The nature of the rocks reveals their origins

Geologists believe that “the present is the key to the past.” After observing lava erupting from a present-day volcano or limestone forming in marine waters, we infer that similar types of ancient lavas or ancient limestones formed in virtually the same ways. This kind of reasoning is used to interpret the origins of all types of ancient rocks, for all the known geological processes that form rocks seem to have been operating since the earth’s beginning.

[Figure 5] shows the many different rock units that have been recognized in Yellowstone National Park. Arranged in a vertical column according to the geologic time intervals in which they formed, these rocks represent a large part of total earth history ([fig. 6]). A generalized geologic map ([plate 1]) shows the distribution of the various units (or groups of closely related units) exposed at the surface throughout the Park area. This map and [figure 5] summarize much of the information that is necessary to interpret the Park’s geologic history—in essence, to provide answers to these two important questions: What were the geologic events that formed the rocks? When did these events occur?

The oldest rocks

If we were to walk backward in time at the rate of one century per step, the first step would return us to 1872, the year that Yellowstone National Park was established. But to return to the oldest recorded event in its geologic history, we would have to walk (at 3 feet per step) some 15,000 miles, or three-fifths of the way around the world! Occurring far back in the antiquity of the Precambrian Era—approximately 2.7 billion years ago according to radiometric dating ([fig. 6])—the oldest event resulted in rocks so crumpled and changed by heat and pressure that their original character is obscure. These rocks, having been transformed from still older ones, are called metamorphic rocks. Considered to form part of the very foundation of the continent itself, they are also commonly referred to as basement rocks.

THE ROCKS of Yellowstone National Park, separated into individual units or formations and arranged according to their geologic ages (see [fig. 6]). A formation is a body of rock that contains certain identifying features (such as composition, color, and fossils) which set it apart from all other rock units. The identifying features of each formation provide valuable clues bearing on its origin. Most formations are given formal names, and usually each formation is thick and widespread enough to be recognized over broad areas. Some, however, change character from place to place, and different names may be used in different areas even though the rocks represent the same geologic time interval. (Fig. 5)

[High-resolution Version]

AGE, IN THOUSANDS OF YEARS ROCK FORMATION OR UNIT
40± to present Stream sand and gravel
Hot-spring deposits
9 to 250± Glacial deposits
60 to 600 Plateau Rhyolite
600 Upper Unit, Yellowstone Tuff
600 to 2,000 Rhyolite and basalt lava flows
2,000 Lower Unit, Yellowstone Tuff
2,000+ Rhyolite and basalt lava flows

KINDS OF ROCKS SHOWN IN COLUMNS
Sandstone or stream sand
Conglomerate, glacial moraines, or stream gravels
Volcanic breccia
Shale
Limestone
Dolomite
Lava flows
Welded tuff
Travertine or geyserite