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.