Igneous intrusions - Introducing Tectonics, Rock Structures and Mountain Belts - page 68

the underworld), and are also divided
into broadly discordant and broadly
concordant types. Broadly concord-
ant plutons include laccoliths and
lopoliths ; these are both roughly
lensoid or disc-shaped, the former
being convex upwards and the latter
convex downwards (Figure 8.1C, D).
Laccoliths are usually around 5-10 km
in diameter, whereas lopoliths are
considerably bigger - up to several
hundred kilometres across. There
probably exist examples of bodies
intermediate between these two types,
or even that vary laterally between
them, so that the distinction may be
rather artificial. Smaller laccoliths
may be indistinguishable from sills.
The largest bodies, known as batho-
liths, have an areal extent of many
tens to hundreds of kilometres. Once
thought to continue down to great
depths in the crust, many have now
been shown to have relatively shallow
roots, although still perhaps several
kilometres in depth. Stocks are a smaller
type of pluton, with a broadly cylindri-
cal shape, and discordant relationships
to the host rocks (Figure 8.1E). Stocks
are typically formed at higher levels of
the crust, whereas those deeper-level
plutons of comparable size that have
been examined in detail have often been
shown to have a shape rather like an
inverted tear drop, as shown in Figure
8.1F; such bodies are known as diapirs .

by the host rocks being actively forced
aside, known as forceful emplace-
ment . To distinguish between these
alternative methods requires an
examination of the structures of the
host rock around the margins of the
intrusion and of the internal struc-
ture of the intrusions themselves.

offshoots, called bayonet structures
(Figure 8.3C, D), that reveal how the
process of intrusion proceeded. Figure
8.2E shows a sequence of events
to explain the observed structure.
Two small magma veins whose ends
overlap are separated by a bridge of
host rock that is subsequently broken
through by the pressure of the magma,
leaving the characteristic bayonet
structure at the margins of the now
broadened dyke; pieces of the broken
bridge may subsequently float into the
magma to be found as inclusions.

Dykes and sills
Most dykes and sills are emplaced by
a process of dilation (expansion) of
the host rocks; that is, the walls of the
intrusion move aside to make way for
the magma ( see Figures 8.2A, B; 8.3B).
This process occurs when the regional
stress field is such that one of the prin-
cipal stress axes, at right angles (or at
a large angle) to the intrusion wall, is
either extensional, or is less than the
lateral pressure exerted by the intrud-
ing magma, as shown in Figure 8.2A.
Sills tend to form at higher levels of the
crust where the least principal stress
(gravity) is vertical (Figure 8.2B), and
dykes rising through the crust may feed
sills, as shown in Figure 8.1A, when they
reach a level where the magma pressure
exceeds the gravitational pressure. Sills
are typically concordant with bedding
in sedimentary rocks but may also
follow faults, as in the example shown
in Figure 8.2C. Here, the sill maintains
a roughly constant depth by alter-
nately following the inclined bedding
and stepping up along the faults.
Careful examination of the walls of
a dyke or sill can reveal small steps or
irregularities that can be matched on
the opposite wall, as shown in Figures
8.2D and 8.3C, from which the direc-
tion of opening of the fissure occupied
by the intrusion can be deduced.
Some dykes exhibit small vein-like

Plugs
These types of intrusion are forcefully
emplaced, usually through fractured
or brecciated volcanic rocks, and are
typically preceded or accompanied
by explosive activity that has blasted
a path through the overlying rock.

Laccoliths and lopoliths
These lens-shaped intrusive bodies are
similar geometrically to sills, only on a
much larger scale, and their emplace-
ment is achieved in the same way, by
dilation of the (usually bedded) host
rocks. Laccoliths are considered to
have formed by the arching up of the
overlying rocks, as shown in Figure
8.1C, in which case the gravitational
load must have been exceeded by the
magma pressure. Lopoliths , on the
other hand, are thought to have formed
by the depression of the floor of the
intrusion (Figure 8.1D) caused by the
weight of the igneous material. Both
laccoliths and lopoliths are typically
formed from basic magmas. Certain
well-known lopoliths are extremely large
bodies; the famous Bushveld complex
of South Africa is about 300 km in
diameter and about 15 km thick, much

Methods of emplacement
As igneous bodies make their way
through the crust, space has to be made
to accommodate them, and this may
be accomplished either by the host
rocks moving aside passively, which is
termed permitted emplacement , or