110 J. D. Dana— Volcanic Action. 
tainly in combined action with the preceding, of extensive 
fractures and outflows, and sometimes in the case of both the 
Mt. Loa summit crater and Kilauea, of large and lofty fountain- 
like throws of the extended lavas. 
(5) The expansion of the moisture in the liquid lava into 
vapors as the lava approaches the surface, thereby expanding 
the lava in this upper part, making it cellular and raising the 
level of the surface. This effect is limited by the amount of 
moisture present, and by the degree of pressure that prevents 
moisture below from taking the form of vapor. 
(6) Underminings, through the discharge of lavas, leading 
often to subsidences and engulfments, and chiefly of the ceutral 
parts of the voleano about and within the crater. 
In 1840, after a time of violent activity over the whole of 
Kilauea, the subsided area included a large part of the floor of 
the crater, it being nearly two miles long and about two-thirds 
of a mile in mean breadth, with the depth nearly 400 feet. It 
left the crater over the sunken floor nearly 1,000 feet deep and 
elsewhere over 600 feet. 
In 1886, after more local activity the subsidence was mostly 
confined to Halema’uma’u and its vicinity (including New Lake) 
the area about half a mile square; the depth of the subsidence 
was small, the depressions that were left being due mainly to 
the escape of the lava and some sinking of the walls of the two 
pits. Owing to the overflowings and intrusions of lavas since 
1840 without equivalent subsidences, the mean depth of Kilauea 
after this last eruption was about 400 feet. There may be great 
empty chambers beneath, and after a time of general activity 
and great discharge, another subsidence like that of, 1840 in 
extent may occur. 
The form of Kilauea and character of the operations now going 
on within it show that the activity (ebullition, etc.) was at some 
time in the past so general over the included area that the sub- 
sidences which followed the eruption affected the whole, nearly 
to the present outer walls; and that under such conditions 
Kilauea took its form and size.* The underminings that have 
since occurred have from time to time caused down-plunges of 
portions of the walls into the emptied spaces beneath and so 
increased somewhat its extent just as the walls of the pits in the 
crater emptied by an eruption continue to fall until the pits are 
filled again with lavas. 
3. Vesuvius and other Volcanoes. 
13. Fundamental forces in Vesuvius and other Voleanoes.— Vesu- 
vius and the Hawaiian volcano differ much in externals, but 
not in forces, or in their mode of action. 
* The plate in Ellis’s Polynesian Researches represents the lower pit in 1823 
(after a great eruption), as reaching at the time very nearly to the outer walls of 
Kilauea. 
