Tafoni in Dronning Maud Land – a unique and mysterious geological feature

A spectacular weathering phenomenon can often be seen in the Antarctic mountains of Dronning Maud Land. Abundant holes and cavities form on rock surfaces in one of the most peculiar expressions of bedrock decay. These holes are called tafoni.

By: Synnøve Elvevold, Ane K Engvik*, Øyvind Sunde and Per Inge Myhre // Norwegian Polar Institute *Main affiliation Geological Survey of Norway

Tafoni are natural ellipsoidal to bowl-shaped holes and cavities in weathered rocks. The holes usually occur in groups and range in size from < 10 cm to > 1 metre, and typically develop on steep exposed rock surfaces. Tafoni are widely distributed around the world and found in diverse climatic zones ranging from hot and cold deserts to moist coastal regions. These strange and interesting landforms form in many different rock types; they are common in sandstone, limestone and in crystalline rocks such as granite and gneiss.

Small pits and huge caves

Antarctica provides a unique opportunity to study tafoni in a pristine environment without the increased complexity that flora and fauna may have on their development. We have observed wide-spread tafoni in central Dronning Maud Land during recent field expeditions. Spectacular examples of tafoni were found on vertical and inclined rock surfaces, as well as on boulders.

Tafoni are also extensively developed on high-altitude plateaus. In the area surrounding the Troll research station, tafoni are typically found in granites and gneisses. These rocks were originally formed as part of a large mountain chain that existed here around half a billion years ago.

The distribution, size and shape of tafoni vary. We think one reason for this is because they change over time, and we observe them at different states of their evolution. The cavities generally occur in groups, are round to oval in shape, and range in size from small pockmarks to huge caves. Intersection of cavities may result in pitted surfaces, which we see at high elevations. Where large caverns have formed we often observe crumbling and flaking of the rock surface, and some hollows are filled with this type of debris. A white coating of salt is locally present on the surface of the bedrock, as well as inside tafoni hollows. And the rock can become hot. Using miniature loggers we have measured surface temperatures as high as 20-25°C on sun-exposed north-facing rock surfaces, even though the air temperature was below zero.

Tafoni – a geological mystery

There has been a great deal of scientific pondering and analysis on how tafoni are formed, but there is no single explanation that can account for all the various occurrences. Although a fair amount of research has been done on tafoni, and numerous explanations have been proposed, the formation of tafoni can still be said to be a geological mystery. Currently, cavernous weathering is considered to be the result of a complex interaction of physical and chemical weathering processes.

Tafoni result from both decay and erosion. Strong katabatic winds, which commonly sweep down the slopes of Dronning Maud Land, facilitate the weathering because they transport sand and gravel particles. Salt inside tafoni is commonly associated with flaking, and this suggests that salt weathering is a contributing mechanism in their development. Salt crystals increase in volume as they form, which produces significant pressure forcing the rock to crumble. Solar radiation can result in large surface temperature fluctuations, and this causes thermal expansion and contraction of the rock. This thermal-induced stress can lead to fatigue and failure of the rock. Temperature variation also plays an important role because it controls moisture availability through drying and wetting cycles.

One aspect we are particularly interested in, is how the structural, textural and mineralogical properties of the rock can potentially influence weathering and cavity development. We have observed that small amounts of radioactive minerals and chemical breakdown of the mineral pyroxene to iddingsite, cause micro-fracturing which facilitates freeze–thaw actions and salt crystallisation. This enhances weathering.

The development of cavities is also a function of the amount of time the rocks are exposed to the physical effects of wind and temperature variations. Recent cosmogenic age dating shows that the highest mountain peaks in the eastern part of Dronning Maud Land have been exposed for the past 3-4 million years.