Welcome to You Ask Andy

Norman Kelchner, Jr., age 14, of Williamsport, Pennsylvania, for his question:

What is the geosynclinal theory?

The name was coined in 1873; the theory that described it was suggested in 1912. The story unfolded as researchers traced the geological history of the earth back through the ages. In the early 19th century, most people were positive that the earth was merely a few thousand years old. So when Louis Agassiz proved that enormous glaciers had.crept over Europe during the past million years, almost nobody believed him. Now we know that he was right    and so were those who found traces of ancient geosynclines.

The Alps stand on a massive hump in central Europe and puncture the sky with their pointed peaks. Their snowy crests, forested slopes and verdant valleys form a panorama of spectacular scenery. A few generations ago, when geologists began decoding the diary of events recorded in the rocks of the earth's crust, the alpine region provided a wondrous research field. The steep slopes exposed strata like multiple rocky sandwiches, some crammed with marine fossils, others with the fossil remains of land dwelling plants and animals.

All this mountainous evidence was pieced together to give the fantastic history of the region, page by rocky page. Obviously those apparently age old peaks were rather young, as mountains go. About 60 million years ago they were just not there. The region was swamped with one or two shallow seas, amid sloping slabs of dry land and volcanic islands. Most of this research was done by European geologists. But two American geologists, Hall and Dana, coined the word "geosyncline" to name the ancient hollow that gave rise to the towering Alps.

Traces of geosynclines were found in the Rockies and most major ranges of the world. A French geologist named Haug assembled all the data and suggested how geosynclines give birth to mountains. Naturally the process moves slowly through many millions of years. The geosyncline may be a long narrow ditch or a sprawling shallow sea. But always running streams flow down its sloping sides and dump their dirty debris in its basin.

Through the ages, massive deposits cover the floor of the geosyncline. Sometimes it rises and, for a while, soggy islands may support land plants and animals. Sometimes its heavy weight sinks and older layers become compressed into solid rocks. This may explain why land or marine fossils are found at different levels high in the Alps.

Crustal slabs surrounding the geosyncline are more rigid than its soggy sediments. Eventually they exert pressure. The pliable deposits push up, bend, crack and fold to form rising mountains. Most modern geologists agree with Haug's geosyncliaal theory, except on one point. He was sure that all geosynclines eventually give rise to mountains. It is now thought that some of them do and others do not.

As the young mountains rise, the weather starts to wear them down. Rains rush down their slopes and erode gullies. Frosts and heat, snows and blowing sands add to the erosion. For millions of years, the mountains outgrow these weathering processes. But at last they reach their full height and the weather spends millions of years wearing them down to gentle hills.

 

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