Tuesday, January 29, 2013

Sand Dunes - Intro to Geomorphology

Geomorphology is the study of land-forms: how did the land get those shapes, what are the processes behind them, and how can we use that information in a practical way? 

For instance, why are the Appalachians really rather low hills compared to the Rocky Mountains? We understand from geologic mapping that the Appalachians were once as high as the Rockies, if not taller. However, they have been under a weathering regime (water and wind erosion) for far longer. The active building phase in the Appalachians stopped about 300 million years ago in the early Permian era – and they have been eroding ever since. The Rocky Mountains, on the other hand, were uplifted between 100 and 65 million years ago. In this case, the rounded shapes simply mean that the Appalachians are much older. There are sedimentary units in Venezuela that are dated about 1.7 billion years old that are weathered remains of even more ancient mountains that no longer exist – they have been weathered flat.

A good example of geomorphology and what it can teach us is exemplified by the following question and answer sequence, sent to the USGS Ask-a-Geologist website. In deserts worldwide there are different kinds of sand dunes, with many different and distinctive shapes. The Empty Quarter of the Arabian Peninsula is loaded with many of the most classic dune-types: Saif (sharp-edged “sword”) dunes, ‘Irq (200-kilometer-long “vein”) dunes, Barchan (huge horn-edged, crescent-shaped) dunes. There are also more complex products like Star and Zibar dunes, typically bridging one dune-type region and another. There are also lag-gravel plains – flat areas covered by small pebbles that keep the diurnal wind-storms from moving the sand beneath them.

Careful examination of sand textures (fine, clay-like particles vs beige quartz sand, for instance), and dune shapes leads to an understanding of the annual wind regimes in these areas. On any given day, there is a diurnal sand storm: at dawn the wind is dead still, but around 10am it starts to pick up. By 5pm, when we would be trying to set up our tents, the wind would be shrieking along at 30-40 knots, blowing stinging sand at eye-level. Every hour we would wake up during the night, because the humidity was ~2% and our throats would be on fire. The first thing we did was brush the dust and sand from our eyes, THEN open them, THEN take a drink from our coolers. The wind would die down to nothing sometime after midnight, and we would find trails of scorpions and dung-beetles (and camel spiders trying to find a way into our tents) that were brand-new and fresh from just the previous hours of the night. 

There are also seasonal changes, and can be more important than you might think. Dunes in the northern Empty Quarter show that there is an annual 10-month wind-pattern that reverses direction and ramps up in speed and intensity in February and March every year. These changes correlate with the Monsoon season in the nearby Arabian Sea. This 2-month-long sand-storm period (the “Khamsin Season”) would have shut down 1990’s Desert Storm war in its tracks. This forced US military planners to rush the start of the fight to reclaim Kuwait from its Iraqi invaders in January, 1991. 

These sandstorms can be truly terrible things, shutting down all human movement for days at a time, stinging your face and literally suffocating unprotected humans foolish enough to try to walk about in it.  I once flew in a Boeing 737 jet on a scheduled flight from Jeddah, Saudi Arabia to ‘Ar-‘Ar in the north near the Iraqi border. The pilot circled the town three times, but we couldn’t SEE the town: ‘Ar-‘Ar was completely obscured by flying sand and dust. The pilot abandoned his attempt to land and we flew instead 500 kilometers farther west to Tobuk, where we were forced to re-plan our entire phosphate resource exploration and mapping effort. A few years later I flew on the single remaining commercial aircraft then owned by Air Mauritania… and learned that the only other Fokker aircraft had crashed just two weeks earlier while trying to land in a sand storm that severely damaged its jet engines. I noted as we landed at Nouakchott (the capitol of Mauritania; the name comes from the Berber language for “place of the winds”) that a sand-storm was already raging. The flight had people standing in the aisles the entire time – it was severely overloaded – and the pilot indicated that he was running out of fuel.

These monster sand storms are not all bad. We finally made it to ‘Ar-‘Ar, but the dust hung in the air for many days. It allowed me to look directly at the setting Sun without eye-protection as we carried out our geophysical well-logging efforts. At that time (1994) there was a huge sunspot cluster that migrated from the upper left quadrant to the lower right quadrant of the afternoon Sun - I sketched it as it evolved over four days in my field notebook. Without using any astronomical instruments, this meant that I could determine the spin axis of the Sun: it is oriented at about 45 degrees to your upper right as you look to the west. I could also roughly determine the Sun’s rate of revolution – about 10 days – using just the naked eye. This must have been possible centuries ago, but I’m unaware of any ancient scientists figuring this out.

Subject: sand dunes
I am displaying a photo below of sand dunes in Saudi Arabia, in the Empty Quarter near UAE (Arabian Peninsula). They are from Google Earth and were 'taken' from 8 km high. I have tried to read your publications on classification of sand dunes so I could identify these dunes, but alas, I failed!!!  
Can you tell me what kind of dunes these are and from which way was the wind blowing to generate them?  
- Hale S

You didn't fail, you're just looking at a transition zone there... in the mid-distance you have merging Barchan dunes (wind is blowing from the lower left of your image, and the “horns” of the Barchan point in the wind direction). In the near distance the Barchan dunes are transitioning to Saif (sword) dunes (the wind is coming from the left side of your image). I actually did a traverse across the Empty Quarter, not too far west of where this image comes from, on an expedition from Yemen to Dhahran to map the Wabar meteorite impact site.  There is an awesome USGS book called "Global Sand Seas" (Professional Paper 1052, Edwin D. McKee, Editor) that will give you quite a bit more information, including provenance (sand source areas) and wind-rose diagrams.

 Thanks for your reply -- now I'm going to think about your answer and try to digest it - on top of a Thanksgiving dinner! That is a LOT!!! 
I thought I recognized your name -- you and Gene Shoemaker investigated the (Wabar) crater and you wrote the paper after Gene died (“The Day the Sands Caught Fire”, by Jeff Wynn and Gene Shoemaker, Scientific American, November 1988 issue). I have read it and enjoyed it. Good job! I would give an arm and a leg -- well, maybe a fingernail -- to spend time in the Empty Quarter, photographing and looking at the area. As you might tell, I am a desert nut!
I found the paper by McKee et al on the Net and have started reading at it...  It is a long one!!Many thanks

The McKee book is great - typical quality for a USGS Professional Paper - Enjoy!  Yes, I would stop everything to jump in on another Empty Quarter expedition.  The Zahid people, the corporation that engineered (literally and figuratively) the three Empty Quarter expeditions I participated in, told me privately that the cost added up to about $10,000+ per seat for each expedition. The Empty Quarter is a truly amazing place; humidity routinely drops to ~2% there. I once completed a magnetic profile survey at Wabar when the temperature at mid-day reached 61 degrees Centigrade (142 F). I am a "desert junkie" myself, now living in the soggy Pacific Northwest.  I also maintain a website about the relatively rare Empty Quarter expeditions: http://www.empty-quarter-expeditions.org