Jeff ' Neel, age 13, of Pfafftown, North Carolina, for his question:
What is the Mohorovicic discontinuity?
The word continue, as we well know, means to go on with whatever you are doing. A discontinuity is a break or change of pace in whatever you were doing over a period of time. Andrija Mohorovicic was a geologist who found a discontinuity in the speed of seismic vibrations penetrating the earth's crust. We can simplify this verbal inconvenience. by calling his discovery the moho. And the word Mohole brings the topic within range of everyday understanding.
A few years ago, the news of project Mohole excited everybody's imagination. At that time, scientists had a plan to drill down all the way through the earth's crust to the mysterious mantle layer below. They selected a spot off the San Diego shore where the crust was known to be no more than five miles or so thick. The fantastic project was named for the Mohorovicic discontinuity, alias the moho.
The 19th century was a great time for geologists. Researchers probed deep into the earth's crust and learned to decode the diary of events recorded in the rocks. One of their most important discoveries was earthquake vibrations passing around and through the planet. In 1875, an Austrian geologist named Suess used this evidence to deduce that the earth is built in concentric layers that get denser from the surface to the center. So far, nobody has refuted this basic pattern.
However, in 1909 Andrija Mohorovicic made the first refinement in the general picture. He tabulated the acceleration of the primary and secondary seismic waves passing down through the earth. The primary or P waves travel faster, accelerating through denser layers right through the core. The secondary or S waves are slower and travel with a transverse or zig zag motion. They move faster through denser solids, though they do not penetrate as deeply as the P waves.
These seismic patterns were known to Mohorovicic, as was the concentric pattern of the earth's layers. It would be natural to assume that the vibrations accelerated at steady predictable speeds all the way down. But Mohorovicic decided to clock them, just in case. At the bottom of the crustal layer he found that P waves travel at 4.3 miles per second, the S waves at 2.4 miles per second. No doubt he expected a speed up as the waves penetrated the denser mantle layer but he got more than he expected.
Suddenly the P waves jumped to 5 and the S waves to 2.9 miles per second. Then, just as suddenly, this speedy acceleration decreased. In the next 1,800 miles down through the mantle, the P waves increased merely to 8.5 and the S waves to 4.5 miles per second. Obviously there was a brief break in continuity at the top of the mantle layer, just below the crust. This is the Mohorovicic discontinuity alias the moho.
Later researchers found other discontinuities at deeper levels. At a depth of 1,800 miles, where the mantle sits on the core, the S waves vanish. The P waves drop to 5 miles per second and veer off course. Another discontinuity occurs at 800 miles from the center of the earth. This one suggests an inner and outer core of different consistencies.