Hazel Rymer

Gravity fields and the interpretation of volcanic structures: Geological discrimination and temporal evolution

Abstract A review of gravity data reflecting shallow concealed structures on volcanoes demonstrates that valuable information may be derived on the geological development of such structures and on the dynamic evolution of active volcanoes. To a first approximation, all recorded examples of anomalous gravity fields associated with volcanoes are concentric with and have sources that lie within the volcanic edifice. Positive anomalies with wavelengths less than 20 km and amplitudes up to ca. 30 mGal, characterise mainly basaltic volcanoes from various tectonic settings, and are caused by a relatively dense intrusive complex/magma body which contrasts with its surroundings either because the body is more mafic than average or, more likely, because near surface, previously erupted materials are uncompacted. Negative anomalies with wavelengths up to 100 km and amplitudes up to ca. 60 mGal, occur over much larger volcanic calderas, many of which have erupted highly silicic pyroclastic ash and pumice; uncompacted silicic caldera infill, with a possible contribution from low-density magma bodies, is responsible for the observed anomalies. There is some evidence for a continuum of gravity anomaly types that corresponds to the geological evolution of volcanic systems generally from primitive rift or subduction-related basaltic andesite to mature, high-level silicic calderas. Repeated microgravity observations over active volcanoes enable magma movements, variations in magma input and changes in magma density to be monitored closely. Dynamic modelling of volcanic systems is providing new evidence on the behaviour of concealed magma bodies and could have considerable potential for predicting eruptive events.

Microgravity change as a precursor to volcanic activity

Abstract In recent decades, systematic microgravity studies over some 20 active volcanoes in Central America, Iceland, Italy, Japan, Papua New Guinea and the USA have provided valuable data on sub-surface mass redistribution associated with volcanic activity. Concurrent data on ground deformation are essential to the unambiguous interpretation of gravity changes. In some instances, gravity and elevation vary along the free-air or Bouguer gradients, implying that there has been no sub-surface mass or density change, respectively. Where there are residual gravity changes after correction for elevation changes, magma movements in sub-surface chambers, feeder systems, vents and fissures (dykes) or water table variations are proposed. Although detailed interpretations depend on local circumstances and the calculations depend on source geometry, in general, the smallest residual gravity changes are associated with eruptions from volatile-poor basaltic vents and at extensional rift zones, whereas the highest residual values occur at explosive, subduction-related stratocones built from volatile-rich andesitic magma. The most intriguing, yet difficult, data to interpret derive from large-volume, infrequently erupting volcanic systems where caldera unrest is now becoming well documented and the ultimate hazards are most severe. Mass increases during inflation followed by limited mass loss during subsequent deflation typify these structures.

Unrest at Campi Flegrei: A contribution to the magmatic versus hydrothermal debate from inverse and finite element modeling

[ 1] We present results from the modeling of ground deformation and microgravimetric data recorded at Campi Flegrei in order to assess the causative phenomena of caldera unrest between 1981 and 2001. We find that residual gravity changes during ground uplift ( 1982 - 1984) are indicative of mass changes in a hybrid of magmatic and hydrothermal sources. During deflation between 1985 and 2001, the inversion of gravity residuals for a single source does not provide convincing results. We then performed the joint inversion of gravity and deformation data for multiple spherical sources and refined source parameters by finite element modeling in order to mitigate against limitations of the analytical solutions. The data recorded during inflation and rapid deflation may be best explained by mass and pressure changes in a deep magmatic source at about 5 km depth and a shallow ( 2 km deep) hydrothermal source. Both sources contribute equally to the gravity changes observed between 1982 and 1984; the contemporary uplift appears to be mainly caused by the shallow source. The subsequent deflation is dominated by a pressure decrease in the hydrothermal source; the magmatic source contributes chiefly to the observed gravity changes. Pressure and density variations within multiple shallow-seated hydrothermal sources provide acceptable fits to the deflation and accompanying gravity changes recorded since 1988. These shallow level dynamics also appear to trigger spatially and temporarily random short-term reversals of the overall mode of ground subsidence since 1985. Our analysis does not support the idea of magmatic contributions to these short-lived periods of inflation.

