Debby VanSickel, age 13, of Mulvane, Kansas, for her question:
Where does all the sand come from?
Nobody has estimated the total tonnage of sand on the earth's .beaches and deserts. This enormous amount of natural material occupies a whole chapter in geological history. The minerals of the earth's crust were formed by a complexity of natural forces, working patiently through eons of time. The story behind all that sand is one of the longest and most dramatic.
Those gritty, golden grains are made mostly of silica, a compound of the elements silicon and oxygen. Silica happens to be the hardest of the earth's common minerals and also the most plentiful. Most sands are formed as erosion frees small crystals of silica from igneous rocks. The first chapter involves the dramatic building of
mighty mountains and fiery volcanos. The second chapter involves the patient wearing away of fire formed rocks.
The story of sand can whisk you from the small samples in a rock collection to the timeless, global activities of the earth's crust. Some of the small rock samples are frozen chunks of assorted fragments. Many granites are mixtures of visible mineral crystals, somewhat like the interesting items in a fruity Christmas cake. As a rule, a great many granules of silica are embedded in these rocks.
The first fierce chapter began miles underground, in the roots of some ancient volcano. Heat and pressure created a deep reservoir of magma a molten mixture of minerals, steam and bubbling gases. The magma erupted and spread rivers of red hot lava on the surface. Most of the frothy steam and gases escaped as the air cooled the molten mixture. As they cooled, the molecules of each mineral tended to attract each other. Silica molecules united in small, hard crystals and an assortment of other minerals solidified around them.
As soon as the new igneous rock was formed, the forces or erosion went to work to wear it down. Its weak spots cracked as summer heat and winter frost forced the rock to expand and contract. Rains and melting snows licked its surface and crevices, dissolving invisible fragments from the softer minerals. Gravity pulled the water downhill, carrying its dissolved chemicals to the sea.
Through ages of time, most of the softer minerals were worn and washed away by patient erosion. But the gritty grains of silica stubbornly resisted. As the other minerals eroded, they were freed from their rocky prison. They gathered in sandy piles and at last the force of gravity could shift them from place to place.
The liberated sand started traveling. It tobogganed down the slopes, blew around the deserts and swept down to the sea with the rivers. On the beaches it joined other sands that had formed as pounding waves eroded grains of silica from rocks at the water's edge. Some of our sands were born yesterday, others are durable old timers.