PAHOEHOE AND AA.
The terms pahoehoe and aa, expressive of the two principal classes of lava are of Hawaiian origin, whence it is evident that the natives early appreciated their peculiarities. They have now taken a fixed place in volcanic terminology in spite of the protest of Professor T. G. Bonney, and have been recognized as existent in other countries and older formations.
Pahoehoe is the most common of the two. The name signifies having the aspect of satin, a shining smooth surface. It is quite hummocky because the liquid stiffens very quickly after exposure. Water is smooth in rivers, but when the temperature falls below the freezing point the surface will bulge much like the lava. Plate 51 represents pahoehoe from the flow of '8o-1 near Hilo. On the right the bulging is more domelike: and it represents a stream later than that on the left. The superposition of the later flow is apparent in the photograph. Standing by this variety of lava as it forms, one sees that it is a stream of liquid material, and if a stick be thrust into it red lava will flow out. On the left side the domes show the wrinkling called ropy structure, convex down stream, because the current has a greater velocity in that direction. The crust is flexible and is modulated by the motion of the liquid beneath. Other figures from Kilauea show the same structure. The surface may be more glassy than the substratum because the more siliceous part is lighter than the rest. Beneath the billowy surface may be vacant spaces, left by lava which has run out, leaving tunnels, which often are lined by stalactites. The rate of the flow is variable, depending on the degree of fluidity and the slope. Most observers overrate the velocity. It took nine months for the flow of '8o-I to reach Hilo, a distance of thirty miles.
The study of the aa has not yet revealed to us its proper nature and origin. All travelers avoid crossing it if possible unless the highway surveyor has pulverized its roughness, when it makes an admirable road. Those who climb Mauna Loa travel miles to avoid crossing it.
Two examples are very prominent ; the first between Kilauea and the Halfway House, Plate 26. It starts from near Puu Ulaula, descends the south slope and when near Kilauea turns to the southwest. For many years this curved ridge figured as the
"flow of 1823," but this label has proved to be erroneous. The roughness of it where traversed by the road for six miles beggars any possible descripton. The fragments are larger than usual, from three to five feet in length. The other illustration is the long embankment below Aina Hou near Humuulu sheep station, situated as if it were a spur from the flow of 1843, with scarcely any descent. It was an older flow, creeping along like a cater pillar, and effectually imprisons the cattle in the mauka grassy area. Plate 5o shows aa from the flow of 1887.
When minutely examined the aa ridge is seen to be composect of bristling ragged rocks, incoherent, with an occasional smoother piece like a bomb. The fragments in the original streams are not cemented together any more than cobble stones in a gravel bank. The blocks may be a few cubic inches to a thousand feet in size, dumped to a height of twenty to forty feet ; they are brittle, not scoriaceous, slightly vesiculate, easily broken down. Professor Dana says, "the reader's conception of it will be feeble at the best if he has not already had a view of chaos." When in motion D. H. Hitchcock thus describes it : "The whole broad front of the then sluggish stream was a mass of solidified lava twelve to thirty feet in height, moving slowly along by breaking and bearing on ward the crushed covering; along the whole line of its advance it was one crash of rolling, sliding, tumbling, red hot rock, no liquid rock being in sight ; there were no explosions, hut a tre mendous roaring, like ten thousand blast furnaces all at work at once. The rough blocks lie piled together in the wildest con fusion, many as large as ordinary houses—and the movement is slow." Others refer to a red heat among the blocks, while the fused rock rarely exudes, but insist upon a heavier molten fluid beneath supporting the bristling fragments, and a rolling action in the front part. I have sometimes compared the onward motion to the flow of burning anthracite coal, when unintentionally by the opening of the grate beneath a quantity of it escapes.