The Wonder Book of Volcanoes and Earthquakes

by Edwin J. Houston

You must not suppose for a moment that this increase in the length of the day is large. On the contrary, it is so small that since the year a. d. 1, up to the present time, the day is only a very small fraction of a second longer.

But it was very different in the earth's geological past, when the inside of the earth was in a molten condition; for then great tides were set up in the melted interior of the earth that not only greatly changed the shape of the earth, but decreased the rate of rotation much more rapidly than it does when the earth's tides are limited as they are now to the waters on the earth's surfaces.

There was, however, at the same time, something going on that tended greatly to make the earth turn more rapidly on its axis. While the originally melted earth was cooling and shrinking, the rate of its rotation was necessarily increasing. As you know, the time of vibration of a pendulum, that is, the time it requires to make one complete to-and-fro motion, is shorter the shorter the length of the pendulum. A pendulum two feet long moves to and fro more slowly than a pendulum one foot in length. In the same way a rotating sphere will make one complete rotation in a shorter time when its radius, which corresponds to the length of a pendulum, is shorter. Therefore, as the earth shrunk, it rotated more and more rapidly, and at last reached a rapidity of motion at which an immense quantity of matter flew off its surface nearest the equator and went out into space, never again to return. It was this mass that constituted the earth's moon.

Necessarily such a tremendous catastrophe was attended by an earthquake as well as by the most fearful volcanic phenomena that the earth has ever witnessed. The terrible catastrophe produced by the explosive eruption of Krakatoa was but as a small drop of rain falling on the earth, when compared with the catastrophe produced when the "five thousand million cubic miles of material left the earth's surface, never again to return to it."

It is not known whether this matter was torn off the earth at a single time or during successive times, but quoting the beautiful language of Professor Pickering:

"We may try in vain to imagine the awful uproar and fearful volcanic phenomena exhibited when a planet was cleft in twain, and a new planet was born into the solar system."

This terrible catastrophe took place at a time not when the earth was a gaseous mass, but when it had condensed into a comparatively small mass not much larger than it is at its present time, and possibly even after it had hardened sufficiently to form a solid crust on its outside.

If you look at a map of the earth on a Mercator's projection, such, for example, as that employed in illustrating the distribution of the world's volcanoes in [Fig. 24], you can see, even without any very close examination, that the great water area of the Atlantic Ocean has its eastern and western shores almost parallel to each other, so that if you conceive the Eastern and Western Continents as being pushed together, they would, except at the south, almost completely fit together, and the same thing is true, if Greenland is pushed towards the northeastern coast of North America. Of course, some portions of the coast would not fit exactly, but then these portions might either have been worn away, or, as is more probable, have been changed in shape by the deposit of immense beds of sedimentary rocks spread over the borders of the Atlantic by the great rivers that empty into it. This is so remarkable a fact that it will be well worth your while to turn to the map mentioned and convince yourself of the proof of what I have just said. As you will see, Europe and Africa would almost exactly fit against South America and North America, while Greenland would even more closely fit against the northeastern coast of North America.

Now, while we do not say that it was so, it has been suggested as just possible that the great depression of the Pacific Ocean represents the spot that was once filled by the moon. That the Eastern and Western Continents, then torn asunder by the great force of the convulsion, were left floating on the surface of a sea of molten matter, a greatly widened crack marking positions they assumed at the end of this cataclysm.

Of course, you must understand that all this is a mere supposition, and that we do not know whether the earth was actually cooled on the outside when this occurred, since it might have still been in a liquid condition throughout. It would seem, however, to have occurred rather recently, since it could not have occurred until the earth shrunk so much that it became so small in radius as to acquire a very rapid rate of motion on its axis.