Bob MacDonald, Jr., age 12, of North Vancouver, B.C., Canada, for his question:
How is clay formed?
A gob of muddy clay is just about the drabbest thing in the world, or so one might think. Actually the various clays are among the most interesting items in the earth's crust. And certainly they are among the most useful. Clay of some sort is the main ingredient in the soils that support the plant world which supports the world of animals. Certain clays also played an important role in human history.
No two clays are exactly alike, though the basic formula is the same. Each formation is a by product of the ceaseless warfare between the weather and the rocks of the earth's crust. Every deposit is made of fine fragments that eroded from a once solid boulder or a large slab of rocky minerals. Rocks expand and contract with the alternating heat and cold of the seasons. This weakens and cracks them.
Seeping rains fill pores and crevices. When this water freezes, the ice expands and widens the cracks. Winds and rains loosen the weakened fragments and sweep them away. This erosion is a slow process but its powdery debris provides the basic. ingredient for the formation of clay. However, its mineral fragments must meet certain specifications to become real clay.
Its fine particles are flat plates, small enough to measure several thousand to an inch. Their dimensions give the clay its special qualities because they overlap and interlock with each other. This is why clay is plastic and moldable, oily to the touch, sticky and slippery. I Then water is added, each tiny platelet holds a film of moisture around itself by electrical attraction. It can slide and glide among its neighbors, which is why wet clay is tacky and slithery. If it is baked slowly to a high temperature, the plates interlock and form a hard ceramic material.
One of the two main ingredients in clay is silica, a mineral compound of silicon and oxygen. The other is aluminia, an earthy compound of aluminum and oxygen. Some clays have little or almost no aluminia and those richest in silicates make the best ceramics. These basic ingredients are contributed by the erosion of feldspars and igneous rocks, such as granites and lava.
Naturally other ingredients become mixed in the erosion process. Iron oxide adds the rusty tints to reddish clays. Small helpings of carbonaceous materials add shades of grey. A pure mixture of moisture, silica and aluminia forms kaolin, a white clay used to make the finest porcelains. These and other various clays are not the final chapter of the story. Dried beds of clay become layers of hard rocky shale and shale may become hard, brittle slate.
In their present form, the various clays are the bulkiest ingredients in the soil. They hold together the humus and decaying organic materials. They work chemically to absorb ammonia and other gases needed by plant roots. However, their agricultural talents are limited. If a soil has too much clay, it becomes hard and stiff: Neither moisture nor tender roots can penetrate it.