Fractures, faults and earthquakes - Introducing Tectonics, Rock Structures and Mountain Belts - page 42

Moreover, later stress conditions may
cause originally extensional joints to
become shear fractures and vice versa .

A B

Unroofing joints
Another variety of extensional joint is
formed as a result of what is termed
unroofing . This occurs when rock
above the present ground level is
stripped off by erosion, thus reduc-
ing the gravitational pressure on the
rock below. The release of this pres-
sure causes the rock to expand, typi-
cally forming sets of joints parallel to
the bedding in horizontal stratified
rocks, or sub-parallel to the present
ground surface in igneous rocks, as
shown by the example in Figure 5.12A.

Figure 5.12 Joints. A. Joint sets in granite tor, Dartmoor, Devon: the main joint set (due to 'unroofing')
is sub-horizontal and roughly parallel to the ground surface. B. Columnar jointing in a horizontal
basalt flow, Staffa, north-west Scotland; note the vertical joints arranged in a hexagonal pattern and
also the prominent horizontal joints.

strained
zone

wrench
fault
splay

normal
fault
splay

reverse
fault
splay

+

-

-

+

-

+

Cooling joints
Contractional joints are associated
particularly with igneous bodies, where
they have formed as the rock contracts
on cooling. In some cases, these joints
form a set of polygonal columns at
right angles to the cooling surface. The
Giant's Causeway in County Antrim,
Northern Ireland, and the island of
Staffa, west of Mull in north-west Scot-
land, are two well-known examples
of this phenomenon. The columns of
Staffa, formed within a lava flow (Figure
5.12B), are vertical and have hexagonal
cross-sections, like a honeycomb.

-

+

- -

+

+ normal

fault
splay

reverse
fault
splay

wrench
fault
splay

A B C
Figure 5.13 Initiation of a fault. A. A strike-slip fault develops within a strained zone, causing the
formation of compressional (+) and extensional (-) areas at the ends of the active fracture. B. Splay
faults may develop at the ends of the active fracture: normal fault splays in the extensional zones and
reverse fault splays in the compressional zones; alternatively, ( C ) some of the displacement may be
taken up by wrench fault splays.

the shear strength of the host rock
is approached. These preliminary
strain effects cause zones of limited
local expansion in the rock within the
strained zone, which can be monitored
to give some indication of impending
failure. When failure does occur, the
elastic element of the strain is suddenly
released causing an earthquake (Figure
5.13A). The release of strain around
the new fracture causes the stress to
increase at the ends of the new fracture
causing an increase of strain there, and
leading eventually to an extension of

the fracture. As Figure 5.13A shows, the
stress fields at the ends of the fracture
are changed as a result of the shear
displacement because one side of
the fracture will experience compres-
sional stress and the other extensional.
This has the effect of encouraging
new fractures to form at an angle to
the existing fracture plane; these are
termed splay faults (Figure 5.13B, C).
If the initial failure occurs within
an existing fault zone, it may spread
rapidly along it over distances of many
tens of kilometres. The major San

Initiation and propagation of a
fault
Faults are caused by sudden brittle
rock failure after a period of increas-
ing strain in the zone around the site
of the future fracture. Initial elastic
strain is followed by limited perma-
nent strain caused by the opening of
small cracks in the strained zone as