John Hammel, age 12, of Indianapolis, Tnd., for his question:

How wide is the continental shelf?

The tide creeps up so far and then retreats. This is the dividing line between land and ocean. Your favorite beach slopes deeper and deeper below the water out to sea. This is the continental shelf that surrounds the big land masses. In most places it gradually slopes to a depth of 75 fathoms. Hence, the continental shelves begin a few feet or inches below the water at tide line and sink,to around 450 f eeta

In Arctic and Antarctic regions, the continental shelves sink to an even greater depth. Perhaps the pressing weight of ice has something to do with this. The depth of the water here is hard to measure. Soundings From a few spots in the Arctic indicate that the shelf falls away to a depth of 100 to 200 fathoms. The shelf off the ioy_ cqatit  of Antartica is deeper still, the deepest in the world.

All over the world, the continental shelves end in a sudden drop. The ocean floor suddenly plunges in sheer cliffs and escarpments anywhere from 12,000 to 30000 feet deep. Below these. rocky tors are the true ocean beds, the huge basins filled with calm, dark mystery.

As you would expect, the continental shelves also vary in width, They tend to narrow off the shores where young mountains are growing. The world's newest mountains are the coastals along the Pacific Coast of North America, Here the continental shelf is no wider than 20 miles or less before it plunges into the deep abyss of the Pacific,

A strong ocean current also seems to go with a narrow shelf. The Gulf Stream presses hard against the shoreline along the southeastern states. The continental shelf begins to narrow south of Cape Hatteras. Off the south coast of Florida it is but a doorstep to the deep ocean basin of the Atlantic

North of Cape Hatteras, the shelf widens to 150 miles. Widest of all is the shelf in the Artie regions. Here the gentle slope runs under the sea for 750 miles before it plunges down the steep cliffs that hold the deep ocean,

Life in these continental shelf waters is vastly different from life in the ocean abysses, It more closely resembles life on the land, In fact„ much of the area has been dry land. From time to time, through the long ages the sea has advanced and retreated. Large areas of the shelves were dry lane' in Ice Age days when so much water was held trapped in the massive glaciers,

Several times paces now under the sea have been clothed in trees and pastures that supported four footed animals, Now the pastures are waving seaweeds the animals gliding fish, Sunlight filters through the shallow waters over the shelves. There are hills and dales, tossing waves storms and changing seasons, In the deep abyss there is no light, no plant life no changing seasons and only a few fishes too weird to imagine.