ERUPTIONS.
The ordinary work of the volcanoes is accumulative. The molten lava has been gathering at the apex of the ascensive column in such amount that there is not strength enough in the containing walls to keep it in place, and it flows away down the mountain side, or else disappears in an unseen abyss. As soon as relief is granted by a discharge the volcano is quiet, and the ordinary work is resumed until there is material enough for an other eruption. There may be said to be four kinds of eruption : first, those escaping from orifices high up the mountain; second, those emitted from fissures quite low down, accompanied by severe earthquakes ; third, those that disappear into the earth, break downs or downplunges; fourth, those that are submarine. In Hawaii flows from the surface, or edge, of the calderas, have never been seen in historic times.
Viewed from a different standpoint the eruptions may be quiet or explosive—the first being where the lava flows like water with out much commotion : the second where the discharges come out from orifices like cannon from ordnance, scattering projectile fragments. Our eruptions are so commonly of the first class that they have been regarded as representative of the Hawaiian type. But the recorded history has demonstrated, as stated by J. W. Judd, that the "two conditions are presented by the same volcano at different periods, and pass into one another by the most insensible gradations." As has been often intimated, a noise or earthquake shock has usually preceded the eruptions when the observer has been located near the place of outbreak. A person in Hilo could not know by observation whether there was 269 any disturbance attending the sudden illumination of the sky in Mokuaweoweo forty miles away. Because he hears and feels nothing he assumes that the action is absolutely quiet, but those who happen to be stationed near the outbreak commonly speak of light or heavy earthquakes. So it has seemed to me probable that earthquakes accompany every outbreak of the first class.
The presence of fragmental materials and heated vapors in the discharges from Kilauea in 1400 and 1790 indicate eruptions as violent as anything recorded from Vesuvius.
Mauna Loa has been the grand theater for lava flows. The first symptom of an eruption there, is the sudden illumination of the sky caused by the reflection of the molten lava. Mokuaweo weo has been empty and quiet, till all at once the end of the as censive column brings in a flood of lava. If the sky is cloudy the light cannot be seen. Within a very few days there is an outburst from some point below on the side of the mountain, and the stream begins to flow, starting from an elevation of from o,000 to 12,000 feet above the sea.
At the point of outbreak what has been called a terminal cone is formed—well seen in Plate 21, known as the Dewey crater, in 1899. These are made of lapilli, and may be seen at the sources of all the historic flows, as well as many more that are pre historic. It would seem that hydrostatic pressure causes the lava to rise from an orifice perhaps hundreds of feet. The jet is high est at first, and disappears when the flow has ceased. The lapilli are simply the cold splashes of the liquid. The various facts relating to the dates, altitudes, duration and other elements of the flows can be seen best from tabular views.
From the tabular view of the eruptions from Mokuaweoweo and Mauna Loa the following facts seem to be established. There are five different features of which three are sometimes combined.
1. Every eruption commences with an influx of melted lava to the pit of Mokuaweoweo. In ten cases there was no farther manifestation of activity. These were in 1849, '65, '72, '73, '73, '75, '76, '8o, '96, 1903.
2. There were eight cases where this lava proceeded from ori fices very high up, from one to three thousand feet below the bot tom of Mokuaweoweo and a considerable time was required to discharge the liquid. These were in 1780, 1832, '43, '52, '55, '59, '8o, '99.