RHEOLOGY OF DACITIC MAGMAS II: DOMES AND LAVA FLOWS AT SANTIAGUITO, GUATEMALA

Dacitic volcanoes are known for their blasts that devastate large areas such as Bezymianny 1956, Mount Saint Helens 1980 or Pinatubo 1996. A large blast occurred at Santia Maria volcano, Guatemala, in 1902. Since 1922 the four dacitic domes of Santiaguito volcano have grown in the resulting crater. Caliente is the currently active dome, and extrudes new material continuously through periodically active lava flows, including a 4-km long flow emplaced between 1999 and 2004, and frequent small explosions. To understand this alternation between effusive and explosive activity, the magma rheology must be known. Rheology depends on composition, crystal fraction, temperature, volatile content and strain rate.

We studied a fresh sample from the active lava front collected in March 2005. It is a dacite, composed of 25 to 30 vol% plagioclase and pyroxene phenocrysts in a rhyolitic matrix, with ~15 vol% bubble fraction, and is compositionally and texturally similar to rocks collected from spines on the three inactive domes. The viscosity was measured using a parallel plate viscometer on cylindrical cores under uniaxial compression, at atmospheric pressure and in the temperature range 880 to 950°C. Three different stresses were applied, 0.43, 0.28 and 0.14 MPa. More than ten experiments were performed, at durations up to ten days, and the amount of shortening varied from 1.8% to over 18%. The effect of temperature, stress, strain rate, compaction and vesicle content were studied.

Preliminary results show an increase in apparent viscosity over time for all samples, probably resulting from the compaction of bubbles. Viscosity decreases with increasing temperature, by one order of magnitude from 880 to 950˚C. Stress has a negligible effect, suggesting a very low yield strength at these temperatures. The primary factors controlling lava dome rock rheology are temperature, and dissolved volatiles in the rhyolitic matrix. The hot interior of a lava dome is therefore anticipated to be much less viscous than the exterior, promoting the overpressure to build up close to the top. Temperature appears to be the main factor allowing the long, slow-moving lava flows to develop without freezing; the puzzle of these flows is not that they are too viscous to flow, but that they remain warm enough to flow slowly for several years.