Geology - Wikipedia
Geology is an earth science concerned with the solid Earth, the rocks of which it is composed, It also provides tools to determine the relative and absolute ages of rocks found in a given location, and also to .. These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. Future research should comprise absolute dates based on relative sequences, integrated In the course of their research in Australia, Chippindale and Taçon ( ) Most of the Harris Matrix applications to South African rock art are isolated efforts should form the basis of archaeological stratigraphy (Harris ). Keywords: South Africa, rock art, Harris matrix, relative chronology. The dating on direct dating in most geographical contexts (although prog- . ings in Arnhem Land, Australia. . non-stratigraphic, criteria, such as manner of depiction. How.
In this context there exist multiple, pre-defined, recognisable traditions of rock art. The Harris Matrix is then used to determine the sequence in which the traditions occur on the rock. Relationships between traditions, not individual figures, are used in the analysis. Individual figures are reduced to the traditions they represent and their superpositions are taken as multiple examples of relationships between those traditions. The Harris Matrix technique appears to produce satisfactory results in these contexts.
The other context to which the Harris Matrix technique has been applied is the single tradition that cannot easily be further sub-divided. Here the technique is used to discover episodes, not supply their order. In South Africa, it has always been the San painting tradition that has been studied e. Loubser ; Russell It is within this single- tradition context that Swart works.
It is unfortunate that it is in this context that the difficulties of using the Harris Matrix in rock art studies arise. The primary difficulties relate to confusion over the unit of stratification.
In ground deposits the unit of stratification is fairly well established: Importantly, such units were originally continuous Harris Significantly, they exist independently of any artefacts they may contain.
The breaks between strata are evident on inspection: The artefacts are thus subsumed by the strata. The Harris Matrix does not establish sequences between artefacts within a single stratum.
Painted units of stratification, on the other hand, are not nearly as clearly defined. Swart uses individual paintings as her unit of stratification. She is right in so far as individual paintings are a potential stratigraphic unit. They, however, are not the only potential unit, or the smallest. One could just as well study the layers of pigment a smaller unit within individual paintings e.
Multiple potential units clearly exist. What researchers have to do is keep in mind the type of sequence they are ultimately trying to construct. In the case of San paintings, Swart and other researchers try to construct a sequence of hypothetical, temporal episodes. This being the case, I argue that the statigraphic unit must be the episode of painting.
If one uses individual paintings as the stratigraphic unit, one is able to construct a sequence of only individual paintings on the rock, not some larger hypothesised episode. Equally, if one were to use layers of pigment as the stratigraphic unit, it would be a sequence of paint deposition that could be constructed. The major difficulty in using the Harris Matrix to determine the sequence of episodes of San rock art is that the hypothesised episodes were originally discontinuous: This original discontinuity has major implications for the application of the Harris Matrix technique.
It is not an easy question to answer and has, I believe, led some researchers, such as Swart, to make a grave error. They have used the Harris Matrix to try to determine the constitution that is, which individual paintings make up an episode of the various episodes of paintings.
The Harris Matrix cannot do this. We need to find a technique that can define episodes strata of paintings before we use the Harris Matrix to find the sequence of those episodes. Finding such a technique is made difficult by the discontinuous nature of the paintings across the surface of a panel.
The lack of physical continuity between elements of the same episode means that there is no way of demonstrating their integrity. All elements of one episode are not necessarily covered by a directly following episode. This sort of discontinuity means that elements of any given episode may be directly superimposed by elements of a number of later episodes, or to look at the situation from the opposite perspective, elements of any given episode can simultaneously directly overlie any preceding episode and, indeed, the rock surface itself.
Consequently, the various sequences of superimposed paintings on the rock face are not necessarily equivalent to the overall sequence of painted episodes. She therefore tries to construct her sequence on the basis of superposition alone, without any reference to the details of the painted image. It is impossible to define an episode of paintings without reference to features of the paintings the artefacts that make up the episode. Not only the subject of the image, but the position of the painting on the rock face and its relation to other paintings is culturally and personally influenced e.
For these reasons, amongst others, I argue that, if researchers wish to apply the Harris Matrix technique to rock paintings, they need to identify episodes of paintings that will form the units of stratification before they seek a sequence.
In the multiple-tradition context the usually clearly differentiated traditions themselves form the stratigraphic units. In the single-tradition context other ways of pre-categorising the paintings need to be found.
This process can work only if paintings are pre-categorised: Swart, however, does not pre-categorise the paintings before finding their sequence and phasing them. This is essentially a post hoc categorisation of the images. But unlike the pre-categorisation I propose, it lacks clearly defined and consistently applied categories.
It is difficult to see how poorly defined post hoc categorisation is superior to clearly defined categories set out in advance. She produces a sequence for one panel at Ngwangwane 8 and two panels at Eland Cave. The most obvious point when comparing the sequences is that they do not match up. Indeed, not one of the three is identical with any of the others—not even the two in the same shelter.
This is primarily because the sequences she constructs represent the order of individual paintings on the rock, not a broader series of hypothetical episodes. This being the case, one should expect that there will be different orders of paintings of different subjects in different panels. Another methodological issue, that of sample, compounds the problem. I have previously argued that when seeking sequences, researchers need to investigate multiple sites in close proximity Pearce In my study of sequences in the Maclear District, I found substantial but not total agreement between the three sites I investigated.
The individual sequences were reinforced by correlations with others, and constrained by cases where what appeared to be sequential categories in one shelter were contemporary in another. Whilst distance alone cannot be said to create differences, in this particular case Aron Mazel has argued that there are iconographic differences between the art in these two areas.
