Fractures
A Structural Geology Storymap
Devil’s Tower photo from Alex Demas, USGS, 2015.
Devils Tower is a national monument in the Black Hills of Wyoming. This tower exhibits columnar jointing, which is a fracture pattern associated with igneous rocks (NPS, 2019). We will learn more about these amazing fractures later on in this story map!
What are fractures & why are they important?
Fractures are a plane of discontinuity, meaning that they are a break or crack in a body of rock. Fractures are a very common outcrop feature and have many applications, such as in understanding rock strength and rock permeability. The effect of fractures on rock permeability and porosity is important for comprehending fluid flow, such as in identifying where water, petroleum, or minerals deposits might be located.
Fractures can occur naturally or be human-induced, such as in the case of fracking, or hydraulic fracturing. Understanding fractures can also help geologists reveal the story behind an outcrop and lead to a better understanding for exploring resources and mitigating contaminants.
Classifying Fractures
Fractures are generally classified into 3 modes based on type of displacement. The image to the right illustrates each mode’s basic movement (Fosson, 2016, 127).
Mode 1 fractures are extensional and result in opening. Joints are a mode 1 fracture that have little to no rock displacement (Fosson, 2016, 125). Mode 1 extensional fractures grow perpendicular to fracture walls and can be filled with a variety of substances that will be discussed later.
Mode 2 fractures are shear fractures, meaning that they have a general slip movement that is parallel to fracture walls. Mode 2 fractures have a sliding displacement that is small and can be thought of as microfractures; they grow by linking these small cracks together (Fosson, 2016, 127).
Mode 3 fractures are also shear fractures, and they have a tearing motion (Fosson, 2016, 127).
The last block at the bottom of the image has a closing movement even though it is not considered a contractional fracture (Wilkerson, 2019). This feature is known as a stylolite and results from rock dissolution rather than loss of cohesion and breaking.
Lastly, fractures that have a combination of these movements are called hybrid cracks. Fractures often have multiple forms of movement and multiple causes for these movements.
The story continues with mode 1 fractures!
As a subset of fractures, the rest of this story will focus on filled-in extensional fractures, how joint sets are classified, and then how some joints form. A few other notable joint features will also be examined at Yosemite National Park in California and at Devil’s Tower National Monument in Wyoming.
What happens when extensional fractures are filled?
When minerals are deposited into the space of a preexisting extensional fracture, they are known as veins (Fossen, 2016, 125). These veins can be used to help explore ore deposits with economic value, such as in the image to the right of gold veins in quartz and granite host rock (Lovett, 2013).
Fractures filled with fluid, such as water or gas, are called fissures. The video on the right is drone footage of an earth fissure in Pinal County, Arizona (Gootee, AZGS, 2017). In the case of earth fissures, these large fractures can damage buildings, injure humans, and disrupt fluid drainage (AZGS).
How are joint systems classified?
Classifying joint sets helps to describe the geometry and relative age of joints compared to other joints in the outcrop. The diagram to the left illustrates the idealized joint system classification (Sorkhabi, 2014).
Systematic joint sets are regularly spaced, through-going, and typically planar; they tend to be older than non-systematic joint sets (Wilkerson, 2019).
Non-systematic joint sets describe irregular joints that may curve and be discontinuous. They are typically younger than systematic joint sets (Wilkerson 2019).
Master joints are long, continuous joint sets that other joints terminate against. This means that they are typically older than the cross joints terminated against them.
Cross joints are joint sets that consistently truncate at a systematic joint. They can also be either systematic or non-systematic, and are typically younger than the master joint sets.
How do joints form?
To review, joints are a mode 1 extensional feature that open perpendicular to fracture walls. In terms of rock mechanics, joints will form when the strength of the rock body is overcome by extensional (tensile) stresses. The result is a brittle break in the body of rock that can form in sets, as discussed above.
Joints form from a variety of processes, with common models including unloading, thermal contraction,
Unloading is a common model for creating joints, and it is based off releasing stress. In this model, as the weight of the overlying rock is removed in a process called unroofing, the layers of rock at lower depth expand vertically upwards towards where the overlying rock was removed.
This decompression of rock creates sheeting joints that grow parallel to the rock surface, as seen in the image of the Half Dome in Yosemite National Park.
Exfoliation occurs when the rock layers fall off, and can be thought of as peeling layers off an onion. The Yosemite Half Dome also exhibits this type of exfoliation in the upper portion of the image from Scott Gediman (LA. Times, 2019).
