The "World" of Slate

Slate Defined

Slates vary greatly in color from black through various shades of gray to greens, reds, and purples of different hues. They vary also in luster from having none to being almost as bright as mica itself. They vary greatly not only in grade of fissility (the property of rocks to split along planes of weakness into thin sheets) but in surface texture, as seen by the unaided eye or determined by touch, and still more in microscopic texture, as seen in thin section. They differ also in their mineral and chemical composition and in their physical properties.

Both slates and schists may have originated in deposits of identical character, but they have undergone different processes. Slate however, should not to be confused with schist which is a rock that may be of identical chemical and mineral composition but is either made up of coarser particles, possesses a wavy structure, or else is marked by both of these features.

For more information about the geology of Slate CLICK HERE.

Since the most extensive use of slate is for roofing purposes, the foremost point of concern is durability. A roofing material is obviously exposed to the most severe weather conditions. Roof temperatures may often reach 140° F., and the exposure offers no protection against the lowest winter temperatures. Moisture, ice, snow, and hail all contribute to the severe conditions. Wind storms deflect the rafters, and where slates are nailed down too tightly, they may strain to the breaking point. Besides the normal weather conditions, slate is exposed to flue gases and the leaching from deposits of soot. Evidently the durability of slate in such exposure depends upon the resistance of the mineral component; hence a study of the composition is important. Strength, elasticity, and density are also important characteristics.

Slate is found in sections of the United States other than the Slate Belt of Pennsylvania, but these locations where slate is quarried is obviously dependent on the specific geology that is necessary to produce slate. When looking at a map of slate locations across the United States, the vast majority of them are found along the eastern edge of the Appalachian Mountains in a region known as the Great Valley.

In addition to the locations for slate quarrying across the United States are also slate regions throughout the world. Some of the most famous slate quarries in the world are found in Wales, a region that produced many of the slaters that worked the slate quarries of the Slate Belt in Pennsylvania and from where the name Bangor originated.