Question

why are volcanoes formed only at ocean continent boundaries and not at continent continent boundaries

Answer 1

Transform and continent to continent plate boundaries do not involve the subduction of a plate. Volcanoes can form at oceanic to oceanic plate, and oceanic to continental plate subduction zones where the presence of water in hydrous minerals which are being subducted can speed melting of some minerals. Other crystals that would be solid at a deeper depth can melt in a high-temperature, subducting environment. The resulting partial melt can carry unmelted material upward, where lower pressure allows more melt to occur. This process can occur all the way up to volcanic eruption. Without subduction there is no melt (except for rifts and hot spots). So they are not formed near Oceans