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Slate is a fine-grained metamorphic rock derived mainly from mudstone and shale.

What is slate?

Slate is derived from fine-grained sediments such as mud or occasionally volcanic ash laid down millions of years ago in layers known as bedding. As the pile of sediments thickened, the original open structure of the mud was compacted into a mudstone or shale.

These rocks are easily split on the bedding planes and are referred to as flagstones. However, for a slate to form, subsequent intense geological forces associated with mountain building are required, during which the minerals present in the original mudstone are metamorphosed.

Some minerals, such as quartz grains, are flattened and stretched, while clay minerals are recrystallised as platy minerals: white mica and chlorite. The quartz minerals give the slate strength and durability, while the platy minerals form cleavage planes, which do not correspond to the bedding planes, but which allow the rock to be split into much thinner slabs suitable as roofing material. Differences in the composition of the original mudstone and the degree of metamorphism affect the quality of the slates thus produced.

natural slate detail

The geology of slate

  • Slate is a metamorphic rock of sedimentary origin.
  • It dates from the Ordovician period (550 million years ago)
  • Is mainly composed of quartz, sericite and minerals of the chlorite group.
  • Has been used as a construction material since the ancient Egyptians.
  • Its colour depends on the mineral structure and size of the mineral grains that define its composition.

Uses of Slate

Slate has a long tradition as a construction material. Slate is a natural, aesthetically pleasing stone. Its strength and outstanding resilience make it one of the most durable and versatile roofing materials. The beauty of slate lies in the fact that it is a natural resource that requires little alteration from its natural form and offers a long lasting, pollutant resistant, fireproof, waterproof material with a warmth and elegance which places it in a league of its own.

Slate is mainly used for:

What is the age of slate?

The age of the slate stone varies depending on the regional geology.
For example in Scotland, Ballachulish slate was formed from sediments laid down approximately 700 million years ago and was metamorphosed 500 million years ago, while Spanish slate from the Orense area was formed from muds laid down over 450 million years ago and was metamorphosed 300 – 350 million years ago.

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