How do mountain folds and faults differ?
Our old earth has a scarred and wrinkled face, spotted with pimples and fiery rashes. The bumpy surface is in slow, but never ending, upheaval. The story of this planetary face lifting is unfolded in the study of earth science. But there are so many details to tell. Sometimes we read of mountain folding and faulting as though the two different operations were one and the same, though actually they are separated by vast eons of time.
Our globe is covered with a rocky crust, somewhat like the skin of an apple. As an apple becomes old and dry, its once smooth skin becomes loose and wrinkled. For reasons we do not fully understand, the crust of the earth also fails to fit smoothly. Its wrinkles are the mountain ranges.
The face of the earth is whipped by winds and washed by gushing rivers, pounded by ocean tides and clawed by icy glaciers. Sand and silty debris from this constant warfare is toted from place to place in billions of tons. This slow shifting upsets the thickness, and more important, upsets the weight of the crust.
There are forces, we are told, inside the earth which work to keep the weight of the rocky skin evenly balanced. The mountains, which grow up and wear down again, may be part of the effort to redistribute the weight of the crust. A large range is born in a long, shallow ditch called a geosyncline.
Through eons of time, countless streams empty their muddy debris into the geosyncline and layers of clay and other sedimentary rocks form on its floor. At last the balances of crustal weight change and forces in or below the crust begin to push up the shallow ditch.
This gentle wrinkle is a sandwich of many different layers of rock and as the mountains grow they bend in loops, one above another. This is a mountainous folding of the earths crust. If the range grows higher, the fold may break under the stress the wrinkle cracks, usually down the middle. Now we have a fault or crack in the rocky layers of the crust.
The fault may slice down miles through the rocky layers. If the pressure continues, one side of the crack may rise high above the other, exposing its rocky layers in a sheer cliff. One side of the fault may slide right over the other, forming one multiple sandwich of rock on top of another, our western mountains have countless folds and faults. The folds have gentle slopes, often covered with soil and trees. The faults show ragged peaks and sheer cliffs striped with rocky layers. There may be stresses below the folds, but the pressures below faulted regions are far more severe. Here we tend to get volcanoes and the unstable regions which cause earthquakes.