Volcanoes develop in areas where the heat from Earth's mantle melts the overlying crust. This occurs along subduction zones, where one of the tectonic plates is dipping toward the mantle, in rift zones, where the crust is thinning (allowing the mantle to move upward) and at hotspots, where a mantle plume is rising into the crust.
Subduction volcanoes are the most widespread form of volcanism on Earth and are concentrated along the Pacific Rim where the Pacific Plate and its associated oceanic plates are dipping beneath the South American, North American, Eurasian and Australian Plates. The Andes, the volcanoes of western Central America and Mexico, the Cascades, the Aleutians and the volcanoes along the western edge of the Pacific (Japan, Taiwan, Philippines, New Zealand) result from this process. Other subduction volcanoes include those along the western and southern edge of Indonesia, the volcanic islands of the eastern Caribbean and Mt. Etna, on Sicily, one of the most active volcanoes on our planet.
Rift volcanoes include those above mid-oceanic ridges and those developing along rift valleys in continental crust. Most oceanic ridge volcanoes are well below the surface of the sea and thus not readily observed; the major exception is the island nation of Iceland, which formed (and continues to form) above the mid-Atlantic ridge. Continental rift volcanoes are seen along and within the East African Rift and the Rio Grande Rift of the American Southwest; they are also scattered throughout the Great Basin of the U.S. where the crust is being stretched (and thinned). Hotspot volcanism occurs across the globe, along the ocean floor and beneath/within continental crust; the Hawaiian Islands, the Galapagos Islands, the Canary Islands, Yellowstone, the San Francisco Peaks of northern Arizona and the volcanic field of northeastern New Mexico are but a few examples.