Formation of metamorphic rocks

Metamorphic rocks

Metamorphic rocks are formed when the appearance, texture and chemical composition of igneous or sedimentary rocks are changed by great heat, pressure or a combination of the two. Marble and quartzite are both metamorphic rocks found in Ireland. Metamorphism refers to the process of changing into a metamorphic rock.

Metamorphism

There are two types of metamorphism: regional metamorphism and thermal metamorphism.

Thermal metamorphism occurs due to heat alone. Rocks are exposed to great heat from magma rising through the Earth's crust, causing them to change.

Regional metamorphism happens due to great heat and pressure over a large area over a long period of time. This usually happens due to plate movement such as when fold mountains are being formed. When plates converge, rocks are put under great pressure, forcing them to compress and buckle, changing them into metamorphic rock. As one plate subducts, it melts and the rock is reformed. Rising magma causes further changes to the rock it comes into contact with.

marble

This happened during the Caledonian fold mountain building period in Ireland. At this time, limestone in Connemara was changed to marble.

Marble is a hard crystalline rock of various colours. It is found in Connemara in Co. Galway, Cork and Rathlin Island in Antrim. White marble is the purest form and it is a result of pure limestone changing form.

However, when impurities exist such as iron oxides, silt, sand and clay, marble will look discoloured. For example, the famous Connemara marble is green and Cork marble is red.

Quartz

Sandstone changes into quartzite when it comes in contact with great heat and pressure. Ireland's sandstone can be found in the Sugarloaf, Co. Wicklow and in Croagh Patrick, Co. Mayo.

During the Caledonian mountain building, sandstone came into contact with magma. Since there were spaces between the grains of quartz in the rock, these were filled up with silica and both were packed together.

Quartzite is a lightly coloured rock and often is white. It is usually found at the peak of mountains.