Temporal trends in lava dome extrusion at Santiaguito 1922-2000

We present a means of extracting lava dome extrusion rates from data provided by the Thematic Mapper and Enhanced Thematic Mapper Plus instruments flown on the Landsat satellites. Extrusion rates derived from the Landsat data for Santiaguito dome complex (Guatemala) during November 1987 and January 2000 of 0.72+/-0.08 and 0.48+/-0.09 m(3) s(-1) compare favorably with ground-based estimates of 0.63+/-0.28 and 0.48+/-0.17 m(3) s(-1). A satellite-data-derived set of 18 extrusion rate estimates between 1987 and 2000, along with ground-based volume estimates prior to 1987, allow us to infer volumes of magma erupted since 1922. These data reveal cyclic extrusion at Santiaguito. Each cycle begins with a 3-6-year-long phase of high extrusion rate (0.5-2.1 m(3) s(-1)), followed by a longer (3-11 years) phase of low extrusion rate (less than or equal to0.2 m(3) s(-1)). The 8th cycle began in 1996 and was still in its high extrusion rate phase during January 2000. With time, however, the durations of the low extrusion rate phases have increased, peak extrusion and time-averaged extrusion rates for each cycle have decreased, and the difference between extrusion rates during the high and low extrusion rate phases of each cycle has decreased. These trends may be the result of depressurization and exhaustion of the source. In this case, the,current eruption may be expected to end around 2020. Alternatively, Santiaguito may be entering a period of persistent activity maintained by a stable magma supply. In this case, we may expect extrusion to become less cyclic and to stabilize at 0.2-0.4 m(3) s(-1). However, a 2 wt% decrease in SiO2 content of erupted products since 1970 appears to be consistent with chamber exhaustion, as well as an increase in block lava flow lengths during 1970-2000.