Metamorphic rock

What Are Metamorphic Rocks?

Metamorphic rocks are formed when existing (parent) rocks are altered by intense heat, pressure, or both, without melting. This causes the rock’s minerals to recrystallise, changing its structure, texture, and appearance.

Metamorphism of Limestone into Marble

Limestone, a soft, grey, sedimentary rock made of calcite and marine fossils, becomes marble through metamorphism. Heat and pressure destroy fossils, remove stratification, and form a hard, crystalline, white rock used in construction and art.

Common Metamorphic Rock Transformations

Several metamorphic rocks form from different parent rocks:
• Granite → Gneiss (banded texture)
• Shale → Slate (fine layers, roofing)
• Sandstone → Quartzite (hard, durable)
• Mudstone → Schist (layered, flaky)
• Basalt → Amphibolite (dark, crystalline)
Each transformation reflects the composition and conditions of the parent rock.

Contact Metamorphism: Heat Only

Contact metamorphism occurs when magma intrudes into the crust, heating surrounding rock. Only heat is involved, no pressure. The surrounding rock “bakes”, forming a narrow zone called an aureole where physical and chemical changes occur.

Regional Metamorphism: Heat and Pressure

Regional metamorphism happens over large areas, especially at convergent plate boundaries, where plates collide. Rocks are buried deeply and compressed, creating high temperature and pressure conditions that alter their form over time.

Irish Example: Caledonian Orogeny

During the Caledonian mountain-building period, the North American and Eurasian plates collided, generating intense heat and pressure. This formed marble in parts of Ireland, including Connemara and Kilkenny, through regional metamorphism.

How Marble Forms

Marble forms when limestone or chalk is subjected to regional metamorphism. The process recrystallises calcite, removes fossil content, and results in a non-foliated, crystalline rock. It often contains large crystals and varied colours, depending on impurities.

Types and Colours of Irish Marble

Irish marble varies in colour due to different mineral impurities:
• Pure white marble: Rathlin Island
• Green marble: Connemara (contains serpentine)
• Red marble: Cork
• Black marble: Kilkenny (contains carbon)
These are used decoratively and have economic and cultural value.

Formation of Quartzite

Quartzite is formed from sandstone through either contact or regional metamorphism. Under high temperatures, the quartz grains recrystallise and fuse, producing a very hard, dense, non-foliated rock resistant to weathering.

Structure and Hardness of Quartzite

Heat causes silica to melt and fuse grains together. The process eliminates pores and layers, producing a rock that is much harder than the original sandstone and nearly impossible to split. This makes quartzite ideal for rugged landscapes.

Quartzite in Ireland

Quartzite forms many of Ireland’s most distinctive peaks, including:
• Croagh Patrick (Mayo)
• Errigal (Donegal)
• Sugarloaf and Lugnaquilla (Wicklow)
Its resistance to erosion explains why these peaks stand out in the landscape.

Other Metamorphic Rocks in Ireland

• Slate: from shale, found in Valentia Island, used for roofing
• Schist: from mudstone, with shiny, flaky layers
• Gneiss: from granite, seen in parts of Mayo
These rocks reflect Ireland’s complex geological history and tectonic activity.

Economic Uses of Metamorphic Rocks

Marble and slate are valuable building materials.
• Marble is used for tiles, flooring, fireplaces, and monuments
• Slate is used for roofing and paving
Their durability, texture, and beauty make them commercially important.

Decorative and Industrial Uses

Polished marble and quartzite are widely used in interior design and architecture. Their strength and aesthetic appeal make them ideal for flooring, countertops, and public buildings, boosting local economies in quarrying regions.