A submarine volcano is a volcano that erupts on the bed of the ocean with its crater covered by the waters. Many of the great volcanic mountains of the world began as submarine volcanoes. A crater first opened on the floor of the ocean, and lava escaping, was heaped up around the opening, until it emerged above the surface as an island. As we have seen, the island of Iceland is believed to have begun in this way. Such, too, in all probability, was the origin of Hawaii, Vesuvius, Etna, and Santorin.

But besides the volcanic mountains that were thrown up during the geological past, there are others that have been called into existence while man has been living on the earth. We will now describe a few islands that have been formed in this manner by submarine volcanic eruptions.

That volcanic eruptions, or at least something that greatly resembles eruptions, occur on the bed of the ocean too far below the surface to permit them to be directly seen from above, has been shown in a number of cases where the captains of vessels have reported that in certain parts of the ocean, jets of water, or steam, and pillars of flame have been seen rising to great heights from the surface of the water, and that in certain regions sulphurous smoke has also been seen. During such occurrences, the water is agitated, as if it were being violently boiled. Moreover, these parts of the ocean are shaken by severe earthquake shocks.

Another evidence of submarine volcanic eruptions is to be found in great quantities of ashes, scoriÆ, or pumice stone, that are seen spread out over the surface of the ocean after the commotions referred to in the preceding paragraph. Still another proof is that parts of the ocean whose waters were previously very deep are found to have suddenly shoaled.

Of course, the best proof is the appearance of rocky reefs or small islands thrown up above the surface of the water, especially where volcanic cones appear. While in many cases the new islands thus thrown up are subsequently washed away by the waves, yet some have continued above the water.

One of the most noted instances of the formation of an island by a submarine volcano was Sabrina, which was thrown up in 1811, in the Atlantic Ocean, off the shores of St. Michael in the Azores Islands. Sabrina had a cone that was 300 feet in height. It did not long remain above the waters, however, being soon washed away by the waves. It is interesting to note that in the same part of the ocean where Sabrina appeared, other islands have appeared and disappeared, at times long before 1811; that is, during the year 1691, as well as during 1720.

Another instance of a submarine island is Graham's Island, that was thrown up in 1831, in the Mediterranean Sea, between the west coast of Sicily and the nearest part of Africa, on which ancient Carthage was situated. The part of the sea where the island was thrown up had previously a depth of 600 feet.

The general appearance of Graham's Island is represented in Fig. 22.

Graham's Island was formed by accumulations of loose scoria and cinders, together with blocks of lava and fragments of limestone. It reached a height of 200 feet above the water, but only remained above the surface for a few months, when it was washed away, leaving a submarine bank some twelve miles in width, that was covered by water of about 150 feet, but which, however, increased rapidly in depth towards the edge until depths of from 1,200 to 2,000 feet were reached.

According to Lyell, on the 28th of June, 1831, before Graham's Island appeared, a ship passing over this portion of the sea felt severe earthquake shocks. On July 10th of the same year, the captain of a vessel from Sicily reported that as he passed near this part of the Mediterranean, a column of water, 800 yards in circumference, was seen to rise from the sea to a height of sixty feet, and that afterwards a column of steam rising to a height of 1,800 feet was seen in the same place. On again passing the same region on July 18th, this captain found a small island about twelve feet in height, with a crater in its centre, that was throwing out volcanic materials, together with immense masses of vapor.

The island thus formed grew rapidly, both in size and height. When visited at the end of July, it had attained a height of from fifty to ninety feet, and was three-quarters of a mile in circumference. By August 4th, it had reached a height of 200 feet, and was then some three miles in circumference. From this time, however, the island began to decrease in size, as the waves began to wash it away. By August 25th, it was only two miles in circumference. On September 3d, it had decreased to three-fifths of a mile in circumference, and continued to decrease until it entirely disappeared, so that in the year 1832, there were, according to measurements, some 150 feet of water over its former site.

The Mediterranean Sea between Sicily and Greece is also especially liable to submarine activity. New islands appear and disappear so frequently that in this region they are almost regarded as common phenomena.

There are many other parts of the ocean where submarine volcanic eruptions are common. This is especially the case in the narrowest part of the Atlantic Ocean between Africa and South America. Here there is a region situated partly above the equator, though for the greater part south of the equator, frequently visited by submarine eruptions, that are accompanied by earthquakes, by the agitation of the water, by the appearance of floating masses of ashes and scoriÆ, as well as by columns of steam or smoke. Floating masses of ashes and scoriÆ sometimes occur so thick as to retard the progress of vessels.

But what forms, perhaps, one of the best instances of a large island formed by submarine eruptions during historical times, is Bogosloff Island in Behring Sea, some forty miles west of Unalaska Island. This island, the position of which is seen on the accompanying map, is known to the Russians as Ioanna Bogoslova, or St. John the Theologian. It is situated in lat. 53° 58' N., long. 168° west. It is said that during the year 1795, some of the natives of Unalaska Island saw what they thought was a fog in the neighborhood of a small rock, which they had known for a long time to project above the sea in these waters. This rock was marked on some Russian chart dated 1768-69. It was seen by Captain Cooke, in 1778, and was named by him Ship Rock.

But it was not a fog that the Unalaskans had seen in the neighborhood of Ship Rock; for, to their great surprise, the fog continued in sight although everywhere else the air was quite clear. Of course, this was a great mystery to the people. During the spring of 1796, one of them, who possessed either greater curiosity than the rest, or greater courage, or both, visited the rock. He returned, telling the strange story that all the ocean around the rock was boiling, and that the mist or fog was caused by the rising steam. What was taking place was a submarine eruption. During May, 1796, sufficient matter had been brought up from below to increase greatly the area of the small rock.

During later years several attempts have been made to visit Bogosloff Island. For example, the island was visited during 1872 and 1873, when it was found to have increased in height to 850 feet. But no appearance of any volcanic crater was to be seen.

During October, 1883, a great volcanic eruption occurred there. Considerable changes were produced in its shape, as well as in the depth of the surrounding water. During this eruption, clouds of steam completely hid the island. Great quantities of ashes obscured the light of the sun. After the eruption, a new island was thrown up near the old one, in a place where the water had previously been deep enough for the ready passage of ships. The new island was about half a mile from the old one. It was conical in form, from 500 to 800 feet in height, and about three-quarters of a mile in diameter.

The new island was visited in 1884 by the U.S. Revenue Marine Steamer Corwin. Lieutenant Cartwell, who visited the island at this time, described it as follows: