When it comes down to it, volcanoes spend most of their existence not erupting. If you look at almost any volcano, it might have a bout of eruption for days to months at a time, then go quiet for decades, centuries or more. So when you think about the activity at any given volcano, you should not only concern yourself with what might be happening when the volcano is actually coughing stuff up (erupting), but also when, at the surface, things look perfectly calm.
There are a number of ways to examine what a volcano was/is doing during these periods of repose. My research is like that of a historian, trying to understand what was going on before eruptions that have already happened. I do this by looking at the evidence of changes in the magmatic system recorded in the crystals that are brought up during an eruption.
There you find the record of intrusions of new magma occurring frequently, even during times when the volcanic system might not erupt for 100,000 years! So the real action at many volcanoes might be happening kilometers beneath our feet.
Now, if you want to be more like a journalist and look at what might be going in at the moments between eruptions, you can turn to seismology. At many volcanoes, there is a constant background din of small earthquakes caused by magma, hot gasses, hot water, or faults underneath the volcano.
They can occur in swarms and across a wide range of depths—from the mantle source of the magma all the way to the surface. You can use the number of earthquakes and their depth to start to weave a story of how magma might be moving underneath a volcano during those quiet periods.
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