After recording what the students already know, explain that the earth's crust is made of rock. Explain to the students that the earth is like a carmel covered apple. The soil is the carmel, the skin of the apple is the crust, the fleshy part of the apple is the mantel, and the core of the apple is the earth's core. As soil accumulates over the years, the older soil is pushed farther and farther down. Eventually, the soil turns into sedimentary rocks.
Sedimentary rocks keep getting pushed further and further down.
In fact, the deeper you dig into the earth, the hotter it gets under ground. So as the sedimentary rock is pushed farther and farther under ground, there is a lot of pressure and heat added to the rock. This melted rock is called magma. Sometimes the pressure is so tremendous under the earth, that the magma will begin to push up through the earth's crust and a volcano is created. When enough pressure is made in the volcano, the magma will erupt from the volcano and lava will begin pouring out of it.