Volcanoes occur in many locations that are not on the Ring of Fire around the edge of the Pacific Plate. At any boundary between two or more of the plates, volcanoes may occur. That's why there are volcanoes in Iceland, for example.
Volcanoes may also occur at any weak or thin area of the earth's crust. In the case of the Hawaiian islands, underwater volcanoes have been erupting for millions of years. As the lava emerges from the underwater volcanic cones, it hardens and adds to the height of the cones. Because this has been happening for millions of years in Hawaii's case, the cones have risen above the ocean level and are now land - the islands of the state, which are still getting taller and larger as lava continues to emerge and cool.
Although most volcanoes occur at converging or diverging plate boundaries, there are several areas on earth where volcanoes occur away from the boundaries. These areas are called Hot Spots. A Hot Spot is an area where the magma comes very close to the earth's surface and forms a plume of magma which causes the volcanic action. Hawaii is located over one of the hot spots. As the plate moves over the hot spot a series of volcanoes occur, this is why there is a chain of Hawaiin Islands. Yellowstone National Park is another area of the earth where a hot spot is located. This is the cause of the many geothermal features in Yellowstone.