Metamorphic Rocks Notes

Metamorphic Rocks

• Rocks changed by heat and/or pressure.
• Examples: Marble (from limestone), Gneiss (from clay-rich rocks).

Formation

• Often occurs at plate boundaries.

Destructive Plate Boundaries

• Oceanic plate subducts under continental plate.
  • Temperature increases: Friction causes a rise in temperature during subduction.
  • Temperature increases $25^{\circ}C$ per kilometer.
  • Known as Thermal Metamorphism.
  • Pressure increases: Weight of overlying rock layers increases pressure.

Continental Plate Collisions

• Folding occurs, forming fold mountains.
  • Pressure changes rock structure.
  • Known as Regional Metamorphism.

Foliation

• Foliated rocks: Striped or banded due to squeezed mineral grains.
• Non-foliated rocks: No bands, form when hot igneous intrusions alter mineral structure without significant pressure.

Examples in Ireland

• Oldest rocks are metamorphic (e.g., Malin Head gneiss - ~490 million years old).
• Caledonian mountain building period: Laurentia and Avalonia collided.
  • Sandstone metamorphosed into quartzite (e.g., Sugarloaf Mountain, Croagh Patrick).
  • Connemara Marble created (Clifden, Co. Galway).