Volcanic eruption prediction: Magma chamber physics from gravity and deformation measurements

One of the greatest remaining problems in modern volcanology is the process by which volcanic eruptions are triggered. It is generally accepted that eruptions are preceded by magma intrusion (Sigurdsson and Sparks, 1978). The degree of interaction between previously ponded magma in a chamber and newly intruded magma determines the nature and rate of eruption and also the chemistry of erupted lavas and shallow dykes. Here, we investigate the physics of this interaction. Volcano monitoring at its most effective is a synergy between basic science and risk assessment, while hazard mitigation depends on reliable interpretation of eruption precursors. The simple and much used Mogi model relates ground deformation (∆h) to changes in magma chamber volume. Gravity changes (∆g) combined with ground deformation provide information on magma chamber mass changes. Our new models predict how the ∆g/∆h gradient will evolve as a volcano develops from a state of dormancy through unrest into a state of explosive activity. Thus by simultaneous measurement of deformation and gravity at a few key stations, magma chamber processes can be identified prior to the onset of conventional eruption precursors.

Gravity−height correlations for unrest at calderas

Abstract Calderas represent the sites of the worlds most serious volcanic hazards. Although eruptions are not frequent at such structures on the scale of human lifetimes, there are nevertheless often physical changes at calderas that are measurable over periods of years or decades. Such calderas are said to be in a state of unrest, and it is by studying the nature of this unrest that we may begin to understand the dynamics of eruption precursors. Here we review combined gravity and elevation data from several restless calderas, and present new data on their characteristic signatures during periods of inflation and deflation. We find that unless the Bouguer gravity anomaly at a caldera is extremely small, the free-air gradient used to correct gravity data for observed elevation changes must be the measured or calculated gradient, and not the theoretical gradient, use of which may introduce significant errors. In general, there are two models that fit most of the available data. The first involves a Mogi-type point source, and the second is a Bouguer-type infinite horizontal plane source. The density of the deforming material (usually a magma chamber) is calculated from the gravity and ground deformation data, and the best fitting model is, to a first approximation, the one producing the most realistic density. No realistic density is obtained where there are real density changes , or where the data do not fit the point source or slab model. We find that a point source model fits most of the available data, and that most data are for periods of caldera inflation. The limited examples of deflation from large silicic calderas indicate that the amount of mass loss, or magma drainage, is usually much less than the mass gain during the preceding magma intrusion. In contrast, deflationary events at basaltic calderas formed in extensional tectonic environments are associated with more significant mass loss as magma is injected into the associated fissure swarms.

Gravity changes and passive SO2 degassing at the Masaya caldera complex, Nicaragua

An understanding of the mechanisms responsible for persistent volcanism can be acquired through the integration of geophysical and geochemical data sets. By interpreting data on micro-gravity, ground deformation and SO2 flux collected at Masaya Volcano since 1993, it is now clear that the characteristically cyclical nature of the activity is not driven by intrusion of additional magma into the system. Rather, it may be due in large part to the blocking and accumulation of gas by restrictions in the volcano substructure. The history of crater collapse and formation of caverns beneath the crater floor would greatly facilitate the trapping and storage of gas in a zone immediately beneath San Pedro and the other craters. Another mechanism that may explain the observed gravity and gas flux variations is the convective overturn of shallow, pre-existing, degassed, cooled, dense magma that is replaced periodically by lower density, hot, gas-rich magma from depth. Buoyant gas-rich magma rises from depth and is emplaced near the surface, resulting in the formation and fluctuation of a low-density gas-rich layer centred beneath Nindiri and Santiago craters. As this magma vigorously degasses, it must cool, increase in density and eventually sink. Five stages of activity have been identified at Masaya since 1853 and the most recent data suggest that the system may have been entering another period of reduced degassing in 2000. This type of analysis has important implications for hazard mitigation because periods of intense degassing are associated with poor agricultural yields and reduced quality of life. A better understanding of persistent cyclically active volcanoes will allow for more effective planning of urban development and agricultural land use.

Volcano spreading controlled by dipping substrata

Most volcanoes grow on slopes, and some tend to fail catastrophically on the downslope side. Many volcanoes also deform by volcano spreading, which may lead to failure. We look at the effect of dipping substrata on the potential for spreading and collapse with analogue models. The dip is found to strongly control the spreading style, rate, and direction. A distinct change from purely radial spreading occurs even with small (<1°) substrata tilt. Structures on the cone and surrounding area are modified according to the underlying dip direction. Spreading becomes concentrated on downslope sectors, where movement is predominantly in the dip direction. The degree of structural realignment is a function of the slope angle. Our models are applied to previously known and new examples of volcano spreading. The effect of dipping substrata in confining spreading to sectors increases the potential for deep-seated sector collapses. This finding provides a mechanism for failure on the downslope side. The incorporation of substrata can create very large volume collapses.

