The striking landscape of Sedona, Arizona, captivates visitors with its massive mesas, buttes, and spires that glow in shades of red and orange. This intense coloration transforms the desert scenery, making the rock formations appear to shift hues throughout the day. The secret to this dramatic visual effect is a microscopic layer of mineral coating that covers the surface of the rock structures. This coating is the result of a long, complex geological process involving ancient sediments, water, and air.
The Primary Cause of the Red Color
The unmistakable red color of the Sedona rocks is caused by the presence of a specific mineral: iron oxide. This compound is better known as hematite, which is chemically identical to rust. Hematite is a reddish-black mineral that forms when iron reacts with oxygen, a process called oxidation.
The original grains that make up the rock, primarily quartz sand, are naturally clear or white. However, a very thin, pervasive film of hematite has stained the surface of nearly every grain. This coating is so fine that only a small amount of the mineral is needed to produce the vibrant, deep-red color seen across the landscape. The intensity of the color varies depending on the concentration of the iron oxide, leading to shades from bright orange-red to a darker rust tone.
The Geological Layers of Sedona
The raw material for Sedona’s famous formations was laid down over a vast stretch of geologic time, primarily during the Permian and Pennsylvanian periods. During this era, the region transitioned between a shallow ancient sea, coastal mudflats, and vast desert environments. These shifting conditions created a layered sequence of sedimentary rocks, each with a unique composition.
The most prominent red formations, which include Bell Rock and Cathedral Rock, are largely composed of the Schnebly Hill Formation. This layer is a thick, dark red sandstone that can be up to 1,000 feet deep in some areas. Below the Schnebly Hill layer are other formations that also display the red hue, such as the Hermit Shale and parts of the Supai Group.
The Coconino Sandstone, which often caps the red rock cliffs, is notably lighter, appearing white or yellow. These distinct layers show how the environment changed, as the Coconino formed later from wind-blown sand dunes. The raw sediments deposited in these environments contained the iron-bearing minerals necessary for the eventual red staining.
The Oxidation and Cementation Process
The process began with the incorporation of iron into the original sediments, present in minerals like iron silicates or hydroxides. Over millions of years, these iron-bearing minerals underwent chemical weathering and dissolution within the sediment layers.
Water played a large role in mobilizing this iron, acting as a vehicle to spread the color. Iron-rich groundwater infiltrated the pore spaces within the layers of sandstone and siltstone, carrying the dissolved iron compounds throughout the rock structure. As the region’s climate became increasingly arid and the iron-bearing water was exposed to air, the final chemical reaction occurred. The dissolved iron oxidized, converting it into the stable mineral hematite, which then precipitated out of the water solution.
The tiny hematite particles permanently bonded to the surfaces of the quartz sand grains, creating a thin, durable coating. This process not only colored the rock but also helped to cement the grains together, a process known as lithification. This specific combination of iron-rich sediments, water movement, and exposure to oxygen in an arid setting permanently fixed the vibrant red color to the rock.