evidence for plume-related magmatic centers associated with vertical and lateral injection of magma over considerable distances (up to at least 2000 km). The abundance of very broadly radiating swarms on Venus supports the notion that the swarms on Earth were radiating over broad sectors at the time of intrusion but were dissected by later events. The Venus data show that a swarm can change from radiating (proximal) to regional (distal) subparallel orientations. An implication for Earth is that many regional linear swarms which do not have a radiating pattern may be due to fragmentation of the swarm during later plate tectonic rifting. Completion of the global classification and census of Venus features, comparison to the terrestrial synthesis, and documentation of the mode of emplacement of dikes in these environments (buffered and unbuffered conditions) should lead to additional general insight into mechanisms of formation and evolution and their relation to plumes.

Hawaiian volcanic rift zones are modeled by representing the rifts and adjacent volcano flanks as long ridges with the geometry of flattened triangular prisms. The intrusion of dikes along the axis of a rift requires a mechanism to generate the appropriate dike-trapping stress field within the prism. Possible factors that affect the state of stress in the prism include multiple dike intrusion along the ridge axis, faulting, and gravitational sagging of the topography. In extreme models with very steep slopes and high Poisson's ratio, corresponding to the gelatin models of rift zones by Fiske and Jackson (1972), results of finite element calculations indicate that gravity-induced stresses are sufficient to trap a dike into propagating within the prism and parallel to the rift zone as proposed by Fiske and Jackson. However, the mechanism does not work for gently sloping flanks or a more acceptable Poisson's ratio of about 0.25. Additionally, trapping stresses in the gravity-loading and density stratification models will not persist after a few dike injection episodes. Therefore in mature Hawaiian rift zones with possibly thousands of dikes, additional processes must act to control the stresses that permit continued dike intrusion and rift persistence. It is proposed that accommodation to dike emplacement occurs by slip on deep faults, possibly of the type proposed for the 1975 Kalapana, Hawaii, earthquake. As suggested by others for this earthquake, the faults could coincide with the contact of the volcano with the seafloor within the weak seafloor sediments. Such faulting not only provides a means for the flanks to adjust continuously to intrusions but also generates the stress patterns needed to constrain future dikes to propagate along the rift axis. Other possible faulting mechanisms, such as shallow gravity slides and normal faulting of the flanks, do not appear to favor rift zone persistence. In this model the horizontal stress generated by a standing column of magma at the time of dike emplacement, the stresses in the ridges, and the fault strength are coupled. This results in a feedback between the maximum height that magma can rise along the rift, fault friction, and fault width. Through this feedback the slope of the volcano flank is controlled by the fault friction. When applied to Kilauea Volcano, the model yields an estimate for the coefficient of fault friction as high as 0.39 assuming normal hydrostatic pore fluid pressure. An implication of this model, supported by other studies, is that rift intrusion and lateral spreading could be major contributors to volcano growth.

Gary S. Puniwai

Papers

Geophysical observations of Kilauea Volcano, Hawaii, 2. Constraints on the magma supply during November 1975–September 1977

The 1977 eruption of kilauea volcano, hawaii