How it Came to Be
The oldest rocks found in the area, although not directly in the park, are granites and metamorphic rocks that represent the original crust of Southern Arizona. These rocks are approximately 1.7 billion years old and belong to an era of geologic times known as Precambrian. The metamorphic rocks are mostly schist resulting from a plate collision at that time, altering preexisting sediments and volcanic rocks.
There is little evidence of what happened over the next billion plus years, as the region was subjected to extensive erosion. Approximately 600 million years ago (MYA), at the beginnning of the Paleozoic Era, gentle rise and fall of the crust, as the Pacific Plate approached the North American Plate, led to deposition of sedimentary rocks: mostly limestones, sandstones and shales separated by extensive periods of erosion. A few scattered outcrops of these rocks can still be seen in the park including around the Sus Picnic Area.
During the early part of the Mesozoic Era, approximately 150 MYA, continued uplift of the region led to erosion of the exposed rocks by streams. The sediments were deposited as floodplains in the shallow water of an inland sea. Today, these sediments make up the Red Hills to the south of the visitor center. The red color is from the iron oxide, hematite, which formed in the oxygen-rich shallow seas of this area. Further evidence of the shallow nature of these waters can be seen in the fossils of petrified wood, clams and even dinosaur leg bones found in the area.
As the Red Beds were being formed, the ancient Pacific Plate continued to descend under the North American Plate, leading to much volcanic activity and mountain building in the west. This event is known as the Laramide Orogeny, which occurred over a 30 million year interval during the latter part of the Mesozoic Era and beginning of the Cenozoic Era. At this time, the Tucson area was subject to extensive volcanic activity, resulting in extensive emission of rhyolite (a light tan fine grained rock) lavas and fiery ash-steam clouds, or nuee ardentes, which were so dense they rolled down the sides of the volcanoes consuming everything in their path. So much material was pumped out from below the surface that eventually the area collapsed, producing a huge depression or caldera at least fifteen miles in diameter! Over time, this caldera was filled with rhyolite, ash deposits (tuff) and brechia, a rock formed from the consolidation of blocks broken from the collapse of the sides of volcanoes. This complex mass of rocks collectively is known as the Tucson Mountain Chaos, and forms the bulk of the rocks which make up the present Tucson Mountains. All can be seen in the proximity of the scenic overlook at Gates Pass. At the same time, nearby areas were intruded by masses of pink granite and quartz veins which bear many of the minerals, mostly copper, silver an gold, which led to the rapid development of southern Arizona during the late 1800’s. One of these intrusions is exposed today at Amole Peak located northeast of the visitor center. No major ore deposits were found in the Tucson Mountains, but the area is dotted with prospector pits and abandoned mines.
After another long period of erosion, renewed activity began with the intrusion of the Wilderness Granite as well as renewed volcanic activity in parts of southeastern Arizona.
What follows is the most commonly accepted theory, but it is still controversial:
The Wilderness Granite was emplaced about six to eight miles below the surface approximately fifteen to twenty miles east of Tucson. This intrusion bowed up this region and at the same time altered the surrounding rocks to a highly mobile state. As arching continued, a huge slab of rocks broke loose and slid west and to its present position along a special type of fault known as a detachment fault. This movement took place slowly over thousands of years. Eventually, the rocks upon which the upper plate rocks slid, solidified and today comprise the Catalina Gneiss which can be seen as strikingly banded rocks along the Catalina Highway. The actual detachment fault and the lower plate rocks can be seen along the loop road at the Rincon Mountain District of Saguaro National Park.
This detachment of the upper plate rocks brought the rocks of the Tucson Mountains to their present site, but this is not the end of our story. This event, however, did end the compressional stage of the Laramide Orogeny. As these stresses relaxed, the entire southwestern portion of the United States became stretched as the Pacific Plate began to pull away from the North American Plate, beginning approximately 20 MYA. The extension of this area produced block faulting, where many blocks separated from other blocks along steep normal faults producing the basins which today surround the Tucson Mountains and other similar mountain ranges in the southwest. At one time, valley floors may have been as much as eight to ten thousand feet below the mountain crests, but today, relief is much reduced as alluvial (stream) deposits of gravel, sand and mud have filled the basins to their present levels.
What does the future hold for the region? Erosion will continue to reduce the relief of the mountains, which may lead to renewed uplift of the mountain fault block and potential future earthquakes. A major earthquake has not occurred in the Tucson region since the 1880’s, but it could happen at any time. However, such events will most likely be few and far between over the next few thousand years!
