How do geologists monitor volcanoes?

Scientists use a wide variety of techniques to monitor volcanoes, including seismographic detection of the earthquakes and tremor that almost always precede eruptions, precise measurements of ground deformation that often accompanies the rise of magma, changes in volcanic gas emissions, and changes in gravity.

True, the standard unit for volume is liters which can be represented by either a lower case "l" or a capital "l"