2.1 Origin and Composition
Convection cells must have developed in the Earth’s Mantle at a very early stage, consequently initiating the differentiation of the elements composing the original magma. The less dense elements, silicon rich, accumulated at the top of the up flow side of the convection cells, just as foam in a boiling pot. Thus, this lighter material concentrated at the surface and consolidated creating the continents which are therefore formed by silicon rich rocks containing an abundance of quartz and are classified as oversaturated (acid). They encompass the granite family, of which the volcanic equivalent is rhyolite. Of the remaining magma, the most common member and the one which forms the oceanic floors, does not have enough silicon for quartz to form, is classified as saturated, and its most common rock family is the gabbro, with basalt as its volcanic equivalent. The rocks with least silicon content are classified as undersaturated (alkaline), and one of its rock types is peridotite.
It is easy to understand that along crustal plate diverging boundaries numerous cracks will form through which the fluid magma from the mantle can flow. Thus, igneous rocks associated with diverging boundaries, if within an ocean and forming its ridge, like the one along the centre of the Atlantic, will have a basaltic composition since its source is also basaltic. If the divergence is within a continent breaking up like the Rift Valley in Africa, the igneous rocks will be basaltic, but only if the magma being tapped is from the mantle.
Along converging boundaries, where the rock masses are under compression, it is not so straight forward, especially since either the two plates are compressing against each other, or the heavier density plate is being subducted under the other. So, I think that in the majority of cases the igneous rocks originate from the melting of the local rocks due to the incredibly high temperatures and pressures caused by the friction developed during compression. Thus, their composition will differ in accordance to their relative location, with basic rocks for the sector close to the subduction trench because they will be fed by oceanic floor rocks. Within continental masses, acidic rocks will predominate.
2.2 Type of Occurrence
2.2.1 Volcanic Rocks
Molten magma is continuously being spewed from the mantle through all sorts of existing fractures. If ejected into the atmosphere, it is known as lava, and the ducts through which the lava pours are the volcanos. Further, because the surrounding atmospheric temperature is markedly lower, the lava will cool very rapidly and the resulting rock will tend to be fine grained. Nowadays volcanos typically have pipe like structures through which the magma flows and as it cools, it creates the well known conic shapes (fig. 1).
Also, they frequently develop lateral vents (fig. 2). However, magma may also outpour along fissures as presently in Iceland and in the past, for example during the Karroo volcanicity (Jurassic), in South Africa.
Lava flows will enlarge the volcanic cone and spread in a fan shape at the base. In the example shown on figure 3 in Tenerife, the fan actually entered into the sea, and that is where the town of Garuchio was built.
Volcanic exhalations may be gentle and fairly continuous, in which case it takes the form of a very plastic fluid termed lava flow, as for example the upper dark layer of figure 4. Or, like the lower layer of the same figure, the out pour may be more violent and have the form of ash, termed pyroclastic, with small fragments predominating, but larger clasts may also be common and in the present case they are easily identified because of their much darker colour.
These pyroclastic explosive bursts are due to the high gas content of the magma, as well as the stage of consolidation of the lava being spewed out. In extreme cases we will have volcanic breccias (fig. 4B)
The appearance of the consolidated lava will also be affected by:
• its degree of plasticity, which, when very high gives a very contorted appearance (fig. 5);
• the rate of cooling which, when very rapid, yields volcanic glass, obsidian (fig. 6);
• high fluidity as well as gaseous content, will cause the lava to be very porous, pumice stone, and the porosity will make these rocks very light (fig. 7).
Further, this porosity will allow water to flow through the hollows and, with time, the substances under solution will precipitate and fill the holes, giving rise to what is known as amygdaloidal lava (fig. 8).
When the size of those hollows is sufficiently large we have the formation of the famous agates and geodes (fig. 9), which will tend to broadly have a spherical shape but may reach quite a considerable size and present a huge variety of internal shapes. The term agate is used when the precipitate is not crystalline, and geode when it is.
