Geological Survey 7th Annual Report Obsidian Cliff, Yellowstone National Park

As the thoroughly glassy forms of many varieties of rocks present much the same general appearance and are with difficulty distinguished from one another, they have all been classed together as obsidian. Thus the term has come to signify any volcanic glass which has a small percentage of water and is almost if not wholly free from porphyritic crystals. But since the more siliceous rocks have a greater tendency to cool as glasses than the basic ones, most of the volcanic glasses belong to acid rocks. They may range from less than 60 per cent, to 78 per cent, of silica, including a few basalts with some andesites and trachytes and all dacites and rhyolites. The rock forming Obsidian Cliff is a rhyolitic obsidian, and on the next page is given a table of chemical analyses of similar rhyolitic obsidian from widely distant parts of the world, together with the analyses of spherulites and lithophysæ found in some of them. The analyses of rhyolite and lithophysæ made by Karl von Hauer are placed side by side for comparison.

Obsidian with more or less porphyritic crystals occurs over large areas in the Yellowstone Park; it abounds in spherulites and lithophysæ, which are also found in the pearlites and rhyolites of the same region. Lithophysæ also occur in the dark, pearlitic rhyolite from the base of the cliff of Shoshone Mesa, near Rock Creek, Idaho, where they were collected by Mr. S. F. Emmons,¹ who says they "are generally hollow and look like a clayey mass formerly filled with gaseous matter which has burst forth leaving a hollow interior. * * * * The centers of some of the lithophysæ are still filled with crystals of quartz and feldspar."

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¹U. S. Geol. Ex. Fortieth Par., vol. 2, Descr. Geol., p. 615.

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A well known locality for obsidian is Mono Lake, California, where a chain of extinct volcanic cones stretches southward from the lake. They are mainly composed of rhyolitic pumice in vast flows, with which are associated large outbursts of obsidian. This obsidian is a black glass without porphyritic crystals. Thin sections of the obsidian from the first and second large craters south of Mono Lake, which were collected by Mr. Arnold Hague, show it to be a clear glass, free from microlites and trichites. Its chemical composition, given in analysis V of the table, which was made for Mr. I. C. Russell, is almost identical with that of the black obsidian of Obsidian Cliff, except that there is only a trace of iron and over 2 per cent. loss by ignition. The obsidian passes into vesicular and bubbly glass and pumice; in places multitudes of minute bubbles produce gray bands through the black glass, which in certain lights show a beautiful luster and heliotrope color. From the highest volcanic cone at the south end of Mono Lake Mr. George M. Wright collected obsidian full of small, blue spherulites, among which are scattered larger ones about an inch in diameter.

Very beautiful, black obsidian, with small lithophysæ and occasional spherulites, occurs at Obsidian Hill and on the Rio San Diego and in other localities in the Tewan Mountains, New Mexico, where it has been collected by Maj. J. W. Powell, Director of the U. S. Geological Survey, and by Mr. William H. Holmes. This obsidian is remarkably transparent in thin fragments, but is intensely black in larger pieces; it is almost free from microscopic secretions, there being only a small amount of black grains, probably magnetite, and a few microscopic feldspars. Chemical analysis, No. VI, of the obsidian, from Obsidian Hill, N. Mex., shows that it has very nearly the same chemical composition as the obsidian of Obsidian Cliff, with slightly more silica and less iron oxide, which may account for the absence of fayalite from the lithophysæ of the former.

Near Coyote Spring, 30 miles north of Milford, Utah, Mr. G. K. Gilbert found obsidian closely resembling that at Obsidian Cliff. It is black, variegated with red and brown, some of it being very transparent in comparatively thick pieces. It is spherulitic and free from porphyritic crystals. Associated with it are pumice, lithoidite, and porphyritic rhyolite. Through the kindness of Mr. George P. Merrill, of the National Museum, the writer has been able to look over the collections of obsidian from the Western States and Territories and some foreign localities and make such notes as he desired.

From Beaver Valley, Utah, Mr. I. C. Russell has collected spherulitic, black obsidian, the spherulites having a curious, ragged form, probably consisting of a cluster of smaller ones. At White Mountain, Utah, a black obsidian occurs which is full of light-gray lithophysæ. Black obsidian without porphyritic crystals has also been brought in from High Rock Canon, Nevada, and a black pearlite from Trout Creek Point, on the crest of Quinn River Mountains, Nevada. This rock is crowded with dark red and brown spherulites of considerable size; it contains 75.50 per cent. of silica.

Black obsidian more or less spherulitic has been found at White Sulphur Springs, Napa County, and in the Pitt River district, California, the latter locality furnishing beautifully banded and mottled varieties.

Glass Butte, Oregon, is formed of a fine, red, brown, and black obsidian, specimens of which were collected by Mr. I. C. Russell. Some of the black is very transparent and clear, with a smoke-brown tinge in thin pieces; through it are scattered small white spherulites.

