Tim O'Keefe, age 12, of Pittsburgh, Pennsylvania, for his question:
How did sand form in deserts far from the sea?
We take it for granted that the golden sand on the beaches is created by the pounding ocean. Actually the smashing, dashing craves only help to do the job by bashing big rocks into small fragments. The winds, the weather and running rivers also help to separate gritty grains of sand from massive slabs of rock. These pounding forces are at work everywhere on the earth. The relentless sand makers of desert regions are mostly wild winds, sharp showers and changing temperatures.
When you let a handful of sand dribble through your fingers, you suspect that it is something rather special. Its hard little grains are not fragments of clay or of any other soft or fairly soft rock. Actually they are made mostly of silica and silica is the hardest of all the common minerals. It rates in class seven on the scale of mineral hardness. The only harder stones are the topaz types in class eight, the gor¬geous corundums of class nine and the glittering diamond of class ten. All the natural minerals harder than sandy silica are gems.
The story of sands of the deserts and beaches begins in the bowels of the earth, when molten magma forms underground. How and then this mixture of melted minerals and steamy gases erupts to the surface or oozes between layers of rock below. It cools and sets solid. As this happens molecules of this or that mineral attract each other and tend to form crystals. Silica molecules often congregate in gritty little grains of golden sand. As the mixture cools to form granites or other igneous rocks, the sandy silica is embedded in masses of felspars, micas and other weaker minerals.
The patient forces of weathery wind and water erosion wear away weaker minerals first. The tough silica crystals are freed and tumble doom in piles of sand. When this occurs under water, the sand may be embedded again in masses of sedimentary sand¬stone.
Vast areas of the continental crust are topped with sandstones and limestones, clays and other sedimentary rocks. Almost everywhere they rest on deeper layers of ig¬neous rocks. In arid regions, wild winds soon erode exposed sandstone and hurl the released grains of hard silica at the faces of tougher rocks. Outcroppings of igneous bedrock are exposed and sandblasted with blowing bullets of hard grit. As the weaker minerals are eroded, more and more sands are freed to join the shifting desert dunes.
Some deserts suffer severe seasonal changes and even in the tropics scorching days are followed by cold nights. Rocks expand and contract. with changing temperatures, which mattes them crack and break into fragnients. This temperature erosion prepares the nay for relentless mind erosion. Mater from snows or brief desert deluges also weaken the arid rocks. 111 these forces of erosion work together to free imprisoned crystals of tough silica and create sands in the dry deserts.