Henry Enns, age 11, of Winnipeg, Manitoba, Canada, for his question:
What exactly is a Geosyncline?
The assorted "geo" words are related to the earth, to its crusty formations and the massive upheavals that cause them. The rest of our earthy word means "having slopes." Earth scientists use the term "geosyncline" to describe a special kind of immense draining ditch with sloping sides.
We know that the earth's major mountains last for hundreds of millions of years and we see few if any changes in them during a human life time. Nevertheless, they do not last forever. Gradually they rise from the ground to their full height and gradually the windy and watery weather wears down their proud peaks to gently rounded hills. This part of the geological event is not hard to imagine. But it is hard to believe that the proud Rockies and even the lofty Himalayas now stand on what once were holes in the ground. A special hollow of this sort in the earth's crust is called a geosyn¬cline.
This cradle of mountains is an immense shallow depression that serves as a drain¬ing ditch 'for streams and rivers over a vast area. Its shape is long and narrow, usually with a length of thousands of miles and a width of hundreds of miles. Its fresh drainage water may or may not be swamped from time to time by an arm of the salty sea. The drainage water from its sloping sides brings down sandy silts, stony gravels and other debris washed along by the streams. From time to time, the geosyncline is choked with layers of these mineral deposits. Later perhaps it is refilled by invading tides or surplus rainfall.
The lifetime of a geosyncline may last for a 100 million years or more. And its history is a series of gradually changing events. The most important feature is the fact that the debris deposited in its basin tends to stay there, adding layer upon layer. This mineral material is heavy and it grows heavier through the ages. Even¬tually it is heavy enough to upset the normal balance of the earth's crust. The global weight of the rocky crust is adjusted by a system of balancing forces called isostasy. The earth tries to keep the weight of its crust evenly distributed around the globe. The accumulated weight of the massive deposits in an immense geosyncline eventually upset the balance, usually in a long narrow area along the edge of a continent.
Earth scientists suspect that at this point the balancing forces of isostasy begin to readjust the weight of the crust. The heavy floor of the geosyncline pushes upward and great crustal blocks may squeeze together on either side. A range of young mountains begins to lift up from its geosyncline cradle. The mammoth operation involves the shifting and shoving, bending and buckling of massive crustal blocks. This triggers the earthquakes and volcanic upheavals that always accompany growing young mountains.
Our western mountains are part of a rugged cordillera system reaching from Alaska to Cape Horn. These ropey ranges arose from a complex series of geosynclines. There were no proud Rockies in Western Canada 150 million years ago. Their place was occupied by a soggy geosyncline. From its eastern brink, it drained silty sands from an arm of the sea that stretched from the Gulf of Mexico to the Arctic Sea. From its western brink it drained rocky gravels and cindery volcanic debris from a youthful range of growing mountains. Its deposits became 20,000 feet thick and their weight helped to trigger the birth of the Rockies.