Introduction

     This website looks at  geological structures such as folds, faults, and joints.  The definition of geologic structure is any feature produced by the deformation of a rock.  A fold is characterized by the bending of a rock.  Faults are fractures along which rock on one side has moved relative to rock on the other side.  A joint is a fracture without movement of rock.  Joints and faults often occur as sets of many parallel fractures.   
 

Folded Structures

    A  fold is a bend in a rock which tends to show up best in layered rocks, although they do occur in unlayered rocks such as granite.  These folds can arch upwards, downwards, or sideways but the effect is always to shorten the crustal rocks.  Folds are usually caused by compressional forces.  Often, oil and natural gas are formed and/or are found within folds.

• ANTICLINE & SYNCLINE FOLDS:  Anticline is a fold that arches upward.  Syncline is a fold that arches downward.  The sides of a fold are called limbs while the backbone is called the axis and occurs where two limbs meet.  There are many types of anticline or syncline folds.  A symmetrical fold is a fold that shows a mirror image on both sides of the axial plane while an asymmetrical fold doesn't have a mirror image.  An overturned fold occurs when the axial plane is tilted and the beds may dip in the same direction on both sides of the axial plane.  An extreme example of an overturned fold happens when the axial plane is horizontal and is called a  recumbent fold.
• MONOCLINE FOLDS:  Monocline is a special kind of fold with only one limb.  This can happen when sedimentary rocks sag over an underlying fault.
• DOMES & BASINS:  Domes are circular or elliptical anticline resembling an inverted bowls.  Sedimentary layering dips away from the center of a dome in all directions. Basins are similar to domes.  They are syncline folds where the layering dips toward the center of the basin.  Domes and basins can be small structures only a few kilometers in diameter but are often much larger.  They result from regional warping of the entire continental crust.

Fault Structures

    A fault is a structure with major displacement of rock material along a crack in a rock.  This can occur when tectonic forces fractures the Earth's crust and rocks on opposite sides move past each other.  The distance that the rocks on opposite sides have moved is called a slip.  Faults can have a single fracture in the rock or numerous closely spaced fractures that are referred to as a fault zone.  Faults can move repeatedly becuase once a fault forms, it's easier to move that fault than to create a new one when tectonic stress is still applied to that region.  Groundwater moves through faults, joints, and fractures and mineral ore deposits often concentrate within faults.  Faults are divided into types based on the relative movement along the cracked rock.  Most faults tend to dip into the Earth at an angle, and therefore have an upper and lower side.  The upper side or roof of the fault is referred to as the hanging wall while the lower side is called the footwall.    

• VERTICAL FAULTS:  Vertical (or dip slip) faults occurs at along the dip of the fault and results in vertical displacement of the opposing rocks.  If the angle of the dip isn't 90 degrees, then the land which appears to be hanging over a lower portion before movement is called the hanging wall.
• NORMAL FAULTS:  Normal faults are when the hanging wall has moved down relative to the footwall.  Normal faults form when tectonic forces stretch the Earth's crust, pulling it apart.  A graben is a wedge-shaped block of rock that drops between a pair of normal faults.  The normal faults around the graben are called horsts.  This resembles two mountains separted by a flat valley.  The mountains are the horsts while the valley in between is the graben.  Normal faults and grabens are typical of divergent boundaries at mid-oceanic ridges and continental rifts where the crust is stretched.  
• REVERSE FAULTS:  Reverse faults occur when the hanging wall has moved up relative to the footwall.  This is due to compressional pressure pushing the crust together and tends to shorten the distance between the crust.  A special kind of reverse fault that is nearly horizontal is called a thrust fault.  It's made of a very low-angle reverse fault. Folds, reverse faults, and thrust faults tend to occure at most convergent plate boundaries where two plates collide.
• HORIZONTAL FAULTS:  Horizontal (or strike slip) faults occur when the fracture is nearly, if not vertical.  This allows the rocks on opposite sides of the fracture to move horizontally past each other.  The infamous San Andreas fault is an example.  A transform plate boundary is a kind of horizontal fault which cuts through the entire lithosphere, often causing folding, faulting and uplifting of nearby rocks.

     A joint is a fracture similar to a fault but without the rocks on opposite sides moving.  They often occur in two sets of cracks intersecting between 45-90 degrees, dividing the rocks into rectangular blocks.  Columnar joints can be formed when hot basalt cools and shrinks.  Parallel joints are caused by tectonic forces sufficient to fracture the rock but not to move it.  Joints are places of weakness in otherwise strong rock.  As such, engineers, miners, and quarry operations tend to be aware of them.  A dam constructed in jointed rock will leak because the water will seep into the joints and flow through the fractures to the side where it's not wanted.    

References

• Dennis, John G.  Structural Geology.  1972.  The Ronald Press Company, New York, New York.
• Thompson, Graham R. and Jonathan Turk.  Earth Science and the Environment.  1995.  Saunders College Publishing: Harcourt Brace College Publishers, Fort Worth, Texas.  Pages 189-198.
• Geologic Structures on the internet.  <http://science.smsu.edu/~mantei/glg110/GeoStruct.html>

       USGS Geological Information