Rocks and Deserts

    This journal is all about desert processes that involve bouncing (sand grains blown by the wind), growing (salt crystals growing as brines evaporate), and fracturing (thermal fracture caused by temperature changes). These processes produce unique features that are found in rocks located in the cold arid deserts of Antarctica and Mars. Listen to my short video introduction below, then take a visual tour of several rocks in the photographs below, and learn about some interesting features found in the rocks and the processes that modify them.

    http://youtu.be/F0FGuXaOByQ

    The following photographs show progressive alteration of Ferrar Dolerite rocks via wind erosion, salt weathering, and thermal fracture. The Ferrar Dolerite is one of the most common rock types found in the cold arid Dry Valleys of Antarctica. It is an intrusive igneous rock formed during the rifting of Gonwana approximately 178 million years ago.

    Wind

    Photograph of progressive wind erosion of Ferrar Dolerite.
    The photograph above shows the effects of progressive wind erosion on Ferrar Dolerite in central Beacon Valley, Antarctica.

    The top left photograph shows the initial effects of wind erosion on a cobble of Ferrar Dolerite in central Beacon Valley. The wind initially produces a ventifact (a rock that has been shaped by wind erosion). Ventifacts show smooth surfaces polished by wind-transported sand grains that bounce along the ground and impact rock surfaces (saltation). Over time, saltating sand grains produce a smooth surface that provides researchers with an indication of the dominant wind direction. Can you tell which direction the wind blows in the top left figure? The answer is not as easy as you might think. First note that the rock shows a dull side on the right and a shiny side on the left. The shiny side is weathered via a process is similar to rust forming on metal). This weathered surface has been removed on the right side of the rock via the erosive impact of saltating sand grains. Hence the dominant winds are from right to left. Top right: in this example, the wind that flow across the rock surface is highly turbulent, creating spiral currents that create elongated grooves and shallow depressions. Here again, erosion and polishing is caused by wind-transported sand grains.

    Salt

    Photograph of rock pits in Ferrar Dolerite.
    This photograph shows the progression of rock pits on the surface on dolerite in central Beacon Valley, Antarctica.

    Over time, the shallow depressions produced by wind erosion at the surface of rocks become sites for minor accumulation of snow. When the rocks are exposed to intense sunlight, they warm and the snow collected in depressions begins to melt. This melt then evaporates leaving behind tiny traces of salt. The amounts are too little to see after a single snowfall, but over millions of years the salt is visible as thin encrustations at the base and sides of pits. As these salts grow, dissolve, and reform, they break down the underlying rock, creating ever deeper pits that attract more snow and more meltwater. In this way, surface pits grow larger over time. Dave and his team have determined that the pits in certain medium-grained dolerite (like the one pictured on the upper left) increase in depth and width by about 10 mm per million year. If correct, the rock pictured here has been exposed on the ground surface for about 5 to 8 million years! Dave also suggested that some of the meltwater migrates around the rock surface and collects underneath the rock. As an eerie test of this hypothesis, I picked up the rock and found a little pile of white salt at the base of the rock. The coarse-grained dolerite (in the photograph on the right) exhibits crystals > 4 mm in size. The pits do not show a steady growth rate over time since the larger crystals are more susceptible to salt weathering. Still, this rock, with its pit up to 10 cm deep and wide, has likely been exposed at the surface for many millions of years.

    Pits in rocks on the surface of Mars have been observed at several locations by the Viking Lander 2 and at rover locations. The pits in the rocks below are approximately 1–2 cm in diameter and may have been formed by the same, or similar, processes that operate on rocks in Antarctica.

    Photograph of pitted rocks on surface of Mars.
    Pitted rocks on surface of Mars. Image modified from Head et al, , 2011.

    Compare the pits on these rocks with those shown above. Is this evidence of snowfall on Mars? What other processes might produce such features? For more information, see Pitted rock surfaces on Mars: A mechanism of formation by transient melting of snow and ice and Antarctic dry valleys:Microclimate zonation variable geomorphic processes and implications for assessing climate change on Mars). Examine the triangular rock in the back right: could this be a ventifact? What would you look for to prove/ disprove this hypothesis?

    Thermal Fracture

    Photograph of thermal fractures on Ferrar Dolerite.
    Photograph of thermal fractures on Ferrar Dolerite.

    Even in the absence of snow, minor melting, and wind, rocks can still undergo physical breakdown in desert regions. The photograph above shows a close-up view of a medium-grained Ferrar Dolerite cobble (cobbles range in size from approximately 2.5 – 10 inches diameter) that shows the effects of thermal fatigue. If you look carefully, you will see that the outer, dark surface at the top of the rock (weathering rind) has been removed in places, revealing a light-colored stain. Unlike the pitted ventifact described above, the responsible process is not wind erosion. Instead, the culmination of thousands to millions of years of temperature changes in the outer few mm of the rock creates stresses that ultimately result in fractures that remove mm-scale flakes from rock surfaces. How does this happen? When heated, most materials expand; and when cooled, materials contract. Alternate heating and cooling of the rock surfaces creates stresses that overtime cause the rock to fracture. These fractures, parallel to the rock surface and only mm deep, ultimately remove thin flakes of weathered rock (the dark portion of the rock above). Once exposed, the “fresh” rock undergoes progressive staining and weathering and the process continues.

    Rock Detective

    Given the above, and thinking outside the box a bit, can you suggest a plausible theory that explains how the coarse-grained dolerite (pictured below) may have been modified? There are many possible answers. Be creative and come up with your own history of rock erosion!

    Photograph of dolerite in central Beacon Valley, Antarctica.
    Photograph of dolerite in central Beacon Valley, Antarctica.

    Date
    Location
    Central Beacon Valley, Dry Valleys, Antarctica