Carolyn stokes, age 13, of Lansing, I11., for her question:

How is beach sand made?

There are dazzling white sands and sands of sooty black. There are brindled mixtures of browns and sands rich in metallic fragments. But most of the beach and desert sands we know best are golden yellow. And every kind of sand has its own long story to tell.

Gritty grains of sand are made of very durable minerals. They may measure anywhere from 12 to 400 to the inch and chances are they can Endure the pounding waves or the wild desert winds for centuries before breaking into still finer fragments. Our favorite stretches of beach sand are made mostly of quartz, and quartz happens to be the hardest of the Earth's common minerals.

It is a compound of the elements oxygen and silicon. Its molecules were formed when the Earth was young, often during violent volcanic activity. Other minerals also formed at such times and mixed together in flowing rivers of molten lava. When the lava cooled, the mixtures of minerals hardened into granites and other igneous rocks.

Granite is a plum cake mixture of assorted minerals, some lighter and some darker, some harder and some softer than the others. Sometimes the mottled rock contains tiny grains of golden yellow quartz. All surface rocks are Exposed to wind and weather, and some of them must face the pounding waves of the sea.

The constant dripping of water wears away the hardest rock. Its softer minerals dissolve first and are washed away. As fragments are lost, the rock crumbles and breaks apart. The hardest fragments remain whole until last and often these tough little pieces are grains of quartz. Boulders break up when desert winds hurl sharp gravel and gritty dust in their faces. Granite crags break up when winter frost and summer heat make their pores and pockets swell and shrink again and again.

Granite crags that face the sea are smashed and bashed by tides and tossing waves. In time, their softer minerals chip off or dissolve and wash away. The grains of durable quartz once buried in the lava are then freed. They slither and slide with the heaving water, down to the level of the beach. Through the weathering of ages, they pile up in deep layers and wide stretches of golden yellow sand.

Some sands contain grains of feldspar, and some are mixtures of gold En quartz and granules of other hard minerals. Dazzling white sands, as a rule, are made from granules of gypsum. Sooty black sands are made from grains of basalt. This hard mineral also took form during ancient volcanic activity.