You may form some idea of the number of volcanoes that have been observed on the moon when I tell you that as many as 32,000 have been seen on that side of the moon that is turned towards the earth.
Now it is an interesting fact that almost all these mountains possess great craters that are not unlike some of the volcanic craters we see on the earth. The volcanic craters of the moon, however, are of very much greater size than those on the earth, many being from fifty to sixty miles in diameter, while some of them are more than 100 miles in diameter. Smaller craters, say from twenty to twenty-five miles in diameter, can be counted by the hundreds.
Like most of the moon's craters, the largest crater more closely resembles one of the pit-craters or calderas on the island of Hawaii. This volcanic crater consists of a huge circular ring with a small irregular peak that rises inside the ring. This peak, by the way, might at first appear to resemble the crater of Vesuvius, which after a long period of inactivity of the mountain during the eruption that destroyed Pompeii and Herculaneum was thrown up inside of what had been left standing of the old crater of Somma. But it has no crater at its summit, and, therefore, resembles rather the irregular pile or rock that rises from the surface of a lava lake in the craters of Mt. Loa or Mt. Kilauea in Hawaii.
Besides the numerous craters to be seen on the moon's surface there are many lines of deep, crooked valleys, known as rills, that may at one time have been the beds of rivers. Besides the rills, there are many straight clefts about half a mile in width, that extend down into the surface of the moon for unknown depths. These clefts can be seen passing directly through mountains and valleys. They are believed to be cracks or fissures in the moon's surface.
On the moon is a great crater called Tycho. It is situated near the moon's south pole. The great crater of Tycho is by far the most prominent object on the moon's surface. It has a system of rays that extend for great distances around its craters.
You will also see if you examine the moon's surface by a powerful glass that there are immense plains called oceans or seas. By an appropriate custom the names of the different craters on the moon are the same as the names of the great astronomers and philosophers that have long since passed from their labors, such as Tycho, Copernicus, Kepler, Plato, etc.
Various explanations have been given as to the origin of the craters on the moon's surface, but without going into a discussion it may be said that they are now generally regarded as having been formed in the main just as were the craters of the earth's volcanoes.
The tremendous size of the moon's craters is of course due to the great decrease in the force of gravity. This would make the craters, approximately, six times as great as the craters on the earth. Professor Pickering points out that while the moon's craters resemble more closely those of Hawaii than those of any other of the earth's volcanoes, yet there is this difference in them: that while the earth's crater floors are generally considerably higher than the level of the sea, the moon's crater floors are generally below the level of the surrounding country. Still, taking them all in all, the craters of the moon's volcanoes resemble those of the island of Hawaii, or again quoting from Pickering:—
"There seems, indeed, to be no feature found upon the moon which is not presented by these Hawaiian volcanoes, there is no feature of the volcanoes that does not also have its counterpart in the moon."