Millions of years ago, fine-grained clay particles settled on the bottoms of lakes and inland seas and formed a soft mud. This later hardened into the mud-rock that is called “shale”.
During this period, the earth’s crust moved and shifted. The layers of shale, covered by beds of other rocks, wore folded up into wrinkles. These were fattened and squeezed so hard that the shale became slate.
The clay particles making up the slate were deposited by the lakhs and seas in layers. Even after the pressure changed the shale to slate, the many separate layers of the deposits remained. And today we can split slate into wide, thin plates because it did stay in layers.
The most common colors of slate are dark grey and black, though it may also be red, green, or various shades of grey. The reason it is chiefly black is that the living matter in the original muds left carbon material.
Slate occurs only where mountain-making pressure and earth changes have been acting upon the layers of old shale.
Slate is used for many purposes. One of its chiefs uses is as a roofing material for homes and buildings of all kinds. Among other things made from slate are blackboards, tabletops, and draining boards.
How Strong Is Slate?
Slate is considered to be in between 2.5 to 4 on this scale in terms of hardness, which means that it is, on average, almost as hard as marble and limestone, but not as hard as granite or natural quartz.