Photos: Jordanie: Roches de Petra / Jordan: Petra Rocks / 2005

Petra is one of the world's most beautiful architectural sites carved from some of the oldest exposed sandstone on Earth (Cambrian and Ordovician sanstone that is 450-550 million years old). However, the sandstone in Petra is typical in that it is comprised of colorless quartz clasts (the particles in sandstone) attached by a matrix binding that is composed of silica (Si), calcium (Ca), iron (Fe), aluminum (Al), manganese (Mn) and minor amounts of others (sodium (Na), lithium (Li), etc.). So the colors are only a function of colorants present in the matrix: browns from iron, yellows from Na and Fe, pinks from Na and Li, etc.

These minor matrix elements (ie. Ca, Mg, Mn, Fe, etc.) have mobilized and separated into what is commonly called 'Liesegang Banding' (banding in color and composition of ores caused by diffusion). However, the banding has been erroneously (and commonly) attributed to its deposition, but in fact when examined (and often simply observed) the original bedding (from coastal fluvial and dunal deposition) is unrelated to these spectacular colors that are not to be seen in the same way anywhere else on Earth.

In fact the colors are covered by a sort of 'drapery' given by recent and secondary deposition of simple calcium carbonates that are relased from the matrix and then redeposited atop and below the sandstone. This drapery is not very durable and is easily broken-off or eroded.

For over 15 years, Professor Thomas R. Paradise (website), Associate Director of the Department of Geosciences of the University of Arkansas, Fahd Center for Middle East and Islamic Studies (website), ) has been studying both lithologic (intrinsic) and climatic and anthropogenic (extrinsic) weathering influences on the Paleozoic sandstones in Petra, an ideal environmental “laboratory” for the study of weathering features, causes, and rates. He has shown that Petra structures, which have been stable since their creation two thousand years ago, are deteriorating at an accelerated rate due to natural and human-induced stone decay processes, most notably due to increasing regional and global tourism.

Source: communication (June 26, 2005) with Tom R. Paradise (personal website) and Thomas R. Paradise, Petra revisited: an examination of sandstone weathering research in Petra, Jordan, Geological Society of America, Special Paper 390, 2005, pp. 39-49. See also "Weathering of Sandstone Architecture: The Deterioration of Sandstone Architecture in Petra, Jordan", by Thomas R. Paradise. On Petra, its geography, its history, and its monuments, see Ancient Route.com.