Mark Jenson, age 13, of Brookfield, Wisconsin, for his question:
What do we know about the earth's mantle?
Suppose we lived on enormous rafts, bobbing gently on the waves and inching slowly around the global ocean. There is nothing frightening in this mildly adventuresome picture. The recent facts we have learned about the earth's mantle are somewhat more adventurous. But the global picture is no more hazardous than life on that huge drifting raft.
Just a few years ago the earth's mantle was described as a rather stodgy layer be¬tween the thin rocky crust and the seething core at the center of the planet. Seismic waves suggested that it is denser than the crust. It was assumed to be hot, plastic or semi plastic but definitely steady and well behaved. My, how those notions have changed:
About 30 years ago, some daring dreamers suggested that the continents drift around the globe like floating rafts. This theory of Continental Drift was pooh poohed, mainly because stupendous energies would be needed to operate the slow motion, global carousel. At that time, nobody dreamed these dynamic resources were in the mantle which was supposed to be a placid, do nothing layer about 1,800 miles thick.
A decade or so ago, researchers from various countries determined to test the new drift theory. Their results shattered the older concepts and masses of evidence proved that indeed the continents are drifting around on global tours. The same evi¬dence also revealed that the stupendous energy needed to operate this global carousel comes up from the mantle.
Earth scientists now think that the mighty mantle acts as a dynamic thermal con¬ductor, between the cool crust and the seething core. Its upper level is rigid and broken into ten major slabs and numerous fragments. These are the tektonic plates that support the continental rafts and enfold the globe like a cracked shell.
Forces from below keep the surface plates moving in definite patterns. On a global scale, the mantle's stupendous thermal energy circulates in convection currents. These patterns resemble boiling water, as it steams up and over, down and around the pot. In the mantle, rising and descending convection currents release their energies at various junctions between certain tektonic plates.
In the ocean beds, some 40,000 miles of submerged ranges mark zones of rising currents. Here masses of molten material erupt from the mantle, cool and solidify. This occurs along the famous Mid Atlantic Ridge and the spreading sea floor pushes apart enormous tektonic plates. With them go the continents. In an average lifetime, the east and west shores o€ the Atlantic may drift six feet farther apart.
This is merely one feature of the dynamic mantle. Another is an irregular layer of hot molten material. It is about 50 miles thick and begins about 30 miles below the rigid plates. Earthquake and volcanic zones channel down and derive their energy from this mysterious level. In other zones, descending currents drag the edges of tektonic plates downward to form plunging trenches in the ocean floor.