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U.S. Department of the Interior Saguaro National Park Service Saguaro National Park Geology of the Tucson Mountains General Setting of the The Tucson Mountains are one of many sediment derived from the erosion of these Tucson Mountains relatively small ranges that dot the south- mountains. The Tucson Mountain block is western United States belonging to the about 20 miles long and up to 7 miles wide Basin and Range Province. These ranges at present, although valley fill covers much are the result of block faulting, which of the lower slopes of the mountain block. occurred about 10-15 million years ago The highest peak in the Tucson Mountains (MYA), and today are separated by basins is Wasson Peak at 4,687 feet, the terminus filled with thousands of feet of alluvial of many popular hiking trails in the park. The Geologic History of Before we begin to look at the origin of the ics, which holds the key to understanding the Tucson Mountains: Tucson Mountains, it is important to look the origin of the rocks and structures General briefly at the rocks which make up the which make up the Earth’s crust. mountains and the theory of plate tecton- The Building Blocks Rocks of the three major classes - (magma) wells up beneath them, and ingenous, sedimentary and metamorphic - coming together along the margins of are found in the park. The ingneous rocks certain continents. Where these plates include coarse grained intrusive rocks such meet, one of three things may happen. If as granite, which cooled deeply within the one plate, usually the oceanic plate, is Earth, and fine grained rocks including denser than the other, it will descend extrusive lava flows and intrusive basalt (subduct) under the other plate and a dikes which cooled much more rapidly. trench will form at that point. As the Sedimentary rocks are formed from the oceanic plate continues to descend deeper, consolidation of sediment derived from the rocks become plastic and then molten, weathering and erosion of preexisting leading to the formation of a chain of rocks and deposited in layers by streams, volcanoes as the less dense magma rises to wind or in the shallow waters of the ocean. the surface. Such is the case today as the The most common of these rocks include Pacific Plate subducts under the North sandstone, shale and limestone. Metamor- American Plate or Asian Plate. On the phic rocks form deep within the Earth other hand, if the two colliding plates are when heat, pressure and chemical fluids of similar densities, neither can descend alter preexisting rocks. These include slate, very far beneath the other. As a result, the marble, gneiss and schist. Specific ex- plates will rise (obduct) high above the amples of these major rock types will be surface. Such is the case with the Himalaya discussed in the following sections as they Moutains in Asia. Finally, if the two plates help to explain the geologic history of the are moving in the same general direction Tucson Mountain District. but at different rates, the plates will sepa- rate along a horizontal or strike-slip fault. The theory of plate tectonics states that the Such is the case today with the San crust is made up of many plates, some of Andreas Fault along the Pacific coast of which are thousands of miles in diameter North America. This very brief description and up to sixty miles thick. These plates are of the theory of plate tectonics will hope- in constant motion, breaking apart along fully suffice in our discussion of the origin the mid-ocean ridges, as molten material of the Tucson Mountains. How it Came to Be The oldest rocks found in the area, al - zona. These rocks are approximately 1.7 though not directly in the park, are gran- billion years old and belong to an era of ites and metamorphic rocks which repre- geologic time known as the Precambian. sent the original crust of Southern Ari- The metamorphic rocks are mostly schist How it Came to Be resulting from a plate collision at that development of southern Arizona during Con’t time, altering preexisting sediments and the late 1800’s. One of these intrusions is volcanic rocks. exposed today at Amole Peak located northeast of the visitor center. No major There is little evidence of what happened ore deposits were found in the Tucson over the next billion plus years, as the Mountains, but the area is dotted with region was subjected to extensive erosion. prospector pits and abandoned mines. Approximately 600 million years ago (MYA), at the beginning of the Paleozoic After another long period of erosion, Era, gentle rise and fall of the crust, as the renewed activity began with the intrusion Pacific Plate approached the North of the Wilderness Granite as well as re - American Plate, led to deposition of newed volcanic activity in parts of south- sedimentary rocks, mostly limestones, eastern Arizona. (What follows is the most sandstones and shales separated by commonly accepted theory, but it is still extensive periods of erosion. A few controversial.) The Wilderness Granite was scattered outcrops of these rocks can still emplaced about six to eight miles below be seen in the park including around the the surface approximately fifteen to twenty Sus Picnic Area. miles east of Tucson. This intrusion bowed up this region and at the same time altered During the early part of the Mesozoic Era, the surrounding rocks to a highly mobile approximately 150 MYA, continued uplift state. As arching continued, a huge slab of of the region led to erosion of the ex- rocks broke loose and slid west and to its posed rocks by streams. The sediments present position along a special type of were deposited as floodplains in the fault known as a detachment fault. This shallow water of an inland sea. Today movement took place slowly over thou- these sediments make up the Red Hills to sands of years. Eventually the rocks upon the south of the visitor center. The red which the upper plate rocks slid, solidified color is from the iron oxide, hematite, and today comprise the Catalina Gneiss which formed in the oxygen-rich shallow which can be seen as strikingly banded seas of this area. Further evidence of the rocks along the Catalina Highway. The shallow nature of these waters can be seen actual detachment fault and the lower plate in the fossils of petrified wood, clams and rocks can be seen along the loop road at even dinosaur leg bones found in the area. the Rincon Mountain District of Saguaro National Park. As the Red Beds were being formed, the ancient Pacific Plate continued to descend This detachment of the upper plate rocks under the North American Plate leading brought the rocks of the Tucson Moun- to much volcanic activity and mountain tains to their present site, but this is not the building in the west. This event is known end of our story. This event, however, did as the Laramide Orogeny, which occurred end the compressional stage of the over a 30 million year interval during the Laramide Orogeny. As these stresses latter part of the Mesozoic Era and begin- relaxed, the entire southwestern portion of ning of the Cenozoic Era. At this time the the United States became stretched as the Tucson area was subject to extensive Pacific Plate began to pull away from the volcanic activity, resulting in extensive North American Plate, beginning approxi- emission of rhyolite (a light tan fine mately 20 MYA. The extension of this area grained rock) lavas and fiery ash-steam produced block faulting, where many clouds, or nuee ardentes, which were so blocks separated from other blocks along dense they rolled down the sides of the steep normal faults producing the basins volcanoes consuming everything in their which today surround the Tucson Moun- path. So much material was pumped out tains and other similar mountain ranges in from below the surface that eventually the the southwest. At one time valley floors area collapsed producing a huge depres- may have been as much as eight to ten sion or caldera at least fifteen miles in thousand feet below the mountain crests, diameter! Over time this caldera was filled but today relief is much reduced, as alluvial with rhyolite, ash deposits (tuff) and (stream) deposits of gravel, sand and mud brechia, a rock formed from the consoli- have filled the basins to their present levels. dation of blocks broken from the collapse of the sides of volcanoes. This complex What does the future hold for the region? mass of rocks collectively is known as the Erosion will continue to reduce the relief Tucson Mountain Chaos, and forms the of the mountains, which may lead to bulk of the rocks which make up the renewed uplift of the mountain fault block present Tucson Mountains. All can be and potential future earthquakes. A major seen in the proximity of the scenic over- earthquake has not occurred in the Tucson look at Gates Pass. At the same time, region since the 1880’s, but could happen nearby areas were intruded by masses of at any time. However, such events will pink granite and quartz veins which bear most likely be few and far between over the many of the minerals, mostly copper, next few thousand years! silver an gold, which led to the rapid EXPERIENCE YOUR AMERICA.

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