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Shale to Slate
Slate is formed by the metamorphosis of a sedimentary rock, shale. The shale rock is exposed to pressure, causing it to change into slate.
Pressure Change
The pressure causes faults in the slate--called cleavage--to form. This results in long, parallel sheets of rock like pages in a book. This is called foliation. When the rock splits, the slate falls apart along those cleavage lines.
Useful Slate
Since slate naturally splits into large flat surfaces, it has long been used for a variety of uses. If you have ever looked closely at a fine fireplace, you have probably seen slate as the base beneath the fire. It is valued for its flatness. It is commonly used for walkways, blackboards, construction and in finer pool tables.
Fate of Slate
Accessible slate itself does not turn or metamorphize into another form of rock. Since it is near the surface, there is no pressure to cause change. Instead, the forces of erosion weather the slate into rock fragments, sand and dust. These small pieces of slate are swept away by wind and rain into low-lying areas. As more and more of these sand and rock fragments gather, the increasing weight forces them together. The cementing action of silicates and carbonates mixed with the slate particles causes the fragments to solidify into a solid mass, and sandstone is born.
Not Just Slate
Shale doesn't automatically turn into slate. It must be kept away from certain chemicals and kept at a certain temperature, or it will become another type of rock as the pressure, heat, and chemical action continue over long periods of time. Shale at first becomes slate, but given enough time, pressure and heat, it will continue to change into phyllite, schist and then gneiss.