Volcanic activity
In early Eocene time, between 55 and 50 million years ago, several large volcanoes erupted in and near Yellowstone National Park. This volcanic activity resulted in the accumulation of the vast pile of Absaroka volcanic rocks ([fig. 5]) which now makes up most of the Absaroka and Washburn Ranges and part of the Gallatin Range, and which covers several other smaller areas in the Park ([pl. 1]).
What special geologic conditions would cause these spectacular eruptions of molten rock at the earth’s surface? Measurements taken in deep mines and oil wells show that the normal increase in the earth’s temperature with depth is about 1°F per 100 feet. This heat is generated by the decay of radioactive elements—chiefly uranium, thorium, and potassium—which are present in at least small amounts in virtually all rocks of the earth’s crust. Ordinarily, enough heat is conducted to the earth’s surface so that the deeply buried rocks do not become hot enough to melt. In some places, however, the heat is not carried off fast enough, and the temperature rises slowly toward the melting point of the rock. Such hot spots may develop (1) because the rocks in those places contain more than an average amount of radioactive elements; (2) because hotter material moves upward from still deeper levels in the earth; or (3) because drastic changes in pressure are brought about by the alternate squeezing and relaxing of mountain-building forces, which in turn substantially affect the melting point of the rocks. Whatever the cause, the eventual result is the accumulation of a huge body of molten rock, called magma, enclosed in a deep underground chamber.
Magma, being a mixture of hot liquids and gases that is lighter in weight than the solid rocks surrounding it, tends to rise toward the earth’s surface. Forcing its way upward, some of the molten material solidifies before reaching the surface and forms bodies of various kinds of intrusive igneous rocks ([fig. 15]). Some of the magma, however, reaches the surface and either pours out as lava or is blown out explosively as rock fragments, ash, and pumice to form extrusive igneous rocks.
INTRUSIVE AND EXTRUSIVE IGNEOUS ROCK BODIES. Extrusive rocks solidified above ground, and intrusive rocks solidified below ground. All features shown occur in Yellowstone National Park. Extrusive rocks are the predominant rock type seen along the Park roads, and the table lists the three principal kinds that are present. (Fig. 15)
| Rock name | Principal rock-forming minerals | Color |
|---|---|---|
| Rhyolite | Quartz,[a] feldspar[] (sanidine). | Light to medium shades of gray and brown. |
| Andesite | Feldspar[] (plagioclase), pyroxene[c] (augite). | Medium to fairly dark shades of brown, red, purple, and gray. |
| Basalt | Feldspar (plagioclase), pyroxene, olivine,[c] magnetite.[d] | Nearly black. |
[a]Clear to light-colored silicon dioxide.
[]Light-colored aluminum silicate minerals.
[c]Dark-colored iron and magnesium silicate minerals.
[d]Very dark colored iron oxide mineral.
The magmas which formed the Absaroka volcanoes erupted mainly through large central vents ([fig. 16]). Most of the eruptions were fairly quiet, with the molten rock welling up to the surface and cascading down the sides of the volcanoes chiefly as viscous lava flows and breccias. Rain, seeping into these porous rocks, caused huge landslides of mud and broken rock to stream down the mountainsides. Hence, many of the rocks seen today are volcanic breccias—jumbled but crudely layered deposits of large and small angular blocks embedded in a sandy matrix, much like man-made concrete except that the rock fragments are considerably coarser ([fig. 17]). Viewed from a distance, however, most of the breccia deposits have a distinct layered appearance ([fig. 18]). The predominant extrusive igneous rock in the Absaroka volcanic sequence is andesite, but basalt also occurs in places ([fig. 15]).
ABSAROKA VOLCANOES and their rocks. Lava (mostly andesite) poured from central vents and formed volcanoes, some steep sided and others broad and relatively flat. As the lava spilled out, much of it quickly solidified, broke up into large angular blocks (breccia), and then either tumbled down the slopes of the volcanoes as individual boulders or slid down in mudflows and landslides. Some of the material was also explosively blown out as rock bombs, cinders, and ash. The more fluid lava (mostly basalt), on the other hand, flowed quietly down the volcanic slopes and onto the surrounding lowlands. The rocks near the volcanic centers therefore include thick crudely layered coarse breccias, thin fine ash and dust falls, and thin to thick lava flows. The volcanoes were repeatedly attacked by erosion, and the eroded material was redeposited by streams and mudflows in widespread layers of volcanic conglomerate and sandstone across the flat-floored valleys and plains between the volcanoes. Forests, which grew luxuriantly in these lowland areas, were repeatedly buried by volcanic eruptions and are now preserved (see inset) as the fossil forests of Yellowstone. (Based on information supplied by H. W. Smedes and H. J. Prostka.) (Fig. 16)
Chiefly lava flows of shield volcano
Chiefly volcanic sandstone and conglomerate of lowland areas
Chiefly volcanic breccias and thin lava flows of cone-type volcano
At times the Absaroka volcanic eruptions were violently explosive, showering the countryside with rock bombs, cinders, and ash. The finer debris that reached the lower slopes of the volcanoes was reworked and carried by streams into the intervening valleys, where it was deposited as sand and gravel ([fig. 16]). Eventually the entire Yellowstone region was choked with volcanic debris, the material from one volcano mixing with that from neighboring volcanoes. Even the mountain masses uplifted during the preceding Laramide orogeny were covered by the vast accumulation ([fig. 18]).
