Andrea Scrima, age 12, of Staten Island, New York, for her question:

What caused the original supercontinent to break apart?

For a long time, this question stopped a lot of scientists from accepting the Theory of Continental Drift. They refused to believe that an original supercontinent broke up and drifted apart. Fortunately researchers went ahead and proved that it really happened and also learned some of the causes.

The whole thing started way back mhen the global map was one world ocean named Panthalassa around one supercontinent named Pan aea. Mountains arose, ice ages came and went. And about 200 million years ago, ominous cracks were cutting deep into the supercontinent.

As the cracks widened, the sea flooded in and divided Pangaea into sections. Gradually the pieces drifted farther apart and became the modern continents.

To grasp how this happened, sae must imagine some of the planetary energies that keep the continents drifting slowly around the globe. The land masses are slabs of light crustal rocks resting on ten lame plates and many small ones. The plates are molded from heavier material in the mantle, lust below the crust.

Pangaea, it is thought, xnas cracked into tectonic elates by convection currents that transport heat through the bowels of the earth. Along the original cracks there are spreading zones that push plates apart and also sinking zones that pull them to¬gether.

Mountainous mid ocean ridges run down the Atlantic and Indian Oceans. Their crests are split apart by rifts, formed by the original cracks in the supercontinent. This is where rising convection currents transport heat from below and erupt lava under the sea. As new rock continues to form in the rift zones, the sea floor grows and spreads. The spreading floors widen the cracks between plates. This pushes The Americas and sections of the Old World farther apart.

Rising convection currents have descending counterparts. The major ones are along cracks in and around the Pacific. In mid ocean a sinking current tends to drag the edges of two neighboring plates into deed trenches and down to the mantle layer below. Near the shores, one ed e may sink while its neighbor shoves up and over the crack. This global pattern shifts the continents by shrinking the Pacific and enlarging the basins of the Atlantic and Indian Oceans.

There are slight variations in the earth's rotation. Some geophysicists suspect that this factor and perhaps the geomagnetic field also helped to break up the original supercontinent. But the major maneuvering seems to have been done by the convection currents. These mighty pushing and pulling forces from below cracked the crusty sur¬face into ten major plates and many small subplates. The same forces are still at work.