Schist is a metamorphic rock. This type of rock has been changed because of great heat or pressure, or both. This usually happens when mountain chains are being made at destructive plate margins. The schist is formed from mudstones and shales which were metamorphosed. This process completely removes all traces of the sedimentary rock's original bedding planes, fossils and so on. The clay particles were recrystallised into mica which often gives schist a shiny character.
The schists in Northern Ireland can be found in the Sperrins and in a small inlier close to Ballycastle in County Antrim. They were formed when original rocks, deposited in pre-Cambrian times between 700 and 600 million years ago, were metamorphosed by a period of mountain building as plates crashed against one another. These were the North American and Eurasian plates and the period of mountain building was during the Ordovician period, between 440 and 500 million years ago. The schist we see today is at least 440 million years old.
|Schist data||Schist sample|