Luis Diego Morales

Dynamics of lava flow fronts, Arenal Volcano, Costa Rica

Abstract Arenal Volcano has effused basaltic andesite lava flows nearly continuously since September, 1968. The two different kinds of material in flows, lava and lava debris, have different rheologic properties and dynamic behavior. Flow morphology depends on the relationship between the amount and distribution of the lava and the debris, and to a lesser extent the ground morphology. Two main units characterize the flows: the channel zone and the frontal zone. The channel zone consists of two different units, the levees and the channel proper. A velocity profile in the channel shows a maximum value at the plug where the rate of shear is zero, and a velocity gradient increasing outward until, at the levees, the velocity becomes zero. Cooling produces a marked temperature gradient in the flow, leading to the formation of debris by brittle fracture when a critical value of shear rate to viscosity is reached. When the lava supply ceases, much of this debris and part of the lava is left behind after the flow nucleus drains out, forming a collapsed channel. Processes at the frontal zone include levee formation, debris formation, the change in shape of the front, and the choice of the flow path. These processes are controlled primarily by the rheological properties of the lava. Frontal zone dynamics can be understood by fixing the flow front as the point of reference. The lava flows through the channel into the front where it flows out into the levees, thereby increasing the length of the channel and permitting the front to advance. The front shows a relationship of critical height to the yield strength ( τ 0 ) surface tension, and slope; its continued movement is activated by the pressure of the advancing lava in the channel behind. For an ideal flow (isothermal, homogeneous, and isotropic) the ratio of the section of channel proper to the section of levees is calculated and the distance the front will have moved at any time t x can be determined once the amount of lava available to the front is known. Assuming that the velocity function of the front { G ( t )} during the collapsing stage is proportional to the entrance pressure of the lava at the channel-front boundary, an exponential decrease of velocity through time is predicted, which shows good agreement with actual frontal velocity measurements taken on two flows. Local variations in slope have a secondary effect on frontal velocities. Under conditions of constant volume the frontal zone can be considered as a machine that consumes energy brought in by the lava to perform work (front advancement). While the front will use its potential energy to run the process, the velocity at which it occurs is controlled by the activation energy that enters the system as the kinetic energy of the lava flowing into the front. A relation for the energy contribution due to frontal acceleration is also derived. Finally the entrance pressure, that permits the front to deform, is calculated. Its small value confirms that the lava behaves very much like a Bingham plastic.

Geophysical Exploration of the Las Pailas Geothermal Field, Rincon De La Vieja, Guanacaste Province, Costa Rica

Geological mapping and geophysical surveys (electrical, magnetometric, and gravimetric methods) were undertaken in the area of the Las Pallas geothermal field at the southern flank of the volcano Rincon de la Vieja, Guanacaste Volcanic Cordillera, in order to obtain a preliminary model of this field. Four rock units of Quaternary age were mapped, namely an ignimbrite unit, an andesitic lava unit, a lahar unit composed of andesitic blocks, and a unit of alluvium and colluvium. A detailed gravimetric survey indicates a positive increase of Bouguer anomalies to the northwest which may indicate a dip of the local basement to the southwest. The local magnetometric map shows a concentration of negative anomalies (up to -600 gammas) within the central region. This coincides with a higher concentration of surface hydrothermal features in this area. The rest of the mapped area is also dominated by negative anomalies which may correspond to a thick hydrothermally altered igneous body at depth.