Abstract
In January 1977 gravity measurements were made at 48 instrument stations on Pacaya volcano, Guatemala. By January 1978 a 200-m-diameter pit crater near the summit had partially filled with incandescent material. Thirty-six of the stations were reoccupied at this time revealing relative changes in gravity from +0.31 to −0.44 mgal. These changes form a conical, −0.75-mgal gravity anomaly centered over the pit crater. Adjusting the temporal gravity changes for the mass filling the crater increases the amplitude of the anomaly. Comparing the relative gravity changes to theoretical elevation changes calculated for idealized magma chambers within volcanoes, using finite element modeling, indicates elevation changes of up to +4 m during inflation could cause the gravity changes. Density changes of −0.25 g/cm 3 within a hypothetical 200-m-diameter, cylindrical magma body could also affect the gravity changes. Because elevation controls are lacking it is not possible to determine the relative importance of either mechanism. This study shows that gravity monitoring may be a practical method of measuring elevation changes preceding eruptions of topographically rugged volcanoes.
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