A volcano is a mountain where lava (hot, liquid rock) comes from a magma chamber under the ground. Most volcanoes have a volcanic crater at the top. When a volcano is active, materials come out of it.The materials include lava, steam, gaseous sulfur compounds, ash and broken rock pieces.When there is enough pressure, it causes the volcano to erupt. The pressure blows off the top of the volcano.The magma comes out, sometimes quickly and sometimes slowly.hese volcanoes are formed by low-silica (i.e. basic) lava. Shield volcanoes are built out of layers of lava from continual eruptions (without explosions). Because the lava is so fluid, it spreads out, often over a wide area.