ATM this returns an incorrect answer for me (at Strasbourg, yet returning Dôme volcano circus in the center of France, 645km) as the closest volcano is actually the Nideck, a 250'000'000 year old volcano, a mere 35km from here.

(Photos, with geological layout at the end)

http://www.passion-escalade-et-montagne.com/2018/04/escapade...

http://www.randoalsacevosges.com/2015/03/le-volcan-du-nideck...