Metamorphic Rock Formation and Composition

Formation

Metamorphic rock refers to any type of rock that is created from a change of pre-existing rocks caused by changing environmental conditions, such as differences in temperature, pressure, and mechanical stress, and the addition or loss of chemical components. The pre-existing rocks (known as protolith) may be igneous, sedimentary, or other metamorphic rocks.The process of metamorphism does not cause the rocks to melt, but instead transforms them into denser, more compact rocks. New minerals are formed either by re-arrangement of mineral components or by reactions with fluids that enter the rocks. Metamorphic rocks are often squished, smeared out and folded (USGS, 2018).

Metamorphism caused by intense heat is referred to as thermal metamorphism e.g. the Sugar Loaf mountain found in Co. Wicklow is comprised of the metamorphic rock quartzite, which was formed by thermal metamorphism. Quartzite forms when quartz-rich sandstone is altered by heat, pressure, and chemical activity of metamorphism. These conditions enable re-crystallisation of sand grains and the silica cement that binds them together.

Similarly, mudstone in Co. Wicklow was metamorphosed to schist approximately 450 million years ago at the onset of the development of the Caledonian mountains as the North American and Eurasian tectonic plates collided. The intense pressure exerted by the collision caused the tectonic plates to buckle and uplift. As this occurred, fault lines in the Earth's crust allowed magma to force its way to the surface, altering the chemical composition of surrounding mudstone rocks.

Similar processes caused by extensive heat and pressure have resulted in the metamorphosis of granite to gneiss in Inishtrahull, several miles off Malin Head in Co. Donegal, and elsewhere in the north-west of Ireland. They originally formed as igneous rocks 1750-1780 million years ago.

Composition

Some of the most common metamorphic rocks include schist, marble, and gneiss. Change in particle size of the rock during the process of metamorphism is called recrystallisation. For instance, small calcite crystals in the sedimentary rock limestone and chalk are transformed into larger crystals in the metamorphic rock marble; in metamorphosed sandstone, recrystallization of the original quartz sand grains results in very compact quartzite, in which the often larger quartz crystals are interlocked. Both high temperatures and pressures contribute to recrystallisation. High temperatures allow the atoms and ions in solid crystals to migrate, thus reorganising the crystals, while high pressures cause dissolving of the crystals within the rock at their point of contact.

Foliated metamorphic rocks including gneiss, phyllite, schist, and slate have a layered or banded appearance that is created by exposure to heat and directed pressure. Non-foliated metamorphic rocks such as hornfels, marble and quartzite do not have a layered or banded appearance.