Hydrothermal Alteration Zones and landscape evolution in the City of Rocks, Idaho 

Charlie Woodruff, Class of 2001

Department of Geology, Whitman College, Walla Walla, WA 99362

Faculty Sponsor: Kevin Pogue


            The City of Rocks National Reserve is located in south-central Idaho near the town of Almo.  The reserve occupies the center of the southern-most structural dome of the north-trending Albion Range, which is located in the Basin and Range geologic province.: The City of Rocks consists of an Oligocene granite pluton, which has been differentially eroded into spectacular fins, domes, and spires.  This 29 ma granite, known as the Almo Pluton, intruded Archean and Proterozoic rocks, which make up the oldest rocks exposed within the Raft River-Grouse Creek-Albion Range metamorphic core complex. The pluton was emplaced at a depth of 10 to 12 km (Miller and Bedford, 1999).Archean gneiss of the Green Creek Complex constitutes the oldest rocks in the National Reserve and is overlain by neoproterozoic metasediments, most notably the Elba Quartzite that forms the hogbacks around the basin of the City of Rocks.The carapace of the core complex consists of deformed Paleozoic metasediments.  The metamorphic core complex underwent drastic uplift and multiple episodes of extension in the Cenozoic that displaced much of the sequence and caused erosion and removal of material overlying the Almo Pluton.  The two extensional events that affected the pluton occurred during the early and late Miocene, the first of which created a west-dipping low angle detachment fault, and the latter which created an east-dipping detachment fault (Miller and Bedford, 1999). the study was to determine if these alteration zones have had significant effects on the overall geomorphology of the City of Rocks landscape

        Recent geological studies of the area conducted by the USGS noted large-scale alteration zones within the City of Rocks Almo Pluton (D. Miller personal communication).  While it was observed that several of the zones were north-trending, information regarding the distribution, age, and composition of the zones was not obtained. This study proposed to thoroughly investigate the zones and using this information, determine the timing and structural controls on the emplacement of the zones.


The hydrothermal alteration zones found in the Almo Pluton are oriented perpendicular to the regional extension during the Cenozoic (Miller and Bedford, 1999) indicating that while the pluton was cooling, joints were forming perpendicular to the regional extension, which was a result of Basin and Range tectonics.  As the pluton was unroofed it depressurized, releasing volatile components.  The low-angle normal faults contributed to the unroofing of the pluton while hydrothermal fluids were released causing mineralization and alteration along north-trending joints.  The fluids traveling along the joints may have also facilitated small amounts of slip parallel to the joint planes.

The hydrothermally altered zones in the Almo Pluton are more easily weathered than the unaltered granite due to the increased muscovite and sulfide content, as well as the occurrence of vuggy quartz (fig. 2).The increased weathering along these zones is responsible for the distribution and orientation of the large-scale fins and spires and žavenuesÓ within the City of Rocks National Reserve (see fig. 3).Thus, the overall landscape evolution of the City of Rocks is a function of the plutonŪs history of unroofing due to uplift, tectonic extension, and differential erosion along north-trending hydrothermal alteration zones, and the visually obvious city žavenuesÓ that dominate the landscape at the City of Rocks are a direct result of this process.