Huong Dinh's Sonoran Field Trip Blog: Weathering, Erosion and Soil

The Sonora Desert is a great place to study the effects of weathering and erosion on granite landscapes. One of the most important processes in the desert is grusification. This is a combination of physical and chemical weathering that breaks down the desert’s granite. The physical nature of this process is driven by salt crystal build-up, which then expands and contracts as the temperatures swing. This process breaks up the rock into chunks which often form piles of smaller rocks known as tors.

World Atlas Photo – Sonora Desert

Once the rocks get smaller, chemical biotite weathering processes oxidize iron. Dissolution breaks the rocks down to further to form smaller tiny rocks or sands known as grus. This material makes up much of the Sonora desert floor.

Arizona Geographic Alliance Photo - Grus

Another important desert process is Alluvial (water based) Mass Wasting as pictured below. Weathering breaks up the rocks along a mountain range. Then grus and clay materials create an unstable base for larger and heavier rocks that sit on top of an impermeable slope. Water from occasional heavy rain storms will saturate the loose materials, and the debris flow is created.

Arizona-Sonora Desert Museum Photo – Alluvial Fan near Yuma, Arizona

Weathering and erosion play an important role in desert soil creation. Alluvial clay and silt particles wash over the basins and mix with wind-blown dust, organic material and rain dropped particles. In the desert, with little water and relatively sparse organic material, this process is SLOW! In fact, it takes 100,000 years or more to develop a full calcic horizon. This white concrete-hard layer varies in depth and thickness. A fully mature calcic horizon, or caliche, may be three feet thick and nearly impenetrable. In relatively moist areas like Tucson, the calcic horizon is deeper (~24 cm). But in very arid regions like Yuma, the calcic horizon may be only a couple of centimeters deep.