joints and jointing The word ‘joint’ has been used by quarrymen and miners since the eighteenth century to express the idea that the parts of a rock mass are joined together across fractures. More correctly, a formerly intact rock mass is now split by joints, rather than being joined by them. Although the word ‘joint’ is used by most geologists, the structure could equally well be called a crack. In contrast to faults, along which rock masses have slid, joints are fractures on which it is not possible to see any evidence for there having been sliding (i.e. shear) along the fracture. A vein is similar to a joint but differs from it in that the crack is filled by minerals deposited from water that formerly flowed through it.
Joints, the most abundant structures caused by Earth movements in sedimentary rocks, are a result of the widespread cracking of rocks in the uppermost 10 km of the Earth's crust, which is brittle. Individual joints are often only a few centimetres apart, and their lengths commonly range from less than a metre to a few tens of metres. Understanding the intricate geometric patterns made by joints and knowing how they were formed is especially worth while because joints commonly influence the underground flow of fluids, such as oil and water, and they can lead to many exposed rock masses being unstable. Joints also profoundly influence the shape of the landscape by controlling, for example, the layout of valleys and ridges and the shapes of rock outcrops. The study of joints allows geologists to determine directions of pressure (stress) that acted on rocks in the past, or are still affecting them at the present day.
A
joint set is a group of parallel joints, and a
joint system consists of two or more symmetrically arranged sets (Fig. 1). A
joint spectrum is a suite of neighbouring joints that are not perfectly parallel but show a continuum of orientations up to a maximum angle of about 45°. It is rare for the members of a natural joint spectrum to intersect at a common node as has happened at the outcrop in northern Spain shown in Fig. 2. A joint that is a plane or nearly plane surface and belongs to a set or spectrum is called a
systematic joint. Most systematic joints are of tectonic origin, that is, they are related to deep-seated Earth movements. By contrast,
non-systematic joints, which are mostly irregular surfaces linking systematic joints (Fig. 1), are of superfcial origin. They are products of processes such as weathering and erosion which act on a rock mass after it has been exposed at the Earth's surface.
The trace of a joint that is younger than one of its neighbours will abut, or terminate, at it. This allows one to deduce the order of formation of joints exposed in an outcrop. The abutting relationship arises because a younger joint propagating through intact rock is unable to jump over the space between the walls of an older joint (Fig. 1). The buffering of younger joints by older ones means that most older joints are of greater dimensions than younger ones. If joint traces cross each other it is not possible to determine which is the older joint; all that can be said is that the walls of one of them must have been welded together before the other joint propagated across it.
Genetic classification of joints
The majority of joints are
extension fractures belonging to single sets of cracks that formed perpendicular to a direction of least stress, commonly a tension (Fig. 3). Some stress fields can also give rise to two sets of joints of roughly the same age. Such
conjugate joint sets (Fig. 3) consist of fractures inclined to the directions of greatest and least stress. Some conjugate joint sets enclose an angle of about 60° with each other and can be interpreted as
shear fractures, that is, fractures along which there was slip, but of such small magnitude that it is not detectable by eye in the field. More commonly, conjugate joints enclose an angle of less than 60° about the direction of greatest pressure. Because these joints show some characteristics in common with extension fractures and some characteristics in common with shear fractures they are often called
hybrid-shear fractures. Most of the joints in the spectrum shown in Fig. 2 are examples of hybrid-shear joints.
Origins of joints
In 1985, Terry Engelder, who had been studying joints in the Palaeozoic rocks of the Appalachian Plateau in the north-eastern USA, concluded that there are four main types of joints in the Plateau and other areas of simple structure.(1)
Tectonic joints form at depth in the Earth's crust and before rocks are uplifted and denuded. The initiation and propagation of tectonic joints are a response to the combined action of stresses of tectonic origin and of water in the crack that has been pumped up to abnormally high pressures as a result of the sediments having been tectonically compacted during Earth movements. Such high water pressure in a small crack-like flaw in a rock enables it to open up and increase in size, becoming a joint in the process.(2)
Hydraulic joints also form at depth, before uplift, and as a consequence of the influence of high water pressures; but, unlike tectonic joints, the cause of the high water pressure is solely related to compaction of sedimentary origin.(3)
Unloading joints are formed close to the Earth's surface after the uplift and exhumation of a sedimentary rock sequence. Although unloading joints are formed at shallow depths beneath the Earth's surface, their orientations are controlled by the directions of deep-seated tectonic stresses at the time of their formation. The idea that some joints are caused by unloading after uplift is not a new one: in 1959 Neville Price had introduced the notion to explain the widespread occurrence of joints in otherwise ‘undeformed’ rocks.(4)
Release joints are also formed in near-surface rocks after uplift and exhumation, but their orientations are largely determined by pre-existing structures. For example, some release joints open up along cleavage planes dating from a time when the rocks were being folded at a depth of a few kilometres beneath the Earth's surface.
Exposed
neotectonic joints are a special category of unloading joints formed in a stress field that has persisted with little or no change of orientation until the present day. Neotectonic joints are useful structures for inferring the directions of present-day stresses where they are not known from geophysical measurements (Fig. 4). Understanding present-day stresses is important because if their orientations and magnitudes are known it is possible to reduce earthquake hazard and predict the underground flow of fluids.
Many of the mechanisms leading to the formation of regionally extensive joints sets in areas of simple structure, such as the Appalachian Plateau, also give rise to joints in structurally more complex terrains, but some joints in such settings are of more local origin, as is explained below.
Relationships between joints and other structures
Many studies of joints have concentrated on their relationship to other structures. An important question to be answered is: are some types of joints associated with particular structures? The answer to this question is a provisional ‘yes’. For example, Jonathan Turner and Paul Hancock found that in the thrusted and folded rocks of the south-western Spanish Pyrenees joints trending at right-angles to thrusts are restricted to the rocks immediately beneath them, whereas joints trending parallel to thrusts occur both above and below thrusts (Fig. 5). These Pyrenean joints, and many others in deformed sedimentary rocks elsewhere, are also symmetrically arranged about the folds containing them.
In addition to wanting to understand geometrical relationships between joints and other structures, geologists have also attempted to determine the relative timing of jointing with respect to folding. Some workers have claimed that joints are early structures that are formed before beds are folded. A powerful argument in favour of this view is that many joints appear to have been rotated with strata during folding. However, where joints cut cleavage planes that are of the same age as the folds, the joints must be younger than the folds.
Joints in igneous rocks
Because they are products of tectonic stresses, many of the systematic joints cutting igneous rocks are similar to those in sedimentary and metamporphic rocks. In addition, some igneous rocks also contain joints related to their cooling history. The best known and most aesthetically pleasing of these joints are
columnar joints bounding long polygonal columns. The columns may be straight or curved, and analysis of their shapes enables successive cooling surfaces to be identified. Columnar joints are best developed in sills and dykes, volcanic vents, and former lava lakes. Some joints in major intrusions, especially those near the margins of plutons, are also thought to be related to stresses generated during cooling or the closing stages of the intrusion of the igneous rock.
Paul L. Hancock
Bibliography
Price, N. J. and and Cosgrove, J. (1990) Analysis of geological structures. Cambridge University Press.
Suppe, J. (1985) Principles of structural geology. Prentice Hall, New York.
