When you decide to visit the desert, you usually have to go off the pavement, onto a dirt road. Sooner or later you arrive in the brightness and space that you came for. And if you turn your eyes from the distant landmarks around you, you may see another kind of pavement at your feet, called desert pavement.
A Street of Varnished Stones
It's not at all like the drifting sand that people often picture when they think of the desert. Desert pavement is a stony surface without sand or vegetation that covers large parts of the world's drylands. It's not photogenic, like the twisted shapes of hoodoos or the eerie forms of dunes, but seeing its presence on a wide desert vista, dark with age, gives a hint of the delicate balance of slow, gentle forces that create desert pavement. It is a sign that the land has been undisturbed, perhaps for thousands—hundreds of thousands of years. To see some examples, have a look at the Desert Pavement Gallery.
What makes desert pavement dark is rock varnish, a peculiar coating built up over many decades by windblown clay particles and the tough bacteria that live on them. Varnish has been found on fuel cans left in the Sahara during World War II, so we know that it can form fairly fast, geologically speaking.
What Creates Desert Pavement?
What makes desert pavement stony is not always so clear. There are three traditional explanations for bringing stones to the surface, plus a much newer one claiming that the stones started out at the surface.
The first theory is that the pavement is a lag deposit, made of rocks left behind after the wind blew away all the fine-grained material. (Wind-blown erosion is called deflation.) This is clearly so in many places, but in many other places a thin crust created by minerals or soil organisms binds the surface together. That would prevent deflation.
The second explanation relies on moving water, during the occasional rains, to winnow out the fine material. Once the finest material is splashed loose by raindrops, a thin layer of rainwater, or sheetflow, sweeps it away efficiently. Of course both wind and water could work on the same surface at different times.
The third theory is that processes in the soil move stones to the top. Repeated cycles of wetting and drying have been shown to do that. Two other soil processes involve the formation of ice crystals in the soil (frost heave) and salt crystals (salt heave) in places with the right temperature or chemistry.
In most deserts, these three mechanisms—deflation, sheetflow and heave—can work together in various combinations to explain desert pavements. But where there are exceptions, we have a new, fourth mechanism.
The "Born at the Surface" Theory
The newest theory of pavement formation comes from careful studies of places like Cima Dome, in the Mojave Desert of California, by Stephen Wells and his coworkers. Cima Dome is a place where lava flows of recent age, geologically speaking, are partly covered by younger soil layers that have desert pavement on top of them, made of rubble from the same lava. Obviously the soil has been built up, not blown away, and yet it still has stones on top. In fact, there are no stones in the soil, not even gravel.
There are ways to tell how many years a stone has been exposed on the ground. Wells used a method based on cosmogenic helium-3, which forms by cosmic ray bombardment at the ground surface. Helium-3 is retained inside grains of olivine and pyroxene in the lava flows, building up with exposure time. The helium-3 dates show that the lava stones in the desert pavement at Cima Dome have all been at the surface the same amount of time as the solid lava flows right next to them. It's inescapable that in some places, as he put it in a July 1995 article in Geology, "stone pavements are born at the surface." While the stones remain on the surface due to heave, deposition of windblown dust must build up the soil beneath that pavement.
For the geologist, this discovery means that some desert pavements preserve a long history of dust deposition beneath them. The dust is a record of ancient climate, just as it is on the deep sea floor and in the world's ice caps. To those well-read volumes of Earth history, we may be able to add a new geologic book whose pages are desert dust.