How are flying drones and unmanned vehicled used for exploring volcanoes?
Here's a partial answer: 

The Trail by Fire expedition 1.5 -- "We're a team of young volcanologists planning to measure the entire flux of gases emitted by volcanoes along the Nazca subduction zone." -- driving through South America has "a fleet of quadcopters with gas sensing and sampling capabilities, establishing a remote presence in the volcanic plumes we encounter along the Trail by Fire."

On their sponsors page they list the science payload for each of three Matrix quadcopters (more details about the mechanisms and some custom kit are on the page), which are used because of the difficulty and hazard of climbing to the vents along the mountain chain and sampling the plumes:


An Ocean Optics Flame UV-DOAS spectrometer for SO2 measurements
A multigas sensor built by INGV (Italy)
A gas collector to bring plume gas to the Delta Ray Spectrometer


There's also a bit of drone footage included in the videos on their blog page.

Note that their goal is specifically to understand the gases: "Our objective is to provide the first accurate and large-scale estimate of the flux of volatile species (H2O, H2, CO2, CO, SO2, H2S, HCl, HF, and more) emitted by volcanoes of the Nazca subduction zone."

(Disclaimer: I only know about this because one of the researchers worked for my institution during last year's initial expedition. A review of the literature may show more published uses of drones and UAVs beyond simply shooting video.)