How does tectonic plate motion relate to the formation of volcanoes?
Formation of subduction zones.
When a denser plate (i.e. composed by oceanic basalt crust) collides with a less dense one (i.e. a continental plate) it will sink below (subduction).
The deeper the plate sinks the more geothermal heat will meet staring to melt and releasing gas.
The accumulation of melted rock and gas is the main source of volcanic activity.
Volcanoes are aligned along tectonic boundaries between tectonic plates forming chains and ridges.
An example is along the pacific coast of North and South America where a series of volcanically and seismically active mountain ridges border the subduction zone of the pacific plates below the american ones.