Carole Diane Riggs, age 9, of Columbus, Ohio, for her question:
Do mountains keep on growing?
We tend to think that mountains live for ever. And they do live a long time. To a mountain, a hundred years is but an hour. Yet mountains do grow, become old and die. In Wisconsin, Minnesota and nearby Canada are the roots of old mountains that poked up 500 million years ago. The weather has worn their proud peaks down to gentle slopes and rolling plains.
The weather is always at war with the mighty mountains. Frost and ice crack up their big boulders. Rain gushes down their slopes, carrying along their stones and pebbles. The wind hurls sand and dust at their cliffs and peaks, wearing them away. Bit by bit, the tops of the mountains slide down the slopes to the valleys.
Meantime the mountains try to keep pace with this havoc. They grow, pushing up from great depths below the ground. And a range of mountains may keep on growing for millions of years. The great Himalayas were pushing up some 60 million years ago and are still growing. The Coastal mange along the Pacific is a million‑year‑old youngster with ages ahead in which to grow.
Geologists know a great deal about the life history of the world's great mountains. But no one is sure of all the reasons which make the old earth hump up her back and form them. Mountain‑making takes place in the earth s crust, the light rocks which form its outer skin. In certain regions wrinkles seem to form in the crusty skin. The folds grow into mountains, wear away, then wrinkle up again.
North America has two great mountain regions. The Appalachians sprawl down the eastern edge and the Rockies, Sierras and Coastals stretch down the western side. So it has been for millions of years and in those places mountains have risen and worn away many times.
Long before the mountains started to grow these regions were hollow troughs in the earths crust. The troughs were filled with shallow seas into which drained turbulent rivers. Sand, gavel and stones wire carried into the troughs. Sea creatures added thick layers of shells to the floor. All this extra weight was added to the earth’s crust. And the earth, it seems likes to keep the weight of her crust fairly even all over. The experts tell us that mountains may well be caused by the stresses and strain of her efforts to keep her weight well balanced.
In any case, the crust of the earth begins to hump. It bends, buckles and sometimes breaks. One sandwich of rocky layers may be pushed to overlap another. The upheaval causes earthquakes and volcanoes. Lava pours up from below, adding new layers to the growing mountains.
Mountains grow slowly, but it is believed that they grow in spurts. And all the time they are growing they are at war with the weather. After millions of years, when the growing is done, the weather wears them down level again.