Similarly, thermal contraction is thought to contribute the sheeting and exfoliation joints seen in Yosemite. Thermal contraction is when rock cools and shrinks, and this may occur as rock uplifts to a higher level in the crust with lower temperature. Collins and Stock (2016) indicate that thermal stress associated with regular day and seasonal cycles helps propagate exfoliation, which can lead to rock falls and harm humans. Their figure on the right illustrates how large these features can be, with the Half Dome located in the upper central portion of the image (Collins, 2016).
The columnar joints seen from the National Park Service's (NPS) image of Devil’s Tower are another example of the effects of thermal contraction. Devil's Tower, located in the Black Hills of Wyoming, is the rock formation that was featured as the cover page of this Story Map. The tower is the largest example in the world of columnar jointing and formed from phonolite porphyry, a rare igneous rock (NPS, 2019).
Details on the exact formation process of Devil's Tower itself are still being discussed, with some theories ranging from it being part of an extinct volcano that then eroded away to forming from the intrusion of magma into the surrounding layers of rock to form the tower (NPS, 2019).
Columnar joints generally form from the cooling of igneous rock (Grippo, 2008). As the molten rock cools and fractures, 5- or 6-sided polygonal columns form. The long axis of the column develops perpendicular to maximum cooling direction around the sides. Grippo's close up image on the columnar jointing at Devil's Tower shows the 5-sided columns.
Other models for creating mode 1 joints include natural hydraulic fracturing, tectonic stresses, and the membrane effect. Hydraulic fracturing increases pore pressure from fluid, which weakens the rock and causes it to fracture. Tectonic stresses includes faults, which create a fracture zone between bodies of rock. While there are many types of faults, we may generally recognize faults as the movement of rock that causes earthquakes, although not all faults do produce earthquakes. The membrane effect is when rock is stretched across a greater radius, and this stretching makes the membrane of rock weaker (Wilkerson, 2019).
Overall, the formation of fractures can have many causes, and these cracks help us better understand how material moves through, or are trapped by rock, aiding in mineral exploration and in comprehending fluid flow. The various types of fractures, their geometry, and their relationships to each other also help geologists better understand how and when these fractures formed. Knowing how fractures grow can also impact human safety, as alluded to with rock falls at Yosemite Park. This amazing geological feature is not only useful for humans, but is also a beautiful work of nature!
Thanks for visiting!
NPS. 2019
Works Cited:
AZGS. 2019. “Earth Fissures & Ground Subsidence”. University of Arizona: Arizona Geological Survey. https://azgs.arizona.edu/center-natural-hazards/earth-fissures-ground-subsidence
Collins, B., Stock, G. Rockfall triggering by cyclic thermal stressing of exfoliation fractures. Nature Geosci 9, 395–400 (2016) doi:10.1038/ngeo2686
Demas, Alex. 2015. Devil’s Tower. United States Geological Survey. United States Department of the Interior. https://www.usgs.gov/media/images/devils-tower-1 .
Fossen, Haakon. 2016. Structural Geology sec. Ed. “Joints and Faults.” Structural Geology, by Haakon Fossen, 2nd ed., Cambridge University Press, 2016, pp. 125–160.
Gediman, Scott and Carlos Lozano. 2019. "Climber is killed in fall from Half Dome cables in Yosemite National Park." Los Angeles Times. https://www.latimes.com/california/story/2019-09-06/climber-killed-in-yosemite-half-dome-fall
Gootee, Brian. 2017. “Using Drone Technology to Examine an Earth Fissure.” Arizona Geological Survey. United States Department of the Interior. https://youtu.be/9xdAnftBKvY.
Grippo, Alessandro. 2008. “Structural Geology and Tectonics”. Santa Monica University. https://homepage.smc.edu/grippo_alessandro/struct3.html
Lovett, R. 2013. “Earthquakes make gold veins in an instant.” Nature. Macmillan Publishers Limited, part of Springer Nature. https://www.nature.com/news/earthquakes-make-gold-veins-in-an-instant-1.12615
NPS. 2019. “How the Tower Formed.” National Park Service. United States Department of the Interior. https://www.nps.gov/deto/learn/nature/tower-formation.htm
Sorkhabi, R. 2014. "Fracture, Fracture Everywhere." GeoExPro. https://www.geoexpro.com/articles/2014/08/fracture-fracture-everywhere-part-i
Wilkerson, M. S. 2019. "GEOS 350: Structural Geology and Tectonics." DePauw University.