• Lava that flows into the sea freezes as it tumbles in and forms very characteristic spherical units, termed pillows. As these pillows fall on top of of those already settled and if the lava is still sufficiently plastic, its lower portion will become sort of squeezed between the ones more solid below (fig. 10).
If, on the other hand the pillows fall on soft ground, their spherical shapes are preserved and they squeeze the paleosol below (fig. 11).
• Lava cooling on land often develop a very characteristic hexagonal jointing, columnar. This occurs both with basalt (fig. 12).
as well as rhyolite (figs. 13).
Because volcanos produce a variety of materials, from lavas to pyroclasts, their settling characteristics give rise to rock assemblages considerably similar to those of sedimentary rocks (Item 6), as nicely exemplified by the rock assemblage at Pico do Arieiro in Madeira. As can be seen (fig. 13B), there are horizontal lava layers with distinct columnar jointing and above those we have a thick succession of “cross bedded” pyroclastic horizons.
Within this upper assortment there are beds that range from poorly sorted (fig. 13C),
to moderately well sorted but rather coarse grained (fig. 13D).
2.2.2. Hypabyssal Rocks
A significant proportion of the magma flowing through the tension cracks will actually consolidate along them. The resulting rocks are termed hypabyssal, that is, intermediate between plutonic and volcanic. The majority of the ducts through which magma flows are narrow and very long (fig. 14). As such, the magmas filling these fissures will cool quite fast and the resulting rocks will predominantly be fine to medium grained. If these intrusives are parallel to the surrounding strata they are termed sills and when cutting across, they are called dykes.
Also, these fractures are a consequence of the breaking away of continental plates, and fracturing of non homogenous brittle materials usually have associated splitting, termed conjugate faulting. Thus dykes tend to occur in conjugate sets (fig. 15).
Hypabyssal rocks occasionally have pipe like forms, many of them corresponding to the volcanos, and they may have considerably large diameters. For those that did not actually reached the surface, their magma will take longer to cool, thus becoming more coarse grained. When they occur along rifting lines and their magma source is very deep, that is from the mantle, it may have an undersaturated composition like the kimberlites (fig. 16),
or they may already show a considerable level of differentiation like the carbonatites (fig. 17).
Volcanic breccias are moderately frequent (fig. 4B), but I think the Boula Igneous Complex in India is a rather unique example (fig. 18)
In fact I put it here rather than with the volcanic rocks, because, according to Augé and Thierry, this breccia was caused by a violent explosion within the magma ducts with the clasts belonging to the intruded, rather than the intruding rock and it must have happened at a considerable depth since the intruding basalt is very coarse grained, often pegmatitic. However the brecciated wall-rock shows very little movement. For example, the position of the very large chromite clast shown in figure 19, is very close to its initial position relative to sector of the chromite lens unaffected by the explosive burst.
Other than the in situ shattering, what we had was the rotation of the clasts within a very hot chamber which partially melted the wall-rock (fig. 20).
2.2.3 Plutonic Rocks
Plutonic rocks are formed by magmatic intrusions at great depths. Since we are dealing with a fluid intrusion, the contacts with the surrounding rocks tend to be irregular (fig. 21).
Further, even though these intrusions occur at great depths, the host rocks are still rather brittle and the magma intrudes through bedding planes and joints, forming a maze of dikes and sills across the host rock in the immediate vicinity of the pluton (fig. 21B).
The other consequence of these intrusions taking place at great depths and the fact that they generally have very large volumes is that, with the exception of the marginal areas of contact, this magma has a very long time to cool, allowing the development of coarse grained rocks. When the magma is rich in volatiles it often has associated hydrothermal pegmatitic (ultra coarse grained) veins, giving rise to magnificently well developed crystals (fig. 22).
2.3 Magmatic Differentiation
Magmatic differentiation was already mentioned (item 2.1) but here I’m just referring to two rather unique examples, the Boula Igneous Complex in India and the Bushveld Igneous Complex (B.I.C.) in South Africa. Both these igneous lopoliths have a basic to ultrabasic composition, meaning that the intruding magma has already had a significant amount of chemical differentiation from the initial mantle magma.
2.3.1 Differential Crystal Settling
While cooling within the intruded chamber, further differentiation took place due to the rate of settling of the various minerals as they crystallised at the top, the coolest area, and slowly dropped to the bottom. The reason why these two cases are so spectacular is because both assemblages consist of a light coloured member, peridotite in India and anorthosite in South Africa, inter-layered with a black member, chromite. Also, the SG of the latter is far higher than either of the other two, thus allowing for a much more clear separation of the respective minerals (figs. 23 and 24).
The similarity between a normal sedimentation process and the crystal settling in these two cases is remarkable. So much so, that initially a school of geology in South Africa believed the B. I. C. to be an assemblage of metamorphosed sediments. Take also the example shown in figure 25. I have never seen such perfect graded bedding in real sediments. In the present case we have granular magnetite forming the base of the sequence with feldspar crystals progressively increasing in quantity upwards, just like in sediments where the heavier clasts are the ones that reach the bottom first.
Another example, still with close similarities with sedimentation, but now with igneous crystal settling characteristics more apparent, is the occurrence observed at the sector of this rock sequence where the locally termed “pyroxenite boulder horizon” occurs. This member of the succession is approximately 50cm above a very well defined and continuous pyroxenite band and consists of a layer of spotted anorthosite, containing scattered coarse grained pyroxenite nodules with an average diameter of 15 cm (fig. 25B).
However, as shown in figure 26, one of the “boulders”, considerably larger than normal, appears to have fallen through the semi fluid mush of the already settled pyroxenite band. Note that the “boulder” was not entirely solid, since it looks as if it is rather frayed at the edges. Both these photos were taken along one of the mine adits, within 2 m of each other, and I think this example is rather useful in helping to understand the notion of a crystal settling environment.
2.3.2 “Pot Holes” Within the Merensky Reef
The Merensky Reef (MR) is a platinum bearing, generaly conformable horizon of the B. I. C.. It is accepted that this band is the first layer after a new magma influx was injected into the settling chamber, raising its temperature and introducing platinum. That is the reason why the MR has a pegmatitic texture, with a much coarser grain than the layer immediately below which is approximately a 3 m thick band of anorthosite known at the Impala Mines as footwall 1 (FW1). This temperature rise also lead to the development of convection currents within the settling chamber, causing irregular whirls that in places disturbed the already settled crystals, developing what are locally termed “potholes”, for which a tentative explanation follows:
Let us start with an example that fits into the frame of the photograph. This one was taken in a stope at the Bafokeng Mine. Unfortunately it is in black and white, but the markings make it understandable, showing quite clearly why these irregularities are called potholes (fig. 26B).
Also, observable in the photo, is that the MR actually consists of a pegmatitic pyroxenite with a discontinuous thin chromite seam at the base as well as at the top, and this is covered by a medium grained pyroxenite. Further, at the centre of the photo, the MR pegmatite has “cut” through the FW 2, a 50 cm thick anorthosite band, and is laying just over 30 cm deep into the FW3. It is also obvious that the MR in the whole of the picture is in fact only the central portion of a considerably larger pothole, since the FW1 is not at all present. That is, this pothole actually reaches a maximum depth of about 4 m at its centre.
The example above is exceptional because predominantly the potholes are much larger, like the one shown in figure 27 where we see only a small fraction of the pothole with, at the right hand side, a MR pegmatitic pyroxenite contacting almost vertically with a mottled anorthosite, which is filling the centre of the pothole, showing a vague suggestion of horizontal layering, a consequence of the latter period of crystal settling.
Finalising, figure 28 is an interpretative cross section along a diamond drill hole that intersected a large pothole and I think helps to understand the situation. M3 and M2 are anorthosites that cover a normal MR (pink band), at the upper portion of the diagramme. Below that, the borehole intersected another mottled anorthosite, interpreted as the inner fill of the pothole. Next comes the MR horizon again, this time consisting of a very thin chromite seam. Following, is a norite footwall below which we have the final segment of MR at the base of the pothole, consisting of a rather thick chromite horizon very rich in platinum.