Geography

by Aaron Reistad

Organ Pipe Cactus National Monument covers 330,689 acres in southwestern Pima County, Arizona.[1]  This monument has received two additional levels of protection since being designated a national monument in 1937.  In 1976, the park was designated a UNESCO (United Nations Education, Scientific and Cultural Organization) Man and the Biosphere Reserve.  The third protection came in 1978 when approximately ninety-five percent of the monument (312,600 acres) were officially designated Organ Pipe Cactus Wilderness.[2]  Although this area is now classified as a desert, about eleven thousand years ago the continental ice cap receded toward the pole, causing temperatures in Arizona and Mexico to rise.   Along with this rise in temperature the geographic setting, which now comprises Organ Pipe, was formed.  This essay addresses the geographic transformation through time, key geographic features, soil which is home to this area as well as earthquakes and current threats which the monument is currently experiencing.

Organ Pipe Cactus National Monument maintains physical features which are not common in other deserts around the world.  Numerous reasons attribute to this fact including, the retreat of glaciers, movement of tectonic plates, and many other significant events allowing Organ Pipe to become physically formed, as it is known today.  In order to understand the geographic features apparent today, it becomes necessary to examine the past.  This region was likely formed on the southern continental margin of North America about 600 million years ago.[3]  As the continents continued to drift, the formation and features of the Organ Pipe are due to the North American plate converging into the Pacific plate along the San Andreas Fault.[4]  During the Mesozoic Period and for a majority of the Cenozoic Period, the Pacific Ocean crust subducted beneath the western edge of North America.[5]  When one tectonic plate converges into another, such as here, a subduction zone occurs where volcanoes are more likely to occur.  In this particular instance, a long chain of large volcanoes were active to the east of the subduction zone and cut right through western Arizona.[6]  According to authors, Richard Felger and Bill Broyles, these volcanoes became especially active during a period of time known as the Middle Tertiary when, “from about 25 to 15 million years ago, all hell broke loose in southwestern Arizona.”[7]  Geologists are able to conclude that due to the recorded layers of lava, ash flows, and ash which fell from the air exposed mainly in the Ajo Mountains but also in various other areas of the monument.  The shape and aspect of landforms are closely related to the rock types that underlie them.  Authors, Broyles and Felger, show an example of this by stating that in the Organ Pipe area, “rounded hills are likely to be made out of Mesozoic granite and metamorphic rocks associated with the uplifted and exposed roots of ancient volcanoes.”[8]

The second topic, which must be discussed during a feature on the monuments geography, are the well-known features of this area.  One of the more well-known areas of the park is Quitobaquito, which is a unique desert oasis that has provided a constant source of water for both humans and animals alike throughout its existence.  The pond is approximately 200 by 260 feet wide, and 5 feet deep[9].  Quitobaquito is the location of several springs and seeps and provides habitat for the endangered Quitobaquito Desert Pupfish, and the Sonoran Mud Turtle.

Apart from Quitobaquito Springs, the Ajo Mountain Range spectacle rises on the eastern side of the park. Most of the mountains in the monument are volcanic in origin and this holds true for the Ajo range. The highest peak in this range, as well as in the park, is Mt. Ajo, which elevates to a height of 4,808 feet.  The western flank of the Ajo Mountains in Organ Pipe is especially lush due to the relatively long and high Ajo Mountains.[10]  According to the authors of Dry Borders, the mountains and landscape evolved, “over the last 15 million years because of the jostling of crustal blocks and earthquake activity associated with Basin and Range crustal extension.”[11]

An interesting piece of information not normally common in a desert setting yet accompanying the geography of the region are the earthquakes experience in Organ Pipe.  The National Park Service’s website about Organ Pipe, relates that over the past year 1,520 earthquakes have been detected by a very sensitive seismic data collecting station located in the monument.[12]  The station has detected earthquakes with very small magnitudes of less than 2.0 up to larger destruction causing seismic events.[13]

The geography of this region is currently facing numerous threats. The common factor behind the majority of these threats comes from human influence.  Although, in 2010 the Superintendent of the park related that most of the park is still undisturbed and maintains a definitive representation of the Sonoran Desert, nonetheless increased border crossing through the monument affects this area.  According to the Superintendent’s 2010 Report on Natural Resource Vital Signs, the most difficult issue faced by managers of Organ Pipe Cactus National Monument is the U.S./Mexico border situation.[14]  In 2010, the Superintendent of Organ Pipe highlighted how the increased human traffic influences the monument stating, “thousands of miles of unauthorized roads and trails, associated damage to soils and vegetation, interruption of natural ecological processes, disturbance to wildlife movements, and an abundance of trash.[15] Along with this, the National Park Service Cultural Landscapes Inventory relates agricultural development along the Mexican border in Sonora has rapidly intensified since the late 1970s.[16]  The Inventory also shows a decrease in the water table as well as, “Agricultural pests, chemical drift, increasing truck traffic, and an increase in vandalism, assaults, and larcenies to visitors and employees has resulted in part because of this development.”[17]

In response to these current threats, Organ Pipe managers have taken specific measures to understand the extent of damage caused as well as ongoing and long-term restoration plans.  Some examples include, Remote-imagery sensing of illegal roads and trails, east/west transects surveyed to document trends in the distribution of illegal trails and vehicle routes, and an assessment of the effects of border activities on threatened and endangered species.[18]  These efforts among numerous others will assist in the maintenance and protection of the Organ Pipe Cactus National Monument area and the geography throughout this magical area.