Fig. 110.—Three successive views to illustrate the growth of the Island of Savaii from the outflow of lava at Matavanu in the year 1906. a, near the beginning of the outflow; b, some weeks later than a; c, some weeks later than b (after H. I. Jensen).
The surface which flowing lava assumes, while subject to considerable variation, may yet be classified into two rather distinct types. On the one hand there is the billowy surface in which ellipsoidal or kidney-shaped masses, each with dimensions of from one to several feet, lie merged in one another, not unlike an irregular collection of sofa pillows. This type of lava has become known as the Pahoehoe, from the Hawaiian occurrence ([Fig. 109]). A variation from this type is the “corded” or “ropy” lava, the surface of which much resembles rope as it is coiled along the deck of a vessel, the coils being here the lines of scum or scoriæ arranged in this manner by the currents at the surface of the stream ([Fig. 123], [p. 124]). A quite different type is the block lava (Aa type) which usually has a ragged scoriaceous surface and consists of more or less separate fragments of cooled lava ([Fig. 131], [p. 130]).
Wherever lava flows into the sea in quantity, it extends the margin of the shore, often by considerable areas. The outflow of Kilauea in 1840 extended the shore of Hawaii outward for the distance of a quarter of a mile, and a more recent illustration of such extension of land masses is furnished by [Fig. 110].
CHAPTER X
THE RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE
VOLCANIC MOUNTAINS OF EJECTED MATERIALS