Lava Flows and Domes pp 208-243 | Cite as

# On the Mechanisms of Lava Flow Emplacement and Volcano Growth: Arenal, Costa Rica

## Abstract

Arenal Volcano is composed of a hierarchical series of geologic units: unit flow, composite flow, lava field, and lava armor. Volume-limited unit flows are emplaced at short time intervals to make up composite flows. Composite flows form lava fields, and lava fields in turn, constitute the lava armor (the volcano). Tephra and lava breccias are selectively eroded from the steep slopes of the volcano by heavy rains and contribute little to the actual shape of the cone. This constructive process has important consequences for the distribution of the age of lava on a composite cone. We show that lava of significantly different ages may be juxtaposed at all scales from the unit flow, to the composite flow, to the lava field, and to the lava armor. These relations are applicable to the time sequential sampling of a composite volcano.

Detailed observations of the dynamic behavior of unit flows indicate that two dimensionless parameters determine the distribution of lava between an active, flowing component and a passive, stationary component. The first parameter, f, is a measure of how much lava the front uses to advance relative to how much it uses to build up levees. The second parameter, q, is the fraction of lava that is able to drain out of the channel when no more lava from the vent feeds the flow. Both parameters have a primary role in determining the final dimensions of a lava flow. These parameters may be calculated from observations of lava flowing onto different topography and at different times after effusion. This data set may allow the prediction of f and q for future flows, and as a consequence, the final flow length along possible flow paths is also predictable.

The development of a thermal structure within the flow plays a critical role in the dynamic evolution of a unit flow. The weight of a cold, highly viscous crust at the surface of the flow actively modifies the stress distribution in the flow and controls the rate of processes such as front velocity, levee formation, and growth of surges. We propose that for a given flux of lava there is a critical channel length beyond which the flow accelerates triggering the separation of the flow from its source near the vent. Thus, the unit flows are volume-limited. Based on this hypothesis we derive a relation for the velocity and position of the flow front at any time after effusion has started, assuming the time functions of f, q, and flow rate are known. We find that the length of a unit flow is directly proportional to f, q, and the flow rate and it is inversely proportional to the cross-sectional area of the channel and to the sine of the slope. These relations also hold for composite flows.

Finally, by making the approximation that a composite flow grows to a constant slope we derive equations for the evolution of lava fields and the growth of the volcanic structure. These relations explain the asymmetric distribution, areal extent, and slope of the various lava fields at Arenal and allow us to infer the position of buried craters and contacts. Remarkably, our model is based on mass conservation and makes no assumption about rheology. With comparable observations this method may be applicable to other volcanoes similar to Arenal.

## Keywords

Lava Flow Frontal Zone Unit Flow Effusion Rate Flow Front## Preview

Unable to display preview. Download preview PDF.

