Cotopaxi is one of the world's great looking volcanoes. It is monitored
seismically and by deformation measurements by scientists at Escuela
Politecnica Nacional in Quito. The last significant eruption was in 1877
when mudflows travelled 10s of kilometers into areas that are now heavily
populated. An interesting student project report about monitoring gases
at Cotopaxi is at:
http://kilburn.keene.edu/Courses/GEOL498/cotopaxi/cotopaxi.html
Hope this information helps.
Chuck Wood, University of North Dakota