I also am interested in what happens to the magma as it rises to the surface. Sometimes crystals settle out or the surrounding rocks melt and are added to the magma. Using all this information I construct the history of the volcanic field. They walk the lava flows and pyroclastic deposits and draw their distribution on maps. They also determine the relative age of deposits by establishing a stratigraphic column.
Old deposits are on the bottom of the stack and young deposits are at the top.
The mapping volcanologists also collects samples which can be used to determine absolute ages. Absolute ages are based on radioactive decay of unstable elements. The finished map conveys a wealth of information and can be used by planners and for hazards assessment. A seismologist from the U.S. Geological Survey. U.S.G.S. photo by C. Thornber, 1994.