© 2016 Elsevier B.V.The Ruiz-Tolima Volcanic Massif (RTVM) is an active volcanic complex in the Northern Andes, and understanding its geological structure is critical for hazard mitigation and guiding future geothermal exploration. However, the sparsity of data available to constrain the interpretation of this volcanic system hinders the application of standard 3D modelling techniques. Furthermore, some features related to the volcanic system are not entirely understood, such as the connectivity between the plutons present in its basement (i.e. Manizales Stock, El Bosque Batholith). We have developed a methodology where two independent working hypotheses were formulated and modelled independently (i.e. a case where both plutons constitute distinct bodies, and an alternative case where they form one single batholith). A Monte Carlo approach was used to characterise the geological uncertainty in each case. Bézier curve design was used to represent geological contacts on input cross sections. Systematic variations in the control points of these curves allows us to generate multiple realisations of geological interfaces, resulting in stochastic models that were grouped into suites used to apply quantitative estimators of uncertainty. This process results in a geological representation based on fuzzy logic and in maps of model uncertainty distribution. The results are consistent with expected regions of high uncertainty near under-constrained geological contacts, while the non-unique nature of the conceptual model indicates that the dominant source of uncertainty in the area is the nature of the batholith structure.