Gina Jones, age 13, of Newport Beach, California for her question:

How does a mountain reveal its age?

Many an aged mountain has a white head and a snowy shawl around its shivering shoulders. Often its sides are wrinkled with rain washed gullies and its rounded peaks are not so tall as they once were. But the most noticeable feature is a calm refusal to participate in the fiery activities of the restless earth crust. An old timer just sits there patiently, while the weather wears down its once lofty peaks.

Few mountains stand alone. I lost of them occur in enormous ranges and their peaks rest on massive crustal ridges. Their ages are reckoned in hundreds of millions of years and their remnants remain long; after they are reduced to gentle hills. The earth reveals the roots of some mountains that arose about two billion years ago. These are the gently rolling, Laurentians of eastern Canada and, so far as we know, they are the oldest mountains with traces still left on any of the continents.     

The real old timers reveal their ages by exposing their roots. These rocky formations were pushed up from deep within the earth, ages ago when an enormous section of the crust uplifted. Much of this material is different from the usual minerals of the surface because it erupted from denser layers far below. Here, for example, we may find some of the densest granites and masses of marble metamorphosed from ancient limestone.     

These basic roots are exposed because of the age old conflict between the weather and every slope on the earth's surface. Countless seasons of rains and melting snows erode young; mountains from the day they first hump their shoulders above sea level. They continue to grow until the earth completes the mountainous readjustment of its crust. During this period, the peaks can outgrow the weathery erosion. After many millions of years, the building project is complete. The earthquakes and volcanos finish their work and the restless young mountains enter a stage of sedate middle age.     

Now the weathery atmosphere hag its way and the crests of the aging mountains begin to wear doom. Season by season, the surface rocks are cracked as they expand and contract in winter's frosts and summer's heat. Rains, winds and melting snows work with gravity to shift countless tons of loose material down the slopes. Gradually the once lofty pointed peaks axe rounded down, exposing their cores and hard rocky roots. When these basic fire formed rocks are near the surface, they reveal the great age of a mountain range.

A field observer has several methods to estimate whether a range of hills is very young or very old. The restless Coastal Range of California betrays its youth because it still is growing. The ancient hills of Minnesota became calm about 400 million years ago. Meantime, the weather has eroded and rounded their crests. The hard minerals o€ their ancient fire formed roots, though not yet completely exposed, are near the surface.