Basaltic lava

“ Dana calls these Hawaiian volcanoes "basaltic," basalt being the prevalent material of which they are formed. Those of the moon may be composed of similar material, or otherwise; but in either case we may suppose they are built up of lava, erupted from vents connected with the molten reservoirs of the interior. Thus we conclude that they belong to an entirely different type, and have been built up in a different manner, from those represented by Etna, Vesuvius, and most of the extinct volcanoes of Auvergne, the Eifel, and of other districts considered in these pages. ”

“ The cavities are filled up with many secondary minerals, such as calcite, chlorite, quartz, epidote, chalcedony: but in microscopic sections the nature of the original lava can nearly always be made out from the shapes and properties of the little crystals which occur in the decomposed glassy base. Even in the smallest details these ancient tuffs have a complete resemblance to the modern ash beds of Cotopaxi, Krakatoa and Mont Pelée. Basaltic tuffs are also of wide spread occurrence both in districts where volcanoes are now active and in lands where eruptions have long since ended. ”

“ Nevertheless, this test will not always avail the geologist; for, although there is usually a prevailing character in lava emitted during the same eruption, and even in the successive currents flowing from the same volcano, still, in many cases, the different parts even of one lava-stream, or, as before stated, of one continuous mass of trap, vary much in mineral composition and texture.
In Auvergne, the Eifel, and other countries where trachyte and basalt are both present, the trachytic rocks are for the most part older than the basaltic. ”

“ One great difference between the more ancient Basaltic eruptions and those of the Lava and Trachyte of existing volcanoes, is that the emission of the former, probably taking place under the pressure of deep water, was not accompanied by the formation of any permanent craters.
In both cases, the fissures through some of which these Eruptions may have issued, are abundantly apparent under the form of Dikes, filled with materials similar to those which form the masses that have overflowed in the Vicinity of each Dike. ”

“ It has been demonstrated, moreover, that the basal-wrecks of no less than five volcanic mountains, each of which must have rivalled Etna in its proportions, existed within this area, and the connection of the lava-currents, which have deluged the surrounding tracts, with these great volcanoes has been clearly proved. It is probable that when more careful and systematic researches are carried on in the other districts, in which widely-spread sheets of basaltic rocks exist, similar volcanic vents will be discovered. ”

“ A lava-current which entered a lake or the sea, however, has sometimes caught up much of the sediment gathering there, and become so commingled with it, that in some parts it is hard to say whether the resulting rock is more igneous or aqueous. Lava which has been squirted up from below into cracks and crevices, and there [Pg 71] consolidated so as to form dykes, sometimes, but not often, produces considerable alteration upon the rocks which it intersects. The basaltic structure is believed to be due to the contraction of lava consequent upon its cooling. ”

“ The mountain is clearly an extinct volcano. It rises probably from a basaltic plateau, for basalt with olivine and pyroxene are seen in it, and red-brown porous lava, with large rounded hornblende crystals and dark-grey tophus, formed of clear little porous fragments of lava and fine ashes, seem to cover its declivity. The tophus, as well as the lava, does not contain any vitreous feldspar, nor is any pumice-stone observable among them, but all the products of the volcano shew that it is a converted basalt. ”

“ While the river Couze has in most cases, as at the site of this ancient bridge, been simply able to cut a deep channel through the lava, the lower portion of which is shown to be columnar, the same torrent has in other places, where the valley was contracted to a narrow gorge, had power to remove the entire mass of basaltic rock, causing for a short space a complete breach of continuity in the volcanic current. The work of erosion has been very slow, as the basalt is tough and hard, and one column after another must have been undermined and reduced to pebbles, and then to sand. ”

“ A second mode of occurrence of tachylyte is in the form of lava flows. Basaltic rocks often contain a small amount of glassy ground-mass, and in the limburgites this becomes more important and conspicuous, but vitreous types are far less common in these than in the acid lavas. In the Hawaiian Islands, however, the volcanoes have poured out vast floods of black basalt, containing felspar, augite, olivine, and iron ores in a black glassy base. They are highly liquid when discharged, and the rapid cooling which ensues on their emergence to the air prevents crystallization taking place completely. ”

