3. Weathering

3.1 General

3.1.1 Initial Stage

Weathering is the chemical alteration caused by the atmospheric water in vapor and/or liquid form, due to the reacting agents carried in solution. Thus, this was the first step in the alteration of the original consolidated rocks forming the initial land masses. Figure 29 shows how the most reactive minerals are preferentially affected.

Figure 29 – Crumbling of weathered material within Flysch shale (Furnas, Alentejo, Portugal).

Also, the less soluble materials of the weathered rock will precipitate in the immediate vicinity, along fissures and/or bedding planes through which the water percolates.  For example, the manganese dendritic growths frequently found on bedding planes and joints (fig. 30).

Figure 30 – Manganese dendritic growths on a quartzite bedding plane (Peninsula Quarry, Cape Peninsula, South Africa).

This differential weathering characteristics can also be noticed in the variability between two different members of a rock outcrop such as in figure 31, where a dolorite dike is totally weathered away and the limestone through which it intrudes, being more weather resistant remains unaltered, thus causing a very sharp topographical contrast.

Figure 31 – Totally weathered and washed away dolorite dyke cutting through a limestone succession (Boca do Inferno, Cascais, Portugal).

3.1.2 Soil Formation

The above example is a rather exceptional case because more often we have reasonably flat areas with uniform rock outcrops where the weathering is widespread, giving rise to the development of soils, as shown in figure 31B. On the right side of the photo, the lowermost member of the rock sequence which has a blotchy wine colour, is covered by a layer of very angular rock fragments and above that we have a layer of fairly unaltered lava. The wine coloured horizon is also a basaltic lava and its colour is caused by the complete weathering of its minerals, which are rich in iron and the blotching indicates mineral assemblages with different chemical composition. If this altered material that is now a soil, had been transported, the blotches would have disappeared because of the mixing during transport. Thus, in this case we have an “in situ” soil.

Figure 31B – Basic lava weathered to soil overlain by a layer of hill slope debris, covered by moderately fresh lava (Tagus estuary, Oeiras, Portugal)

3.2 Special Aspects

3.2.1 Weathering Cups

The difference in reactability of the various minerals constituting a rock give rise to interesting surface features. One such case is the formation of cup shaped holes on granite outcrops on mountain tops (fig. 31C). I presume these remarkably circular depressions are caused by the weathering of the feldspars and other chemically unstable minerals into clays, initially in single rain drops size cups. At the next rain stage, the very light clay washes away, enlarging the cup and exposing newer levels of rock to weathering. In the mean time, the much more weather resisntent quartz grains present get loosened and acumulate at the bottom of the cup as shown in the picture.

Figure 31C – Granite weathering in the form of very circular cups (see keys for scale) (Serra de Freita, Arouca, Portugal)

When the rain is stronger, these quartz grains will also be washed away, thus allowing the continuation of the enlargement of the cup, which can eventually reach significant dimensions like the ones at Marinieche in France, as was shown to me by Jean Jacques Espirat, another geologist. In fact, after his communication, I changed my mind about a depression in a granite boulder, with a mouth diameter of just over 1 meter, which I photographed in the vicinity of the Orange River (Agrabis Falls), in South Africa (fig. 31D). I had originally interpreted it as being a river pothole, at a site which, although with an elevation somewhat higher than the present river bed, I presumed it was where the river previously flowed. Perhaps another point in favour of this new interpretation is the fact that river potholes tend to be deep and have a proportionally narrow mouth, as against these weathering cups which appear to be shallow and have a relatively large mouth.

Figure 31D – Very large weathering cup (diameter over 1 m) (Agrabis Falls, Orange River. South Africa).

3.2.2 Boxwork Weathering

The larger the surface area being affected by the weathering, the more effective it will be. That is, the more cracked the original rock, the sooner it will become weathered, since the cracks greatly increase the area exposed. Cracks develop, for example, due to the influence of the temperature variations from night to day, which cause consecutive expansions and contractions giving rise to numerous little cracks. The weathering enhancement along these cracks often forms linear ridges where the more soluble material is washed way, giving rise to a surface texture with the obvious name of boxwork (fig 32),

Figure 32 – Boxwork weathering on flisch sandstones (Cabo Sardão, Portugal).

which comes in all sizes, and is quite noticeable in iron rich sediments (fig. 33).

Figure 33 – Large scale box-work weathering from a Fe rich horizon (Castro Verde, Portugal).

Another example is the spectacular weathering sequence initiated with the alteration of olivine to serpentine (fig. 34).

Figure 34 – Serpentine after olivine, notice the initial formation of a box-work texture (view approximately 100 x70 cm) (Boula, india).

This is followed by a marked increase in the box-work texture, and the alteration of serpentine to breunerite (fig. 35).

