274 S. Powers — Vbioanic Domes in the Pacific. 



rising magma reaches the surface. After the throat of the 

 volcano ifl partly cleared by these explosions, the lava appears 

 at the surface as homhs and usually as a mass of incandescent 

 blocks which is elevated by the injection of magma from 

 beneath. A monolithic mass of rock may be thrust through 

 the dome, and, if so, by expansion and explosions of gas it 

 becomes surrounded by a pile of debris. Once the dome 

 appears at the surface, gas appears to play only a minor role 

 in furthering the uplift, but a major role in causing the destruc- 

 tion of any projecting portions of the dome or even of the 

 entire mass. As in the cases of Perry Peak and McCullqch 

 Peak at Bogoslof, the gas rising from the deeper seated portions 

 of the magma may apparently become confined under a 

 hardened crust in such a quantity that the whole top is 

 blown off. 



Vicosity is the principal factor which determines whether 

 the magma shall appear as a flow or a dome. At Santorin in 

 1866 a dome was formed, but the lava finally burst out the 

 side as a flow, whereas at Colima in 1869 and at the crater near 

 Pauline Lake, Oregon, the flow came first, and an increas- 

 ing degree of viscosity finally led to the formation of a dome. 

 Contrasted to these domes built by magmas of low fluidity, 

 and frequently great porosity, stand the Kilauean uplifts 

 caused by basalt, which, when liquid, is one of the least viscous 

 of magmas. The frozen crust of a lava lake was elevated about 

 600 feet at one time, the floor of older lava surrounding an 

 open lava lake was raised at another time, and still later a pile 

 of talus in the form of an inverted cone was raised into a debris 

 cone in the center of which the lava lake finally appeared. 

 The uplifted masses at Kilauea and at New Mountain formed 

 at Usu in 1910 differ from the other domes because consoli- 

 dated lava or talus instead of newly intruded magma formed 

 the uplifted mass. The principle of uplift, however, appears 

 to be the same, because in each case new lava appeared later at 

 the surface. 



