Gordon A. MacDonald

HAWAIIAN PETROGRAPHIC PROVINCE

The lavas of the Hawaiian Islands range from mafic picrite-basalts and melilite-nepheline basalts to salic trachytes. Olivine basalt, by far the most abundant type, is regarded as representing the parent magma of the Hawaiian province. Closely associated with the olivine basalts are basalts, and picrite-basalts with many large phenocrysts of olivine. Further differentiation results in eruption of andesine andesite, oligoclase andesite, picritebasalt with abundant large augite phenocrysts, and more rarely trachyte. Following a long period of quiescence there have been erupted on some islands nepheline basanite, nepheline basalt, melilite-nepheline basalt, “linosaite,” and a third type of picrite-basalt. The mineral and chemical composition of the various rock types, and their distribution on the individual islands, are described. Starting with olivine basalt as the parent magma means of deriving the other rock types are considered. It is concluded that crystal differentiation has been the principal process, although assimilation of limestone may also have been important. The parts played by gaseous transfer and selective remelting are difficult to evaluate, though both probably operated to some extent. Comparison of the types of igneous rocks in the Hawaiian province with those recorded from other Pacific islands shows that the rocks throughout the true Pacific Basin are, for the most part, closely similar, and suggests essential uniformity of parent magma and petrogenic processes throughout the Pacific Basin.

Eruption of Lassen Peak, Cascade Range, California, in 1915: Example of Mixed Magmas

The pumice of the 1915 eruption of Lassen Peak is conspicuously banded. Light bands are dacite; dark bands are andesite. They represent two distinct magmas which were imperfectly mixed at the time of eruption; the portions of different composition were drawn out into bands by flowage. Similar, but less obvious and possibly less extreme, inhomogeneity exists in the lava flow of the same eruption. In both lava flow and pumice, the phenocrysts were out of equilibrium with the enclosing magma and were reacting with it. Numerous feldspar phenocrysts that originated in the dacite portion show clouding, which resulted from fritting of their outer parts, and a more calcic outer zone; however this appears to be related only in small part to the addition of the basic magma, represented by the dark streaks in the pumice. These feldspars probably are remnants of remelted granitic rocks of the underlying sialic basement admixed with basaltic magma rising from still greater depth. Phreatic explosions in 1914 probably resulted from distension of the Lassen dome by the rise of new magma.

PHYSICAL PROPERTIES OF ERUPTING HAWAIIAN MAGMAS

Field measurements and estimates of temperature, viscosity, gas content, and specific gravity of erupting Hawaiian lavas are assembled largely in tabular form. Hawaiian tholeiitic basalt magmas as they reach the surface have temperatures of 1050°–1200° C, viscosities of 2–4 × 10 3 poises, and gas contents ranging from 0.5 to 2 weight per cent. Below the level of vesiculation they have a specific gravity of about 2.73. Flows continue in motion to apparent temperatures less than 800°.

The 1959 and 1960 eruptions of Kilauea volcano, Hawaii, and the construction of walls to restrict the spread of the lava flows

A summit eruption of Kilauea volcano took place in Kilauea Iki crater, immediately adjacent to Kilauea caldera, from November 14 to December 20, 1959. Approximately 61 million cubic meters of lava was poured into the crater during 16 short eruptive episodes. Following each of the later episodes nearly as much lava drained back into the vent as had been poured out.On January 13, 1960, eruption commenced on the east rift zone of Kilauea about 40 kilometers east of the caldera. This eruption continued until February 19, liberating 113 million cubic meters of lava, and burying approximately 5.6 square kilometers of land surface, including much agricultural land and most of the village of Kapoho. Several walls were built, to try to prevent or lessen the southward encroachment of the lava. These walls were dams, impounding the lava, not diversion barriers. Lava overflowed the dams, but it is believed that the amount of southward spreading of the lava was sufficiently reduced to more than justify their construction; and it was demonstrated that such walls, properly constructed, will adequately withstand the thrust of even thick lava flows of the type characteristic of Hawaiian eruptions.