Figure 35 – Breunerite after serpentine with a distinct box-work texture (view approximately 100 x70 cm) (Orissa, India).

3.2.3 Jointing in Weathering

Much better defined cracks (joints) develop when buried rocks are released from their surrounding pressure. Mining for example, when the ore and associated waste is extracted, relieves the surrounding rock body from its original tension. Figure 36 shows the cracks developed by the pressure release caused by the mining of this dunite pipe. Humidity concentrated along these cracks thus accelerating the weathering which altered the dunite into magnesite, now framing the joints so distinctly.

Figure 36 – Magnesite after dunite, developed along the pressure release joints around the dunite pipe mined for platinum (view approximately 2.5 m high) (Bushveld Igneous Complex, South Africa).

On a much larger scale, are the joints formed during the uplifting of rock masses caused by isostatic adjustment. A good example is seen in granite outcrops, since they are formed at great depths and thus at very high pressures. Erosion will eventually bring these large igneous rock masses to the surface reducing dramatically the surrounding pressure and causing them to break into large blocks. Since the weathering will be most intense at the corners of these blocks, these tend to become rounded, as nicely demonstrated in figure 36B.

Figure 36B – Small scale exfoliation in a dolorite dike (Cascais coast, Portugal)

Hence the decomposition becomes more concentric causing a pealing effect, like an onion (exfoliation). In a grand scale like in a granitic land surface, this can give rise to the very characteristic geomorphology of upstanding huge egg shaped solid granite boulders, known as inselbergs (fig. 37).

Figure 37 – The most famous inselberg in the World (Sugar Loaf – Rio de Janeiro).

A very unusual effect of the decompression on a granite body and consequent enhancement of the exfoliation by weathering is shown in figure 38 where the granite boulders resemble piles of pancakes. It is possible that this is a consequence of the lamination effect, caused by prior strong shear pressures on the granite mass.

Figure 38 – Unusual exfoliation in granite (granite pile approximately 1 m high) (Serra de Montesinho -Trás-os-Montes. Portugal).

3.2.4 Sink Holes

Perhaps the most devastating weathering effect, is the one giving rise to sink holes. In acidic waters, limestones and dolomites are incredibly soluble rocks and thus very easily weathered. The effect on outcrops, termed “karst”, gives rise to a very irregular surface with rather deep hollows (fig. 39).

Figure 39 – Typical example of karst topography (Cabo Carvoeiro, Portugal).

This weathering action continues under the surface anywhere above the ground water table. Thus, where the water table is sufficiently deep, the limestone will continue weathering above that and caves will form (fig. 40).

Figure 40 – Inside the Sterkfontein Caves (Transvaal, South Africa).

Figure 41 shows the difference between solubility levels. Notice the marked contrast between the very weathered, soluble, limestone bed, above, and an insoluble chert horizon below.

Figure 41 – Highly weathered limestone horizon (above), and unweathered chert band below (view approximately 60 x40 cm) (Sterkfontein Caves,Transvaal, South Africa).

If the conditions remain constant for long periods, the weathering of the limestone will continue, the caves will increase in dimensions, reaching a stage when the ceilings, generally in the form of a vault, will no longer support the mass of ground above, and will collapse, forming sink holes. In the mining town of Carletonville, South Africa, there are/were 4 deep gold mines which pumped to the surface in excess of one million cubic meters of water per day, in order to maintain the underground workings sufficiently dry to enable the mining operations. This continuous extraction of such vast volumes of water caused a dramatic drop in the elevation of the water table which was originally close to the surface, but fell, in places, to more than 500 m below.

After a few years, numerous sink holes developed and some of the mining villages had to be evacuated because of the collapse of some of the constructions and the unfortunate death of some of the inhabitants. Even small sink holes can cause significant damage like the one shown under the railway line (fig. 42).

Figure 42 – Small sink hole under a railway line (Bufulsfontein Mine – Stilfontein – S. Africa).

On the other hand, particularly interesting is the double sink hole shown in figure 43, where the initial stage caused the partial collapse of a large portion of the dome, but somehow the central sector only collapsed at a later stage. For a matter of scaling, the markings on the right are car tracks.

Figure 43 – Reactivated large sink hole (Carletonville, S. Africa).

Much more frightening though, is what occurs in Slovenia where the village of Skocjanske (fig. 44),

Figure 44 – the village of Skocjanske suspended amongst limestone arches separating different sink holes (Slovenia).

is located on top of arch remnants separating old sink holes (fig. 45). Having lived in Carletonville, I do not understand how a village can continue being inhabited under such conditions.

Figure 45 – View from below, of a sink hole face (escarpment approximately 10 m high) (Slovenia).