The Arizona Geology Virtual Tourist: Part II



The Arizona Geology Virtual Tourist: Part II

Why does scenery vary from place to place? — Geologic Map of Arizona

Introduction

In interpreting the landscapes and provinces of Arizona, our eyes can only get us so far before we run up against some fundamental questions: what type of rocks are these, and how old are they? A useful piece of such information would be a map showing what rocks are actually present. Such a map is called a geologic map, such as the Geologic Map of Arizona. This map shows the general distribution of different types of rocks across the entire state. More detailed maps covering a smaller area, like a single mountain range, also exist for some areas.

Materials

• Geologic Map of Arizona (1986), Compiled by S. Reynolds and Published by Arizona Geological Survey, Tucson

• Or the on-line version accessed via the Arizona Geology Virtual Tourist Web Site, at .

How to Read and Use a Geologic Map

A description of what the Geologic Map of Arizona shows is available at . A short summary is given below.

What does a Geologic Map Show?

• A geologic map shows the types of rocks and ages of rocks that are exposed at the surface. Different types of rocks are identified by different colors and labels.

• The capital letters at the start of each label indicate the age of the rock, such as “J” for Jurassic. If there are two capitol letters, then that rock unit may span both time periods, or the age of the rock unit may be imprecisely known.

• The smaller, lower-case letters in the label indicate the type of rock (“g” for granite”) or the name of the geologic formation (“Kaibab Formation”)

• The colors also indicate the type and age of rocks.

• The oldest rocks are colored brown on the map, and are labeled with capital X or Y, to indicate they are Precambrian (older than 600 million years).

• The next youngest rocks are colored purple and blues, and have age labels (P,MD, C) for various parts of the Paleozoic (600 to 240 m.y.)

• The next youngest rocks are mostly colored green, and have age labels (Tr, J, and K) for various parts of the Mesozoic (240-66 m.y.), a time during which the Dinosaurs lived.

• Rocks deposited after the Dinosaur became extinct are labeled “T” for the Tertiary (66-2 m.y.) or “Q” for Quaternary, the most recent period of geologic time (in the last 2 m.y., or during and after the ice ages).

• If two areas on the geologic map have the same color and label, they are of similar type and age.

• Geologic maps also show other geologic features, such as faults (fractures in rock along which there has been movement).

The Big Picture — Geology of Arizona’s Provinces

• Use the Geologic Map of Arizona to see what types of rocks are most common in each of the three regions. What rock types are present in more than one region? What rock types are mostly found within a single province?

• Summarize your main conclusions about what types and ages of rocks are dominant in each province.

• Does this information help explain the differences and similarities between the regions? Write your conclusions and further questions that arise.

Using Geologic Maps to Anticipate Natural Hazards and Resources

• We can use geologic maps to anticipate, and help predict, natural hazards such as volcanic eruptions and earthquakes. Also, natural resources, such as groundwater, petroleum, and copper deposits, are closely related to their geologic setting, and we can use geologic maps to identify areas that are most likely to contain these types of resources.

Volcanoes in Arizona?

• Does Arizona have any volcanoes or any risk of volcanic eruptions? Geologic maps help us predict which areas are most likely to have dangerous volcanic eruptions. Such areas would be places where volcanic eruptions have formed volcanic rocks recently (within the last several million years). Recall that on the geologic map, the most recent period of geologic time is indicated by the capital letter “Q”, for Quaternary (the most recent period in geologic time).

• Using the legend of the map, suggest which areas in Arizona are most likely to have volcanic eruptions and present your reasons.

Earthquakes in Arizona?

• Geologic maps also show other geologic features, such as fractures in the rock along which the rocks have slipped with respect to one another, such as during an earthquake. These fractures, known as faults, are shown on the geologic map with a heavy line. When a fault slips, it cuts across any nearby rocks. A fault that has moved recently may cut relatively young rocks (any “Q” map units). If a fault has become inactive (doesn’t slip anymore), it may have become buried by rock units deposited after the faulting. In this case, the fault will be shown ending when it reaches this younger, burying rock unit.

• Using these principles, suggest which areas in Arizona are most likely to have earthquakes based on the geologic map.

Energy and Mineral Resources

not done yet

Where to Site the Colony

• Using everything you have observed, propose to the captain where she should place new colonies. Discuss advantages and disadvantage of each regions. If you need additional information, outline what types of information you need and how it could be attained.

GEOLOGIC MAP OF ARIZONA

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. On the legend, the rock are listed from youngest to oldest.

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, 416 W. Congress, Tucson, AZ 85701. The price of the map is ~ $5.00, plus shipping and handling costs. Phone: (520) 770-3500; Fax: (520) 770-3505

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