Aspects of the deterioration of sandstone masonry in Anasazi dwelling ruins at Mesa Verde National Park, Colorado, USA

Mesa Verde National Park, a World Heritage Site, encompasses a large concentration of Anasazi dwelling ruins in southwestern Colorado. These ruins, constructed of sandstone masonry, have been exposed for ca. 750 years to an environment which has been virtually isolated from human impacts. In the past 30 years, increasing public visitation of cultural sites in the area, construction and operation of large industrial facilities such as power plants and smelters, and rapid regional population growth have ended this isolation. National Park Service site managers became concerned that local air chemistry and microclimatic conditions may have changed sufficiently so as to affect the rate and mode of deterioration of the sandstone masonry of the ruins. Airborne pollutant concentrations, microclimate variation, and parameters of sandstone masonry deterioration were studied from 1984–1988. The investigation focused on mechanisms by which pollutants, especially acid precipitation and associated air pollutants, can cause acceleration of natural weathering processes. Although pollutant concentrations in the arid southwestern United States remain low relative to the levels occurring in northeastern North America, relatively sudden recent changes may lead to alteration in the microstructural and chemical composition of the masonry. The presence of moisture at the stone surface strongly affects the rate of deposition and reaction of certain airborne pollutants. Water from direct precipitation or from condensation on exposed surfaces thus enhances susceptibility to pollution-enhanced deterioration. Present rates of erosion of sandstone surfaces in cliff-dwelling walls were not found to be different from inferred long-term rates of surface recession. Walls exposed to direct input of precipitation, however, erode much more rapidly than protected sites. Although the mineralogy of the sandstone suggests potential susceptibility to acid deposition enhanced weathering, present pollutant concentrations have not yet impacted sandstone masonry deterioration rates.