Volcanoes are hills, mounds or sheets of relatively localized igneous rock assemblages made up of pyroclastic rocks, lava flows, and intrusions in varying proportions. Volcanoes differ notably in their geometry, volume, and relative amounts of pyroclastic rocks and lava flows, with differences mostly dependent upon eruptive mechanisms and rates of extrusion. These in turn depend chiefly upon the magma composition. Chemical composition of magma is also responsible for, or can be correlated with, physical properties such as volatiles and viscosity, which govern to a large extent the nature of many pyroclastic eruptions.