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Rock varnish | Photo by Eclipse Chaser/Pexels
You have probably noticed the dark, coated look of rocks in desert landscapes.
The most common colors are deep black, rust-red, orange and warm brown. These coatings are called desert varnish. They form over thousands of years as a thin layer of clay minerals, manganese and iron oxides gradually builds up on exposed rock surfaces.
But what is desert varnish, why does it form and why does it matter?
Key takeaways
- Desert varnish is a thin coating of clay minerals, manganese and iron oxides that builds up on rock surfaces.
- It forms when dust and minerals settle on rock and bacteria may help bind them to the surface.
- It grows incredibly slowly, often just a few micrometers per thousand years.
- The color tells you what’s in it: black means more manganese, red or orange means more iron, and tan or brown means a mix of both.
- Scientists still debate whether bacteria are essential to the process or just along for the ride.
- Ancient petroglyphs were carved by chipping through the varnish to expose lighter rock underneath.
- You don’t have to wait thousands of years to get the look. A reactive color treatment can create a similar weathered appearance in a fraction of the time.
What is desert varnish?
Desert varnish is a thin, slow-growing coating that forms on exposed rock surfaces.
The name suggests that it only forms in deserts, but rock varnish can develop in a range of climates, including dry deserts and more temperate regions. For this reason, scientists often use the broader term rock varnish.
Rock varnish forms where exposed rock collects dust and minerals from the surrounding environment. Bacteria may also help bind manganese and iron oxides to the surface. Over thousands of years, these materials build into coatings that range from tan and reddish brown to deep black.
Scientists have identified at least 14 types of rock coatings. Desert varnish is one of them, and its exact composition can vary depending on the environment where it forms.
What is desert varnish made of?
Desert varnish is made mostly of windblown clay, along with manganese and iron oxides that give the coating its color.
| Component | Approx. share | What it does |
|---|---|---|
| Windblown clay minerals | 60 to 70% | Forms the bulk of the coating and binds it to the rock |
| Manganese and iron oxides | 20 to 30% | Create the color: manganese darkens, iron reddens |
| Silica and trace elements | Remainder | Includes rare elements that record environmental history |
Desert varnish on cliff in Escalante, Utah | Photo by Donald Giannatti/Unsplash
One of its most unusual features is its high manganese content. Manganese concentrations in desert varnish can be 50 to 300 times higher than those in the surrounding soil.
That difference raises an important question: how does so much manganese end up concentrated in such a thin coating?
Dust and minerals from the surrounding environment settle on exposed rock over time. Bacteria may help bind manganese and iron oxides to the surface, allowing the coating to build up layer by layer. Exactly how this process works is still debated, particularly the role bacteria play.
How does desert varnish form?
Desert varnish forms as dust and minerals settle on exposed rock and gradually accumulate on the surface. The process happens over three general stages:
- Dust and minerals settle on the rock. Wind and water carry these materials to exposed rock surfaces.
- Iron and manganese oxides accumulate. These minerals gradually build up and bacteria may help bind them to the rock.
- The coating develops over time. Continued mineral buildup and chemical changes create the dark, weathered appearance associated with desert varnish.
The process is extremely slow. Over 1,000 years, desert varnish may grow by only 1 to 40 micrometers. That’s far thinner than a human hair.
The color depends on the minerals in the coating:
| Varnish color | Dominant mineral |
|---|---|
| Black | Manganese oxide |
| Red or orange | Iron oxide |
| Tan or brown | A mix of manganese and iron |
Depending on the local mineral composition, some rocks develop almost black coatings, while others take on red, orange, brown or tan tones.
Is desert varnish alive? The bacteria question
Scientists debate how much bacteria contribute to the formation of desert varnish. One question is whether microbes actively help build the coating or whether it forms mainly through chemical processes.
The debate goes back to a 1981 study by researchers Ronald Dorn and Theodore Oberlander, published in Science, which proposed that bacteria play an active role in varnish formation.
Since then, researchers have continued to study how much of the process can be explained by microbial activity and how much results from inorganic chemistry.
A 2021 study from Caltech provided further evidence that desert varnish may preserve chemical traces of microbial activity. The findings suggest microbes may be involved, but scientists are still figuring out what role they play, if any.
Why does this matter so much to scientists?
The answer changes how scientists understand the coating. If desert varnish forms mainly through chemical processes, it is a slow mineral process. If microbes contribute to its formation, the coating may also preserve evidence of microbial activity in harsh environments.
That possibility also matters in the search for life beyond Earth.
If microbial activity can leave recognizable chemical signatures in rock varnish, similar coatings could potentially help scientists identify evidence of past life elsewhere. This is one reason researchers have taken an interest in rock varnish-like coatings observed on Mars.
Petroglyphs in Moab, UT | Photo by Chris Kofoed/Unsplash
Where is desert varnish found?
Desert varnish forms on stable, erosion-resistant rock, often sandstone, in arid and semi-arid regions.
You can find it across the Colorado Plateau and on the Messak Settafet plateau in the Sahara Desert. Similar coatings also occur in coastal, semi-arid and some temperate environments, showing that desert varnish is not limited to deserts.
Mars has also shown potential rock varnish-like coatings, giving researchers another reason to study how these coatings form on Earth.
Why is sandstone commonly coated with desert varnish?
Sandstone is durable enough to remain exposed for long periods, giving varnish time to build up. Its surface also provides conditions that can support the accumulation of the minerals and materials that make up the coating.
Softer rocks may erode too quickly for a coating to remain intact. Harder, smoother surfaces may accumulate less material. This helps explain why sandstone cliffs and canyon walls often show some of the clearest examples of desert varnish.
