Ty Schuiling, age 13, of San Bernardino, California, for his question:
How are mountains built?
Our busy planet is engaged continuously in remodeling its wrinkled surface. Small projects, such as lakes, are minor events. Others re form vast areas of the earth's crust and span vast eons of geological time. The biggest and most dramatic of these projects is mountain making.
San Bernardino basks amid a scenic geological wonderland of fruitful valleys, stretches of mineral rich desert, rocky, sun baked slopes and jagged snow capped peaks. The dramatic formations run generally crosswise, underlining the massive Sierra Nevada that parallels the coast of California. :.'Trey are the Transverse Ranges and their formation is linked to the dynamic mountain making that created the Sierra Nevada. The uplift of the Sierra Nevada into a mountain chain began late in the Miocene Epoch, almost 20 million years ago, but it was another great era of uplift in the middle of the Pleistocene Epoch, less than one million years ago, which raised the majestic peaks of the range to their present towering heights.
The major modeling of this planetary scenery is typical of most mountain making. Slow, shattering activity occurs in the earth's crust. Under large plains, the crust may be 15 to 20 miles thick, under massive ranges it may be 40 miles thick. Yet geological measurements show that the average weight of the crust, thick or thin, is roughly the same. What's more, it balances the weight of the heavier sea beds.
The earth attempts to maintain this globe wide crustal balance but weather and water continuously shift vast masses of rocky silt downhill, adding its weight to the hollows. The weathery atmosphere and the weight balancing forces within the crust are engaged in eternal warfare. The opposing factors direct the slow rise and decline of massive chains such as the Sierra Nevada system with all its minor ranges.
A major mountain chain stands on the site of an ancient ditch called a geosyncline. Busy streams dumped weighty deposits into its shallow waters, triggering a gradual read¬justment of crustal balances. The sides of the geosyncline rose and the entire region up¬lifted. The crustal layers compressed and wrinkled, bent and cracked, buckled and folded into overlapping piles like tumbled sandwiches.
These massive upheavals generated stupendous crustal heat and pressure. The region shuddered with earthquakes, volcanoes erupted new layers of rocky lava. The growing chain was readjusting its crustal weight back to normal. Then the violent phase subsided. But meantime the watery weather had begun its counterattack. It wears down the lofty peaks. Rocky debris washed down their slopes adds extra weight to the hollows. This creates the necessity for crustal readjustments in the far future.
Earth scientists cannot explain fully why our global mountain chains form along certain well defined belts. They suspect that this problem involves forces within deep levels below the crust. One theory suggests that heat from below wells up and sinks in convection currents. These pockets would likely cause weak areas in the rocky crust. This theory may or may not explain why restless, mountainous regions tend to favor definite zones on the global map.