Fig. 107.—Diagrammatic representation of the structure of the flanks of lava volcanoes as a result of the draining of frozen lava streams.
Protected from any extensive consolidation by its congealed cover, the lava within a stream may all drain away, leaving behind an empty lava tunnel, which in the case of the Hawaiian volcanoes sometimes has its roof hung with beautiful lava stalactites and its floor studded with thin lava spines. Later lava outflows over the same or neighboring courses bury such tunnels beneath others of similar nature, giving to the mountain flanks an elongated cellular structure illustrated schematically in [Fig. 107]. These buried channels may in the future be again utilized for outflows similar in character to that of Kilauea in 1840.
Fig. 108.—Diagram to show the manner of formation of mesas or table mountains by the outflow of lava in valleys and the subsequent more rapid erosion of the intervening ridges. R, earlier river valley; R’R’, later valleys.
While the formation of lava stalactites of such perfection and beauty is peculiar to the Hawaiian lava tunnels, the formation of the tunnel in connection with lava outflow is the rule wherever a dissipation at the end has permitted of drainage. A few hours only after the flow has begun, the frozen surface has usually a thickness of a few inches, and this cover may be walked over with the lava still molten below. At first in part supported by the molten lava, the tunnel roof sometimes caves in so soon as drainage has occurred.
Fig. 109.—Surface of lava of the Pahoehoe type.
Wherever basaltic lava has spread out in valleys on the surface of more easily eroded material, either cinder or sedimentary formations, the softer intervening ridges are first carried away by the eroding agencies, leaving the lava as cappings upon residual elevations. Thus are derived a type of table mountain or mesa of the sort well illustrated upon the western slopes of the Sierra Nevadas in California ([Fig. 108]).