One of the purest obsidians known, according to Professor Zirkel,¹ occurs in the form of kernels and balls in the pearlite of Grass Cañon, Nevada, which has been described by Mr. Arnold Hague.² The obsidian balls are from half an inch to an inch in diameter and are associated with a white, pumiceous tufa inclosing fragments of pearlite. They are intensely black in color and in thin section are gray with few black, trichitic lines. The obsidian from Cerro de las Navajas, Mexico, as already mentioned, corresponds in many ways to that of Obsidian Cliff (compare analyses VII and I). It is associated with liparite and bluish-gray, spherulitic lithoidite and carries occasional crystals of sanidine and lithophysæ. Red and black obsidian is found at the same place. At Cerro Pelado, southwest of Cerro de las Navajas, spherulitic obsidian with lithophysæ is associated with lithoidal and spherulitic rhyolite and pearlite. Similar obsidian is found at San Juan de los Llanos, Puebla.

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¹U. S. Geol. Ex. Fortieth Par., vol. 6, Microscopical Petrography, 1876, p. 210.

²Ibid, vol. 2, Descr. Geol., p. 784.

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In Equador, near Guamani, occur spherulitic obsidian flows. The glass is banded with dark layers which are older than the spherulites. It contains a few porphyritic crystals of sanidine and biotite and is associated with lithoidite and pumice.³

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³J. Roth: Monatsber. k. preuss. Akad. Wiss., Berlin, 1874, p. 379.

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An obsidian lava rich in spherulites is described by G. vom Rath⁴ from Antisana in the Andes. The microscopic characters of the Spherulites correspond closely to those of Obsidian Cliff.

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⁴Zeitschr. Deutsch. geol. Gesell., Berlin, vol. 27, 1875, pp. 296-302, 341-343.

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The Lipari Islands, north of Sicily, are famous for the abundance of obsidian found there. It is partly spherulitic, with lithophysæ, and is intimately associated with pumice. Analysis IX, published by Abich, is of obsidian from this locality. At Monte Guardia a gray, porous, and pumiceous rock contains exceedingly numerous, thin strips of black obsidian, which are parallel to the direction of the lava flow.

Another celebrated locality is Iceland, where the obsidian, judging from its chemical composition, belongs rather to dacite. The flow at Hrafntinnuhryggr is banded by alternating layers of crystalline and glassy rock.

It is not very abundant in Hungary, but occurs in the Caucasus, on Teneriffe, Ascension, and Guadeloupe, in Java, and Japan.

In New Zealand,¹ on the southeast shore of Rotorua Lake, spherulitic obsidian is found which is dark-grayish brown in thick masses; in thin splinters it is as clear as water or has a light tinge of smoky gray. In it lie sharply defined, small spherulites with a bluish-gray, waxy luster. The obsidian has 75.03 per cent, silica and the spherulites have 74.55 per cent.

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¹F. Zirkel: Petrographische Untersuchungen über rhyolitische Gesteine der Taupo-Zone; F. v. Hochstetter: Geologie von Neu-Seeland, 1864.

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Black obsidian is very abundant on Tuhua Island, Bay of Plenty, tuhua being the South Sea Islander's word for obsidian. Pieces often have a silvery or iridescent coating like that produced on artificial glass by the decomposing action of the atmosphere. Its silica percentage is 74.91.

Behind the village of Totara, on the southeast shore of Taupo Lake, stands a vertical wall of lithoidite with most remarkable lamellar structure and regular columnar parting. The lamellae are of almost microscopic thinness, principally in two colors, a grayish-black and purplish-gray, of which there are many lighter and darker shades, alternating with one another like the banding of an agate. Occasionally bubble-like distensions are observed, in the neighborhood of which the lamellae of the rock thin out. The cavities are mostly flattened in the plane of the lamellæ. The interior is often interrupted by partition walls, which are coated with minute crystals, probably of quartz and feldspar, with hornblende and thin flakes of mica. The rock corresponds very closely to the lithoidite at Obsidian Cliff, except for a few porphyritic crystals of quartz and feldspar and a slightly lower percentage of silica, 70.67.

CONCLUSION.

In conclusion, we see that obsidian of nearly the same chemical composition and microstructure occurs in all parts of the world; that it is generally accompanied by like modifications, passing into pumice on the one hand and into lithoidal or porphyritic rhyolite on the other; and that in most instances it is more or less spherulitic. But the obsidian flow a part of which forms Obsidian Cliff is especially remarkable for its extent and thickness, which exceed those of any other described locality, unless surpassed by some occurrences in Mexico. It is, so far as we are able to learn, the only occurrence of rhyolitic obsidian in which a distinctly columnar structure has been developed. It is entirely free from porphyritic crystals and abounds in spherulitic structures and lithophysæ of great beauty and perfection. The absolute freshness of the rock and absence of secondary alteration permit the study of these forms of crystallization without confusion with decomposition-products or subsequent metamorphism, common in older and more crystalline rocks, or even in recent lavas which have been attacked by solfataric or hot-spring agencies, as those in many parts of the Yellowstone Park have been, where the effects of secondary alteration are easily recognized. They leave no doubt that the spherulites and lithophysæ, in all their complexity of form and structure, are of primary crystallization out of a molten glass, which was gradually cooling and consolidating, and that since its solidification, no alteration, chemical or mechanical, has taken place.