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.
This is the legend for the volcano activity animation. Each value of the bar has two colors: the brighter color represents currently active volcaoes, while the darker color represents previously active volcanoes.
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 188.8.131.52.0