Magma movements in Etna volcano associated with the major 1991–1993 lava eruption: evidence from gravity and deformation

The 1991–1993 eruption was probably the largest on Mt. Etna for 300 years. Since then the volcano has entered an unusually quiescent period. A comprehensive record of gravity and ground deformation changes presented here bracket this eruption and give valuable insight into magma movements before, during and after the eruption. The gravity and deformation changes observed before the eruption (1990–1991) record the intrusion of magma into the summit feeder and the SSE-trending fracture system which had recently been active in 1978, 1979, 1983 and 1989, creating the feeder dyke for the 1991–1993 eruption. In the summit region gravity changes between 1992 and 1993 (spanning the end of the eruption) reflect the withdrawal of magma from the conduit followed more recently (1993–1994) by the re-filling of magma in the conduit up to pre-eruption levels. In contrast, in the vicinity of the fracture zone, gravity has remained at the 1991–1992 level, indicating that no withdrawal has occurred here. Rather, magma has solidified in the fracture system and sealed it such that the 1993–1994 increase in magma level in the conduit was not accompanied by further intrusion into the flanks. Mass calculations suggest that a volume of at least 107 m3 of magma has solidified within the southeastern flank of the volcano.

Hazard assessment during caldera unrest at the Campi Flegrei, Italy: a contribution from gravity-height gradients

Hazard assessment and risk mitigation at restless calderas is only possible with adequate geophysical monitoring. We show here how detailed long-term micro-gravity and deformation surveys may contribute to hazard assessment at the Campi Flegrei caldera (CFc) in Italy by evaluating gravity–height change (Δg/Δh) gradients obtained during ground inflation and deflation between 1981 and 2001. Such gradients provide a framework from which to assess the likelihood and type of volcanic eruptions. Our new analysis of unrest at the CFc allows us to separate ‘noise’ during the gravity survey from the signal of deep-seated magmatic processes. This facilitates identification of the dynamics within the magma reservoir beneath the CFc. We found that magma replenishment during rapid uplift between 1982 and 1984 was insufficient, probably by one to two orders of magnitude, to trigger an eruption similar to the 1538 Monte Nuovo eruption, the most recent volcanic eruption within the CFc. Furthermore, our interpretation of Δg/Δh gradients for the ongoing period of deflation since 1984 suggests that eruptive volcanic activity is not imminent. Short periods of minor inflation associated with large gravity changes since 1981 are interpreted to reflect noise, which to some degree is probably due to sub-surface mass/density changes within shallow hydrothermal systems beneath the CFc, indicating no risk of eruptive volcanic activity. We propose that monitoring Δg/Δh gradients at restless calderas is essential as a caldera develops from a state of unrest to a state where volcanic eruptions have to be anticipated. Adoption of this method for the several tens of restless calderas world-wide will provide early warning of changes in or increase of activity at these supervolcanoes.

Detecting volcanic eruption precursors: a new method using gravity and deformation measurements

Abstract One of the fundamental questions in modern volcanology is the manner in which a volcanic eruption is triggered; the intrusion of fresh magma into a reservoir is thought to be a key component. The amount by which previously ponded reservoir magma interacts with a newly intruded magma will determine the nature and rate of eruption as well as the chemistry of erupted lavas and shallow dykes. The physics of this interaction can be investigated through a conventional monitoring procedure that incorporates the simple and much used Mogi model relating ground deformation (most simply represented by Δ h ) to changes in volume of a magma reservoir. Gravity changes (Δ g ) combined with ground deformation provide information on magma reservoir mass changes. Our models predict how, during inflation, the observed Δ g /Δ h gradient will evolve as a volcano develops from a state of dormancy through unrest into a state of explosive activity. Calderas in a state of unrest and large composite volcanoes are the targets for the methods proposed here and are exemplified by Campi Flegrei, Rabaul, Krafla, and Long Valley. We show here how the simultaneous measurement of deformation and gravity at only a few key stations can identify important precursory processes within a magma reservoir prior to the onset of more conventional eruption precursors.