Winter ended around here about two weeks ago. I’m already wearing shorts, riding my bike to work, and leaving the windows open at night. My ash tree quickly put on leaves, and my flowers are blooming. Perhaps this weather excuses my lack of blogging.
With the good weather, I went for a drive. Not all of Arizona is a winterless wonderland. In fact, nearly a quarter of the state can see snow well into May. This is one of those rare regions where one can experience drastically different climates in the span of a one-hour car ride, all thanks to topographic relief.
Three geographic zones categorize Arizona’s geology and provide distinct boundaries that sometimes influence the region socially and politically. Narrow, steep mountain ranges separated by broad, flat valleys define the Basin and Range zone In the south and southwest third of the state. The difference in elevation between the mountain tops (ranges) and valley floors (basins) may be as much as six thousand feet. Valleys are hot, dry and barren, whereas mountains are cooler and sometimes forested. Typically, metamorphic or igneous rocks form the mountain cores, with tilted sedimentary rocks making up the mountain flanks. Accumulations of sediment thousands of feet thick make up the valley floors. The photo below shows the Basin and Range zone near Phoenix, looking from South Mountain, one of the “ranges”.
The Colorado Plateau encompasses the northeastern third of Arizona. This region is characterized by high elevation, relatively flat topography, and horizontal sedimentary rocks. The layers of the Grand Canyon (pictured below) nicely illustrate these sedimentary rocks, and the flat canyon rim mimics the flat topography typical of the Colorado Plateau. Because the plateau is more than 4000 feet higher than the valley floors in southern Arizona, climate is drastically different between the two zones. It’s not uncommon for a Phoenix resident to lounge by the pool on a warm sunny day, while inhabitants on the plateau suffer through snow, freezing temperatures, and gale force wind.
Between the Colorado Plateau and the Basin and Range zones exists the aptly named “Transition Zone”. Here exists a chaotic mix of topography that exhibits some characteristics of both the Basin and Range and Colorado Plateau. Rocks may be of igneous, metamorphic, or sedimentary origin, and whose structure may be flat-lying or tilted, folded, and faulted. The transition zone has both mountains and valleys, but not the broad, flat valleys of the Basin and Range. Interestingly, the boundary between the Colorado Plateau and the Transition Zone is a rather abrupt escarpment called the Mogollon Rim. Here I am standing on the edge of the rim, looking south toward the Transition Zone. To the north (behind the camera) the land is flat and slopes gently south for a hundred miles. Behind me, you can see the broken mix of hills and valleys that make up the Transition Zone.