THE ASCENSIVE ACTION IN THE LAVA COLUMN.
Perhaps the first suggestion of the uplifting action came from Prof. C. S. Lyman in 1846, following observations two years earlier by Mr. Coan. In 1844 Mr. Coan found that the lower pit formed in 1840 had been filled up, ostensibly by overflows of lava. Two years later (June, 1846) this pit was nearly obliterated, and there were wide canals of liquid lava intervening between the black ledge and the area of the lower pit. By July, 1846, the pit was filled up and Mr. Lyman concluded that the interior had been elevated, in some parts above the black ledge, which had remained stationary. When the pit was depressed many blocks of lava had fallen into it, making a talus on the floor after elevation. These fragments formed a ridge inside of the canal (shown in Plate 3o) higher than the black ledge from which the blocks had fallen. This phenomenon he attributed to the "combined effect of re peated overflowing together with the upheaving agency of sub terranean forces." Mr. Coan subsequently noted further move ments in the same direction.
In 1848 Mr. Coan describes the formation of a crust over the lake of Halemaumau, which was soon raised into a dome two or three hundred feet high from which one could look out upon the surrounding country beyond the outer wall. This dome was said to have been "formed by the compound action of upheaving forces from beneath, and of eruptions from the openings forming successive layers upon its external surface." This dome collapsed in 1855.
Meanwhile the interior area continued to rise, and streams of lava to flow over the black ledge adjacent to the walls, while the original talus of 1846 is discernible upon the map of Mr. Brigham made in 1866. It was very nearly a plain in 1868 when two-thirds of the floor fell down as much as six hundred feet, leaving a lower pit comparable with that figured by Drayton in 1840, and a black ledge encircling the outer walls a hundred feet higher. Thus the removal of the subterranean support of the mass that had been slowly rising for thirty-eight years caused the disappearance of the columnar block.
The subsequent filling of the lower pit till the breakdown of 1886 was mostly effected by flows from Halemaumau, but the "crag walls" and circular debris about the principal lake up to two hundred feet altitude are to be ascribed to the ascensive column.
Three months after this collapse, the cone within Halemaumau began to rise. Several figures illustrate these changes. The level upon March 6 was three hundred and eighty feet below the datum point at the Volcano House, the highest point attained by the black ledge thus far in the history of Kilauea. (For convenience the fig ures showing the variations in the altitude of the ascensive column are calculated as below this datum, with the minus sign.) March 7 the lowest point of the depression was nine hundred feet. See
the sections i and 2 in Plate 38. The beginning of the rise was more rapid than it was later, viz., one foot per day. In October, i886, Plates 37B and 38, the depression had been filled up, in some measure by lava flowing. hut more particularly by elevation of the debris to be on a level with the rim of Halemaumau—three hun dred and thirty-five feet. Professor Dana saw the cone in August, 1887, and declared it to be rising slowly. In July, i888, by Mr. Dodge's measurements, Plates 38, 39A, the highest point was one hundred and sixty-two feet. There are no measurements to indi cate how much the cone rose later, for nearly a year. In May, 1889, the floor and the cone fell eighty feet. But it commenced to rise again till the highest point was attained of about eighty five feet, just before the collapse of March I, 1891. The hori zontal outline of this column, or of Halemaumau, is shown in Plate 43 in its relation to the earlier outline of i886. The falling of the floor to nearly five hundred and fifty feet indicates how much the ascensive column was depressed.
After this depression the refilling of the pit was occasioned chiefly by the accumulation of lava ; but on March 21, 1894, there was a sudden elevation of the north wall to the extent of eighty feet, which must be referred to another ascensive movement, though it does not seem to have been very important, as it had mostly disappeared before the collapse of July 8 to 11 following.
The presence of fault planes parallel to the outer wall of the caldera or to Halemaumau as seen on the map of 1863, or to the outline of Kilauea iki, would suggest some relation to ascensive columns. Hence a careful study will tend to increase the num ber of these columns in all the calderas. They illustrate also the sympathy discernible between the volcanoes.
A column of basaltic material rising between circular walls re minds one of the obelisk or spine forced upwards in the late eruptions from Mont Pelee in Martinique and at Bogosloff in Alaska. These were not permanent because of exposure to denudation ; but if they had been protected like the ascensive columns in Kilauea, they might have endured much longer. It is conceivable that the spine may sometimes be elevated from one side as a horizontal block, so as to be like the elongated rectangu lar mountains now projecting above the floor of Mohokea. They may bear kinship also to laccoliths.
The force that elevates these columns must be the pressure of the deep seated mobile lava seeking an outlet, and of course inti mately related to the general theory of volcanic action. Some have spoken of this action in Kilauea as if the column were float ing on the surface of the subjacent lava. When more lava is supplied the column rises; and after the removal of the liquid the debris will sink back, following it downwards as far as possible.