CHARACTER OF HAWAIIAN VOLCANIC ACTIVITY
Composition of lavas. A volcano may be defined as an opening in the earth's crust from which molten rock or gases, or generally both, are liberated at the surface. The "molten" rock is in reality not a simple melt, but a complex mutual solution of silicates and oxides. It appears always to contain gases, either dissolved in it or merely enclosed in it as bubbles. Also, the molten rock often contains solid crystals of various minerals, which have separated from it by crystallization. This hot liquid, with its dissolved gases and suspended gas bubbles and solid crystals, is called magma, and when it is poured out onto the earth's surface it is called lava.
Lavas vary greatly in composition. In considering their composition we generally deal only with the solid material which is left after the lavas cool and harden. Physical difficulties have prevented us from capturing for analysis a sample of magma complete with its enclosed gases, and we do not yet know the relative proportions of gas to the things that remain in the solidified lava. For this reason, in making chemical analyses of lavas we generally disregard the gases, and list only the substances present in the solid state. These are usually reported as oxides of the various chemical elements present (Table 1).
The most abundant oxide in lavas is silica. Its proportion ranges from about 40 to 75 percent. In general, as the percentage of silica increases so does that of the alkalies (soda and potash), and as it decreases there is an increase in the percentage of iron oxide, magnesia, and lime. The great preponderance of Hawaiian lavas are near the silica-poor end of the scale, and are relatively rich in iron, magnesia, and lime. Table 1 shows the average proportions (in weight percent) of the principal oxides in the commonest lavas (olivine basalt) of Kilauea and Mauna Loa, and also of a type (andesite) common on Haleakala, which is slightly richer in silica and considerably richer in alkalies.
The commonest lava of all the Hawaiian volcanoes is the type known as olivine basalt. Its most abundant constituent is the light-colored mineral labradorite, which is a variety of plagioclase feldspar (see vocabulary page 60) containing more lime than soda. It generally comprises nearly half of the crystallized rock. Next in abundance is pyroxene (see vocabulary). A little iron oxide (magnetite) and titanium-iron oxide (ilmenite) also are present. The grains of all these minerals are commonly too small to be seen readily without the aid of a magnifying glass. However, another mineral, olivine (see vocabulary ), commonly forms larger-grains of a bottle-green or brownish-green color, in a stony matrix composed of other minerals. Olivine grains may reach a length of half an inch, although they are generally smaller. When they are large enough and free of cracks or other flaws they can be cut into handsome semiprecious gems, known to the jewelry trade as "peridot." Loose crystals of olivine are fairly abundant in the ash of the explosions of 1924 around Halemaumau, in Kilauea caldera. Large black crystals of augite (a pyroxene) occur in the lavas and cinder cones of Mauna Kea and Haleakala.
The olivine basalts grade into rocks known as andesites, richer in silica and alkalies than the basalts (Table 1), and generally lighter in color. Andesite is unknown at Kilauea and Mauna Loa but is abundant in Mauna Kea and Haleakala. The commonest type of andesite of the Hawaiian Islands goes by the special name of "hawaiite."
There are two principal types of lava flows in Hawaii. Pahoehoe is the type characterized by smooth, ropy, or billowy surfaces (Plate 5); whereas aa has a very rough, spiny or rubbly surface, but a massive interior (Plate 4). The two types intergrade, and a pahoehoe flow may change over to aa downslope or toward its margins, although the reverse change from aa to pahoehoe never occurs. Analyses of congealed fragments of the two types of lava from the same flow show them to be essentially identical in chemical composition. The difference between them seems to result largely, if not wholly, from the state of the enclosed gas at the time the lava froze to immobility.
Aa flows are fed by open rivers of lava located near the center line of the flow. From the river, some lava spreads laterally to feed the moving margins of the flow, but a larger quantity moves downhill to feed the actively advancing flow front. At the end of the eruption the fluid lava may partly drain out of the river, leaving a distinct channel lower than the surface of the adjoining flow. Such a channel is crossed by the Mauna Loa Strip Road at 5,500 feet altitude, and similar channels are seen where the highway crosses several of the historic lava flows on the western side of the island.
The feeding rivers of pahoehoe flows soon crust over and develop more or less continuous roofs, and thenceforth the lava stream flows within a tube of its own making. Many smaller tubes branch off of the main tubes and feed the front and sides of the flow. At the end of the eruption most of the molten lava may drain out of the main tubes, leaving open tunnels commonly as much as 10 to 20 feet in diameter, which often resemble subway tunnels with arched roof and nearly flat floor. The floor is the final congealed surface of the lava stream in the tube. "High lava marks" along the walls commonly mark the stages of lowering of the stream surface as the supply of molten lava decreased. All of these features are well shown at the Thurston Lava Tube, 3 miles southeast of Park Headquarters toward the Chain of Craters Road.
Nature of Hawaiian eruptions. It has already been said that the outstanding characteristic of Hawaiian volcanic activity is its gentleness. A few violent explosions are known, both before and since the beginning of the historic record, but they are rare and abnormal. Generally the lava issues quietly from its vent, bubbling out gently to form flows or lakes, or spouting a few hundred feet in the air as great fountains of liquid lava (Plate 7). The Hawaiian volcanoes lack the explosive character of many other volcanoes, which throw out great quantities of rock fragments and turn day into night over hundreds of square miles with their high-flung dark cloud of ash. Even the rare explosive eruptions of Hawaiian volcanoes are puny indeed compared with such explosive cataclysms as the eruption of Krakatoa in the East Indies in 1883, or the eruption of Katmai in Alaska in 1912.
Why should the Hawaiian volcanoes erupt so quietly as compared with most other volcanoes? The answer lies in the greater fluidity of Hawaiian lavas and in their lower gas content, which appears generally to be only about 0.5 to 1 percent by weight. The three factors that govern the viscosity of any lava are: its chemical composition, its temperature, and the amount of gas that it contains. In general, the lower the silica content of the lava the less viscous it is, and also the higher the temperature and gas content the lower the viscosity. If the lava is highly viscous the gas enclosed in it has difficulty in escaping, and must build up a high pressure before it is able to force its way out. When it finally escapes, it does so with explosive violence. On the other hand, if the lava is less viscous the contained gas is able to escape easily, without explosion. We already have seen that the Hawaiian olivine basalts are relatively low in silica. This, together with high temperature, (generally 1100° to 1180°C., or 2000° to 2150°F.) appears to be the cause of the great fluidity and consequent quiet eruption of Hawaiian lavas.
One evidence of the high fluidity of Hawaiian lavas is the formation of tree molds and tree casts. Where fluid pahoehoe surrounds a tree the lava is chilled against the tree trunk and hardens, preserving many of the details of the bark. The tree burns completely or in part, and the charcoal and other debris is eventually washed away by rain and blown out by wind, leaving a cylindrical hole in the lava where the tree once stood or lay. Molds of this sort, formed by engulfment of large koa trees, can be seen at Kilauea about 2 miles west of Park Headquarters. Elsewhere, as near Napau Crater, lowering of the flow surface by liquid lava draining away leaves the molds projecting as much as 10 feet above ground level (Plate 16).
The high fluidity of Hawaiian lavas results not only in lack of explosion but also in high speeds of flow. Some Hawaiian lava flows advance several miles in a single day, and rates of flow as high at 35 miles an hour have been measured in the main lava rivers. In comparison, most lava flows in other areas creep ahead only a few feet an hour or a few feet a day.
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