The Essence of a Geologic Map

A geologic map is a graphic representation of the rock units and geologic features that are exposed on the surface of the Earth. On the Geologic Map of Arizona, the colors show the distribution of different types or ages of rock. For example, the light-blue color in northern Arizona represents areas where limestones and sandstones of Permian age (about 250 million years [m.y.] old) are exposed. The adjacent pink colors show areas where more recent (less than 4 m.y. old) basalt flows and cinder cones are present. By using the geologic map, a traveler could predict what type of rock, and therefore what type of scenery, would be present in an area, before actually going there. For example, the light-bluish-green areas in northeastern Arizona are composed of the Chinle Formation, which forms variably colored landscapes of the Painted Desert and Petrified Forest. The adjacent darker green color indicates where the Navajo Sandstone and many beautiful slickrock canyons are present. The large yellowish-green area in the center of northeastern Arizona shows exposures of sandstones and shales that host large coal deposits. The gray colors in much of southern Arizona show the areas, mostly in the valleys, that are underlain by relatively recent deposits of sand and gravel.

In addition to showing different types and ages of rocks, most geologic maps depict geologic features, such as faults, folds, and volcanoes. The Geologic Map of Arizona shows faults (breaks in the rock produced by a slipping motion) with heavy lines. Notice that faults are nearly absent from northeastern Arizona, but are more common in the rest of the State. A geologic map gives clues to the age of a fault because it shows which rock units are cut by the fault and are therefore older than the fault, and which rock units were deposited across the fault after it was formed. Knowing the age of a fault is important for assessing the potential for earthquakes along that fault: The more recently the fault has moved, the more likely it is to move again and produce earthquakes.

Uses of a Geologic Map

Geologic maps are used in four major ways: (1) to discover mineral and energy resources; (2) to assess the potential for natural hazards, such as earthquakes and volcanoes; (3) to evaluate the suitability of an area as a construction site; and (4) to convey information about the geologic history of an area.

A geologic map is one of the most important guides for finding essential mineral resources, such as copper and petroleum, the source of gasoline and many synthetic materials, including textiles. A mineral commodity is commonly formed in a specific environment and is therefore associated with one or more specific types of rocks. For example, most gypsum, which is used in the construction of wallboards, is formed by the evaporation of water in lakes or shallow lagoons. The logical place to look for gypsum, therefore, is an area containing rocks that represent these ancient waterways.

An area's potential for geologic hazards may also be assessed from a geologic map. Areas that contain recent volcanic rocks (less than several m.y. old) are more likely to have volcanic eruptions than regions that lack such rocks. Likewise, earthquakes are most likely to occur in areas where faults displace recent rocks or deposits, such as river gravels. High concentrations of radon gas are commonly associated with rocks, such as particular types of granite, that have an anomalously high uranium content.

Determining the suitability of a site for a large construction project, such as a bridge, requires an adequate knowledge of the rock types that compose the foundation. A geologic map shows the surface rocks types and suggests which rock types lie at depth.

Geologic maps are an effective way to convey large amounts of geologic information about an area. The geologic history of an area is recorded by the types of rocks deposited there and by the geologic relationships between those rocks. A geologic map might show, for example, that a limestone was formed under the sea, but was later uplifted and covered by lava flows. This is exactly what the Geologic Map of Arizona implies by the proximity of the pink and light-blue areas in northern Arizona.

Geologic Setting of Arizona

Arizona has a complex geologic history that spans 1.8 billion years (b.y.) and resulted in the formation of three geologic provinces: the Colorado Plateau, Transition Zone, and Basin and Range Province. The Colorado Plateau in northern Arizona is a region of broad plateaus and mesas composed of picturesque sedimentary rocks deposited during the Paleozoic and Mesozoic Eras (570 to 245 m.y. ago). On the Geologic Map of Arizona, the Colorado Plateau includes the large region shown in light blue and various shades of green. The southern boundary of the Colorado Plateau is the Mogollon Rim, which is represented on the map by the southern limit of the light-blue color. The Plateau is incised by deep canyons, such as the Grand Canyon and Canyon de Chelly, which are illustrated on the map by the purple and brown colors that represent deeper rocks exposed in the canyons. Large extinct volcanoes, such as San Francisco Mountain and the White Mountains (shown in pink and red) are present along the edge of the plateau.

The Basin and Range Province of southern and western Arizona is characterized by alternating mountain ranges and broad valleys, most of which were formed by block faulting during the last part of the Cenozoic Era (15 to 5 m.y. ago). The mountain ranges contain rocks of various types and ages that have been extensively folded and faulted during the Mesozoic and Cenozoic Eras (100 to 15 m.y. ago). The intervening valleys, shown in gray and light yellow on the map, are generally underlain by thick sequences of consolidated sediments (mostly gravel, sand, and silt) that are the main aquifers for the region.

The third province, the Transition Zone, is between the Basin and Range Province and the Colorado Plateau and has geologic characteristics intermediate between the two. It contains narrow, sediment-filled valleys and broad, high mountain ranges mostly composed of rocks of Proterozoic (late Precambrian) age (1.0 to 1.8 b.y. old). The Transition Zone is shown on the map as a brown and purple belt that trends northwest across the center of the State. The yellow represents sedimentary deposits in the valleys.

Ordering Information

The Geologic Map of Arizona is published by the Arizona Geological Survey, a State agency in Tucson. The price of the map is $5.00, plus shipping and handling costs. Contact: Arizona Geological Survey, 416 W. Congress St., Tucson, AZ 85701 Voice: (520) 770-3500; Fax: (520) 770-3505. Copies may also be obtained at local map stores.

Good example of what a geologic map is trying to show

Neat example of what a geologic map is trying to show

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