Petrography of the Samoan Islands

Tutuila Islands consists of five overlapping shield volcanoes. The lavas of all are predominantly olivine basalts, but picritic basalts, olivine-poor basalts, and olivine-free basalts also are present. Two flows of oligoclase andesite are found in the easternmost (Olomoana) volcano; both andesine and oligoclase andesites are present in the westernmost (Taputapu) volcano. The pre-caldera lavas of Pago Volcano are predominantly olivine basalts, with less abundant olivine-poor basalts, and picritic basalts rich in olivine phenocrysts. The same types are present in the caldera-filling lavas, but the olivine-poor basalts are more abundant; andesites and olivine-free basalts also are present. The post-caldera lavas include olivine basalts, olivine-poor basalts, picritic basalts, andesine andesites, and oligoclase andesites. The picritic basalts differ from those in the earlier groups in containing numerous augite phenocrysts in addition to those of olivine. During the period of eruption of the caldera-filling and post-caldera lavas, there were extruded several masses of quartz trachyte. The last eruptions on Tutuila were of limburgite-basalt. Specimens from Ofu include olivine basalt, basaltic andesite, and picritic basalt with many phenocrysts of augite. Similar olivine basalts and a picritic basalt were collected on Tau. The oldest (Pliocene?) lavas of Upolu are preponderantly olivine basalt, with smaller amounts of olivine-poor basalt, picritic basalt, and basaltic andesite. At least one mass of trachyte is present. The picritic basalts contain phenocrysts of both olivine and augite. With one possible exception, all the Pleistocene lavas of Upolu are olivine basalts. The exception is a specimen of basaltic andesite which may belong with the older lavas. The Recent lava flows also are olivine basalts. The lavas of Savaii are predominantly olivine basalts, but picritic basalts containing augite and olivine phenocrysts also are known. All available chemical analyses of Samoan rocks are listed. The suite is alkalic, near the boundary of the alkali-calcic group, with an alkali-lime index of 50.5. Differentiation was probably largely by crystal settling.

A Further Contribution to the Petrology of Haleakala Volcano, Hawaii

Sixteen new chemical analyses of the later rocks of Haleakala Volcano, on the island of Maui, Hawaii, add to the differentiation picture for that volcano. The early rocks of the volcano are tholeiitic. These are followed by dominant hawaiites with less abundant alkalic olivine basalts, picrite-basalts of ankaramite type, and a few mugearites. Still later rocks, separated from earlier ones by a profound erosional unconformity, include some hawaiites and ankaramites, but are dominantly alkalic olivine basalts (basanitoids) containing as much as 16.5 percent normative nepheline, some of them transitional to ankaramite. The progression toward ultramafic, strongly undersaturated rocks (nephelinites), characteristic of the post-erosional lavas of other Hawaiian volcanoes, appears to have just begun at Haleakala.

CONTRIBUTION TO THE PETROGRAPHY OF HALEAKALA VOLCANO, HAWAII

The volcanic rocks of Haleakala Volcano comprise, from oldest to youngest, the Honomanu, Kula, and Hana volcanic series. The Honomanu volcanic series consists largely of olivine basalt, with less abundant basalt and picrite-basalt containing large phenocrysts of olivine. In the Kula and Hana volcanic series there are some basalts and olivine basalts, but the characteristic rocks are andesites and picrite-basalt containing abundant phenocrysts of augite as well as olivine. New chemical analyses of six specimens of Kula lavas are given. Five are andesites, and one a picrite-basalt. Each of the six rocks is described in detail. A variation diagram for Haleakala rocks illustrates the alkalic character of the series.

PETROGRAPHY OF IWO JIMA

The rocks of Iwo Jima are augite and augite-hornblende trachyandesites, distinctly more alkalic than the ordinary andesites of Japanese volcanoes or the average world andesite. They are described, and chemical analyses are listed. Fumarolic gases have altered some rocks to a mixture of opal and clay minerals and have cemented tuff with opal, calcite, zeolite, and iron oxides.

Interlaboratory comparison of some chemical analyses of Hawaiian Volcanic Rocks

The comparability of chemical analyses of Hawaiian volcanic rocks by different laboratories is investigated. The comparability is reasonably good for most purposes, but small differences in compositions of rocks determined by different laboratories should not be used as a basis of classification or to support petrogenic deductions.

PETROGRAPHY OF THE WALLIS ISLANDS

The lavas of Uvea Island, in the Wallis group, include olivine basalts containing colorless monoclinic pyroxene, less abundant olivine basalts containing titaniferous pyroxene, and oligoclase andesite. Palagonite tuff builds cones along the eastern coast. Of the lesser islands in the Wallis group, Nukuatea is built largely of palagonite tuff, with a flow of olivine basalt; Nukufetau is composed of olivine basalt flows and cinders; Luaniva is composed largely of olivine basalt with a little tuff; and Fugalei of olivine basalt which filled the bowl of a tuff cone, most of which has been eroded away. Other islets represent the emergent edge of the barrier reef and are composed of poorly to well-consolidated calcareous sand. The lavas of the Wallis Islands belong to the alkaline suite of the central Pacific volcanoes, rather than to the calc-alkaline suite of the circum-Pacific belt.