Arkansas provides many great opportunities to explore nature, as the "Natural State" you will not want to forget your camera, a walking stick, and a buddy or two to go out and enjoy nature.
In this Earth Cache lesson you'll lean about Fissures in limestone and the geological factors that help form springs
After completing this Earth Cache you will have a better understanding of:
- How a fissure is created
- compression and tension
- the three classes of fracture modes
- natural hydraulic fracturing
In geology a fissure is a enelogated narrow fracture in stone. A fracture is any separation in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces. A fracture will sometimes form a deep fissure or crevice in the rock.
Fractures are commonly caused by stress exceeding the rock strength, causing the rock to lose cohesion along its weakest plane. Fractures can provide permeability for fluid movement, such as water or hydrocarbons. Highly fractured rocks can make good aquifers or hydrocarbon reservoirs, since they may possess both significant permeability and fracture porosity.
Causes
Fractures in rocks can be formed either due to compression or tension. Fractures due to compression include thrust faults. Fractures may also be a result from shear or tensile stress. Some of the primary mechanisms are discussed below.
Modes
First, there are three modes of fractures that occur (regardless of mechanism)
Mode I crack – Opening mode (a tensile stress normal to the plane of the crack)
Mode II crack – Sliding mode (a shear stress acting parallel to the plane of the crack and perpendicular to the crack front)
Mode III crack – Tearing mode (a shear stress acting parallel to the plane of the crack and parallel to the crack front)
Tensile fractures
This deformation creates propagation of fractures into previously unfractured rock, when the rock is subjected to tensile stress. In the case of a homogenous stress field, the crack will propagate in the direction perpendicular to the least principal stress.
Engineers studying elasticity found that the stress due to a load distant from the region of interest is concentrated around material flaws. A. W. Griffith took this example and applied the concept of stress concentration to the ends of fractures. Griffith asserted that all material have micro cracks or flaws where the stress concentration naturally occurs, and that these were the flaws where, under increasing stress, fractures propagate. As a result, these pre-existing flaws were named Griffith cracks, and allow the conceptual assumption that rocks are relatively weak.
From this assumption, tensile fracture development may be examined.
The first form is in axial stretching'. In this case, remote tensile stress is applied, allowing micro cracks to open slightly throughout the tensile region. As these cracks open up, the stresses at the crack tips intensify, eventually exceeding the rock strength and allowing the fracture to propagate. This can occur at times of rapid overburden erosion. Folding also can provide tension, such as along the top of an anticlinal fold axis, where the tensile forces associated with the stretching of the upper half of the layers during folding can induce tensile fractures parallel to the fold axis.
Tensile fracturing may also be induced by applied compressive loads along an axis, which results in longitudinal splitting. In this situation, tiny tensile fractures form parallel to the loading axis, however the load forces any other micro-fractures closed. To picture this, imagine an envelope, with loading from the top. A load is applied on the top edge, the sides of the envelope open outward, even though nothing was pulling on them. Rapid deposition and compaction can sometimes induce these fractures.
Another tensile fracture mechanism is hydraulic fracturing. In a natural environment, this occurs when a combination of rapid sediment compaction and thermal fluid expansion causes the pore fluid pressure to exceed the pressure of the least principal normal stress. When this occurs, a tensile fracture opens perpendicular to the plane of least stress.
Tensile fractures are almost always referred to as joints, which are fractures where no appreciable slip or shear is observed.
Faults and shear fractures
Faults are another form of fracture in a geologic environment. In any type of faulting, the active fracture experiences shear failure, as the faces of the fracture slip relative to each other. As a result, these fractures seem like large scale representations of Mode II and III fractures, however that is not necessarily the case. On such a large scale, once the shear failure occurs, the fracture begins to curve its propagation towards the same direction as the tensile fractures. In other words, the fault typically attempts to orient itself perpendicular to the plane of least principal stress. This results in an out-of-plane shear relative to the initial reference plane. Therefore, these cannot necessarily be qualified as Mode II or III fractures.
An additional, important characteristic of shear-mode fractures is the process by which they spawn wing cracks, which are tensile cracks that form at the propagation tip of the shear fractures. As the faces slide in opposite directions, tension is created at the tip, and a mode I fracture is created in the direction of maximum principal stress. It is important to point out that pore fluid pressure has a significant impact on shear stress, especially where pore fluid pressure approaches lithostatic pressure, which is the normal pressure induced by the weight of the overlying rock.
Subcritical crack growth
Sometimes, it is possible for fluids within the fracture to cause fracture propagation with a much lower pressure than initially required. The reaction between certain fluids and the minerals the rock is composed of can lower the stress required for fracture below the stress required throughout the rest of the rock.
Did You Know?
Hydrogeology (hydro- meaning water, and -geology meaning the study of the Earth) is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust. The term geohydrology is often used interchangeably. Some make the minor distinction between a hydrologist or engineer applying themselves to geology (geohydrology), and a geologist applying themselves to hydrology (hydrogeology).
Requirements: (please do not put your answers in the log)
Now that we learned a little bit about fractures, take a moment to study ground zero. With the information above, and information at GZ you should be able to answer the following questions. You have 24 hours to send me an email (found in my geocaching profile) with the correct answers. Failure to do so will result in the removal of your "Found it log". Please add the GC code and title in the subject line. If you are sending answers for multiple people in your group, please add all the names in your email to ensure they don't have their log deleted :) Photos with you/group of the area would be greatly appreciated.
1 In your own words define how fissures are created.
2 With the information provided at Ground Zero what is coming out of this fissure?
3 Fractures are created by what two types of force?
4 According to the information provided at ground zero what is the source of this Fissure?
5 In your opinion is Natural Hydraulic Fracturing taking place at Ground Zero? Explain your answer.