Geology Reference
In-Depth Information
Faults are classified into two major categories based on the primary direction of move-
ment along the fault plane. If the rocks move vertically (up or down) relative to one an-
other, they create a
dip-slip fault.
If they move horizontally relative to one another, they
create a
strike-slip fault.
Dipping and slipping
Dip-slip faults are the result of rocks breaking apart and moving up or down next to each
other along a tilted fault plane. This movement creates two separate blocks of rock: a
hanging wall
above the fault plane and a
footwall
below the fault plane.
The names
hanging wall
and
footwall
for the blocks of a dip-slip fault come
from mining terminology. Important minerals and metal ores are often concen-
trated in veins that follow fault fractures. When miners would set up to work re-
moving these materials from the vein, they hung their lanterns on the rocks above
the vein, or fault (thus, the hanging wall), and they stood at the rocks below the
fault (thus, the footwall).
A
normal fault
is a dip-slip fault where the hanging wall block slips downward in relation
to the footwall; in other words, the two sides of the fault move farther apart as illus-
trated in Figure 9-13. Normal faults like this are common at mid-ocean divergent bound-