MASSIVE BEDS OF BRECCIA of the Absaroka volcanic rocks along the road north of Dunraven Pass. This breccia formed part of a steep-sided volcanic cone, of which Mount Washburn is a remnant. (Fig. 17)
Closeup view shows very coarse character of the breccia, with large rock fragments imbedded in fine ash, dust, and sand. Nearly all the rocks are of andesitic composition, consisting chiefly of feldspar and pyroxene. Most common colors are medium to fairly dark shades of brown, red, purple, and gray.
Absaroka volcanism, however, was not a simple, continuous process—the eruptions were intermittent, the many volcanoes were not always active at the same time, and between eruptions there were long periods of quiescence during which the erupted material was deeply eroded. The repetitive nature of the eruptions is best illustrated by the famous fossil forests of Yellowstone. Here is striking evidence that enough time elapsed between eruptions for widespread forests to become established on the lower slopes of the volcanoes and in the broad valleys between them. Judged from the great size of some of the now-petrified logs ([fig. 19]), several hundreds of years must have passed before another volcanic outburst smothered the forest. Many different forest layers have been recognized in the Specimen Ridge area as well as in several other places throughout the Park.
As the Absaroka magma rose from deep underground, some of it squirted, like toothpaste, into the layered Paleozoic and Mesozoic sedimentary rocks through which it passed. These relatively small masses of molten rock material slowly cooled and crystallized to form intrusive igneous rocks such as diorite ([fig. 20]). The resulting intrusive bodies, called sills, dikes, stocks, and laccoliths, depending on their form, are most abundant in the Gallatin Range and in the vicinity of the East Entrance ([pl. 1]). At the conclusion of volcanic activity, the last of the rising magma solidified in the main conduits to form slender, somewhat cylindrical bodies of rock called volcanic necks that probably conform closely to the shape of the original conduits. The circular intrusive rock body at Bunsen Peak ([fig. 21]), now exposed to view because erosion has stripped away the lava and volcanic breccia that once completely buried it, represents either a volcanic neck or a small stock that solidified directly beneath a volcano.
MASSIVELY LAYERED BRECCIAS, conglomerates, and sandstones of the Absaroka volcanic sequence at Barronette Peak, as viewed from the road near the Northeast Entrance; the ridge is 3,000 feet high. These rocks, deposited as part of an alluvial plain between volcanoes, once filled the Yellowstone region to a level higher than the top of Barronette Peak, but erosion since late Tertiary time has stripped the volcanics from much of the Park area. The volcanic rocks (Eocene in age, [fig. 5]) rest directly on Paleozoic sedimentary rocks along the line indicated. During the Laramide orogeny, in Late Cretaceous and early Tertiary times, the region was folded and uplifted into mountains. Thousands of feet of Mesozoic and Paleozoic sedimentary rocks were then eroded off the rising mountains before the Absaroka volcanic rocks were deposited, (Fig. 18)
GIANT PETRIFIED TREE TRUNKS in Yellowstone’s fossil forest. The enclosing rocks, part of the Absaroka volcanic sequence that forms Specimen Ridge, are approximately 50 million years old. Many of the tree trunks are still upright, having been smothered and buried in their original positions by breccia, ash, and dust from nearby volcanoes. It is evident that more than one “forest” is represented in this view. Prof. Erling Dorf, of Princeton University, counted a total of 27 different forest layers in the rocks now exposed at Specimen Ridge. He also determined that the most common kinds of trees were sycamore, walnut, magnolia, chestnut, oak, redwood, maple, and dogwood. The nearest living relatives of many of these trees are now found in the warm temperate to subtropical forests of the southeastern and southern United States. (National Park Service photograph.) (Fig. 19)
Mount Washburn is the north half of one of the ancient Absaroka volcanoes ([fig. 26]), and many of the rocks and other features related to this volcano, which characterized this great period of volcanism, can be seen along the road between Canyon Village and Tower. In roadcuts just south of Dunraven Pass several thin igneous dikes cut through volcanic breccias. These dikes radiate outward from the nearby central core of the volcano, which lies east of the highway in the vicinity of Washburn Hot Springs. From Dunraven Pass northward for 2-3 miles, the road is lined with lava flows and very coarse breccias that accumulated close to the volcanic neck ([fig. 17]). Farther north toward Tower Falls, breccias and conglomerates predominate, but the average size of individual rock fragments decreases gradually northward away from the center of eruption. Beds of sandstone then begin to appear in the sequence, having been deposited mainly by streams that drained the north slope of the volcano.
IGNEOUS ROCK. Closeup view of intrusive igneous rock (diorite) from the Electric Peak stock in the Gallatin Range; Electric Peak is pictured in [figure 37]. The rock is composed chiefly of light-colored quartz and feldspar and dark-colored iron and magnesium silicate minerals. (Fig. 20)
At the end of Absaroka volcanism, approximately 40 million years ago ([fig. 6]), all of Yellowstone lay buried beneath several thousand feet of lavas, breccias, and ash ([fig. 18]). The landscape must have appeared as a gently rolling plateau, drained by sluggish, meandering streams and dotted here and there by volcanoes still rising above the general level of the ground. This plateau surface, however, probably stood at a maximum of only a few thousand feet above sea level, for animals and plants now found as fossils in the Absaroka volcanic rocks indicate that warm-temperature to even subtropical climates existed during the volcanic period ([fig. 19]).
BUNSEN PEAK, a roughly circular body of intrusive igneous rock, is the eroded remnant of either the “neck” of an Absaroka volcano or a small stock that solidified directly beneath a volcano. The peak rises approximately 1,200 feet above a flat plain (foreground) that is covered by flows of younger basalt. The Yellowstone Tuff, formed by volcanic ash and dust exploded from the central Yellowstone region to the south, underlies the basalt. When erupted, the volcanic debris (as well as the basalt lava) flowed around this high-standing peak. (Fig. 21)