Geology and Soils in Deserts of the Southwestern United States - Design with the Desert: Conservation and Sustainable Development - page 37

In addition, mountain ranges (e.g., the Sierra Nevada of California) cause clouds to cool
and lose water as rain as they head inland from the oceans and climb over peaks. The
resulting “rain shadow” contributes to the persistence of the deserts in Arizona and Nevada
by decreasing the water available for precipitation on the leeward side of the ranges (see
Chapter 3). Deserts can also occur near coastlines due to cool ocean air heating up as it
travels inland, leading to increased evaporation. A more detailed discussion of the character
and geographic distribution of deserts appears elsewhere in this topic (see Chapter 1).
2.3 Geological Processes in Deserts
2.3.1 Fundamental Geological Concepts
Lithified geological materials at the Earth's surface are products of the rock cycle . 3 Igneous
rocks are crystallized directly from molten magma, either quickly as the products of
eruptions at the surface or slowly following intrusion into preexisting rock deep below
the surface. Uplifted and exposed igneous rock is constantly being reduced to smaller
particles, or eroded, by continual mineralogical and chemical changes (Figure 2.1).
Sedimentary rocks are formed of lithified layers of these eroded materials after deposition
and accumulation in basins by wind and/or water. Sedimentary rocks can also form, under
the right conditions, directly from chemical precipitation of elements like calcium and
sodium. Subjecting igneous and sedimentary rocks to elevated temperature and pressure
creates metamorphic rocks with different minerals and structure than the original rocks.
While the term “rock cycle” implies a circular progression through the different rock
types, it is more like a network of potential pathways, for example, metamorphic rocks can
be eroded to form sedimentary rocks, sedimentary rocks can be melted and recrystallized
to form igneous rocks, and metamorphic rocks can undergo additional heat and pressure
to form new metamorphic rocks.
The theory of plate tectonics explains the large-scale geological processes that actively
shape the Earth's surface today and provides the larger conceptual framework within
which the rock cycle operates. Rather than having a continuous outer shell of rigid rock
material, the surface of the Earth is formed of numerous interlocking plates that comprise
the continental and oceanic outer crust of the planet. 4 These rigid plates are supported by
viscous mantle material, which enables them to interact with each other in a variety of
ways in response to movement of the underlying mantle.
The three major types of plate boundaries—divergent, convergent, and transform—are
recognized. Divergent boundaries form where new magma erupts along rifts in the older
crust (e.g., the Mid-Atlantic Ridge) where new oceanic seafloor crust is being formed and
the Atlantic Ocean is widening. Convergent boundaries occur when crustal plates collide;
if both plates are formed of continental crust, the collision can create high mountain
ranges. A good example of a convergent boundary is the Himalayas, which are formed
by the ongoing collision of the Indian and Eurasian plates. If an oceanic plate collides
with a continental plate, the denser oceanic crust is driven downward into the mantle to
eventually remelt and form new magma. This process is called is ubduction and typically
forms a deep trench along the boundary together with volcanoes on the continental plate
above the descending oceanic crust. The Andes in South America are created by volcanoes
that erupted above the descending Nazca oceanic plate. Finally, transform boundaries
occur when two plates move tangentially against each other, with neither plate overriding