Desert Rains

The Namib Desert in Africa is one of the driest places on Earth. The coastal town of Swakopmund receives less than 20 mm of rain each year. However, since 2010 rainfall has been above average in many parts of sub-Saharan Africa. These tropical-sourced systems occur as severe storms that cause local flooding. River banks burst and flood waters erode structures in their path.

A large flood occurred during May of 2011 when Swakopmund received double the annual average rainfall in one day (42.2 mm). Desert floods are exceptional geomorphological events that leave a physical mark in the landscape that can be preserved in the regions geological record.

One of the more subtle geological signatures of rainfall are raindrop imprints. These are small circular to sub-circular, shallow impressions formed on fine-grained siliciclastic, soft sediment surfaces. They are formed by the impact of falling rain. They may be associated with mud cracks and terrestrial fossils.

In August 2011 Dr Mary Bourke visited a dry river channel south of Swakopmund, Namibia where the effects of the recent flood were clearly visible. Although the floodwaters had ceased to flow, the flood sediment in the river channel had desiccated forming an extensive polygonal network of desiccation cracks (Image a). The surface of those sediments had raindrop imprints preserved (Image b). The mud cracks and raindrop impacts will be preserved in the geological record if the surface is rapidly buried, either by another fluvial deposit or by drifting aeolian sands.  An example of preserved features is shown in Image c. If the features are not preserved by sediment burial they will be subject to wind erosion and disappear.

a) Desiccated river bed sediments south of Swakopmund, Namibia. b) Raindrop imprints formed by rainfall on soft muddy river sediments. c) An example of rainfall imprints preserved in the geological record. Image from University of Tennessee
a) Desiccated river bed sediments south of Swakopmund, Namibia.
b) Raindrop imprints formed by rainfall on soft muddy river sediments.
c) An example of rainfall imprints preserved in the geological record. Image from University of Tennessee

Authored by Dr Mary Bourke, Earth and Planetary Surface Processes Group, Geography, TCD

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