Volcanoes mostly occur at plate margins. They can also occur at hot spots and in rift valleys. The most active volcanic zone is the Pacific Ring of Fire around the Pacific plate. Here plates are colliding or separating. At active points on these plates volcanic mountains are formed, e.g. Nazca Plate and South American Plate, where plates separate, e.g. Pacific and Nazca Plate and in rift valleys, e.g. African Plate, Kenya. The most active area on the globe is the western rim of the Pacific Plate – stretching from Russia to New Zealand.Several findings relating to earthquakes and volcanic activity can be explained by plate tectonics. If you follow news stories of earthquakes, you may begin to wonder why such places as California or Japan have so many earthquakes, whereas the north eastern United States or Western Europe have so few.
The fact is that earthquakes occur along belts and the vast majority of these belts lie on the boundaries of the earth’s tectonic plates. The same is true of volcanoes and locations known for earthquakes, such as the Philippines, have many volcanoes.The Hawaiian Islands are located near the centre of the Pacific Plate. These islands of Hawaii experience plate movement, helping to form the volcanoes that created those islands. At the south of the island group, many volcanoes are still active, while those to the north are mainly dormant. The reason for this is that the Pacific Plate is moving north over a stationary lava source (mantle plume) below Hawaii.
The south islands remain above it, while the northern islands have moved away.The Oceanic crusts are tend to be youngest at the centre of ocean basins and along mid-ocean ridges, or along submarine ridges where new seafloor is created. There is much volcanic activity here as massive fissure eruptions frequently occur. Crusts of oceanic and continental type differ by their thickness and by composition. Basalt, an igneous rock (from the cooling of magma), makes up the SIMA, whereas much of the continental crust (SIAL) is made up of granite, also an igneous rock. Where the crust is thin volcanic activity is more clearly seen.