The earth is made up of several different layers. At its center is a solid metal core that is surrounded by a liquid metal core, and both layers are extremely hot. The next layer is the mantle, which is mostly solid and only slightly cooler. The top layer of solid rock is the earth’s crust, the surface on which we live.
The mantle, though not as hot as the earth’s core, is hot enough that some rocks start to melt. This molten rock, called magma, is less dense than the surrounding rocks in the mantle. Due to this difference in density, the magma is pushed upward with great force, similar to a helium balloon rising up through the denser surrounding air. The magma is called lava when it reaches the earth’s surface. When the lava, hot gases, ash, and rock fragments escape from deep inside our planet through holes and cracks in the crust, the lava cools and hardens and eventually starts to build up and form a volcano.
Volcanoes can only form in certain parts of our planet. The earth’s crust is broken into plates that “float” on the denser mantle below. These plates are constantly moving and most earthquakes and volcanoes occur where two plates come together or move away from each other. Occasionally volcanoes can form right in the middle of a plate. These so-called “hotspot” volcanoes are created when a narrow stream of hot mantle rises up from deep inside the earth and melts a hole in the plate so that the magma can ooze upward. The Hawaiian islands, for example, are a result of hotspot volcano formations near the center of the giant Pacific plate.
--Evelyn Füri, graduate student, Geosciences Research Division