The nature of cracks

(from a geologic perspective)

Harmon D. Maher Jr., Dept. of Geography and Geology, University of Nebraska at Omaha, 68182-0199

2007

This page was created for students engaged in early undergraduate research (EUR) on chalcedony veins in western Nebraska and South Dakota. The research is funded by a NSF STEM grant to the University of Nebraska at Omaha.

Introduction: I am using the term crack here simply because it is widely understood. This is in essence a research project about cracks. It turns out that cracks are quite interesting geologically (more on that below). Most people know a crack when they see one, but that doesn't mean they understand or have thought much about cracks - not, at least, in the way we will. For our purposes a crack is a break (a brittle discontinuity) where the sides pull apart as the crack opens, breaking bonds to create the crack. It is a tensile fracture. This can be distinguished from a break where the sides slide past each other, a shear fracture. Faults are shear fractures. Tensile and shear fractures are fundamentally different features, although they commonly occur together.

Simplified sketches of tensile versus shear fractures.

If you develop expertise in any field you develop vocabulary to go along with it. 'tensile' and 'shear' are words we will use a lot. Types of tensile features found in geology include joints, veins, dikes, sills, fissures, crevices, and crevasses. For the Inuit there are many words for snow and ice, and for the geologists many words for cracks and fractures. Again, more words that may not mean much at this point, but through experience, discussion and reading they can take on more and more meaning.

The earth is all cracked up. Any outcrop shows rocks to be fractured. For a road cut or a mine some of the fractures may have be caused by the excavation process, but many of the fractures were there before - they are of geologic and human origin. Below are some images to demonstrate the ubiquity and diversity of cracks in rocks. It will be helpful in this research project and in learning geology to train your eye to start seeing crack patterns, and your mind to start thinking about them.

This is a peak in the Snowy Mountains of Wyoming. It is composed of very old quartzites (once quartz rich sandstones) that have been deformed and metamorphosed. The quartzite layers form the large relatively flat cliff faces steeply dipping toward the viewer. They are intensely cracked by relatively straight fracture sets known as joints. Climbers are especially attuned to these joint sets. The form of this mountain is due to fracture controlled erosion.

To the south and east of Laramie, Wyoming is a terrain underlain by granite. The granite is fractured in a semi-regular pattern as can be seen here. The orientation of the joints is highly non-random. Erosion of the homogeneous, but jointed granite results in a very distinctive landscape.

Above are some jointed basalts (volcanic flows) from Giant's Causeway, Ireland. Classic columnar jointing is developed here. Note that not all the columns are perfectly hexagonal. Also note how subhorizontal fractures segment the columns along their length. These fractures have a distinctive disk shape, and are both convex upward and downward. The mechanics of the hexagonal columns are well studied, but those of the dish shaped fractures are which segment the columns are to my knowledge not.

This is the river bed of the Niobrara river near the Norden bridge, and the field of view is several feet. These are Tertiary sediments somewhat similar to those we will be conducting research on. The pattern of cracking shown here is distinctly different that shown above. It is on a much smaller scale and of a much more complex and irregular manner. It could be considered polygonal. Note also how there are fracture patterns within fracture patterns.

Significance of cracks: Scientists often study a particular phenomena simply because they find it personally interesting, if not fascinating. That is good enough reason for me to study patterns of cracks in the earth. However, science is definitely a community activity, and so one can ask why cracks might be of more general interest to the scientific community, if not beyond? How might understanding cracks be useful? For the engineer the answer is simple - cracks are associated with failure and there is a strong desire to avoid failure. Fatigue consists of many small cracks accumulating in a material with time and use so that what was strong becomes weak. Eventually the little cracks start to link and can form a big crack, and if that big crack is in your airplane wing, well then ... Engineering literature on cracks and cracking is extensive. Similar reasons exist for understanding cracks in rocks. Cracks abound in rocks, and control much of their behavior. If you build a dam they determine how the ground will settle, or if the adjacent mountain side will slide into the dam reservoir (which has happened). If you cut into a hard rock mountain side cracks can determine whether the slope will be stable. They represent a path along which fluids can move along in the ground, and two very important fluids to humanity that migrate in the ground are water and oil. Of course it is not only water that moves, but dissolved material in the water. Magma can also move along cracks. Since cracks are easier to erode along they also shape much of our landscape. This is very apparent in Arches National Monument in Utah, where erosion along a pervasive set of roughly north-south oriented planar cracks has produced an amazing array of rock fins, cliff faces, and, or course, arches. Some of these crack patterns are related to the tectonic forces inside the earth, and so they give witness to past crustal movements. So cracks are of interest for a variety of reasons, and the geologic literature on them is also extensive, although much remains to be understood.

How to describe a crack: Basic aspects of cracks that bear information are their geometry, orientation, and fill mineralogy and textures. In our study we will focus on orientation, but think about the other aspects also. Another web page describes how orientations are measured. The internal textures are particularly striking in the case of the chalcedony veins we will be studying.

Diagram of geometric aspects of cracks that can be described. the aspect ratio is a particularly Some cracks can have very blunt ends.

As drawn here the surface of the fracture feature is smooth. In reality there can be patterns on the fracture surface, and some of these patterns can indicate the propagation direction. One of the better examples of this is conchoidal fracture, a ribbed and curved fracture surface seen in obsidian (volcanic glass). Most people probably would not have thought there was so much to describe about a simple crack or fracture.

Crack growth: If you have ever watched a small chip on your windshield grow into a nice long fracture you have some experience with crack growth. Driving on dirt roads as much as I do has given be more experience with this that I would like. In this particular case, the crack often grows in spurts. Some cracks can grown at supersonic speeds, others grow slowly. Some cracks can branch as they grow, while others will terminate, often against another crack. The cracks grow by opening of the tip. A migrating tip propagates with time.

Questions: Asking fruitful questions is a crucial skill in science. Below are some questions that might be fruitful with respect to understanding cracks.

Some additional resources: