Road Trip: The Central Highlands of Arizona. Introduction

I haven’t posted much about the geology of Arizona, where I grew up and wandered the back ways while studying Geology at Arizona State University because, unfortunately, most of these excursions occurred before the internet and are nothing more than dim memories, supported by fuzzy photographs enclosed in aging plastic as part of my photo album collection. Many of those albums were damaged by water during storage. Recent trips to the Grand Canyon State were in a motor home, which isn’t conducive to unplanned photo stops on busy highways. The good news is that I am back for a few days and ready to explore one of the state’s most interesting provinces, the Central Highlands.

Figure 1 reveals how Arizona can be divided based on the most-recent geological event in its history, the problematic uplift of the Colorado Plateau during the last 80 million years by approximately 2.5 km (about 8000 feet). Today, Flagstaff is located at an elevation of 7000 feet while Phoenix, located in the Basin and Range province sits at a mere 1000 feet above sea level. The Transition Zone (Fig. 1) reflects how the earth’s crust adjusted to this 6000 foot difference in elevation. (Note that I am using the older terminology — Central Highlands — mostly as a nod to this area being identified as a distinct geographic province long before its tectonic underpinning was understood.) It’s also a nod to the motivation for this blog, in this case The Roadside Geology of Arizona, by Halka Chronic, published in 1983 by Mountain Press Publishing.

This post is an introduction to the Central Highlands. I will be driving (in a car and not a motor home) from Phoenix, in the Basin and Range province (see Fig. 2) on Interstate 17 to the middle of the volcanic complex shown in a burnt orange in Fig. 1, and then follow AZ-69 NW to Prescott, before returning to Phoenix on AZ-89 by a more westerly route.

The dramatic change in elevation from Phoenix to Flagstaff suggests that the earth’s crust was deformed substantially when the Colorado Plateau was uplifted (to the right in Fig. 3), much of the difference accommodated by faulting.

Blocks of crust the size of mountains slide up and down along the endless number of faults that characterize the Central Highlands, which is why its geological name is the Transition Zone (see Fig. 1), from low elevation to high. Most of the rocks we will see are Precambrian gneisses and granites. In other words, sedimentary rocks originally deposited far more than 500 Ma in the past, buried and subsequently altered by extreme heat and pressure more than a billion years ago, when they were penetrated and partially melted by molten material from deep within the earth (at least ten miles below the surface). Much later (within the last 100 million years) magma filled the faults crisscrossing the Central Highlands and flowed onto the ancient surface to form volcanoes and lava flows.

I will cross this Transition Zone at two locations, separated by about 40 miles, and attempt to identify variations in the rocks caused by a number of geological factors. That is the (hypothetical) purpose of this road trip.

There are six kinds of rocks and sediments we will be looking for on this trip, and we expect to find them in wildly confusing juxtapositions because of so many faults and the immense spans of time represented by the rocks, not to mention taking different routes on our journey.

1. Quaternary gravel
2. Quaternary and Tertiary lava flows
3. Tertiary stream deposits of sand, silt and gravel with rounded pebbles
4. Tertiary lake sediments of horizontal, whitish, fine-grained rock with layers of volcanic ash
5. Precambrian granite
6. Precambrian gneiss and schist

I will try to show examples of these with photographs but…well, roadside geology is a lot more dangerous than it used to be, with so many vehicles on the roads, and everyone in such a hurry…not to mention reading my blog on their cellphones (I wish).