By early Eocene time, about 20 million years after they had begun, the deformational forces relaxed. But the effects of the giant earth movements were to last for a very long time. Crustal disturbances of such magnitude commonly produce conditions deep within the earth which, in places, gives rise to intense volcanic activity; one such place was Yellowstone.

CROSS SECTIONS SHOWING GEOLOGIC STRUCTURES in Yellowstone National Park. These illustrate the possible rock relationships that might be seen along the faces of vertical slices of the earth’s crust, if it could be cut and pulled apart (much like slicing a cake and looking at the different layers). The locations of the sections are shown on the geologic map, [plate 1]. Reverse faults and most folds originated during the Larimide orogeny, and normal faults originated chiefly during Pliocene and later times. The arrows indicate the relative movements of fault blocks. Geologic symbols: Qs, Quaternary superficial deposits; Qb, Quaternary basalt flows; Qy, Quaternary Yellowstone tuff; Tav, Tertiary Absaroka volcanic rocks; Mzr, Mesozoic sedimentary rocks; Pzr, Paleozoic sedimentary rocks; pCr, Precambrian metamorphic (“basement”) rocks. (Based partly on information supplied by E. T. Ruppel and J. D. Love.) (Fig. 14)

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.