Seismic,deformation, and gas activity (unrest) typically precedes volcanic eruptions. Tracking the changes of this activity with monitoring data makes it increasingly possible to successfully forecast eruptions from stratovolcanoes. However, and this is not the case for monogenetic volcanoes (normally the result of a single magmatic pulse). Eruptions from these volcanoes tend to be small but are particularly difficult to anticipate since they occur at unexpected locations,and there is very limited instrumental monitoring data.
Source: phys.org