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.
Fig. 22. Graham's Island—a Recent Volcanic Island
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.