YELLOWSTONE NATURE NOTES

YELLOWSTONE'S FIERY PAST

A visitor confining his trip to the main loop roadway will encounter a diversity of rock types which surprisingly enough are almost identical in their chemical composition. In canyons and roadcuts extrusive volcanic products in the form of breccias, agglomerates, ash and welded tuffs are readily seen. In places these are interbedded with, or more commonly overlain by more quiet flows of rhyolite and basalt.

Beginning some 40 million years ago during the Tertiary period and extending up to the Glacial epoch this basin, hemmed in by mountain peaks, became the resting place for vast accumulations of volcanic products.

The earliest volcanic activity was of the explosive type and the observations of Dorf (1939) and Rouse (1937) indicate that beginning in the Eocene and extending into mid-Tertiary time these events happened; extrusion of andesite breccia, followed by basalts and an erosion interval with the whole sequence repeated a second time giving a maximum thickness to these deposits of 6,500 feet in the eastern part of the park. Outcrops of these rocks may be seen throughout the Absaroka Mountains and isolated remnants occur in the Gallatins to the northwest and in the Tetons to the south. Within the park the Washburn Range stands today as an island of breccia surrounded by the later plateau flows. The source of all this volcanic material and its mode of emplacement has been an enigma to geologists. Various features within the park such as Mt. Washburn, Mt. Sheridan, Electric Peak, and the "Crandall Volcano" on Hurricane Mesa east of the park have all been suggested as source areas. It seems likely that there were many sources and their locations may never be known. The crude stratification of the deposits is suggestive of emplacement as mud flows.

At times the volcanic fires subsided long enough for forests to grow and then these were buried under showers of ash, petrified, and the process repeated again and again. Today we can count the remains of 27 forests of petrified trees on the slopes of Specimen Ridge, proof of the rhythmic alternation between times of quiescence and explosive volcanic action. Man, accustomed to thinking in terms of his conventional time span of "three-score and ten," finds his imagination taxed to comprehend the time factor of the fossil forests alone. Yet when placed in their proper framework of geologic time, these events are seen to represent but one grain of sand on the beach of time.

Explosive volcanic action gradually subsided and rivers and streams began their relentless task of dissecting the landscape. Gullies were deepened into valleys and these in turn became canyons. An erosion cycle was in progress but never carried through to completion because of renewed volcanic action of a different type; extrusion of the welded tuffs and rhyolite and basalt flows of the plateau.

The first detailed study of the rhyolite plateau was undertaken by members of the U. S. Geological Survey in 1883. Results of their investigation appear in Folio No. 30 and in Part 2 of Monograph 32 of the U. S. Geological Survey. Considering the difficulties of travel and the hardships under which these men labored we can today only pay tribute to the prodigious amount of work they accomplished and the soundness of their basic geologic deductions. However, the earth sciences have made progress in recent decades and with refined mapping techniques and new tools of petrographic research it is apparent that the rhyolite plateau is much more complex than the earlier maps suggest.

F. R. Boyd (1957)¹ has made an outstanding contribution to the knowledge of Yellowstone geology. By careful field work and detailed petrographic studies he has shown that much of the "rhyolite" is in reality a welded tuff which he believes was erupted as incandescent tuff avalanches, emplaced in a rapid sequence of eruptions and deposited over the previous erosion surface. The present outcrop of these deposits encircles the Madison, Pitchstone, and Central Plateau covering an area of 1,000 square miles in Yellowstone and at least 1,000 square miles west and southwest of the park. Emplacement of the welded tuffs was followed by block faulting which depressed the central and southwestern part of the park creating a basin bordered in places by scarps up to 3,000 feet high. Faulting of the basin was followed by extrusion of flow after flow of rhyolite lava with minor amounts of basalt.

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¹From thesis abstract by permission of F. R. Boyd.

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It is obviously difficult to assign geologic ages to extrusive volcanic rocks because the high temperatures destroy organic material and consequently the rocks are devoid of fossil material. The welded tuffs are believed to be Pliocene in age and some of the volcanism probably continued into Quaternary time.

FIG. 5. Showing generalized outcrops of Tertiary extrusive volcanics. Unshaded areas within the park represent pre- and post-Tertiary outcrops. Based on work of F. R. Boyd (1957) and interpreted by the author.

By plotting the areal distribution of these volcanic products and estimating their thickness, Boyd concluded that the basin had been filled with 600 cubic miles of rhyolite and welded tuff, making Yellowstone one of the great volcanic areas of the world.

Geologists know that deep-seated batholithic injections of granite have been recognized over most of the world and are commonly found to occupy the cores of folded mountain ranges. They are normally associated with areas of great crustal deformation. The Yellowstone region is notably deficient in granite and apparently has not been subjected to great deforming stresses since the early Tertiary. Almost fifty years ago Daly (1911, p. 63-67; 1933, p. 142-143) suggested that the rhyolite plateau is the foundered crust of a roofless batholith. By comparing Boyd's (1957) petrographic analyses of the plateau rocks with similar associations from other parts of the world, Hamilton (1959) concludes that the Yellowstone plateau is more likely a collapse feature technically known as a lopolith.

The Geophysical Laboratory of the Carnegie Institute of Washington, D. C., has had a long standing interest in hydrothermal research in the park. In 1929 and 1930 they sponsored the drilling of two test holes, one near Old Faithful in the Upper Geyser Basin and a second in the Norris Geyser Basin. Fenner (1936) summarizes temperature and pressure data from these test holes and projections of an abnormally high thermal gradient of 20°C. per one hundred feet is suggestive of an igneous mass of unknown size existing at depths perhaps less than one mile below the surface.

Allen and Day (1935, p. 512) make a conservative estimate of a heat loss from the thermal areas equal to about 220,000 kilogram calories of heat per day. This requires a cooling and recrystalization of rocks below the surface of about one cubic kilometer every 50 years or some 200 cubic kilometers in the past 10,000 years.

What exciting food for speculation awaits today's Yellowstone visitor! To realize that he is viewing here a unique spot in the world where the magmatic hearths are yet warm and molten rock may exist less than a mile below the surface.

As one reconstructs the volcanic past it becomes easier to see why hot springs, geysers and earthquakes should be associated in Yellowstone.

FIG. 6. Theoritical East-West Cross Section of Yellowstone Park Illustrating Faulting of Plateau and Intrusion of Lopolith.