DYNAMICS OF SMALL VERTICAL ERUPTIONS FROM SANTIAGUITO VOLCANIC DOME GUATEMALA, 2007-2008

Santiaguito is an open vent dacite lava dome which has erupted since 1922 at an average rate of about 0.5 m3/s. In the last three decades extrusion is focused on the Caliente Vent region, located within the 1902 explosion crater of Santa Maria. The dacite lava is extruded vertically from a cylindrical conduit (~90m diameter) in abrupt movements typically about every 30-90 min, and up to 150 min, along the conduit rim which generate vertical explosions that rise from slip fractures while the conduit suddenly moves upward. During the past few years the extrusion rate has been far below average (0.1-0.2 m3/s), and is apparently still decreasing. After vertical movement through the conduit, the lava flows flow slowly down the slope of the dome as a block lava flow, which gives rise to hot rock avalanches and occasional block and ash flows.

In this study, we imaged the vertical explosions with a conventional video camera and calibrated the height and distance to map the dynamics of the eruption clouds from a observation point 6.4 km S. Each conduit movement gives rise to vertical clouds which resemble a series of 2-7 closely spaced thermal clouds. The cloud sequences could be tracked for 1-6 minutes and rose to heights of 100 to more than 1500m above the vent at rise velocities of 6-14 m/s. We measured the dynamics of these clouds on several dates from July 2007 to March 2008. The data shows 2 common patterns of thermal succession; type 1, a weakly buoyant thermal followed by several increasingly stronger thermal clouds and type 2, an initial thermal cloud of high buoyancy followed by more thermals with similar dynamics. The source buoyancy of the individual thermals are controlled by the surface area of ash which comes from the slip fractures and hot gas which is emitted. We do not know if the distinct thermals represent different source events or emanations coming from different depths along the conduit margins.

During the two years of observations the intensity of explosions has decreased, which correlates with thermal data which is consistent with other data that suggest that eruption rate is decreasing.