Diaclase

Granite flowering with diaclase (Ptasie Skały, in the Giant Mountains, Poland)

System of two conjugated diaclase families. Cretaceous of Cuenca, Spain (Fm. Dolomías tableadas de Villa de Vés). Photo perpendicular to the stratification plane.

It outlines the two families of diaclase. The red line would indicate the direction of compression (high angle bisectriz).

A joint (from the Greek «διά» dia, through, and klasis, break) is a fracture in the rocks that It is not accompanied by sliding of the blocks that it determines, except for a minimum transversal separation. They are thus distinguished from faults, fractures in which there is slippage of the blocks. They are brittle deformation structures of rocks that are very abundant in nature.

Characteristics of a joint

The orientation of a joint, like that of other geological structures, is described by two parameters:

• Direction: angle forming a horizontal line contained in the plane of the diaclase with the north axis - south.
• Buzamiento: angle formed by the diaclase and an imaginary horizontal plane.

The joints do not have to be generally flat, nor respond to any regular geometry, so the indicated parameters may vary from one point to another.

Socket associations

Joints do not usually appear isolated, but rather associated with faults and folds. When, as usually happens, there are two or more sets of joints, one speaks of a joint system or "joint system". The simplest are:

• Parallel diaclase system: all the diaclase have equal direction and digging.
• Diaclase system cut: the diaclase have different directions and dives and therefore cut into certain points. The most common case is that of conjugated diaclase families, with two or three predominant directions of diaclase produced by the same tectonic phenomenon (detention or compression).

In order to discriminate between compression and tension joints, the main axes of local or regional deformation must be studied, since the joints themselves do not provide sufficient information (striations or displacement). In the case of extension joints, the direction of the most noticeable family is usually perpendicular to the direction of extension, and in compression joints, the direction of the bisector of the acute angle of the intersection of joints.

Mechanisms

The Bodies on the island La Gomera are the result of spinal disjunction in basalt, a type of diaclase produced by cooling.

Decompressed curved decompression in an exhumed granite pluton.

The formation of joints is due to many different causes, including directed forces such as those that cause faulting or folding of the terrain. One of the most frequent causes of jointing is the decrease in the volume of the material (increase in density), which in turn can occur for different reasons:

• Dehydrationas in sediments that remain in the air after being submerged.
• Coolingas in the case of basaltic columns. They are formed by basaltic tails, which, once solidified the lava, by the subsequent cooling, are divided into prismatic columns (columnar disjunction). La Calzada de los Gigantes de Ireland, or Los Organs de La Gomera are one of the many known examples of this case.
• Re-Christalization. The passage of time favors, in geological materials, a reordering of the molecules that together expands the extension of the crystalline networks, increasing the density of the material, which is compensated, as in previous cases, with the formation of cracks.
• Decompression. It is another important cause of demonstration, such as the one that affects a granite pluto that erosion is uncovered. This is how the formations that are called berruecos or berrocals originate.