Greetings!This Earthcache is designed to teach you a little bit about fractures, which are also called joints. They are some of the most common features in geology. I hope you enjoy your lesson here!

What is a fracture, and what causes them?

Fractures are surfaces along which rocks or minerals have broken, thus generating two free surfaces where none existed before. Fractures are commonly caused by stress exceeding the rock strength, causing the rock to lose cohesion along its weakest plane.

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, but it is usually synonymous with fracture.

Types of joints

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.

Nonsystematic joints are curved and/or 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.

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.

The Earthcache

Standing at the posted coordinates, you will see an excellent example of fracturing, with some very prominent examples. Study them, and then answer the questions below. Send those answers to me via a message through my profile. Failure to send the required answers will result in your log being quietly deleted.

1. Define the term fracture.

2. What causes fractures?

3. At the western side of the corner of the rock face, you’ll notice a very prominent joint. Approximately how long (tall) is it? How wide is it at its widest point?

4. Based on what you know about fracture types, what type of joint do you think this is?

5. Do you think that this rock structure is in danger of collapse, due to this joint and the others near it? Why or why not?

6. Can you estimate the size of the most prominent joint?

7. Is this the only formation in this area with joint/cracks of this magnitude?

7. A picture of yourself at the site would be nice, but it’s not required.