Lava movements within the caldron of Kilauea.—The craters of these mountains are the largest of active ones, each being in excess of seven miles in circumference. In shape they are irregularly elliptical and consist of a series of steps or terraces descending to a pit at the bottom, in which are open lakes of boiling lava. Enough is known of the history of Kilauea to state that the steep cliffs bounding the terraces are fault walls produced by inbreak of a frozen lava surface. The cliff below the so-called “black ledge” was produced by the falling in of the frozen lava surface at the time of the outflow of 1840, the lava issuing upon the eastern flank of the mountain and pouring into the sea near Nanawale. Since that date the floor of the pit below the level of this ledge has been essentially a movable platform of frozen lava of unknown and doubtless variable thickness which has risen and descended like the floor of an elevator car between its guiding ways ([Fig. 102]). The floor has, however, never been complete, for one or more open lakes are always to be seen, that of Halemaumau located near the southwestern margin having been much the most persistent. Within the open lakes the boiling lava is apparently white hot at the depth of but a few inches below the surface, and in the overturnings of the mass these hotter portions are brought to the surface and appear as white streaks marking the redder surface portions. From time to time the surface freezes over, then cracks open and erupt at favored points along the fissures, sending up jets and fountains of lava, the material of which falls in pasty fragments that build up driblet cones. Small fluid clots are shot out, carrying a threadlike line of lava glass behind them, the well-known “Pelé’s hair.” Sometimes the open lakes build up congealed walls, rising above the general level of the pit, and from their rim the lava spills over in cascades to spread out upon the frozen floor, thus increasing its thickness from above ([Fig. 103]). At other times a great dome of lava has been pushed up from the pit of Halemaumau under a frozen shell, the molten lava shining red through cracks in its surface and exuding so as to heal each widely opened fissure as it forms.
Fig. 102.—Schematic diagram to illustrate the moving platform of frozen lava which rises and falls in the crater of Kilauea.
At intervals of from a few years to nine or ten years the crater has been periodically drained, at which times the moving platform of frozen lava has sunk more or less rapidly to levels far below the black ledge and from 900 to 1700 feet below the crater rim. Following this descent a slow progressive rise is inaugurated, which has sometimes gone on at a rate of more than a hundred feet per year, though it is usually much slower than this. When the platform has reached a height varying from 700 to 350 feet below the crater rim, another sudden settlement occurs which again carries the pit floor downward a distance of from 300 to 700 feet.
Fig. 103.—View of the open lava lake of Halemaumau within the crater of Kilauea, the molten lava shown cascading over the raised lava walls on to the floor of the pit (after Pavlow).
The draining of the lava caldrons.—The changes which go on within the crater of Mokuaweoweo, though less studied than those of Kilauea, appear to be in some respects different. Here every eruption seems to be preceded by a more or less rapid influx of melted lava to the pit of the crater, this phenomenon being observed from a distance as a brilliant light above the crater—the reflection of the glow from overhanging vapor clouds. The uprising of the lava has often been accompanied by the formation of high lava fountains upon the surface, and the molten lava sometimes appears in fissures near the crater rim at levels well above the lava surface within the pit.
Although in many cases the lava which has thus flooded the crater has suddenly drained away without again becoming visible, it is probable that in such cases an outlet has been found to some submarine exit, since under-ocean discharge effects have been observed in connection with eruptions of each of the volcanoes.
