Google Maps is a great resource, particularly in satellite view. My favourite way to enjoy it is via the Chrome extension “Earth View from Google Maps“. This pops up a gorgeous image in every new tab. Many show human landscapes, but every now and then one appears that catches this geologist’s eye. This post is the first in a series exploring and celebrating these images.
This view is of the Rub’ al Khali or ‘Empty Quarter’ of the Arabian Peninsula – the largest sand desert in the world.
In dry environments sand is moved not by water but by wind. The characteristic landform is the sand dune. Common in deserts on earth, they are also found on Mars and even comets.
The shapes of dunes depends on the supply of sand, but above all the wind. Wind strength and direction, averaged over the year, will determine the shape of a dune. Various types of dune are recognised. The ones in this image are complex. Further east of here the dunes are clearly linear features, but here the lines are broken up into loops reminiscent of arabic script.
Look carefully and you’ll see that there are dunes upon dunes. The surface of the large sand bodies are covered in ridges and patterns with a wide variety of shapes. These themselves may have small ripples on, only visible if you visit in person.
This image is from the ‘Grand Erg Oriental’ in the Sahara desert in Algeria. It shows star dunes, which form when the wind is variable and simply piles the sand up into mounds 100s metres tall.
Both these areas of sand (by chance) sit above oil-fields, the oil-bearing rocks sitting deep under the ground. Ancient desert sands are of interest to oil geologists as the grains are very round and form sandstones that can contain a lot of liquid.
Ancient ‘aeolian sandstones’ are basically fossilised sand dunes and are often red. The ‘red sandstones’ of the UK, much Triassic sandstone in Europe and the classic Navajo Sandstone in the US are of this type.
Next time you see a red sandstone with big swooping cross bedding, think of these pictures.