volcanic activity was quite recent, during the current crescentic barchans, snaking siefs, or star shaped rhourds. Some, known as singing dunes, give off a tuneful rustling sound as sand grains brush against each other. It is a siren song for travellers who are not wise to the ways of the desert.

As wonderful as anything in the Sahara are the jagged remains of numerous extinct volcanoes. In the remote interiors of Algeria, Libya and Chad, they form landscapes that could belong on Venus. The volcanic activity was quite recent, during the current era of geological time, and its cause is something of a mystery. It may reflect localized heating of the continental crust, by thermal "plumes" rising through the underlying mantle from as deep as its boundary with the Earth's metallic core.

AYERS ROCK
Ayers Rock in central Australia is said to be the largest geological feature of its kind in the world. Rising to 348m above the surrounding plain and some 9km around its base, it is a monolith created by gradual weathering away of all the rocks that previously enclosed it. It consists of a type of sandstone called arkose, in layers that were originally laid down on the sea floor about 500 million years ago, then lifted up and tilted to near vertical by tectonic stresses.

Why this particular block of arkose survived the destruction of everything around is not entirely clear. Scientific explanations have to compete with more fanciful ones involving ley lines, magnetic forces and mysterious lights. In the aboriginal tradition it was created as it stands, shaped in the time before time, by spirits in the form of animals.

Today's solitary monolith is about 30km away from its nearest counterparts, a cluster of dome shaped hills called the Olgas. It must have taken many tens of millions of years to develop, probably by weathering down of an ancient tableland that has

 

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