Marc Lacroix, Age 10, Of West Warwick, R.I., for his question:

Are the rockies growing or decreasing?

The mighty mountains of the world seem to be fixed forever. But their steep slopes, their lofty peaks and snow crowned heads are slowly changing. We do not notice the changes because the lifetime of a mountain range lasts through hundreds of millions of years.

The proud Rockies are part of a vast system of mountains that runs down the western side of North America and stretches southward through Central and South America. The system of ups and downs reaches from the arctic to the antarctic, and its story began perhaps half a billion years ago. In our lifetime we can behold the splendid rockies at the peak of their greatness. In the future, say 100 million years from now, we might notice that they had decreased.

The Rocky Mountains form one of the youngest ranges of North America. For almost half a billion years this region of the earth was a long, narrow ditch, stretching from alaska to southern California. The hollow was fi11ed with a shallow sea and for countless ages it gathered the waters from countless muddy streams. Layers of silty sand and mud were dumped on the floor of the ditch and gradually added masses of extra weight to the earth's crust.

While this was happening, the northern appalachians humped up their backs through New England. About 100 million years later the southern appalachians arose. Long after the eastern mountains had grown tall, the western region became restless. The Sierra Nevadas began to arise about 150 million years ago. Some 50 million years later the shallow, silty ditch farther west began to change.

There were shuddering earthquakes, and fiery volcanoes erupted in rivers of molten lava. The crust of the earth heaved and trembled, cracked and buckled. This birth of the Rockies began about 100 million years ago.

As the young mountains grew taller, streaming rains ran down their sloping sides. They washed down dust and gravel, gnawed away rocky slabs and toppled tons of baby boulders. But this mountains grew faster than the weather could wash them away. Now the mighty range seems to have reached its full height. The weather of the future will work many hundreds of millions of years to wear down its towering peaks and level its sloping sides.

It took about 100 million years for the Rockies to reach their full height. As the ground uplifted, there were earthquakes and landslides. Great slabs were shifted one above another like rocky sandwiches. Erupting volcanoes added layers of lava and built massive cones and some of these volcanoes still smoulder. Heat from their seething lava, still trapped in the ground, causes the hot springs and the steamy geysers of Yellowstone National Park.