Metamorphic rock Metamorphic rock Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The original rock (protolith) is subjected to heat (temperatures greater than 150 to 200 °C) and pressure ( or more), causing profound physical or chemical change. The protolith may be a sedimentary, igneous, or existing metamorphic rock. Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface. They are classified by texture and by chemical and mineral assemblage (metamorphic facies). They may be formed simply by being deep beneath
Aberllefenni Slate Quarry Aberllefenni Slate Quarry Aberllefenni Slate Quarry is the collective name of three slate quarries, Foel Grochan (sometimes misspelt as Foel Crochan), Hen Chwarel and Ceunant Ddu, located in Aberllefenni, Gwynedd, North Wales. It was the longest continually operated slate mine in the world until its closure in 2003. Foel Grochan is the quarry on the eastern side of the valley, facing Ceunant Ddu and Hen Gloddfa on the west; all three were worked as a single concern throughout their history. Technically all three of these are mines, not quarries, since all rock extraction takes place underground, though they are often