Iralise L. Miller, age ll, of Philadelphia, Pa., for her question:
How do they make slate?
In olden times the children doodled their pictures and wrote their school notes on flat slabs of slate. The classroom blackboard was a large slab of slate stuck on the wall. Nowadays our blackboards and bulletin boards, our scratch pads and note books are created from assorted man made materials.
Today We can buy man made flag stones and floor coverings that copy the beauty of natural slate. But real slate stone is made by Mother Nature, and the job takes millions of years. Our deposits are not very plentiful, and hard, smooth slate is an expensive building stone. Millions of years ago, slate making in the earth's crust began in Pennsylvania and Vermont, in Maine and Maryland.
Slate is classed as a metamorphic rock which means that it has been remade from some other rocky mineral. Its story began in the ancient past, and for millions of years the deposit was merely a layer of oozy mud. The silty layer may have been dropped fragment by dirty fragment in a flooded valley. It may have been a carpet of silty sand and fine fragments of clay that settled on the floor of a shallow sea.
In any case it was the dregs or sediment from some supply of muddy water. hater the water disappeared. Perhaps an old sea dried up or a flooded river changed its course. The layer of fine muddy particles also dried up and became a solid deposit of shale, which is classed as a sedimentary rock.
Shales or mudstones formed in many places where fine particles settled to the bottom of old seas and rivers. But in a few places dramatic events were going on in the earth's crust and shale deposits were changed or metamorphosized into slate. In. Most cases the rocky layers of the crust were involved in mountain making. Under terrific heat and immense pressure the fragments of the shale were re-crystalized. This completely changed the nature of the shale mineral to slate mineral.
A deposit of slate still has the flat layered features of muddy shale but it is much harder and more durable. Its fine, densely packed crystals are made mostly of clay and mica type minerals, and its slabs can be cleaved into smooth, flat layers. It has taken the earth more than 800 million years to make some of her slate deposits from the ancient mud.
Slate gray is the dull, dark color of a rainy day but not all the earth's slate is gray. some deposits are charcoal black and some are tinged with green. A few natural slates are colored a dark, rich red. Natural slate may be used in switchboards and other electrical gadgets. But most of our supplies are cut into roofing tiles and into tiles and flagstones to make durable floors for busy hallways and outdoor patios..