There are more than 1500 active volcanoes on Earth. 95% of these are located at the boundaries of two plates (zones of divergence or of subduction) . The remaining 5% are located  above thermal plumes, in those regions known as “hot spots”.
It is estimated that 80% of these volcanoes lie under the oceans, located along the length of the oceanic ridges. They are at the origin of the material that makes up the ocean floor.
This volcanic activity is one of the numerous phenomena supporting the theory of Plate Tectonics. The gases, ashes and rocks expelled in eruptions are carefully studied in order to better understand the internal composition of the Earth.
Note:  These animations are not necessarily drawn to scale.