Geology

The McDowell Mountains and the surrounding smaller peaks and desert are the result of billions of years of geological processes. Although the visible landscape is usually produced by the most recent geological events, the rocks in that landscape can be much, much older than the events that created what we see today. The basic rock underlying the Preserve is ancient granite formed between 1.4 and 1.7 billion years ago. This rock was mostly underwater for more than a billion years during which time it became covered with a thick layer of sediment that became sedimentary rock. After the creation of the Rocky Mountains about 65 – 75 million years ago (MYA) lifted central Arizona above sea level, this thick layer of sedimentary rocks on top of the granite began to erode away.

The major geological process that shaped the local landscape was the formation of the basins and ranges. Between 8 and 15 MYA, volcanic activity beneath the crust stretched the surface of central and southwestern Arizona and it began to crack in many places, forming parallel north – south cracks. Earthquakes and continuing volcanism caused some of the regions between cracks to slide downward by as much as 4 miles while other areas did not. The lower areas are now called basins and the higher areas are called ranges. The McDowells, the Phoenix Mountains, and the Sierra Estrellas are parallel, roughly north-south ranges created during this period. The valleys in between, including metropolitan Phoenix, are examples of basins. The basins have mostly filled up with sediment eroding off of the ranges in the millions of years since they were formed. In the McDowells, almost all of the sedimentary rocks have eroded away, leaving mostly granite.

Geology of the preserve  Prepared by Dan Gruber

This material describes the general understanding of the geology of our area. For purposes of discussion, we describe events with certainty even though we have only indirect knowledge about and evidence of them. This is a highly simplified description of complicated processes. For more information, we suggest reading Geology of Arizona by Nations and Stump, A Natural History of the Sonoran Desert by the Arizona – Sonora Desert Museum, Roadside Geology of Arizona by Chronic, and Guide to Rocks and Minerals by Mottana, Crespi, and Liborio.

I. ROCKS

There are three basic types of rocks:

Igneous —
These are rocks that form as molten rock cools and solidifies. Molten rock is called magma. Magma can reach the surface via volcanoes or oceanic ridges and cool and harden quickly in the air or under water. This kind of igneous rock usually has a fine-grained structure and is generically called basalt or lava. Magma that cools and hardens slowly below the surface is coarse-grained and is generically called granite.

Sedimentary —
Any kind of rock can erode through the action of wind, water, earthquakes, etc. and form sediment. Also, the skeletons of marine organisms like coral can accumulate into sediment. When sediment becomes deep enough, the pressure exerted by the material’s weight welds together the sediments into sedimentary rock. Coarse-grained sediment like sand becomes sandstone. Fine-grained sediment like dirt or mud becomes shale. Coral and other microscopic skeletons become limestone.

Metamorphic —
Any kind of rock that has been subjected to heat and pressure but that hasn’t actually melted can become metamorphic (“changed”) rock. Under the right combination of heat and pressure, limestone changes into marble, sandstone changes into quartzite, shale turns into mica or slate or schist, and even basalt (lava) can turn into greenstone.

II. THE PRESERVE AND VICINITY

The visible landscape is usually produced by the most recent geological events. But the rocks in that landscape can be much, much older than the events that created what we see today. This is very much the case with everything we see in central Arizona. We’ve taken a chronological approach to describing the geological history of the area, which means that the forces that shaped the visible landscape will be discussed last.

The Mazatzals, the McDowells, and almost everything else in this vicinity share the same basic geological history and were subjected to the same basic geological processes. The basic rock underlying all these formations is ancient granite, igneous rock formed between 1.4 and 1.7 billion years ago. This rock was mostly underwater between the time of its formation and the creation of the Rocky Mountains about 65 – 75 million years ago (MYA), when central Arizona was lifted above