Metamorphic rocks are those rocks which have been changed by metamorphic processes, typically heat and pressure increases due to one or more physical factors-such as plate tectonic movements of the Earth’s crust-that create a new environment for the rock. The new environment makes the rock “unstable” and the atoms in the minerals rearrange themselves to become more stable to the new conditions, creating a new rock.
Solid-state mineral change
Metamorphic processes are generally slow and long term taking place slowly over many thousands of years. Importantly, all metamorphic mineral changes occur while the rock is in solid state, that is, at temperatures and pressures before the rock reaches its melting point. Different grades of metamorphic change can occur as temperature or pressure changes. For example, the sedimentary rock shale can be changed to slate with slight temperature and pressure increases, while the most severe temperature changes produce granite.
Texture of the rock
The main mineral changes that occur in metamorphic rocks are in the texture of the rock and how the mineral grains or crystals are aligned. Both heat and pressure changes can impact a rock’s texture because minerals are stable only over a certain range of temperatures and pressures. Mineral texture in metamorphic rock is described as either foliated (aligned mineral grains produced by directed pressure) or non-foliated (randomly arranged grains produced by all-round elevated temperatures or rocks that were originally composed of spherical grains).
Slate is a common example of a foliated metamorphic rock at the lower end of metamorphic grade (or change) where the original clay particles experienced pressure and realigned, producing a rock that is denser and hence harder and more resistant to erosion than the original rock. At the other end of the spectrum is gneiss, consisting of alternating bands of quartz, feldspar and micas. With increasing metamorphic grade, the intensity of the foliation increases.
Formation of coarse grain crystals
During metamorphic change the increased temperature often produces a coarser crystal. A small number of large crystals has greater stability than a large number of small crystals and, during thermodynamic change, metamorphic rocks tend to amalgamate internal crystals to produce a coarser-grained rock. This is especially true of the higher-grade metamorphic rocks that are typically coarser-grained.
Formation of new minerals (polymorphs) from crystal realignment
As the temperature changes, minerals become unstable and their atoms transform to new minerals that are stable at the given temperature. One of the most important mineral changes occurs among mineral polymorphs, or crystals that exist in more than one structure. The education unit Metamorphic Rocks explains how a common mineral, alumino-silicate (Al2SiO5) exists in three different polymorphs. The high pressure–low temperature polymorph is kyanite; the low pressure phase is andalusite, while the high temperature polymorph is sillimanite. Each of these minerals occurs naturally over specific ranges of temperature and pressure. Thus the minerals present within a metamorphic rock can help deduce what the temperature was at the time of formation.
Formation of new minerals from hydrothermal fluids
Additionally, hydrothermal solution can cause the formation of new minerals. Gases carry minerals through rocks and, when the hydrothermal solution comes into contact with a rock, deposits are made and new minerals crystallized. Hydrothermal fluids can carry dissolved calcium dioxide, sodium, silica, copper and zinc. Igneous minerals, such as olivine and pyroxene, are changed into metamorphic minerals such as serpentine by hydrothermal fluids.
Summary
The mineral changes in metamorphic rocks are fascinating and can reveal a lot about the original rock, the conditions during the change and what environmental factors influenced the process of rock formation. Mineral changes in metamorphic rocks can be spotted by observing the texture and granularity of the rock, as well as looking at the crystal polymorphs that have resulted, which will again reveal the different conditions that would have been present for mineral change to occur.
