Some geological notes for the climber in South Sinai
(by Andrea Anati)
The South Sinai massif is part of the Arabo-Nubian massif. Its mounts are mainly magmatic, that is, originate from the magma, or the molten rock below the earth's crust.
Towards the end of the Precambrian, around 600 million years ago, large bodies of molten magma began intruding upwards toward the surface, cooling, and thus solidifying into rock.
The many kinds of rock found in the climbing region of the South Sinai massif, formed by the cooling process mentioned, may be classified into three main groups:
1. Plutonic granites, were formed deep beneath the surface. The cooling was relatively slow, and their crystals are typically large. Jabel Rabba, Jabel Safsafa and Jabel Er-Deir are examples of mounts of this type of granite.
The typical structure of a Sinai granite mount is:
The upper 1/4 - round and smooth dome top.
The middle 1/2 - steep cliffs.
The lower 1/4 - Talus and scree.
2. Volcanic rocks, where formed of magma which erupted to the earth's surface, cooled quickly, and their crystals are too small to be distinguished by the naked eye. J. Katerina, J. Musa and (the upper portion of) J. Abbas Basha are examples of mounts of volcanic rocks.
The typical structure of a Sinai volcanic mount is:
The upper 1/3 - steep, fractured, sharp edges
The lower 2/3 - Talus and scree
3. Dykes, where formed by magma which broke its way upwards in cracks and fissures. It cooled rather quickly, but at a slower rate than volcanic rocks. Dykes are typically of crystal size between plutonic granites and volcanic rocks. Dykes can be seen in great numbers and in a variety of sizes throughout the South Sinai massif. Sample.
The typical structure of a dyke is either a gully with vertical walls and fractured base, if its erosion is more intense than in the hosting rock, or an outcrop, if its erosion is lower than in the hosting rock
Other types of rock, such as metamorphic, sandstones, or limestone, although found in other areas of Sinai, are quite absent from the South Sinai massif, and their structure is not relevant in the context of the present climbing logbook.
After their formation, these magmatic rocks were deeply eroded, and then covered by layers of sedimentary rocks for most of their subsequent lifetime. These sediments were formed either at the bottom of a rather shallow ocean, in which case limestone layers and their relatives are found, or on the continent itself, in which case sandstone is found. In the following sketch: + is a granite, v is volcanic rock, and dark stripes are dykes.
It was only relatively recently, during the Miocene (about 20 million years ago), that stresses and motions in the crust brought about an uplifting of the area which today comprises the South Sinai massif, accompanied by two important phenomena: 1. Erosion of the sedimentary layers and thus exposure of the magmatic rocks, and 2. Creation of faults, firstly connected with the formation of the Gulf of Suez on the west, and then with the formation of the Gulf of Aqaba on the east.
The granite is composed mainly of three minerals: Feldspar, Quartz and Mica. The Feldspar and the Mica portions, with the addition of water, may convert, under favorable conditions, into clay minerals, in which case the rock texture disintegrates, as if the cement between the bricks of a wall is removed, and the wall then looses its strength.
As the Sinai granite has not been exposed for long, and the climate is very dry, not much of the Sinai granite has undergone this disintegrating process, except in locations where humidity tends to persist. On the whole, whatever little erosion occurs in Sinai, is chiefly mechanical, rather than chemical.
At exposed surfaces the humidity is low, and whatever friable material is formed - it is soon removed by winds and rains. Exposed surfaces in Sinai, therefore, are in most cases fairly solid.
In gullies and dry waterfalls humidity is maintained for prolonged periods of time after a rain event, and the disintegration is therefore considerable. However, in gullies and dry waterfalls the disintegration materials is flushed away by the next water stream, and solid rock is left behind.
In sloping walls, wherever an irregularity of the surface forms a pocket, water is retained. The erosion increases the size of the pocket until it touches the neighboring pocket, and creates a layer of friable material beneath the surface, covering large areas of the wall, with typical "coves" and "bridges" imbedded in it. This kind of granite is the least reliable one, from a climber's point of view, although its many bridges do offer protection points to pass a sling through. See example of this structures (Safsafa NE wall "Afrika" part)
Onion-like structures are fairy common in Sinai, seen mostly as "belts" separating between "slabs".
The NE wall of J. Safsafa (on both sides of the Boustan Gully), The "Little Rabba", and the W wall of J. Rabba (near the Um-Surdi pass), are examples of such onion-like structures. This feature is not yet well understood, and is often attributed to weathering, rather than to some process which might have occurred during the magma solidification.
Exfoliation due to onion-like structures, to the extent which might endanger climbing, is not reported to occur in the area.
From a climber's point of view, onion-like structure provide most of the overhanging "belts" in the area, with many interesting and challenging passages.
Example of typical belt (Safsafa NE wall "Afrika" part)
Much of a climber's protection in Sinai is placed in parallel slots, and consequently Friends, Tricams and Camlocks come very handy. Stoppers and Hexes, on the other hand, become less and less popular with climbers of Sinai routes as years pass by and experience is gained. Pitons are rarely used, but in those cases they are solidly placed and fit well the crevices.
Chimneys are common, and mostly very solid, as the walls of a dyke were "baked" during the dyke's formation by the molten magma. On the other hand chimneys in Sinai tend to be smooth and without fractures, and therefore poorly protected; in other words - classic chimneys. Example of typical chimney and small gully (Safsafa NE wall "Boustan" gully)