Wadi Rum Geology
This short article is about the geology of the Rum area. It describes the various rock types, formations and the stratigraphic section. Since it is for climbers, it also refers to climbing styles, typical holds and anchoring techniques.
תוכן עניינים
Introduction
The Rum area is located east of the Jordan/Dead Sea rift, on the farthest side of the Edom ridge, and near the Saudi-Arabian border. The area is featured by fairly steep mountains, in most cases, nicely separated from one another either by deep canyons, or by wide, flat, sandy rifts.
The overall nature of the cliffs, their steepness and prominence hint at a geologically young region, criss-crossed with impressive joint/crack systems The geological section of the region is pretty straightforward. Three rock types are easily seen. The lower part of the section is made of granite. On the granite, comprising most of the columnar section, there is a fairly hard, fairly dark sandstone. On top of that there is a softer, whiter sandstone.
Those three main geological units can be easily seen on the east face of Jebel Rum:
- The base of the wall is made of Granite.
- Dark, brownish-red sandstone forming the main, impressive wall. Most of the section is made of this type of sandstone.
- White, soft sandstone, forming the curvy domes and the summit plateau.
The formation of Wadi Rum and the cliffs surrounding it are a by-product of the formation of the Jordan/Dead Sea rift, in a series of geological events.
The Geological Section
Wadi Rum itself, as well as the village Rum, are set at an altitude of approximately 900m above sea level. The highest summit in the region, Jebel Rum, is 1754m above sea level. The stratigraphic column is approximately 850m thick. On the east wall of Jebel Rum the thickness of the main units are:
- Granite: 40-50m
- Dark sandstone: ~550m
- White sandstone: ~220m
The second highest mountain in the region, Jebel Um-Ishrin, is only one meter lower than Jebel Rum. Its west wall shows the same rock types, but with different thicknesses:
- Granite: 15-20m
- Dark sandstone: ~550m
- White sandstone: ~250m
The reasons for the differences will become obvious once we discuss the formation of the region.
The General structure of the region
The foundation rock is granite, which is a plutonic rock. That means it solidified from magma (molten rock) deep in the earth's crust. On the Granite there are layers of sandstone, which is a sedimentary rock. The sandstone is made of sand, originating from eroded granite. This whole complex is tilted at an angle of approximately 15° towards the west.
The Rum area is split by a few dry ravines. The two main ones are Wadi Rum and Wadi Um-Ishrin, which are parallel to one another. Both ravines flow from north to south. They define the two prominent massives: The Jebel Rum and the Jebel Um-Ishrin Massive. These are split by a large number of smaller canyons and Siqs, most of which follow joint lines and crack systems with an east-west orientaion.
The formation of the geological section
The plutonic infrastructure
The first stage is the formation of the granite foundation. The solidification of the plutonic rocks from magma, and its uplifting and exposure, were followed by a long, tectonically inactive period. During periods with little tectonic activity, the main process affecting the geomorphology is erosion. That, aided by gravitation (everything just falls down) results in an almost flat plain, called the peneplane (from the greek word "pene" which means almost). Similar low-relief plains representing the final stage of fluvial erosion during times of extended tectonic stability can be seen all over the globe. These are usually the oldest rocks we can find in any region.
The vast plateaus of the peneplane were the product of wide and large rivers, transporting large volumes of alluvium and scree, and eroding the hills into flats.
The sandstones
On top of the granite plain, Dark sandstone (From the Cambrian era, ~600 million years ago) and white sandstone (from the Ordovician and Silurian eras, each ~100 million years younger.
Sandstone is a continental sedimentary rock. That means it is formed in a continental environment. The Nubian sandstone of Rum mostly consists of quartz sand grains, which are basically white in color. The sandstone gets its colors from various metal oxides (mostly Iron) that are a part of the cement, holding the sand grain together to form the rock.
The colored sandstone is called the Um-Ishrin formation, and it can also be found on the other side of the rift, near Eilat, in Timna, and in the Sinai peninsula.
The white sandstone is called the Dissi formation, and is named after the village Dissi, located on the border of the Rum region, north-east of Rum.
Joint sets
Joint sets are formed when the rock yields to stress. Large joint sets are the result of regional stress regimes, and the joint planes are formed perpendicular to the main compressive stress direction. Wadi Rum features numerous joint sets, and the two main sets are oriented north-south (demonstrated by Wadi Rum itself, and the walls on both it's sides, Wadi um Ishrin etc.) and east-west (demonstrated by the Large Siq, on Jebel Rum, the canyons that split the Um-Ishrin massive etc.).
