Little drops of water, little grains of sand, Make the mighty ocean and the pleasant land.
Until recently scientists knew little more than the nursery rhyme about why the earth has oceans and land, or why most of its high land masses are concentrated on one side, opposite the Pacific Ocean. If the earth condensed from gases or from meteorlike particles, as many theories have held, it should be smooth and symmetrical with a deep layer of water covering the whole surface.
Within the last few years a wealth of new information—from seismology, astronomy and nuclear physics—has been brought to bear on the...