“ Now, for the first time, vast masses of basalt and dolerite are extruded both from vents of eruption and fissures; and, owing to their extreme viscosity, spread themselves far and wide until they reach the margin of some uprising ground of old Palæozoic or Metamorphic rocks by which the volcanic plain is almost surrounded. The great lava sheets thus produced are generally more or less amorphous, vesicular and amygdaloidal, often exhibiting the globular concentric structure, and weathering rapidly to a kind of ferruginous sand or clay under the influence of the atmosphere. ”

“ After the outflowing and consolidation of these volcanic streams, the action of running water cuts them down, cleaving beds of immense depth through their substance. Mr. Poulett Scrope, speaking of the solidified streams of basalt, in the volcanic district of Southern France, observes:—
"These ancient currents have since been corroded by rivers, which have worn through a mass of 150 feet in height, and formed a channel even in the granite rocks beneath, since the lava first flowed into the valley. ”

“ To the conglomerate and trachyte succeeded black basalt, the first dispread in layers full of bubbles, the latter forming regular prisms, placed like a colonnade supporting the spring of the immense vault, an admirable specimen of natural architecture. Between the blocks of basalt wound long streams of lava, long since grown cold, encrusted with bituminous rays; and in some places there were spread large carpets of sulphur. A more powerful light shone through the upper crater, shedding a vague glimmer over these volcanic depressions forever buried in the bosom of this extinguished mountain. ”

“ On the weathered faces the streaky layers may be observed to have been broken up, and their disconnected fragments have been involved in ordinary basalt wherein this flow-structure is not developed, while large blocks and irregular masses are wrapped round in a more decomposing matrix. There can be no doubt that in such cases we see the effects of the disruption of chilled crusts, and the entanglement of the broken pieces in the still fluid lava. ”

“ This is too hard a nut to crack, in our current ignorance of rock under sea bottom muds. But Stübel’s insistence on a coating of lava armor plate over both continents and sea bottoms as the earlier volcanism, and an external veneer on the earth, is unavoidable. If it were all basalt like the present oceanic volcanoes, we should find basalt in continents underneath the granites. We do not do so. If it were all light weight granitizing by segregation of silica, we should find commonly granite and obsidian fragments within oceanic lavas. We do not do so. ”

“ On consideration it seems improbable that the great sheets of augitic lava, such as cover the surface of the land of Bashan, are altogether the product of the volcanic mountains which appear to be confined to special districts in this wide area. Some of the craters do indeed send forth visible lava-streams, but they are insignificant as compared with the general mass of the plateau-basalts; and the crater-cones themselves appear in some cases to be posterior to the platforms of basalt from which they rise. ”

“ Here and there an obelisk, either wholly or half detached from the cañon-wall, juts out like a buttress. Farther down, these projecting masses stand like a row of columns upon the left bank. Above them, a solid capping of black lava reaches out to the edge, and overhangs the river in abrupt, black precipices. Wherever large fields of basalt have overflowed an earlier rock, and erosion has afterward laid it bare, there is found a strong tendency to fracture in vertical lines. The immense expansion of the upper surface from heat seems to cause deep fissures in the mass. ”

“ The surface over which the lava flowed seems to have been mainly terrestrial. Here and there, between the successive sheets of basalt, the leaves, stems, and fruit of land-plants, sometimes in most perfect preservation, may be observed, together with the remains of insects and fresh-water fish. Distinct relics of old river-channels can be recognized which have been buried under streams of lava. Among the deposits left by these streams the uppermost layers are commonly dark with decayed vegetation, while layers of coal are found here and there between the basalts. ”