Because of these differences, and aside from the methodological flaws with the Harris Matrix technique, it is unlikely that sequences in the two sites will be similar on the grounds of variation across space.
Significantly, none of the sequences is closely matched to any other. Some broad trends are evident in the various sequences, but I suggest that they are largely a product of preservation rather than cultural practice.
The trend Swart Given the well known fugitive nature of white pigment, it is hardly surprising that this colour is found only in more recent paintings.
Similarly, shading is unlikely to be found in old, faded paintings where some colours have disappeared. With isotopic dates, it became possible to assign absolute ages to rock units, and these absolute dates could be applied to fossil sequences in which there was datable material, converting the old relative ages into new absolute ages. For many geologic applications, isotope ratios of radioactive elements are measured in minerals that give the amount of time that has passed since a rock passed through its particular closure temperaturethe point at which different radiometric isotopes stop diffusing into and out of the crystal lattice.
Common methods include uranium-lead datingpotassium-argon datingargon-argon dating and uranium-thorium dating. These methods are used for a variety of applications. Dating of lava and volcanic ash layers found within a stratigraphic sequence can provide absolute age data for sedimentary rock units that do not contain radioactive isotopes and calibrate relative dating techniques.
These methods can also be used to determine ages of pluton emplacement. Thermochemical techniques can be used to determine temperature profiles within the crust, the uplift of mountain ranges, and paleotopography. Fractionation of the lanthanide series elements is used to compute ages since rocks were removed from the mantle. Other methods are used for more recent events. Dendrochronology can also be used for the dating of landscapes.
Radiocarbon dating is used for geologically young materials containing organic carbon. Geological development of an area[ edit ] An originally horizontal sequence of sedimentary rocks in shades of tan are affected by igneous activity. Deep below the surface are a magma chamber and large associated igneous bodies. The magma chamber feeds the volcanoand sends offshoots of magma that will later crystallize into dikes and sills.
Magma also advances upwards to form intrusive igneous bodies. The diagram illustrates both a cinder cone volcano, which releases ash, and a composite volcanowhich releases both lava and ash. An illustration of the three types of faults. Strike-slip faults occur when rock units slide past one another. Normal faults occur when rocks are undergoing horizontal extension.
Reverse or thrust faults occur when rocks are undergoing horizontal shortening. The geology of an area changes through time as rock units are deposited and inserted, and deformational processes change their shapes and locations. Rock units are first emplaced either by deposition onto the surface or intrusion into the overlying rock.
Deposition can occur when sediments settle onto the surface of the Earth and later lithify into sedimentary rock, or when as volcanic material such as volcanic ash or lava flows blanket the surface.
Igneous intrusions such as batholithslaccolithsdikesand sillspush upwards into the overlying rock, and crystallize as they intrude. Deformation typically occurs as a result of horizontal shortening, horizontal extensionor side-to-side strike-slip motion. These structural regimes broadly relate to convergent boundariesdivergent boundariesand transform boundaries, respectively, between tectonic plates.
When rock units are placed under horizontal compressionthey shorten and become thicker. Because rock units, other than muds, do not significantly change in volumethis is accomplished in two primary ways: In the shallow crust, where brittle deformation can occur, thrust faults form, which causes deeper rock to move on top of shallower rock. Because deeper rock is often older, as noted by the principle of superpositionthis can result in older rocks moving on top of younger ones.
Movement along faults can result in folding, either because the faults are not planar or because rock layers are dragged along, forming drag folds as slip occurs along the fault.
Deeper in the Earth, rocks behave plastically and fold instead of faulting. These folds can either be those where the material in the center of the fold buckles upwards, creating " antiforms ", or where it buckles downwards, creating " synforms ". If the tops of the rock units within the folds remain pointing upwards, they are called anticlines and synclinesrespectively. If some of the units in the fold are facing downward, the structure is called an overturned anticline or syncline, and if all of the rock units are overturned or the correct up-direction is unknown, they are simply called by the most general terms, antiforms and synforms.
A diagram of folds, indicating an anticline and a syncline. Even higher pressures and temperatures during horizontal shortening can cause both folding and metamorphism of the rocks. This metamorphism causes changes in the mineral composition of the rocks; creates a foliationor planar surface, that is related to mineral growth under stress. This can remove signs of the original textures of the rocks, such as bedding in sedimentary rocks, flow features of lavasand crystal patterns in crystalline rocks.
Extension causes the rock units as a whole to become longer and thinner. This is primarily accomplished through normal faulting and through the ductile stretching and thinning. Normal faults drop rock units that are higher below those that are lower. This typically results in younger units ending up below older units.
Stretching of units can result in their thinning.
In fact, at one location within the Maria Fold and Thrust Beltthe entire sedimentary sequence of the Grand Canyon appears over a length of less than a meter. Rocks at the depth to be ductilely stretched are often also metamorphosed. These stretched rocks can also pinch into lenses, known as boudinsafter the French word for "sausage" because of their visual similarity. Where rock units slide past one another, strike-slip faults develop in shallow regions, and become shear zones at deeper depths where the rocks deform ductilely.
Geologic cross section of Kittatinny Mountain. This cross section shows metamorphic rocks, overlain by younger sediments deposited after the metamorphic event. These rock units were later folded and faulted during the uplift of the mountain. The addition of new rock units, both depositionally and intrusively, often occurs during deformation.