What is a Fault?
In simple terms, a fault is a fracture (like a break or a crack) or grouping of such fractures, in an otherwise solid mass of rock. Not every fracture, break, or crack in the earth's crust is a fault. What characterizes a fault, is movement along the fracture. Faults are often associated with earthquakes and indeed, when the movement along a fault is sudden and abrupt, the energy released can send out tremendous shockwaves. Faults can range in size from mere inches to hundreds of miles.
Types of Faults
There are several ways faults can be categorized depending on the direction of movement along the fracture. For this cache, we will put them into three broad categories:
- Normal Fault - A fault which is neither parallel to the ground, nor is at a right angle (perpendicular) to the ground. The angle created between the fracture and the ground is less than 90 degrees. This angle is known as the "dip" of the fault. So a normal fault is also a type of "Dip-slip Fault." The movement which occurs in a Normal Fault is that of the rock above the fault line moving, or "slipping," in a downward direction.
- Reverse Fault - Also a type of "Dip-slip Fault," and almost identical to the Normal Fault listed above. The key distinction is that in the case of a "Reverse Fault," the rock above the fault line moves or "slips" in an upward direction as opposed to a downward direction.
- Strike-slip Fault - Also known as a "Transform Fault." The movement which occurs along this fault can be described better as "sliding" rather than slipping. Whereas with a Normal Fault and a Reverse Fault, motion occurs at an angle, with a "Strike-slip Fault" motion is horizontal.
To summarize, faults are charcaterized by the direction AND angle of which movement occurs.
Identifying a Fault
You're staring at a rock outcropping which is said to contain a fault but have no idea where the fault actually is. How do you actually identify a fault in the field? There are several methods geologists use to determine the location of faults:
- Smooth, slick, shiny rock surfaces which appear polished. These surfaces are called "Slikensides." Adorning the Slikenside are parallel scratches resulting from the rock on rock scraping which occurred during the movement along the fracture.
- Ground, crushed, pulverized, or powdery rock created by friction. To be exact, the friction which occurred when the two sides of rock scraped against each other during movement. Also known as "Gouge."
- An abrupt change of color/texture between the rock above the fault, also known as the "headwall," and the rock below the fault, also known as the "footwall." Sometimes both the headwall and footwall can appear similar, while the rock along the fault itself has a distinctly different visual appearance.
- A series of deep embossed grooves in the surface which are parallel to the direction of movement.
- Fragments of different rock pieces joined together by a finer-grained sand mixture to form a new blended rock. This mixture of differing rock fragments is called "Breccia." Imagine mixing concrete where coarse pieces are bound together by the mortar, this can be an effective way to visualize breccia.
Some faults contain multiple visual clues, whereas other faults contain very few. Not any one item from those listed above positively identify the location of a fault as most can also occur due to various other geological phenomena. However, when used together, the above CAN offer clues to where a fault MAY lie.
Sabino Canyon's Fault
Currently inactive, the Sabino Canyon Fault influenced the formation of Sabino Canyon itself. The fractured rock along the fault influenced the course of Sabino Creek to trace along its route. The weakened rock along the fault's path, facilitated the erosion of the hard gneiss which composes this part of the Catalina front range into the rugged, desert canyon we see today.
Logging Requirements
- From the posted coordinates, walk up and down the road a few hundred feet in either direction. Which of the item(s) listed in the Identifying a Fault section above do you see which will support the existence of a fault along where you are walking?
- Standing exactly at the posted coordinates, how high above the road do you estimate the fault to be? When viewed from the posted coordinates, does the fault appear to be more of a straight line, or better described as a curve?
- From the section above on Types of Faults, can you identify which of the three types of fault this is? What evidence did you find to support your conclusion? If you were not able to identify the type of fault at this location, do you feel it is because you aren't an experienced geologist, or do you feel there may be another reason? If you feel it is for another reason, explain a bit more.
- Do NOT post any pictures of the answers with your log.
Please note that answers should be sent within a reasonabe amount of time, which for the purpose of this cache is one week. When logging as part of a group, the person sending the answers needs to include the name of all members of their group. Geocaching is based on the honor system so while I'm not going to require any photos, I trust that when you log this cache as a find you will have personally visited the location.