Geology and Soils in Deserts of the Southwestern United States - Design with the Desert: Conservation and Sustainable Development - page 38

the other. The classic example of this type of plate boundary is the San Andreas Fault
system in California, caused by northwest motion of the Pacific plate against the North
American plate. Frequent and sometimes strong earthquake activity is associated with
both convergent and transform boundaries.
While the oldest exposed bedrock in the southwestern United States formed billions of
years ago, the desert landscapes we live in today are mainly the result of climatic changes
that only took place between 10,000 and 8,000 years ago (see Chapter 6). The fundamental
concept here is that of “deep time” or the realization that the landscape contains an exten-
sive geological history of past climates and tectonic events recorded in the rocks that may
be quite different from the current environmental conditions. In desert environments,
water and wind are the major natural agents that move unconsolidated materials across
the landscape, with episodic events like flash floods capable of altering the local land-
scape significantly over time periods of hours to days. In addition to these natural agents,
humans have become major agents of landscape change in desert regions by converting
arid lands to agricultural and urban/suburban land uses.
The basic concepts presented here will be expanded upon in the following sections,
including some discussion of the role humans play in geological processes. An overview
of the major geological events that formed the state of Arizona, which includes sections of
the Sonoran, Chihuahuan, and Mojave Deserts, provides a sense of the region's geological
history, in which the current desert landscapes are the most recent chapter. Other desert
regions of the United States have similarly complex geologic histories.
2.4 Physiography and General Geological History of Arizona
Arizona can be broadly divided up into three physiographic provinces that have distinctive
geological and geographic characters. These provinces are the Colorado Plateau Province,
the Transition Zone, and the Basin and Range Province (Figure 2.2). 5 The Colorado Plateau
Province forms the region at the adjoining corners of Arizona, Colorado, New Mexico, and
Utah and is characterized by high plateaus, deeply incised canyons, and isolated buttes.
In northern Arizona, the exposed rocks of the Plateau are mostly horizontal Paleozoic and
Mesozoic sedimentary strata between 550 and 66 millions of years old that have not been
greatly folded, faulted, or metamorphosed. Subsequent volcanic activity on the Plateau pro-
duced a large stratovolcano in Arizona—the San Francisco Peaks—and numerous basaltic
lava flows, with the most recent eruptive activity taking place less than 1000 years ago.
The Transition Zone, also known as the Central Highlands Province, defines a west-
northwest trending band across central Arizona bounded by the Colorado Plateau to the
north and the Basin and Range Province to the south. It is comprised mainly of uplifted
and eroded Proterozoic igneous and metamorphic bedrock ranges between approximately
1.8 and 1.4 billion years old and erosional remnants of younger Colorado Plateau rocks
(Figure 2.2).* Unlike the “layer-cake” Colorado Plateau strata, rocks in the Transition Zone
are more extensively faulted and form rugged mountain masses.
The Basin and Range Province is located primarily in southern and western Arizona, and
the Phoenix and Tucson metropolitan areas are located within this province. Proterozoic
and younger igneous, metamorphic, and sedimentary rocks are exposed in dominantly
* For detailed discussions of Proterozoic geology in Arizona, the interested reader is referred to Karlstrom. 6