The Keeling Curve?

Serious question, why do climatologists and Al Gore use the C02 readings on top of the worlds largest shield volcano? The Keeling Curve is made from the measurements on top of Mt. Loa or something like that in Hawaii, but seriously, volcanoes are by far the largest emmiter of Co2 by far. Is that fair to use that information? and if so why was there no Co2 spike when it erupted in the 1980's? just curious