Igneous intrusions - Introducing Tectonics, Rock Structures and Mountain Belts - page 66

Igneous intrusions
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Igneous intrusions

Igneous intrusions occur in a wide
range of sizes and shapes. For the
present purpose, it is useful to subdivide
them into major and minor intrusions,
although there is no clear-cut division
between them. Minor intrusions include
the familiar dykes and sills, which are
typically narrow in width, but may
extend for many kilometres in length.
The major intrusions include very large
bodies, especially of granite, whose
outcrop extends for tens or hundreds of
kilometres in both length and width.
The geological structure of intru-
sions is important in several ways. As
igneous magmas make their way into
and through the Earth's crust, their path
is partly controlled by the structure
of the host rocks, which determines
both the shape and the location of the
resulting intrusive body. Intrusions may
also affect the host rock by imposing
an additional stress field on it. Moreo-
ver, the process of intrusion may also
produce structures, such as planar and
linear fabrics, within the body of the
intrusion itself, as is now explained.

orientation of planar or linear elements
such as crystals or inclusions. These
fabrics will normally be parallel to the
walls or roof of the intrusion and are a
useful guide to the direction of magma
flow. Elongate phenocrysts within still-
liquid magma are often oriented in this
way. The outer parts of an intrusion that
are already solid may become fractured
and injected by a later magma pulse,
which may form intrusive dykes or veins
in the older material, or break off and
incorporate pieces of it as inclusions.
Linear fabrics in dykes are interesting in
providing evidence for the propagation
mechanism. For example, many long
dykes appear to have been propagated
laterally away from their source, since
the direction of magma flow as indi-
cated by the linear fabric is oblique or
even horizontal, rather than vertical.

inclined sheet-like bodies cutting
structurally homogeneous rock such as
granite would also be termed dykes.
Sills, by contrast, are generally con-
cordant with the host rock structure
(Figure 8.1A); for example, they are par-
allel to bedding in sedimentary rocks,
although in detail they may exhibit short
discordant sections, as in the sill shown
in Figure 8.2C. Sills in undeformed or
weakly-deformed bedded rocks are thus
horizontal or gently tilted, and gently-
dipping igneous sheets in structurally
homogeneous rocks would also be
regarded as sills. To summarise, there-
fore, dykes in their undeformed state are
discordant and/or steep, whereas sills
are concordant and/or gently inclined.
Most dykes and sills are in the
range 1-100 m in thickness, although
both thinner offshoots and much
thicker examples also occur. One of
the largest known dykes - the Great
Dyke of Zimbabwe - is up to 11 km
thick and extends for around 600 km.
Plugs are steeply inclined or verti-
cal discordant bodies (Figure 8.1B),
typically with a roughly cylindrical
shape; they are formed in volcanic
complexes as the feeder bodies of vol-
canoes and often occur within volcanic
vents . Plugs vary from a few tens of
metres to around 1 km in diameter.

Classification of igneous bodies
The structural classification of igneous
bodies is based on their size, shape and
structural relationship to their host rock.

Minor intrusions
The main types of minor intrusion
are sheet-like bodies with a long and
narrow outcrop, comprising the dykes
and sills , and tube- or pipe-shaped
bodies, usually known as plugs . Dykes
are distinguished by their discordant
(cross-cutting) relationship to their
host rock (Figure 8.1A). They are also,
typically, steeply inclined, unless sub-
sequently affected by folding. Steeply

Structures formed by magma flow
Since magma bodies cool and solidify
from the outside inwards, the outer por-
tions are frequently solid or partially
solid, while the magma further within
the intrusion is still wholly liquid and
moving relative to the outer parts. This
relative movement may, therefore,
create planar or linear fabrics within
the intrusion, caused by the parallel

Major intrusions
Igneous bodies that are large in all
three dimensions are referred to as
plutons (named after the Greek god of