The metamorphosis of rocks generally occurs under pressure and high temperatures. When exposed to these conditions the composition of the rock can also change. There are other conditions that can change the composition of the rock as well.
When gases, hot liquids and pressures are present, the pores of a rock can become filled. These liquids and gases can cause the rock's chemical composition to change. Generally, chemical change is a rare thing. When it does happen, it is usually a result of the addition or loss of water and carbon dioxide. Instead, metamorphic change generally deals with the new arrangement of the rocks.
Another way that the composition can change is when the rock is exposed to magma. This magma contains new minerals which, when in contact with the rock, can cause color changes. Some of the minerals that can be carried along include sulfur, copper, sodium, potassium, and silica. When these minerals interact with the rocks, a process called metasomatism, takes place to change the composition of the rock. This is the process that copper and lead go through.
When exposed to high temperatures, chemical bonds within the rock are able to break and reform. This can cause a rock's chemistry to change during metamorphism. As temperatures change, new minerals are able to transform as they become unstable. Many minerals are only stable within a certain temperature range. The observation of the minerals in a metamorphic rock can actually help geologists determine an approximate temperature at which the mineral was formed. The high temperatures can also help these scientists determine the speed that the metamorphosis took place because certain reactions will only take place at certain temperatures. By examining the minerals, the temperature that the process occurred can also be determined.
Pressure can also change the composition of metamorphic rocks. Certain minerals are only stable under a specific range of pressure. If the pressure goes beyond this range, the minerals will change. The changes are most common when minerals have the same chemistry but different structures. This, as with temperature, can help determine the pressure that occurred at the time of the formation of the rock.
Generally quartz sandstone and limestone do not change composition because of their simple chemistry, even through they may recrystallize. Other rocks, such as shale, can go through compositional changes due to their complex chemical composition. Shale can be changed with the greatest diversity. It is able to change into, in order, slate, phyllite, schist, and then gneiss.
Some of the compositional changes that rocks undergo do not appear physically, so through the knowledge of how certain minerals can change, given the proper environment, can help geologists determine the process that the ending material went through to be formed.