Volcanoes form at two different types of boundaries between tectonic plates on the Earth's crust: subducting and constructive. Subducting boundaries appear where one plate slides beneath the surface of the other while constructive boundaries slide along one another from side to side rather than colliding. Also, other hot spots on the crust lead to volcano formation as well.
Many of the world's volcanoes sit on the "Ring of Fire." This rim runs up the west coast of North, Central and South America, down the eastern side of Asia, through Indonesia, and then down along New Zealand. This rim follows the boundaries of the Pacific Plate, which is the bed of the Pacific Ocean, with other continental plates. Along the "Ring of Fire," earthquakes are also common as the plates collide with one another.
Eruptions of volcanoes are often as difficult to predict as earthquakes. At places where plates join or when cracks form in plates, magma can ooze up through the fissure that forms; however, at points where one plate slides under the other, the mantle can melt, pushing magma upward and forming pressure under the crust. When the pressure gets high enough to create a crack in the crust, the magma spews forth, creating a volcanic eruption.