Alessandro Grippo, strcutural geology 2

Alessandro Grippo, Ph.D.


part 3, geologic structures: FRACTURES, or JOINTS

Fractures, or JointsLast Updated  •  January 18, 2014   
What is a fracture, or joint?
Fractures are surfaces along which rocks or minerals have broken, thus generating two free surfaces where none existed before.

Fractures are among the most common of all geologic features. Their study is important because fractures provide information on what kind of stress caused them (history of deformation) and also because they alter the characteristics of the rocks in which they occur: for instance a fracture would weaken a rock (and we need to know that if we build a dam, or a tunnel), and would allow fluids to move through it (and we need to know that if we are looking for oil or gas, or if we are dealing with groundwater).

Fractures in general can also be described as self-similar, or having a fractal geometry. This means that any part of a fracture viewed at any scale looks similar to the fracture seen at any other scale. Such a characteristic allows us to better comprehend the way fractures influence the physical properties of the rocks in which they are found.

If many fractures occur in the same area and have a similar orientation, they are referred to as a set of fractures. Individual extension fractures are referred to as joints, and a group of them is called a joint set. The term joint can be at times misused and the way we intend it here is simply synonymous of fracture.

Is there more than one type of joint?
Yes, there are several ways in which a joint or a joint set can develop:

  • Systematic joints are roughly planar, parallel to each other, usually regularly spaced
  • An example of vertical (planar) joints: Muley Point, Utah

    An example of vertical (planar) joints: Muley Point, Utah

    An example of vertical (planar) joints: Muley Point, Utah
    Here we can see vertical joints crossing the individual layers

    An example of vertical (planar) joints: Muley Point, Utah

  • Nonsystematic joints are curved and irregular
  • Joint systems form when two different joint sets occur in the same rock
  • Joint sets and systems are found almost everywhere in rocks and can sometimes show up as linear fractures, or lineaments
  • Sheet joints, or exfoliation joints are curved fractures characteristic of intrusive igneous rocks, such as the classic example of Half Dome, in Yosemite National Park, California
  • An example of sheet (exfoliation) joints: Half Dome, Yosemite NP, California

    Half Dome, Yosemite NP, California sheet (exfoliation) joints: a detail

  • Columnar joints are typical of extrusive igneous flows and shallow tabular intrusive igneous rocks. These fractures separate the igneous rock into pentagonal to hexagonal columns. Classic examples are Devil's Postpile in California, Devil's Tower in Wyoming, Giant's causeway in Ireland, the Alcantara narrows in Italy.

An example of columnar joints: Devil's Tower, Wyoming

A detail of the columns and the relative joints

The hexagonal cross-section of a broken column

Can joints be found associated with other structures?
Yes, fractures, or joints, can often be found as subsidiary features related to faults and folds. In this case, the fractures can provide information about the origin of the deformation.

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