Why are some rocks red?
My daughter is enthralled with the red rocks found around St. George, Utah and Sedona, Arizona. Gazing over the red Navajo Sandstone of Zion National Park she asks, “why aren’t there red rocks around Kingman, AZ?”
I explain that the rocks around Kingman are of metamorphic and igneous origins and the iron is not oxidized, which causes her to roll her eyes. The answer isn’t simple. The red color is due to the oxidation of iron- the iron losing three electrons as it combines with oxygen. This new compound is called hematite (Fe2O3)- the name comes from the Greek word for blood.
Iron in nature is initially tied up in many minerals including biotite, ferromagnesium minerals and sulfides. When rocks weather, the iron-bearing minerals breakdown and release the iron. As the iron is washed downstream, carried with the sand and silt and deposited in an ocean, some of it is oxidized. If the river that’s carrying the detritus to the ocean forms a shallow delta where there’s abundant oxygenated water, the iron is further oxidized and because hematite is a strong pigment, it coats the sand and silt a red color. Once iron makes it to deep water where there’s little oxygen, it’s no longer oxidized and the material takes on a green or gray color.
So why is there so much red sandstone in Utah and Arizona? This area was once a large desert of sand dunes near an ocean, similar to the present-day Sahara Desert. But this ancient landscape was much larger and offered a perfect place to form dunes with oxygenated water that converted the iron to hematite.
Red is not the only color of the beautiful sandstone cliffs in southern Utah and northern Arizona. Depending of the source of the iron, the oxidizing conditions and what happens to the iron after it’s deposited, the colors might be red, yellow, brown or a bleached white. But the most beloved color is red- the color of hematite. Yellow color indicates the mineral jarosite, and brown the mineral goethite /go thite/
In Mohave County, the reddest colored sandstones and shales can be found in the Arizona Strip country, north of the Grand Canyon. Places like the Vermillion Cliffs and Parashant National Monument have similar formations to those found in Zion- the Navajo Sandstone through the Moenkopi Formation. Near Kingman, the few places with red rocks are found near mining areas where the colors are more yellow, orange and brown and in people’s front yards- decorative rock. In all of these places, the origin of the color is iron.
In the Mineral Park area of the Cerbat Mountains and the Blake Ranch Rd - Wilson Ranch Rd area of the Hualapais, the soil is stained reddish-orange from the oxidation of pyrite. If you drive south on Blake Ranch Rd notice that the road starts out a tan color. As you approach Wilson Ranch Rd, the soil becomes a distinctive reddish-orange. Continuing south, the soil turns tan again. The reddish-orange color tells you there is mineralization nearby. As pyrite weathers, the iron goes into solution and is quickly attacked by oxygen. If the pH of the solutions is very acid (low pH), jarosite is formed and the rocks are stained yellow. If the solutions are somewhat acid, goethite is formed and the color is reddish-brown. If the iron bearing mineral contained copper, the oxide would include some hematite which brings joy to an exploration geologist. The mixture of these three minerals gives the distinctive reddish-orange soil color.
The study of the different iron oxides is very important in the exploration for copper deposits. It’s called leached cap interpretation. As a cub geologist I’d carry a color chart for iron oxides and compare the chart to every rock. With experience, I can now distinguish the colors of interest to exploration geologists from those that just look pretty. If I could only convince my daughter that the color she’s in love with is not of interest to her dad.
However, the cliffs of Zion aren’t only red but also orange, yellow and white. What happened to the red color? The sandstone is very permeable (allows water to pass through). Over millions of years after the sandstone was deposited, water passed through. If the water was chemically reducing, the hematite (iron) would pick up electrons and lose the oxygen, making the iron soluble. In this way, the hematite (red color) is washed out of the sandstone. If the washing isn’t complete, there can be remnant zones, or swaths, of red in otherwise white rock. The iron that was washed out travels to a place where it’s again oxidized, precipitating in various shapes including round concretions, called “Moki Marbles,” also in streaks, pipes and other shapes. The iron concretions are found in white sandstone where the hematite pigment was washed out.
Red and white sandstone cliffs of Zion.
Too bad this process can’t be replicated in our washing machine. We haven’t found a way to get the red out of the clothes we wear while 4-wheeling in red rock country. I even have an official “Dirt Shirt” T-shirt that my wife bought in Moab. It’s that distinctive red-brown color to match my unofficial rust-red socks and pants.
