Albert Sauceda, age 11, of Santa Maria, California, for his question:
What makes blackboard chalk sometimes squeak?
This startling event occurs from time to time in every classroom. Group attention is focused on the blackboard as a soft stick of chalk moves smoothly along. Then suddenly the chalk utters a sharp squeak setting everybody's teeth on edge.
The major ingredient in a stick of white chalk is calcium carbonate, soft enough to rub off on a blackboard. However, almost always there are impurities in the substance, including a few grains of silica. The silicates are among the hardest of nature's natural minerals. They can scratch most other minerals. A gritty crystal of silica refuses to rub a soft powdery line on a blackboard. When pushed, it scratches along with a high pitched squeak.
Most of the world's chalk was formed during the Cretaceous Period, the Age of Chalk, that began about 130 million years ago and lasted some 70 million years. This last chapter of the Mesozoic Era saw the decline and departure of the dramatic dinosaur clan. The giant reptiles, however, had nothing to do with chalk making. This patient work was done by myriads of one celled creatures that lived in the warm seas. These protozoa are classified as the foraminifera, a scientific term meaning the window makers. The foraminifera built shells of calcium carbonate around their soft little bodies ¬with tiny window holes to keep in contact with the watery world outside.
Invisible fragments of calcium and carbon are dissolved in sea water. The foraminifera extracted these chemicals and made them into calcium carbonate, the building material used to create their houses with windows. As they lived and died, their limy little shells sifted down to the sea bed. There they mixed with the sand and other substances that carpet the floors of the oceans. Through later ages, the lands and seas changed. In some areas, calcium deposits rose above the water and became dry beds of chalk, limestone and even marble. Thick chalk beds were left high and dry in Kansas. In southern England, others became the famous White Cliffs of Dover.
These deposits provide the material we use to make blackboard chalk and dozens of other chalky items. The basic ingredient is white, powdery soft calcium carbonate. But through millions of adventurous years, both under the sea and on dry land, one could hardly expect the basic material to remain absolutely pure. The natural chalk deposits are grimy with dust and mud. Stones and gravel are mixed with the chalky ingredient. So are countless fine crystals of silica and other hard minerals. In refining this material to make blackboard chalk, manufacturers try to remove all the impurities. But a few gritty little crystals get through the screening processes.
Other limy deposits formed by foraminifera went on to other adventures. Many were uplifted, dried and compressed into massive beds of limestone. Certain layers of limestone later became involved in mountain making, volcanic activity and other immense movements of the earth's crust. They were altered completely by heat and pressure. The rough limestone became smooth marble. Various impurities tinged the marble with lacy lines and decorative blotches of assorted colors.