Geology Reference
In-Depth Information
Figure 2.12
Igneous xenoliths in granitic melt, Sierra Nevada, USA.
Where two different magmas come together in a juvenile (molten) state a
great variety of textures can result where there are inclusions of one magma
type within another. These can be angular to rounded, wispy and irregular and
show a variety of stages of mixing and mingling together. This process is of
particular importance and will be dealt with in detail in Chapter 9.
2.5 Secondary or Late Stage Outcrop Structures
2.5.1 Joints, faults and shear zones
Joints
are usually the most obvious features of an igneous rock outcrop where
there are several sub-parallel sets of cracks or fractures, varying from a few
centimetres to a few metres apart, known as
joints
. The origins of joints in
intrusive igneous bodies are complex, but may be attributed to (i) the contraction
that occurs when a volume of magma crystallises, producing joints that may
bear a simple geometrical relationship to the intrusion walls and which may,
therefore, be useful in deducing the shape of the body; (ii) post-intrusive regional
tectonic stress, producing joints that usually extend beyond the intrusive body
and may be matched to joint patterns in the surrounding country rocks; and
(iii) expansion due to the removal of the confining pressure of overburden. This
will widen pre-existing joint sets, and possibly increase their numerical density,
