What is Slate
The word slate is sometimes used to describe any stone that splits into flat slabs and is used as roofing tiles. True slate, however, is a very distinctive dark grey, brittle metamorphic rock that flakes into smooth, flat sheets. It is created when shales and clays are altered by low-grade regional metamorphism at low temperatures and moderate pressures.
Getting its name from the Old German word for break, slate is essentially metamorphosed mudrock that has been strongly compressed deep underground, but in a low-grade regional metamorphic environment, away from the most intense metamorphism. Conditions such as these are found deep at the root of fold mountains, where the convergence of tectonic plates slowly but surely crushed rocks deep down. Most slates occur in old mountain chains, like the Appalachian Mountains of the USA, or Snowdonia in Wales. They tend to be Precambrian or Silurian in age. Occasionally, though, they form in more recent fold mountain chains, like the Alps.
Slate varies enormously in colour, though it is normally dark grey, or purplish or greenish grey. But it is always easy to recognize because of the way it cleaves into the flat sheets that make it so useful for roofing. This distinctive ‘slaty’ cleavage develops when mudrock is metamorphosed. As the rock is compressed, water is squeezed out and the rock is compacted. All the tiny clay and silt grains are not simply recrystallized as mica and chlorite but are reoriented at right angles to the pressure. This alignment occurs partly because just as the layered nature of clay crystals means they can be moulded, they can also be pressed flat, and partly because new crystals grow in this direction.
Metamorphism usually destroys most of the original sedimentary structures, so the cleavage planes are entirely unrelated to bedding planes and are probably at an entirely different angle. Fossils are usually destroyed by metamorphism, too. Only when the pressure is pretty much at right angles to the bedding are fossils preserved – though often rather dramatically flattened. Because slate’s cleavage is produced by the same forces that fold mountains, slate cleavage usually clearly marks out the pattern of folding in the formation and the direction of compression.
This makes it a very useful rock when studying the tectonic history and structural geology of an area.
Ref: The illustrated guide to rocks & minerals, John Farndon.