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FAULTING Direction of
Rock layers do not always respond to stress by folding. Rocks near dip of fault Strike of fault
the surface are cooler and under less pressure, so they tend to
be more brittle. A sudden stress on these rocks may reach the
rupture point, resulting in a cracking and breaking of the rock
structure. If there is breaking of rock without a relative displace-
ment on either side of the break, the crack is called a joint. Joints Ore vein
Hanging wall
Footwall
are common in rocks exposed at the surface of Earth. They can be block along fault block
produced from compressional stresses, but they are also formed
by other processes such as the contraction of an igneous rock Hanging
while cooling. Basalt often develops columnar jointing from the wall
contraction of cooling, solidified magma. The joints are parallel Footwall
and evenly spaced, resulting in the appearance of hexagonal col-
umns (Figure 19.8). The Devil’s Post Pile in California and Devil’s A
Tower in Wyoming are classic examples of columnar jointing.
When there is relative movement between the rocks on either
side of a fracture, the crack is called a fault. When faulting occurs,
the rocks on one side move relative to the rocks on the other side
along the surface of the fault, which is called the fault plane. Faults
are generally described in terms of (1) the steepness of the fault
plane, that is, the angle between the plane and an imaginary hori-
zontal plane, and (2) the direction of relative movement. There
are basically three ways that rocks on one side of a fault can move
relative to the rocks on the other side: (1) up and down (called
dip), (2) horizontally, or sideways (called strike), and (3) with ele-
ments of both directions of movement (called oblique).
One classification scheme for faults is based on an ori-
entation referent borrowed from mining (many ore veins are
associated with fault planes). Imagine a mine with a fault plane
running across a horizontal shaft. Unless the plane is perfectly
vertical, a miner would stand on the mass of rock below the
fault plane and look up at the mass of rock above. Therefore,

B
FIGURE 19.9 (A) The relationship between the hanging
wall block and footwall block of fault. (B) A photo of a fault near
Kingman, Arizona, showing how the hanging wall has moved relative
to the footwall.

the mass of rock below is called the footwall and the mass of
rock above is called the hanging wall (Figure 19.9). How the
footwall and hanging wall have moved relative to one another
describes three basic classes of faults: (1) normal, (2) reverse,
and (3) thrust. A normal fault is one in which the hanging wall
has moved downward relative to the footwall. This seems “nor-
A B mal” in the sense that you would expect an upper block to slide
down a lower block along a slope (Figure 19.10A). Sometimes
FIGURE 19.8 Columnar jointing forms at right angles to the
surface as basalt cools. (A) Devil’s Post Pile, San Joaquin River, a huge block of rock bounded by normal faults will drop down,
California. (B) The Devil’s Tower, Wyoming. creating a graben (Figure 19.10B). The opposite of a graben is a

482 CHAPTER 19 Building Earth’s Surface 19-6