Rocks are made of minerals – combinations of chemical elements that crystallize at specific temperatures and pressures. Metamorphism occurs when minerals within existing igneous, sedimentary or metamorphic rocks are altered by heat, pressure or the chemical action of gases and liquids. This can happen during tectonic events such as continental collisions and mountain building. Metamorphism can cause structural change, where the existing minerals are reorganised into layers or collect in specific areas within the rock. It can also cause chemical change where existing minerals break down and new ones form in the solid state as atoms are exchanged.