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 to 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 by either deep canyons, or by wide, flat, sandy rifts.
The over-whole nature of the cliffs, their steepness and prominence hint at a geologically young region, criss-crossed with impressive joint/crack system. 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 and occupying 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 are obvious, once we discuss
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 are split by a large number of smaller canyons and Siqs, most follow joint lines and crack systems with east-west orientaion.
The formation of the geological section
The plutonic infrastructure
The first stage is the formation of the granite foundation. After the solidification of the plutonic rocks from magma, and it's uplifting and exposure, a long tectonically inactive period followed. During periods with little tectonic activity, the main process affecting the geomorphology is erosion.
That, helped by gravitation (everything just falls down) results in an almost flat plane, called the peneplane (from greek: pene-almost). Similar low-relief plain 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 peneplain 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 is mostly consisted of Quartz sand grains, which are basically white in color. The sandstone gets its colors from various metal oxides that are a part of the cement, holding the sand grain together to form the rock, mostly Iron oxides.
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, that is 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 plains 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 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 and the opening of the red sea. The result is that all the sedimentary rock, as well as the plutonic infrastructure, are inclined to the east, causing the outfrops 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, is
Regional tilting
The next stage is the inclination of the whole region towards the east. This is strongly related to the movement of the whole Arabian plate northward along the Jordan/Dead sea transform fault-line and the opening of the Red sea as a young ocean. The results are that the whole complex of plutonic as well as sedimentary rock mass is tilted towards the east. This is the reason why stratigraphic sections differ from one another. The more you go west, the more granite you'll see. 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. More to the east, 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 still goes on and constantly morphing the relief we see today, is Erosion. Erosion forms both the large scale feature such as cliffs, siqs and Wadis, as well as smaller scale features like holds, chicken-heads, sand-clocks, spike 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. When raid does come, ii happens in brief, powerful bursts of rain, 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, become a secondary and indirect influence on erosive processes
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, Those weakness lines are joints of the already existing joint sets tat were mentioned before. This explains 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 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 plain, where the other side came off. On the joint plain
התמוטטויות של בלוקים גדולים משאירות במקום הגושים שנפלו מצוקים תלולים. הסלע השחור הקשה שמכסה פייסים תלולים בראם (כמו הפייס של ג'בל אום-אג'יל, או הפייס של הקיר המרכזי) הוא מישור של סדק, שאחד הגושים שלו התמוטט. על מישור הסדק ניתן לראות במקרים רבים דגם של עיגולים קונצנטריים שהוא עקבות היצירה של מישור הסדק המקורי.
בשלב מאוחר יותר זרימה של מים עוזרת לפורר ולפנות את הגושים שנפלו. בשעת אירועים שיטפוניים עוברות בסדקים העיקריים כמויות גדולות של מים בזמן קצר, ובאנרגיה גבוהה. שיטפונות אלו מסלקים כל סלע רופף שקיים בסדקים, ושוטפים את המלח מהם. בתוך סדקים כאלו, כמו בסיק הגדול, הבלייה כתוצאה ממליחות על הסלע היא משמעותית פחות.
Small Scale Erosion
בליה בקנה מידה מקומי תלוי ומושפע במידה רבה מתנאים מקומיים: סדקים קטנים, קוי חולשה ותלילות של המצוק באיזור מסויים משפיעים כולם במידה רבה על זרימת המים המועטים שנמצאים שם. דווקא על קירות מאד תלולים, אפילו שליליים, המים מתנקזים למרזבי זרימה ואינם שוטפים את כל הקיר. במקומות שבהם זורמים פחות מים, מצטבר מלח, והוא מאכל את הסלע. התהליך דומה לזה המתרחש לקירות בתים הסמוכים לים, שגם עליהם מצטבר מלח מרסס הגלים.
הבליה מהירה ביותר על קירות שיש עליהם קוי זרימה מובהקים, שכן בקוי הזרימה הללו עוברים כל המים, ושאר הקיר אינו נשטף לעולם. האיזורים שמתבלים הופכים שקועים, וקוי הזרימה של המים נשארים בולטים מן הקיר. התופעה המתקבלת נקראת טפוני (Tafooni - חלונות ביוונית) והיא טיפוסית לכל סוגי הסלעים בסביבות מדבריות.
כאשר שני חלונות (או יותר) כאלה מתחברים, הם יוצרים פיצ'רים טיפוסיים. אם החיבור הוא ראשוני, רק נוצר חור ביניהם, מקבלים שעוני חול, או גשרים אנכיים. גשרים אלה הם עיגונים מצויינים לגולש אחרון, ומצויים לרוב מתחת לצנירים וגגות קטנים.
כאשר הבליה מתקדמת יותר, מתקבלים פייסים שעליהם מעין מדפונים עם שוליים בולטים, השאריות של פני הסלע המקוריים. אחיזות אלו עשויות להיות שונות בטיבן באופן דרמטי בין איזורים שונים על הפייס, וככל שהשולים של האחיזה בולטים יותר, יש יותר סיכוי שהמטפס ימצא את עצמו מתעופף עם האחיזה ביד.
בשלב מתקדם עוד יותר מתבלים כמעט כל פני הסלע המקוריים, ואת השאריות אנו מכנים בשם ראשי תרנגולות (chicken heads). גם גושים בולטים כאלה עשויים להיות עגינה מצויינת (אם מקיפים אותה ברצועה, למשל) או כזו שאפשר לתלוש ביד.
תרמו לדף זה: מיכה יניב ואחרים...
