Intra-crater activity, aa-block lava, viscosity and flow dynamics: Arenal Volcano, Costa Rica

Abstract

Arenal Volcano has exhibited an almost continuous effusion of lava since September 19th, 1968. Present intra-crater activity is characterized by a relatively quiet rate of effusion accompanied by rhythmic pulsing of the lava due to degassing phenomena. At the vent, the main lava type is aa, which is seldom associated with lava aplastada (flattened lava). This latter lava is formed within the zone of major activity and exhibits smooth, slightly curved, reddish-brown oxidized surfaces which develop irregular cracks on cooling. The genesis of this lava type is due to gas oxidation and rhythmic pulsing. The additional stresses produced by the latter phenomenon (combined with surface tension forces) would hinder the formation of aa lava and generate flattened lava. The dynamic development of lava flows can be explained in terms of three continuous cyclic phases: early, mature and collapsing. A single flow can be divided into four structural units: the upper part (unit a), the middle-upper vesiculated part (unit b), the nucleus (unit c), and the fragmental base (unit d). Rheologically these lavas behave like a Bingham plastic characterized in the field by plug flow. While the 1968–1973 lava flows are characterized by an advanced but incomplete transition from aa to block lava, present flows generally undergo the entire process. In this case the transition zone for a mature flow is located within the middle-lower part of the volcanic cone, reaching an approximate length of 300 m. Transition occurring during the mature phase has been investigated in detail. It is related to the progressive brittle deformation of the middle-upper part (unit b). Fracturing takes place along the side of the plug where the shear strain is higher as well as within the central part where it is due to topographic discontinuities systematically reflected in the mechanical behavior of the rigid plug. Viscosities calculated on the basis of field measurements at 50–200 m from the crater give values of 108 poise. These values are consistent with experimental results obtained in the range of extrusion temperatures (1100–1150°C) and are in good agreement with experiments on suspension rheology in basalts for a similar temperature range and degree of crystallinity. The high viscosity of Arenal lavas can be explained in terms of their high crystallinity (70–80%), lower extrusion temperatures and higher silica content in respect to basalts. The existence of two interstitial immiscible melts also contributes to increase viscosity.