Volcanoes, then, are openings or vents which communicate with the melted rock within the earth, and the conical form of volcanoes is owing to the deposits of volcanic matter as it falls from the opening called the crater. If we let a small spade full of mould run through our hands, or from the spade, it will form a small cone, the heavier particles sliding to the base at a certain slope. Thus the volcano builds its own hill, and inside the crater we find cones from which smoke and steam issue. These cones within the cone are the points of issue of vapour and smoke, miniature volcanoes making up a whole.
Fig. 709.—Eruption of Vesuvius, August 26th, 1872.
The signs of eruptions are much the same, and usually occur a couple of days before the actual outbreak takes place. First smoke is perceived, perhaps, and the escape of various noxious gases accompanied by earthquakes occur. Now the eruption may commence and blow away the summit of the mountain, as in the case of the commencement of the catastrophe of A.D. 79, when the whole side of Vesuvius was torn away, and continuous showers of ashes fell for days and nights, burying everything, while the hot lava poured down the sides. Stones and ashes with vapours are hurled into the air. Clouds of steam are formed, and vivid electrical discharges take place in these clouds, while water dashes down, carrying stones (“volcanic bombs”), and reflected lurid flames from within are cast on the steaming clouds, which look like fiery columns. Then the lava issues in a white, hot, steady, irresistible stream, covering everything, and burning up all vegetation.
Fig. 710.—Birth of a volcano.
New volcanoes are continually in process of formation, and at Santorin for hundreds of years volcanic action has been busy in forming islands. These violent efforts of Nature frequently give rise to earthquakes, which are the most destructive of natural convulsions. The records of late occurrences are fresh in the minds of all readers, and need not be specified. The slow subsidence and gradual upheaval of the land is still going on, but we are frequently startled by the account of a rupture of the ground or the destruction of a portion of a city.
The motion of the earthquake is generally in a direct line, and undulating. Sometimes what are termed vertical shocks arise and destroy solidly-built edifices. Mountains have been overturned by earthquake shocks, and trees have been twisted round. Sometimes the ground yawns into enormous fissures. The sea is tossed into great waves and encroaches upon the land, and when the sea recedes the recession of the water is followed by a more terrible invading wave sweeping all before it. Earth tremblings often occur far away from volcanoes, and without any visible connection with volcanic action. There are many aspects of land and water which the student of geography will remember, but which need not be separately treated of. We must, however, refer to plains, plateaus, and lakes. The mountains also play a most important part in Physical Geography and in “Climatology,” as they collect the vapours for rain, and make the valleys fertile, and thick with vegetation. We have spoken of the mountains under Geology, and the various formations and strata will be found enumerated there, but now we have to do with the mountain chains in their physical aspect as regards their shape and appearance on the globe.
Fig. 711.—Earthquake fissures.