Volcanic (extrusive) igneous rock, e.g. basalt, forms when magma reaches the earth’s surface, at which point it is called lava. It cools quickly out in the open or under water. It is a dull brown or black colour with microscopic crystals. In Ireland basalt formed in the Giant’s Causeway 90 to 65 million years ago when the crust split apart and the Atlantic Ocean began to form. This rifting allowed basic magma to reach the surface and pour over the land, forming the Antrim Plateau.
5.2 Sedimentary rocks
Sedimentary rocks are made from the build-up (accumulation) of layers of inorganic or organic sediments. Each layer (strata) is deposited on a lower layer, squashing out water, minerals and air. As the sediment layers are buried they become compacted and eventually cemented together by silica or calcite to form solid rock. This process of turning sediment into stone is known as
lithification. Horizontal joints called bedding planes separate the layers.
Sedimentary rocks are classified according to their mode of formation: Weblinks
1. Organically formed 2. Inorganic or mechanically formed 3. Chemically formed
Weathering and erosion of land
Rivers transport sediments eroded from land to the sea