J. D. Dana — Cause of the form of the Mt. Loa Dome. Ill 



III. The form of Mt. Loa a consequence of its Eruptions. 



Mt. Loa differs from almost all other volcanic mountains in 

 having a double curvature in its profile, owing to the flattening 

 and widening of its summit, and the spreading of its base. It 

 is the flattened summit which gives so vast bulk to a mountain 

 of its altitude. 



This peculiarity I attributed in my Expedition Report, to 

 the positions of the prevailing outflows, on the ground that dis- 

 charges of lavas about the base tend to widen and flatten the 

 base and give a single concavity to the profile on either side'; 

 that discharges at the summit, especially if in short streams, 

 serve to elevate the summit and make still more pronounced 

 the single concavity ; but that discharges over the upper slopes 

 and not over the summit, tend to widen the upper part and 

 flatten the summit so as to produce a convexity in the profile 

 above. 



The outflows of the century have had the distribution required 

 to produce the actual form. Part are basal ; another large part 

 start just below the summit, and none of much size from the 

 vicinity of the summit crater. The double curvature so pro- 

 duced, however, is mostly confined to the eastward and sonth- 

 southwestward slopes, the chief directions of expansion by basal 

 outflows ; moreover, the widening in the former direction owes 

 much, beyond doubt, to the eruptions of Kilauea. Further : 

 owing to the absence nearly of cinder -ejections, the summit fails 

 of the most common means of growth in height w T ith tapering 

 top ; and this is a prominent source of the difference between 

 it and most other volcanic mountains. 



Another cause tending to modify the shape of the mountain 

 is that producing fractures and subsidences. Its effects are seen 

 about the great craters, and still more pronounced about the 

 borders of the island. The former action aids in making summits 

 broad and flat, while the latter works directly against the widen- 

 ing of the coast region. It makes the greatest fractures, nearly 

 parallel with the coast and drops the coastward block ; it thus 

 tends to shorten the radius of that part of the mountain and 

 put precipices into its profiles, increasing thereby the mean slope. 

 Two such walls in southern Hawaii, cross the road betw T een 

 Keauhou and Kilauea, one about a mile and a half from the 

 coast and the other three miles ; they are marked features be- 

 fore the traveler in his ride from the coast to the volcano. 

 These faul tings seem to be a reason for the concavity in the 

 southern coast-line from Keauhou westward, and for the short 

 distance in that direction between the summit and the coast. 

 Other great fault-planes exist ; but the government map of the 

 island should be completed before the facts can be satisfactorily 

 discussed. 



