The Effect of Climate on Rocks of the Superstition Mountains L. Dahlman

What effect has climate had on the life and landscape? The climate of the Superstition Mountains, like the rest of central and southern Arizona, is considered mild, despite its high summer temperatures and low annual rainfall. Vegetation in the region is almost exclusively cactus and shrubs, with thin ribbons of riparian plants tucked along ephemeral waterways.

Weathering in a dry climate often results in angular landscapes Weathering in a dry climate often results in angular landscapes. As exposed rocks don’t experience the same level of chemical or mechanical weathering that occurs in climates with more rain and vegetation, solidified volcanic rocks generally form cliffs in the Southwest. This photograph from March of 2010 shows desert vegetation thriving across the lowlands following a wet, El Niño-enhanced winter.

The region receives just 3 to 11 inches of precipitation per year, with 50% of the annual total occasionally arriving in a single storm.

Mild, region-soaking storms move through the region from the Pacific Northwest during winter. Sporadic, violent thunderstorms often pelt the area with raindrops and hail in the summer.

In the Southwest, the longest instrumental climate records are only around 100 years long. Image from http://ldstalk.com/1919/mormon-history-pioneer Settlers began populating the region and keeping records of weather in the early 1900s.

Paleoclimate records, deduced mostly from tree rings in Northern Arizona, provide clues about the region’s climate over the past 1,200 years. Annual growth rings can indicate conditions such as temperature and precipitation.

Quantitative tree ring studies have been used to reconstruct flow levels of the Colorado River, a proxy for precipitation across the southwest.

The “take-home” message of those studies is that precipitation has varied up to 20% over the past millennium Wetter than average Long term average Drier than average 800 1000 1200 1400 1600 1800 2000 Year

Resulting in repeated cycles of drought conditions that occasionally last for decades 50 years 50 years 50 years 120 years Wetter than average Long term average Drier than average 800 1000 1200 1400 1600 1800 2000 Year

Archeological evidence indicates a widespread migration or exodus of indigenous peoples from the area around 1400, and drought may have been the driving factor. Though the observed magnitude of climate variability made it difficult for humans to live in the region, it is not likely to have a measureable effect on weathering of rocks in the Superstitions.