Since that time the new island has again sunk beneath the waves. But it will probably rise again, or another island somewhere in its neighbourhood will take its place, for a great new submarine ridge of volcanic rocks is forming in this neighbourhood and has been forming for many hundreds of years. The Pribyloff Islands are known to be volcanic from the materials of which they are composed, and sprang up above the waves in the same way.
CHAPTER XV
THE CAUSES OF EARTHQUAKES
Now that we have before us some of the examples of the changes which historic earthquakes have brought about in the face of the country, it is easy to see what an important effect they must have exerted in geological history. But there are still weighty questions to be answered about earthquakes. We have seen that an earthquake can contort, upset, and twist the surface strata of the earth as easily as we can crumple a sheet of cardboard. We have yet to find whether the crumpling of the strata is always produced by earthquakes, or whether an earthquake is the culminating symptom that some agency is at work crumpling the strata. Let us try to imagine an example on a small scale. Suppose we take the top of a pill-box, and, holding it in the crook between our thumb and forefinger, compress it very tightly on all sides. What will happen? The lid of the pill-box, being subjected to stress or strain on all sides, will presently buckle and crack. We shall have produced an earthquake on a small scale, and there will be an earthquake fracture—perhaps an earthquake fissure. If the whole pill-box had been used for the purpose of our experiment, and had been packed to the brim with ointment or thick liquid, and if it had been squeezed in a vice instead of in our hands, then perhaps we should have provoked still more striking symptoms of an earthquake. The ointment might have broken out through the lid. Perhaps even tiny jagged holes or craters would have been formed in the lid. Thus we see how strain may produce earthquakes. Take some more examples. Suppose a cork is very tightly fixed in a wine bottle, and in order to get it out we employ a very powerful lever corkscrew. The neck of the bottle, under the effect of the too powerful pressure put on the inside surface of the glass, will crack or break. Similarly if we screw down a microscope too hard on a slip of glass the glass will often crack suddenly. Both these instances recently occurred within the writer's experience, and few readers can have escaped noticing one or other of them. The breakage in these instances is always caused because a strain is set up somewhere in the glass—there is more pressure at one point than another, and the glass, unable to resist this unequal pressure, gives way.
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The Track of an Earth Wave
Showing portion of a street in San Francisco after the terrible earthquake of 1906. The resemblance of the break in the ground to the appearance of a stationary wave should be noted.
What happens when it gives way? To answer this question we had better carry our minds to the example of the pill-box lid. If the top of the box were of very brittle material, like pottery or glass, then after the breakage we know that most likely one of the broken pieces would be a little higher than the other—would perhaps overlap it. That is what we often see when examining the geological earth strata. One stratum, instead of lying evenly with another where a crack has occurred, rests a little above it or below it. This inequality or unevenness geologists call a fault. Now we can easily see that whenever, and by whatever causes, a fault is produced, there is probably at the same time an earthquake. The fault cannot be produced without a great and shaking disturbance. Mr. John Milne, the most distinguished of British authorities on earthquakes, says that all large earthquakes originate from the formation or extension of these "faults" or great cracks in the strata.