The N-S Joint sets are older, had more time to erode and become wider, and owe their existence to regional stress regime that preceded the Jordan/Dead sea rift. The E-W joint sets are perpendicular to the rift, and are due to the same stress that causes the translocation along the rift
regional tilting
The next stage in the formation of the area as it is today, is a tilting of the whole region to the east. This inclination is associated with the northward movement of the whole Arabian plate, along the Jordan/Dead sea transform fault-line and the opening of the Red sea as a young ocean. The result is that all the sedimentary rock, as well as the plutonic infrastructure, are inclined to the east, causing the outcrops to show more of the lower part of the section on the west of the area, and more of the top of the section in the east. The Granite, at the bottom of the section, shows more at the further west you go. As you progress to the east, you see less and less of the Granite at the base of the section, as it plunges into the underground.
In the Rum region, on the west faces of Jebel Rum, the granite outcrops reach some 80-100m, on the east face, it is only about 50m. Eastward still, on the west face of Jebel Um-Ishrin (on the east side of the Wadi, the Granite outcrops drops to ~20m only, and on the other side of the Um-Ishrin massive, on the east faces, there is no more granite to be seen, as it is all buried in the sand of the Wadi Um-Ishrin.
Further to the east, the Granite cannot be seen at all.
Erosion
The last stage, which is ongoing and is constantly morphing the relief we see today, is Erosion. Erosion forms both the large scale features such as cliffs, siqs and wadis, as well as smaller scale features like holds, chicken-heads, sand-clocks, spikes and threads.
Holds, Anchors and more
We already mentioned the distinction between large and small scale features. These features also differ in the exact process of erosion that forms them.
Erosion processes strongly depend on climate. Arid environment erosion is not (contrary to common beliefs) caused by water or wind. The main factor in desert erosion is salt. Salt is a highly corrosive substance, and the lack of water in dry regions means that the salts cannot be washed away. It remains on the surface of the rock and slowly corrodes it.
If we look at the yearly distribution of precipitation in Rum, we'll find that it does not rain a lot, maybe 2-3 rainy days a year. The rare rainfall events are brief, powerful bursts, after which the rock dries quickly. This nature does not allow for water to become a significant factor in erosion.
The amount and temporal distribution of precipitation does not allow for the constant removal of the salt from the rocks either. In this way, water, or the lack of it, becomes a secondary and indirect influence on erosive processes.
The Corrosiveness of salt can be seen on walls close to the see, where there is always some salt water in the air. The walls facing the sea are always more eroded. That is because there is constant supply of fresh salt to do the job.
Large Scale Erosion
Large scale erosion affects the whole region. These processes take advantage of existing large weakness lines and works them. In most cases, these weakness lines are joints of the perviously mentioned, already existing ,joint sets. This explaines the general layout of the region, where the two large wadis (Rum and Um-Ishrin) are parallel to one another, and so are the east and west faces of both massives. The two large Rock masses (Jebel Rum and Jebel Um-Ishrin) are divided into smaller mountains by a number of narrow canyons and siqs, all parallel to one another and perpendicular to the main wadis.
The crumbling of large blocks leave steep walls in their stead. The smooth, black rock covering some of the sandstone faces in Rum are actually the remaining joint plane, where the other side came off. On the joint plane itself, a pattern of concentric circles is common. This pattern is the marking of the progress of the fracturing process, which generated the joint.
At later stages, the salt corrosiveness works on the surface and breaks smaller chunks from the main block. The smaller the clocks - the more surface area there is for the salt to work on.
The flow of water can help evacuate the fallen blocks. It doesn't rain much, but when it does, a flash-flood event occurs. All the water is channelled into cracks and flow there with high energy. The strong current removes all the loose rock and wash the salt from the walls. In these areas, such as the Big Siq, salt induces erosion is less significant. On the other hand, since most of the water flow in cracks, the rest of the rock is almost never washed.
Small Scale Erosion
Small scale erosion is local and is highly influenced by local conditions: thin cracks, broken area and wall steepness affect the local flow regime. It is interesting to note, that on steeper walls, even overhanging sections, the water prefer to flow in "drain pipes" rather that n distribute evenly on the wall. This means that on most of the wall, the salt remains, and erosion is more intensive.
The erosion is especially rapid on walls with obvious drain lines. The water flow through them, and leave the rest of the rock face always dry. The eroded areas becomes a depression, and the drain lines remain protruding. The phenomenon is called Tafooni (which mean "windows" in Greek) and is common to all rock types in arid, desert areas.
When two such depressions grow larger, they can join to form typical features. When the barely connect, sand-clock, or vertical threads appear. these threads make excellent rappel anchors, and usually appear under small overhanging sections.
As erosion progresses, more of the rock face is missing, leaving the original joint plane as holds with protruding edges. These holds can differ dramatically in quality between different areas on the face. The more protruding the edges, the bigger chance there is that the climber will shoot off the wall, with a chunk of rock in his hand.
At the last stage of erosion, most of the rock face becomes depressed. In stead of dark, compacted, solid rock, we get lighter, sandy rock, with protruding bits that are all that's left of the original joint plane. These are called chicken-heads. Chicken-heads can sometimes serve as an anchor, usually by wrapping a webbing around it. Chicken-heads can also be very loosely connected pieces of junk...
Contributions to this page were made by Mica Yaniv and others...
