This animation represents cumulative global volcanic activity over a 36-year span, from 1960 through 1995. Volcanoes occur near but not on tectonic plate boundaries. If a plate boundary is a convergent boundary, where one plate is subducting under another, then volcanoes occur on the top plate, over the area where rock from the subducting plate has melted, is rising, and has broken through to the surface. The Mt. St. Helens eruption is visible in this animation starting in March, 1980.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 8.0.0.0.0