Stephen McInnis, age 11, of High Point, North Carolina, for his question:
How come the earth's rocks are arranged in layers?
Somebody once sent Andy a book on this topic. The author had an original theory, oh yes, to explain the crustal rocks. He was positive that the layers fell down one at a time from the earth's dense, ancient atmosphere. Needless to say, our geologists, who have their scientific feet on the ground, did not agree with this one man theory. This all goes to show that you can't believe everything you read.
If a person wishes to be picky, he could point out that mountain peaks may not be arranged in neat rocky layers and that jumbled assortments of rocky debris are found at many medium and lower levels. True. However, a serious student is aware that immense forces have been modeling and remodeling the earth's crust through billions of years. The general pattern tends to form rocks in orderly layers but one would expect all this restless activity to leave a few untidy corners here and there. In time, they too are molded into uniform layers.while various collections of rocky debris are left elsewhere.
Our globe is almost 8,000 miles through and the thickness of its crustal shell varies from five to 40 miles. The thick lower level is a densely packed layer of heavy basalt type rocks. It forms the solid floors below the oceans and a durable platform below all the continents. Under the sea, this basic layer is carpeted with deposits of shells and oozy slime that continuously settle down through the water. The continents cover it with massive shields made of somewhat lighter rocks and minerals.
Minerals are chemical elements and compounds. Their number is limited but the earth's immense building activities blend them into a countless variety of rocks. Volcanos and growing mountains create and recreate vast deposits on land and under the sea. On land, the erosion of wind and weather shift masses of surface material, creating new surface deposits. Ocean sediments tend to form in flat layers and in time erosion tends to level the hilly bumps on the continents. New layers are formed on older layers as restless earth activities bring changing conditions.
The leveling process is assisted by gravity, pulling down everything on the surface with equal force. In time, gravity also causes heavier rocks to sink below lighter deposits. The weight of the surface rocks tends to crush the lower, older deposits into flat layers. Meantime, different surface deposits are forming. Seas invade the land, creating sediments of silt and shells. When they retreat, these deposits become beds of limestone and sandstone. Rivers change their courses, leaving muddy deltas that form layers of clay. Maybe these areas are covered later with seething lavas and still later by silty flood waters. Gradually the assorted layers of the earth's crust form one upon another. Each is a page of geological history, revealing how and when it was formed.
The most dramatic earth activity is diastrophy, or mountain building. As thick crustal blocks are pushed up, their neat layers are humped out of shape. The crust bends and cracks and immense rocky sandwiches are shoved one upon another at all angles. But for those in the know, these scrambled and crumpled slabs also tell the same stories. Each layer was formed under certain conditions and later deposits were formed on top of it.