“ In comparing together volcanoes that are in present activity, and others in which the crater and the streams of emitted lava are too distinct to permit a doubt of their having arisen from the same cause, differences are observed that only have arisen from great differences in the circumstances under which the eruption has taken place. In many of the ancient volcanoes, we find the emitted streams are arranged in prismatic forms, constituting basalt, and frequently passing into what under other circumstances would be styled trap by the Wernerians. ”

“ It descends by a twist or step, and then lies on the inclined surface of the tuff which dips towards the agglomerate and seems to pass into that rock. Further east this basalt increases in thickness and forms the lowest of the basalt-sheets of the cliff. The lava that commences on the west side of the agglomerate (d) is a massive jointed basalt, which, though not seen at the vent, appears immediately to the west of it and rapidly swells out so as to become one of the thickest sheets of the locality. ”

“ Necks of Tuff or Agglomerate with a Central Plug of Basalt or other Lava.—It has often happened that, after the explosions in a vent have begun to decrease in vigour, or have at last ceased, lava has risen in the chimney and finally sealed it up. In such cases the main mass of the rock may consist of tuff or agglomerate, which the enfeebled volcanic activity has been unable to expel from the orifice, while a plug of basalt, dolerite, or even more basic material, of much smaller dimensions, may have risen up the pipe in the centre or towards one side. ”

“ In looking at the great basalt-escarpments of Antrim, the Inner Hebrides or the Faroe Islands, and in following with the eye the successive sheets of lava in orderly sequence of level bands from the breaking waves at the base to the beetling crest above, we are apt to take note only of the proofs of regularity and repetition in the outflows of molten rock and to miss the evidence that these outflows did not always rapidly follow each other, but were separated by intervals of varying, sometimes even of long duration. ”

“ We see that the streams of earthy matter, which issue in a state of fusion from active volcanos, are spread around their craters in sheets of many kinds of lava; some of these so much resemble beds of basalt, and various trap rocks, that occur in districts remote from any existing volcanic vent, as to render it probable that the latter also have been poured forth from the interior of the earth. We further find the rocks adjacent to volcanic craters, intersected by rents and fissures, which have been filled with injections of more recent lava, forming transverse walls or dikes. ”

“ The volcanic series begins with a line of great dacite domes including the geburite-dacite of Macedon, which is associated with sölvsbergites and trachy-dolerites. The eruption of these domes was followed by that of sheets of basalt of several different ages and the intrusion of some trachyte dykes. The oldest basalts are associated with the Oligocene lake deposits; and fragments of the large lava sheets of this period form some of the table-topped mountains in the Highlands of eastern Victoria. ”

“ A great intrusive crystalline mass, of the same chemical composition as the first-extruded lava, is found to be rent asunder and penetrated by a similarly crystalline mass having the composition of the lavas of the second period. Thus, in the volcanoes of the Western Isles of Scotland, which are reduced by the action of denudation to this condition of basal wrecks, we find that rhyolites, trachytes, and andesites were ejected during the earlier periods of their history, and basalts during the later periods. ”

“ The rhyolitic debris is often mixed with pebbles of basalt. Sometimes it becomes so fine as to pass into pale clays.
(δ) Breccias of non-volcanic materials.—These, the most exceptional of all the fragmentary intercalations in the plateaux, consist almost wholly of angular blocks of rocks which are known to underlie the basalts, but with a variable admixture of basalt fragments. They are due to volcanic explosions which shattered the subjacent older crust of rocks, and discharged fragments of these from the vents or allowed them to be borne upwards on an ascending column of lava. ”

“ Precipices rise at times two thousand feet, sheer as a wall. Elsewhere gentle slopes of powdery lava, moss-tinted, connect rim and water with a ruler line. And between these two extremes are found every fashion and kind and degree of lava wall, many of them precipitous, most of them rugged, all of them contorted and carved in the most fantastic manner that imagination can picture. Caves open their dark doors at water's edge. Towered rocks emerge from submerged reefs. A mimic volcano rises from the water near one side. Perpetual snow fills sheltered crevices in the southern rim. ”