If you are going to work with slate, or use it as a roofing material, it is a good idea to know in basic terms what slate is.
The American Standards of Testing and Materials (ASTM) defines slate as,
“Slate is a micro-granular crystalline stone derived from argillaceous sediments by regional metamorphism, and characterized by a perfect cleavage entirely independent of original bedding, which cleavage has been induced by pressure within the earth”.
In simplified terms Slate is a sedimentary rock. It was formed hundreds of millions of years ago, while the mass continent of “Pangaea” existed. Slate is made up primarily of Quartz and Mica. Roofing slate in particular, is referred to as “Mica” slate. This slate was formed in clay silts, and due to geological pressure formed into separable rock suitable for roofing material.
Slate is hard, durable, and long lasting, and because of its composition is a great material for roofing.
One interesting fact, is that because slate formed on the mass continent of Pangaea, Canadian slate, American Slate, English Slate, and Spanish slate are all very similar! When these slates originally formed, they were only separated by hundreds of miles. Today due to continental drift, these slates are thousands of miles apart, but are still virtually the same product.
This basic information is pivotal in understanding slate. To learn more about slate, a geology class is a great place to start, along with books such as The Slate Bible, by Joseph Jenkins.
Remember, if you work with slate or are looking to put a slate roof on your house, use only the best – Georgetown Slate
Jenkins, Joseph. The Slate Roof Bible. Grove City: Joseph Jenkins Inc., 2003.