## References

- Angevine CL, Turcotte DL, Ockendon JR (1984) Geometrical form of aseismic ridges and seamounts. J Geophys Res 89: 11287–11292CrossRefGoogle Scholar
- Baloga SM (1987) Lava flows as kinematic waves. J Geophys Res 92: 9271–9279CrossRefGoogle Scholar
- Baloga SM, Crisp J (1987) Leveed lava flows on Mars. Proceedings of IUGG XIX Conference IAVCEIGoogle Scholar
- Baloga SM, Pieri D (1986) Time-dependent profiles of lava flows. J Geophys Res 91: 9543–9552CrossRefGoogle Scholar
- Becker GF (1885) The geometric forms of volcanic cones and the elastic limit of lava. Am J Sci 30: 293–382Google Scholar
- Bennett FD, Raccichini S (1977) Las erupciones del Volcan Arenal, Costa Rica. Rev Geograph Am Centr Costa Rica 5–6: 7–35Google Scholar
- Borgia A, Linneman SR, Spencer D, Morales LD, Brenes JA (1983) Dynamics of lava flow fronts, Arenal Volcano, Costa Rica. J Volcanol Geotherm Res 19: 303–329CrossRefGoogle Scholar
- Borgia A, Poore C, Carr MJ, Melson WG, Alvarado GE (1988) Structural, stratigraphic, and petrologic aspects of the Arenal-Chato volcanic system, Costa Rica: evolution of a young stratovolcanic complex. Bull Volcanol 50: 86–105CrossRefGoogle Scholar
- Cigolini C, Borgia A (1980) Consideraciones sobre la viscosidad de la lava y la estructura da las coladas del Volcan Arenal. Rev Geograph Am Centr Costa Rica 11–12: 131–140Google Scholar
- Cigolini C, Borgia A, Casertano L (1984) Intra-crater activity, aa-block lava, viscosity and flow dynamics: Arenal Volcano, Costa Rica. J Volcanol Geotherm Res 20: 155–176CrossRefGoogle Scholar
- Dragoni M (1987) A dynamic model of lava flows cooling by radiation. Proceedings IUGG XIX Confernece IAVCEI 2: 416Google Scholar
- Gregg DR (1956) Eruption of Ngauruhoe 1954–1955. NZJ Sci Technol 37: 675–688Google Scholar
- Guest JE, Kilburn CR J, Pinkerton H, Duncan AM (1987) The evolution of lava flow fields: observations of the 1981 and 1983 eruptions of Mount Etna, Sicily. Bull Volcanol 49: 527–540CrossRefGoogle Scholar
- Hallworth MA, Huppert HE, Sparks RS J (1987) A laboratory simulation of basaltic lava flows. Modern Geol 11: 93–107Google Scholar
- Lacey A, Ockendon JR, Tbrcotte DL (1981) On the geometrical forms of volcanoes. Earth Planet Sci Lett 54: 139–143CrossRefGoogle Scholar
- Malin MC (1980) Length of Hawaiian lava flows. Geology 8: 306–308CrossRefGoogle Scholar
- Mavridis H, Hyrmak AN, Vlachopoulos J (1986) Deformation and orientation of fluid elements behind an advancing flow front. J Rheol 3: 555–563CrossRefGoogle Scholar
- Melson WG, Saenz R (1968) The 1968 eruption of Volcano Arenal: preliminary summary of field and laboratory studies. Smithson Cent Short-Lived Phenomena Rep 7–1968Google Scholar
- Melson WG, Saenz R (1973) Volume, energy and cyclicity of eruptions of Arenal Volcano, Costa Rica. Bull Volcanol 37: 416–437CrossRefGoogle Scholar
- Milne JFGS (1878) On the form of volcanoes. Geol Mag 5: 337–345CrossRefGoogle Scholar
- Milne JFGS (1879) Further notes upon the form of volcanoes. Geol Mag 6: 506–514CrossRefGoogle Scholar
- Pieri DC, Baloga SM (1986) Eruption rate, area and length relationships for some Hawaiian lava flows. J Volcanol Geotherm Res 30: 29–45CrossRefGoogle Scholar
- Pinkerton H, Wilson L (1987) Factors affecting the length of lava flows. Hawaii Symposium on How Volcanos Work p 203 (abstract)Google Scholar
- Reagan MK, Gill JB, Malavassi E, Garcia MO (1987) Changes in magma composition at Arenal Volcano, Costa Rica, 1968–1985: real time monitoring of open-system differentiation. Bull Volcanol 49: 415–434CrossRefGoogle Scholar
- Rose W (1961) Fluid-fluid interfaces in steady motion. Nature 191: 242–243CrossRefGoogle Scholar
- Shteynberg GS, Solov’yev T (1976) The shape of volcanoes and the position of subordinate vents. Izv Earth Phys 5: 83–84Google Scholar
- Wadge G (1978) Effusion rate and the shape of aa lava flow-fields on Mount Etna. Geology 6: 503–506CrossRefGoogle Scholar
- Wadge G (1983) The magma budget of Volcan Arenal, Costa Rica, from 1968 to 1980. J Volcanol Geotherm Res 19: 281–302CrossRefGoogle Scholar
- Wadge G, Francis P (1982) A porous flow model for the geometrical form of volcanoes - critical comments. Earth Planet Sci Lett 57: 453–455CrossRefGoogle Scholar
- Walker GPL (1972) Compound and simple lava flows and flood basalts. Bull Volcanol 35: 579–590CrossRefGoogle Scholar
- Walker GPL (1973) Length of lava flows. Philos Trans R Soc Lond 274: 107–118CrossRefGoogle Scholar
- Williams H, McBirney AR (1979) Volcanology. Freeman, Cooper, San Francisco, 397 pGoogle Scholar
- Wood CA (1978) Morphometric evolution of composite volcanoes. Geophys Res Lett 5: 437–439CrossRefGoogle Scholar
- Wood CA (1982) On the geometrical form of volcanoes - comment. Earth Planet Sci Lett 52: 451–452CrossRefGoogle Scholar