Larry Skelton, age 14, of Heber, Utah, for his question:

How does a coral atoll form?

The story of the changing earth can be read in its rocks and formations. Some of the details have been set down by an assortment of very odd earth science reporters. Among these are the diligent corals that built reefs and lonely atoll isles in the midst of the deep ocean.

The structure of the typical coral atoll was figured out by Charles Darwin about 130 years ago. Darwin was a shrewd observer, and some of his theories later proved to be true. He was correct about the structure of the coral atoll, though scientists had to wait more than 100 years to prove it. Darwin suspected that the coral rock was a crusting on the top of an undersea mountain. Modern scientists drilled almost a mile through the coral surface of Eniwetok Atoll and struck the solid foundation of an ancient volcanic mountain.

Through the ages many, volcanoes erupt on the deep floors of the Pacific and other oceans. And through the past million years four ice ages have frozen and then freed vast quantities of the world's waters. Sea levels fell during the glacial periods and, rose when the massive ice melted again. The tops of many extinct volcanic cones were submerged and exposed many times as the water level inched higher or lower.

Little, soft bodied corals thrive in tropical seas where the water is no colder than 70 degrees Fahrenheit. They live and construct their rocky apartment houses in the sunny upper leyel no deeper than 150 feet. In warm seas coral building began on many peaks that reached near or above the surface. As the sea level subsided the coral crustings were left as island atolls, high and dry above the water.

A young coral atoll may be a circular reef because the deep crater of the cone is not encrusted. The bashing waves from prevailing winds may tear dawn one side of the circle leaving the familiar curved atoll with its sheltered lagoon. The structure of coral atolls may reveal long chapters of the earth's history, of changing temperatures and sea levels and of the comings and goings of the bitter ice ages.

The sea dwelling, soft bodied coral animals still build their stony apartment houses in our warm oceans. Most of this rock formation now occurs within about 200 miles of the equator. But the ragged remains of ancient coral reefs have been found in our chilly Arctic waters. This proves that in times past these northern seas were as warm as our tropical waters.