Messak Settafet plateau | Image credit: NASA Earth Observatory/USGS
Before paper, there was patina
Long before scientists studied desert varnish, people were using its dark surface to create rock art. By carving through the coating, they could create images that would remain visible for thousands of years.
How is desert varnish connected to ancient rock art?
Many petroglyphs, or rock carvings, were made by chipping away the dark varnish to expose the lighter rock underneath. The contrast made the carved lines visible and preserved them on the rock surface.
New Mexico’s Petroglyph National Monument contains about 25,000 rock carvings attributed to Puebloan people and early Spanish settlers. Many show the contrast between the dark varnished surface and the lighter rock exposed by carving.
The Wadi Mathendous archaeological site in the Libyan Desert also contains ancient engravings of wildlife, including rhinoceroses, crocodiles, giraffes, and elephants. Some of these petroglyphs are estimated to be around 8,000 years old, dating to a period when the Sahara was much wetter than it is today.
Petroglyph National Monument | Photo credit: NPS/Dale Kissner
What rock varnish can teach us
Desert varnish may preserve clues about past environmental conditions. Researchers have studied its layers for evidence of changes in climate and other environmental factors, although the extent to which varnish can reliably record those changes is still being investigated.
It also offers a possible record of microbial activity in harsh environments. If microbes contribute to varnish formation, the coating may preserve chemical evidence of life on exposed rock.
Can you recreate the desert varnish look?
Yes! You can achieve a similar earthy, weathered appearance in a fraction of the time.
That’s where Natina comes in. Our patented Reactive Color Treatment develops a natural, earthy finish in weeks. Unlike paint or stain, Natina’s non-toxic, environmentally safe treatment reacts with the material to create a low-maintenance finish with natural color variation.
Natina helps reduce visual contrast across a range of applications, including highway infrastructure, guardrails, retaining walls, wildlife crossings, utility structures, bridges and landscape projects.
| Desert varnish | Natina Reactive Color Treatment | |
|---|---|---|
| Time to develop | Thousands of years | Up to 8 weeks |
| Where it works | Stable, hard rock; primarily arid climates | Rock, concrete and steel; wide range of climates |
| Appearance | Red, orange, brown and black tones influenced by mineral composition and exposure | Natural earth tones with rich browns, gray undertones and organic color variation |
| Maintenance | Naturally occurring; maintenance not applicable | Low maintenance; no painting or sealing required |
Natina is already being used on projects ranging from guardrails and retaining walls to wildlife crossings and park features. Examples include a rock bench in Yosemite and a retaining wall in Shoshone.
“I was very pleased with the results of the solution and found it to be appropriate and beneficial for this type of use.” — Brad Lewis, Yosemite National Park
Explore how Natina’s Steel, Rock and Concrete Color Solutions are being used on projects across different industries at natina.com/projects.
A natural finish in a fraction of the time.
Natina offers a practical way to reduce visual contrast while creating rich, earthy tones on galvanized steel, rock and concrete. With a low-maintenance finish and natural color variation, Natina helps projects meet aesthetic goals without relying on paint or stain.
Have a project in mind? Request a quote or connect with our team to explore how Natina can help.
Frequently Asked Questions
What is desert varnish?
Desert varnish is a naturally occurring coating that forms on exposed rock surfaces over thousands of years. It contains clay minerals, manganese and iron oxides, along with other materials from the surrounding environment.
What is desert varnish made of?
It consists mainly of windblown clay minerals, manganese and iron oxides. It can also contain smaller amounts of silica and other elements.
How long does desert varnish take to form?
Desert varnish forms extremely slowly, often taking thousands of years. In some environments, the coating grows by only a few micrometers per thousand years.
What causes desert varnish to form?
Dust, moisture, minerals and microorganisms can all contribute to the formation of rock varnish. Manganese and iron oxides gradually accumulate on exposed rock surfaces over long periods.
Is desert varnish caused by bacteria?
Scientists still debate the role bacteria play. Some research suggests that bacteria help bind minerals to the rock, while other research suggests the coating could form through inorganic chemical processes. A 2021 Caltech study provided evidence supporting microbial involvement, but the question remains open.
Is desert varnish only found in deserts?
No. Similar rock coatings can form in semi-arid, coastal and some temperate environments. Scientists often use the broader term rock varnish because the coating is not limited to deserts.
Why are some desert rocks black?
Dark or black varnish is associated with higher concentrations of manganese oxides. The mineral composition of the coating affects its color, which can range from tan and brown to red, orange, and black.
Why is desert varnish important?
Desert varnish can provide information about past environmental conditions and microbial activity. It also has cultural significance because many ancient petroglyphs were created by removing the dark coating to expose lighter rock underneath.
How do you recreate desert varnish?
Reactive color treatments can create a similar weathered appearance on materials such as steel, rock and concrete in a matter of weeks rather than thousands of years.
What types of projects use natural-looking rock or steel finishes?
These finishes can be used on highway infrastructure, retaining walls, guardrail systems, wildlife crossings, utility structures, parks, bridges and landscape projects where construction calls for a more natural appearance.
How does Natina help achieve a desert varnish appearance?
Natina develops natural, earthy tones on galvanized steel, rock and concrete. The treatment reacts with the material to create a mottled finish with organic color variation. Full color development can take up to eight weeks, helping new structures achieve a more natural appearance without waiting for nature to do the work.
Sources
- National Park Service, Desert Varnish
- Colorado.edu, Mystery of Desert Varnish
- Earth.com, Messak Settafet Plateau in the Sahara Desert
- UC Riverside, The Edge Institute, Desert Varnish Study
- Dorn, R.I. and Oberlander, T.M., “Microbial Origin of Desert Varnish,” Science, 1981
- Caltech, 2021 study on the microbial origins of desert varnish
- NASA, rock varnish-like coatings on Mars
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