Slate

Background |  Name |   Sources |  Uses |  Substitutes and Alternative Sources

MII Photos

Background

Slate is a foliated metamorphic rock derived from the metamorphism of shale. It is formed by regional metamorphism from tectonic plates colliding with one another creating immense pressure. Its foliation does not coincide with the layering or foliation of the original shale. Foliation in regionally metamorphosed sediments runs perpendicular to the direction of the forces of metamorphism.
Geologists recognize that metamorphism occurs in different grades. The grade represents the amount of pressure and heat involved in the metamorphism of a particular rock. Slate represents low-grade metamorphism of shale. As the pressure increases, the grade of metamorphism increases through a series of different rock types. With increased metamorphism the crystals in the rock become larger. The mineral grains in slate cannot be seen with the naked eye. However, increased metamorphism causes the mineral grains in slate to grow resulting in a higher-grade metamorphic rock called phyllite. The larger mineral grains give phyllite a shiny appearance. As the grade of metamorphism increases phyllite becomes schist, which has easily-identifiable quartz and mica grains. Further increased metamorphism results in a still higher grade metamorphic rock called gneiss (pronounced nice). If the metamorphism increases to an even higher grade, gneiss partially melts into magma (liquid rock) and upon cooling becomes migmatite. It is impossible to tell the difference between metamorphic migmatite and igneous granite in hand specimens. They are differentiated by the geologic environment in which they are found.

Slate has a dull appearance and occurs in a number of colors including light and dark gray, green, purple and red. It is not unusual for pyrite crystals to form in slate. Pyrite forms due to iron minerals present in the original shale from which the slate formed.

Name

Slate is found worldwide in geologic settings where the continental crust is compacted and folded by the collision of two continental plates.

In the United States, slate is abundant in the so-called Slate Belt of eastern Pennsylvania in the Appalachian Mountains. Slate is quarried in the Green Mountains of Vermont. Vermont is the largest producer of slate in the United States. The slate deposits of Vermont run westward into eastern New York State where it is quarried in the town of Granville. This region is locally known as “Slate Valley.” Some smaller occurrences of slate have been important in the U.S. The slate found in Monson, Maine was abundant early in the 20th century, but has been nearly depleted with only one quarry still working. President John F. Kennedy’s grave marker is made from the dark slate quarried in Monson, Maine.

Worldwide, significant slate occurrences are found in Wales, England, Italy, Portugal, Germany, Brazil and China. Production of slate in China for export throughout the world is growing rapidly each year.

Uses

A very small amount of slate is crushed and used for road construction, concrete mixes and other construction purposes. In these instances, it is used locally when it is more expensive to import other crushed stone products such as limestone or granite.

Of all the dimension stone applications of various rock types, slate represents only 1% of dimension stone applications. This represents approximately 15,000 tons of slate used annually in the United States. Slate’s foliation allows it to be broken into sheets of any desired thickness. Therefore, for centuries it has been used for roofing and for pavement stones around homes, buildings and gardens. The same feature made slate a most suitable material for making pool table tops. Until the invention of “white boards” and erasable markers, slate was used in schools as chalkboards, both small sizes for individual students and large wall-sized sheets for teachers. The days of slate chalkboards are nearly gone.

Substitutes and Alternative Sources

Slate roof tiles are extremely expensive, but also extremely durable. Homes shingled with slate seldom need to have the roof replaced, except for individual shingles due to storm damage or after many, many years of use. Because of the expense, shingles made of a variety of other materials are more commonly used in construction today. Some use wooden shake shingles. Others use terra cotta tiles. Other options include corrugated plastics and metal roofing. Perhaps the most common are asphalt shingles which are made of paper soaked with bitumen and covered with granular aggregate.

Any kind of rock that forms a flat surface can be used for yard and garden decorations and walkway stones.

By Darryl Powell

Darryl Powell is a 1984 graduate of the University of Rochester, Rochester, New York with a B.S. in Geological Sciences. For six years Mr. Powell was an Adjunct Professor at Finger Lakes Community College in Canandaigua, New York, where he taught “Introduction to Physical and Historical Geology.” He creates Earth Science educational kits for NeoSCI, Inc., a science education resource company based in Rochester, New York. He also writes Earth Science materials for NewPath Learning, Inc. based in Victor, New York. Mr. Powell is the founder of Diamond Dan Publications, a small company focused on creative activity books and activity-based learning games about rocks and minerals, based in Manchester, New York.