In contrast to basaltic strombolian eruptions, vulcanian eruptions are most often associated with andesitic to dacitic magma. The high viscosity of these magmas makes it difficult for the vesiculating gases to escape. This leads to the build up of high gas pressure and explosive eruptions. The ejected lava fragments do not take on the aerodynamic shapes common to Strombolian eruptions. This is partly due to the higher viscosity of the erupting magma, but also because the ejecta often incorporates a high proportion of crystalline material broken away from the rock plugging the throat of the volcano.
Vulcanian deposits contain large blocks and bombs near the vent.
These resemble a crusty loaf of bread broken by deep cracks that often expose a frothy interior. The interior of the bomb, however, continues to vesiculate on the ground, which leads to expansion of the interior and cracking of the brittle outer crust. Although blocks and bombs are common in proximal deposits, the bulk of Vulcanian deposits is very fine grained and dominated by ash . The abundance of ash indicates a high degree of fragmentation, which can only be generated by magmas with high gas contents. In some cases, these high gas contents are derived from heated meteoric water.