Physical Geography- Sedimentary and Metamorphic Rocks

Explain the formation of sedimentary rocks and explain briefly how they may be transformed into metamorphic rocks, with reference to Irish examples you have studied.

Sedimentary rocks were formed millions of years ago when the remains of animals and plants built up and compacted and cemented to form solid rock. Sedimentary rocks form in layers . This is because over time the rock fragments and remains of plants and animals build up in stratified layers. Each stratified layer is separated from the next by bedding planes, There are 2 types of sedimentary rocks: organic and inorganic. Limestone is an organic sedimentary rock. It is the most common rock type in Ireland and formed from the cemented remains of fish and other sea creatures. Limestone is made up of bedding planes containing vertical cracks called joints, which make the rock permeable. The Burren in Co. Clare is a well-known limestone region. Sandstone is an inorganic sedimentary rock. Sandstone is formed when rock sediments laid down in layers and over time each successive layer is compressed to the next layer. Most of the sandstone in Ireland is old red sandstone and was formed approximately 400 million years ago. Sandstone can be found in the Comeragh mountains, Co. Waterford and the Macgillycuddy Reeks, Co. Kerry.

Metamorphic rocks are formed when the appearance, texture and chemical composition of igneous or sedimentary are changed by great heat or pressure, or both. This can happen in 2 ways. Thermal metamorphism occurs when rocks are exposed to the heat of magma alone and this changes their composition. Regional metamorphism occurs due to great heat and pressure over a large region due to colliding plates and rising magma. With metamorphism, limestone is changed into marble. This is a hard crystalline rock of various colours and can be found in Connemara, Co. Galway. Sandstone is changed into quartzite. This is a light-coloured rock, often white, and can be found in the Great Sugarloaf Mountain, Co. Wicklow and Croagh Patrick, Co. Mayo.