Volcanic activity produces ground deformation and gravity changes in response to geodynamic processes within the crust. Many of these precursors are measurable with present-day technology like precise surveying techniques or "high-technology" as those use in satellite-based geodesy (e.g. Global Positioning System). It is usually assumed that vertical deflection and geoid height needed for comparing such as techniques, are not significantly affected by the intrusion process. In this work, we have tested theoretically this assumption and applied to active zones with different crustal structures that resemble layered media, namely Lanzarote (Canary Islands, Spain) and Long Valley Caldera (California). Considering the geoid as an equipotential surface of the gravitational field we have used the elastic-gravitational deformation model, proposed by Rundle and Fernández, to compute geoid and vertical deflection changes produced by a magmatic intrusion in the crust. This technique represents the geoid and vertical deflection due to a point source, which therefore can be used as Green’s function with which to convolve an arbitrary distribution of subsurface mass or pressure change. The results show that the magma intrusion radius should be of approximately 1 km for the effects on both geoid undulations and vertical deflection not to be negligible. This radius would decrease for shallow intrusions. The pressure effects computed with the model, if we considered realistic pressure changes values, would be always negligible.The research of M.Ch. and J.F. has been conducted as part of Research Projects AMB96-0498-C04-04 and AMB99-1015-C02-01. The research conducted by M.J.S forms part of project AMB97-0706. The work of J.B.R. forms part of project DE-FG03-95ER14499. This research has been also partially supported with funds from a New Del Amo Program project.Peer reviewed