The state of stress in the wall rock adjacent to a magma chamber will depend upon the pressure of the magma fluid, the actual shape of the chamber, the depth below the surface and density of the overlying rock burden, fluids that may be present within fractures or pore space of the wall rock, and any applied far-field stresses. If, however, the geometry of the magma chamber can be approximated as a fluid-filled cylinder, and assuming the country rock to be elastic, isotropic and infinite, then an exact solution exists for the radial and tangential stresses around the chamber. It is critical to understanding the stability and failure of tunnels and boreholes. Failure around vertical boreholes, known as borehole breakouts, has also proven to be one of the most useful indicators of the orientation of the present-day maximum and minimum horizontal stress directions in the shallow crust.
Breakouts take the form of an elongation of the borehole cross-section.
For many engineering problems, such as an air-filled vertical shaft, these solutions are appropriate. Borehole breakouts are equally common in regions of tectonic shortening, extension and strike-slip faulting. Shallow crustal magma chambers are probably generally emplaced via large-scale dikes or dike swarms. However, many volcanoes are roughly circular in form, and they are capped by summit craters that are also commonly circular. Eroded volcanoes often reveal roughly cylindrical stocks beneath their centers.