Gravity Studies at Cerro Machín Volcano, Colombia

A gravimetric study was performed at the Cerro Machin volcano (4° 29’ N, 75° 22’ W), Tolima Department, Colombia,  to obtain a density distribution of the volcanic edifice and its basement. This study was divided into three main sections.  The first section consisted of gravimetric measurements on  the volcano, which were performed with a Scintrex Autograv CG-5 gravimeter. In the second section, a complete Bouguer anomaly was obtained by applying gravimetric corrections  to the field data, such as instrumental drift, latitude, free-air, Bouguer, and topographic corrections. For the third section, we used the GM-SYS extension of Oasis Montaj to obtain a forward model of the subsurface density distribution that  allowed us to explain the source of the gravimetric anomaly. As the main results for this study, we determined that the  field-obtained Bouguer anomaly ranged between -87 mGal and -29 mGal in the study area. The Oasis Montaj density  model allowed us to infer an elongated dacitic complex at  the top of the distribution with a mean density of 2300  kg/m3, with the presence of a volcanic conduit of 2400  kg/m3 at the base. The seismicity in the area suggests that  the gravimetric anomaly caused by the dome and its  surrounding materials may be related to a large weakened zone at the interface between the volcanic edifice and the  metamorphic basement, provoked by fault activity,  interaction with the hydrothermal system and the ascent of  hot, fluid material to the surface. This study suggests one  interpretation of the Machin dome volcanic complex and  encourages further gravimetric studies and modeling over a wider area.