The New Student's Reference Work/Volcanoes
Volca'noes, vents in the earth's crust from which lava is ejected. The lava may flow out quietly, or it may be ejected forcibly. In the latter case much or all of it may be solid. Small fragments of solid lava are called cinders, but if they are as small as particles of fine sand or dust, they are called ashes. Volcanic ash does not, however, imply combustion. It is nothing more nor less than powdered lava. Besides the lava which issues from volcanoes, either in the liquid or solid form, many gases or vapors escape from the vents. Among the latter vapor of water is most abundant. Steam is, indeed, the principal force in the violent expulsion of materials from volcanoes of the explosive type. Chlorine and sulphur and various compounds of these substances are among the commonest fumes escaping from volcanic vents. Carbonic acid gas is also one of the common gases. Many of the gases are noxious, so that it is sometimes difficult to approach the openings whence volcanic products issue.
The solid material and the liquid lava which escape from volcanoes accumulate about the vents, and build up volcanic cones. In the top of a volcanic cone there generally is a depression, the crater, in the bottom of which the vent is situated. When a volcanic cone becomes high, the lava may break through its sides instead of flowing over the top. Cones built up by lava flows have low slopes, often less than 5°, and rarely so much as 10°; cones built up by cinders often have steeper slopes, but rarely more than 25° or 30°. When a volcano ceases to be active, it is said to be extinct, if the quiescence be permanent. When it becomes extinct, however, it really ceases to be a volcano. When the activity of a volcano is temporarily suspended, the volcano is said to be dormant; but it is often difficult to tell whether a volcano is extinct or only dormant. Vesuvius was thought to be extinct until the time of its destructive eruption in 79 A. D. When this occurred, it was seen that the volcano had been dormant only instead of dead. A volcanic vent often continues to give off vapors and gases long after the lava ceases to issue. Vapors and gases also escape, and often in quantity, during periods when no lava is ejected.
When volcanoes cease to be active, their craters may be occupied by water, giving rise to crater lakes. Such a lake, Crater Lake, exists in Oregon. Volcanic cones retain their perfect form for a short time only. Erosion by rain and melting snow soon modifies them. Volcanic cones in all stages of degradation occur in many mountainous regions. Mt. Shasta in California, Mt. Ranier in Washington and San Francisco Mountain in Arizona may serve as examples of volcanic mountains in process of degradation. In southern California and northern Arizona there are volcanic cones so recently formed that they have been modified scarcely at all by erosion. In many of them the craters are still preserved. These fresher cones are largely of cinders.
Volcanoes are often associated with earthquakes, and the violent eruptions of volcanoes are often the direct cause of earthquakes. In many cases, however, it may be true that the two phenomena, the earthquakes and the volcanic eruptions, are to be referred to a common cause rather than either to the other. In the explosive eruptions of Vesuvius the quakings are felt for considerable distances from the crater. In many cases of violent eruption the old cones are partly or wholly blown away. Even large parts of the islands where they occur may be demolished. Outside the present cone of Vesuvius there is a remnant of an older cone, partially destroyed in a violent eruption subsequent to its formation. A large part of the Island of Krakatoa was blown away in the eruption of the volcano of the same name.
The great destructiveness of volcanic action is more commonly due to the material blown out than to the lava, which flows out. The flow of lava usually is slow, and in most cases it flows but a short distance before it congeals. But the solid matter may be widely distributed. It was by ashes ejected from Vesuvius that Pompeii, with its 2,000 people, was buried in 79 A. D. Torrents of rain, due to the condensation, of the escaping water-vapor, often fall with the ashes, converting them into a sort of hot, fluid mud, and this is sometimes most destructive in its flow. In the Krakatoan eruption of 1883 it has been estimated that bits of pumice and dust were sent up into the air 20 miles. Sent up into the upper air by the violent explosion, some of the dust was carried by air-currents completely around the earth. Large blocks of lava are sometimes hurled miles from the volcano whence they are ejected.
The number of active volcanoes is estimated to be between 300 and 400. About one third of the active volcanoes are situated on the continents, the others on islands. Most of the volcanoes on continents are relatively near their borders, though extinct volcanoes occur at great distances from the coasts. Continental volcanoes are, on the whole, more numerous about the borders of the Pacific Ocean than about the Atlantic. Many islands are really nothing more than the crests of volcanic cones which have been built up above the surface of the water. There are doubtless very many volcanic cones the tops of which are still below sea level. Of such submarine volcanoes little is known. The lava which issues from volcanoes comes from the interior of the earth, but from what depths is unknown. Active volcanoes are more numerous in regions where the formations are relatively young than where they are old. They are thought to occur in regions where the crust of the earth is in movement, that is, where it is either sinking or rising, rather than in regions where it is stable.
No existing volcano seems to have been active for a period of time which would be considered long, as geologists reckon time, though many of those now known have been active since the beginning of the historic period. There however, is reason to believe that all existing volcanoes will in time cease to erupt and new ones will doubtless come into existence. Volcanoes have been more numerous at some stages of the earth's history than at others, but some of the periods of great volcanic activity were early in the earth's history, and some late. On the whole, it cannot be affirmed that volcanic activity has increased or decreased as the history of the earth has advanced.
References: Judd's Volcanoes; the topic Volcanoes in text-books on geology; Russell's Volcanoes of North America; Geikie's Earth Sculpture and Dana's Volcanoes.
R. D. Salisbury.