metamorphic rocks Marble and Quartzite Physical Geography

Definition: Marble rock is a hard crystalline rock that is formed when limestone undergoes a transformation due to regional metamorphism.
Regional Metamorphism: This process occurs when rocks are altered by the simultaneous application of both heat and pressure over a large geographical area.
Formation Environment: It is typically formed at destructive plate boundaries. This occurs when plates converge at subduction zones, causing buckling rock layers to push limestone deep into the Earth's crust.
Contact with Magma: When the limestone is pushed deep, it comes into contact with cooling magma, which facilitates the change into marble stone.

Recrystallization: During the process of metamorphosis, the calcite crystals originally present in the limestone are re-crystallized.
Density and Structure: This recrystallization results in a denser rock composed of interlocking calcite crystals.
Crystal Size: The scale of the transformation is proportional to the heat and pressure applied; greater levels of metamorphosis produce larger calcite crystals.
Properties: Marble is classified as a coarse-grained, resistant rock. It is highly durable and capable of being polished to a high shine.

Role of Purity: The final colour of the marble rock is directly dependent on the purity of the parent limestone.
Pure White Marble: * Formed from pure white limestone with no impurities. * This variety is rare. * Named Example: Pure white marble is found in Carrara, Italy.
Variables of Colour (Impurities): Impurities in the limestone cause various colorations: * Iron Oxide (Rust): Gives the marble a pink or red colour. Found in Co. Cork. * Serpentine (Copper Ore): Gives the marble a green colour. Found in Connemara, Co. Galway. * Carbon: Gives the marble a dark grey or black colour. Found in Co. Kilkenny.

Timeframe: Irish marble was formed approximately $450 \text{ million years ago}$ .
Orogeny: It formed during the Caledonian orogenic (mountain-building event).
Tectonic Activity: Formation occurred when the North American and Eurasian plates were converging. This convergence caused the rock to fold and allowed magma to rise into the buckling rock layers.
Economic Uses: Due to its resistance and aesthetic appeal, marble is highly valued as a decorative stone. It is used for: * Kitchen worktops. * Fireplaces. * Gravestones.
Example: Green marble from Connemara, Co. Galway.

Parent Rock: Sandstone is the parent rock of quartzite.
Type of Metamorphism: Quartzite is a metamorphic rock formed specifically due to thermal metamorphism.
Thermal Metamorphism: This refers to the process where rocks are changed primarily as a result of exposure to extreme heat.
Mountain Folding Context: Quartzite forms during periods of mountain folding. Layered sandstone is buckled and pushed deep underground by converging tectonic plates, where it comes into contact with cooling magma.

Superheating and Chemical Change: As the grains of the porous sandstone are superheated, they undergo a chemical change to become quartz.
Cementation: The spaces between these new quartz grains are filled with silica, which acts as a cementing agent to solidify the rock.
Resulting Characteristics: The intense heat from the surrounding region creates a new rock that is non-porous and significantly more resistant than the original sandstone.
Appearance: Quartzite is a pale-looking, medium-grained resistant rock, often appearing white in colour.

Decorative/Household Use: Polished quartzite is increasingly used for countertops. It serves as a more affordable alternative to marble.
Construction and Infrastructure: Crushed quartzite is used primarily for: * Road surfacing. * Railway ballast.
Locations in Ireland: Quartzite is commonly found on the peaks of major mountains. It was revealed on these peaks due to the weathering of the surrounding fold mountain material. Specific locations include: * The Sugarloaf, Co. Wicklow. * Croagh Patrick, Co. Mayo. * Mount Errigal, Co. Donegal.
Example: Mount Errigal, Co. Donegal.