What is the name of the device that measures volcanic activity?
Not sure what you are after here. There are so many different devices. A seismograph is used to detect and record earthquakes. At a volcano it is used to record the tremors caused by lava moving within it. Stress meters, tilt meters, various global positioning devices, laser range finders and a whole host of others are used. Probably the most informative and useful device is the gas analyser. This gives an indication of the types of gas, their concentration and volatility etc.
