Monitoring and Studying Volcanoes

Ground deformation (swelling, subsidence, or cracking) is measured with a variety of techniques, including Electronic Distance Meters (EDM), the Global Positioning System (GPS), precise leveling surveys, strainmeters, and tiltmeters.
Instruments to measure sulfur dioxide and carbon dioxide can be mounted in aircraft to determine the quantity of gas being emitted on a daily basis. Such instruments can also be used in a ground-based mode.
Most volcanoes provide various types of warnings before eruptions begin. Although an explosive eruption could occur without warning, some premonitory events more likely will precede the next eruption.
Ground deformation (swelling, subsidence, or cracking) is measured with a variety of techniques, including Electronic Distance Meters (EDM), the Global Positioning System (GPS), precise leveling surveys, strainmeters, and tiltmeters.
Restless volcanoes can be very dangerous places, but it's possible to work safely around them if you're properly prepared.
Hot lava samples provide important information about what's going on in a volcano's magma chambers.
The type of equipment and techniques we use to study volcanoes depends on the particular volcano topic we are investigating and on the experiment we are conducting.
Images record volcanic activity and provide a basis for comparison. Each observatory collects and archives their own images. Here are the images from the Volcano Hazards Program. 
Field observations by experienced volcanologists go hand in hand with more sophisticated equipment and techniques to form a complete system for monitoring volcanoes.
The United States and its territories contain 169 geologically active volcanoes, of which 54 volcanoes are a very high or high threat to public safety [National Volcano Early Warning System (NVEWS)].